Biomechanical Strategies during Oldowan and Acheulean Stone Tool Production
by Erin Marie S. Williams
B.A. in Anthropology, 2000, Grinnell College M.A. in Anthropology, 2007, The George Washington University M.Phil in Hominid Paleobiology, 2008, The George Washington University
A dissertation submitted to
The Faculty of The Columbian College of Arts and Sciences of The George Washington University in partial fulfillment of the requirements for the degree of Doctor of Philosophy
May 15, 2011
Dissertation directed by
Alison S. Brooks Professor of Anthropology and Brian G. Richmond Associate Professor of Anthropology
The Columbian College of Arts and Sciences of The George Washington University certifies that Erin Marie S. Williams has passed the Final Examination for the degree of
Doctor of Philosophy as of February 28, 2011. This is the final and approved form of the dissertation.
Biomechanical Strategies during Oldowan and Acheulean Stone Tool Production
Erin Marie S. Williams
Dissertation Research Committee:
Alison S. Brooks, Professor of Anthropology, Dissertation Co-Director
Brian G. Richmond, Associate Professor of Anthropology, Dissertation
Co-Director
Peter W. Lucas, Professor of Anthropology, Committee Member
Adam D. Gordon, Assistant Professor of Anthropology, University at Albany—SUNY , Committee Member
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Dedication
This dissertation is dedicated to three sets of educators:
To my teachers and professors, starting with Geneva Ballard and Anne Miller in the nursery school at church, for my formal education,
To Bella-Boo, Charlie-Choo, John-J, Greggy-Goo and ‘Ria-Roo Sprenkel and C-Bear,
Anna-Banana, Katie-Did, and ‘Tuffer Gass, for my informal education, and
To my mom and dad, Sara Lou and David Williams, for teaching me most of what resides in between the two.
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Acknowledgements
I would like to begin by thanking my primary graduate school advisor, Dr. Alison
S. Brooks. Alison’s Paleolithic Archaeology class was the first class I took in graduate school, and her early assistance and support gave me the confidence to apply to the
Hominid Paleobiology Doctoral Program. To me, Alison is the consummate scientist and role model. She cares a great deal for all of her students and treats everyone in the scientific community with dignity and respect. Alison has walked me through a number of joyful and trying experiences while I have been at GWU, and I do not think it is unwarranted to say that I would have not made it this far, or perhaps even into the program, without Alison’s guidance.
I would also like to thank Dr. Brian G. Richmond, who may have been surprised when he realized that I’d snuck in and stuck myself among his students, as well. Brian is one of the best teachers I have ever had the privilege to study with and I know that he has exercised enormous patient teaching me about biomechanics, anatomy, and physics during my time at GWU, for which I am very grateful. I would also like to thank Brian for always cushioning his critiques with at least one positive statement before getting to the “issues,” and for talking me back from the cliff’s edge when I thought that I had erased NPR’s FTP site and possibly taken down their website, as well.
Dr. Peter W. Lucas served as the third adviser in my advisory trifecta. Peter helped me look at things from unique perspectives and challenged me to investigate the underlying causes governing why systems work as they do. He has consistently been a source of
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encouragement and support and he is the only academic I know that gets as excited about experiments as my father does about life in general. I was sad to see Peter pick up and move to another continent and I will miss his sly jokes and watching him doodle on his tablet.
Dr. Adam D. Gordon has also acted as one of my main advisors since the collection of my pilot data. Adam has an amazing ability to take the most difficult concepts and slice them up into manageable pieces that make sense to me. He also has a mysterious way of guiding students to craft better experiments without dictating protocol from on high, which helped me learn far more than I would have otherwise. I am grateful for the enormous amount of the time he dedicated towards my education and dissertation. I also appreciate
Adam’s good humor and his continual reminders that THIS IS FUN!
The four people listed above formed the core of researchers that helped guide me through the dissertation process. They all read multiple drafts of grants and manuscripts, listened to dozens of podium and poster presentations, and let me find the right answers on my own time. I have been lucky to learn from them and their help has meant a great deal to me.
I would also like to thank Dr. Daniel Schmitt, who served as one of my external committee members. Dan is another teacher that manages to make difficult concepts less intimidating. He has been supportive of me and my research from the early stages, and his excitement made me more excited. Dan has been willing to discuss my questions and to loan me rather expensive equipment on what has turned out to be a long term basis. I am grateful for all that Dan has done for me.
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Dr. Dietrich Stout also served as an external committee member, and helped usher me through the final stages of the process. I am grateful for Dietrich’s thoughtful comments and discussions of my research.
I am also grateful to Dr. Bernard Wood, first and foremost for all of his hard work over the years as the Director of the Hominid Paleobiology Doctoral Program. Bernard is not one to mince words or to leave Ts uncrossed, which means that manuscripts that pass across his desk come out cleaner (notwithstanding the abundance of red ink) and presentations that he previews paint a coherent picture and are free from spelling and grammatical errors. I appreciate the time he takes with all of us making sure that we are
“stage ready.” I am particularly grateful for his help in securing the postdoc I received and for his help at the end of my dissertation process.
I am endlessly grateful to the faculty and students of The Center for the Advanced
Study of Hominid Paleobiology and I am proud to be counted as one of their peers. The people that make up our department support one another personally and professionally in a variety of ways, and they made the long hours far more enjoyable than one may imagine they would be. I would like to thank our faculty and postdocs for all of the instruction they gave me and for making graduate school comfortable and fun, including: Dr. Robin
Bernstein, Dr. Shannon McFarlin, Dr. Chet Sherwood, Dr. Muhammad Spoc(ter), and Dr.
Erin Vogel. I would particularly like to thank Chet and Erin for their helpful discussions regarding cognition and statistics, respectively. I would also like to thank all of my fellow
Hom/Pal students, past and present, for creating an atmosphere free from competition and full of mutual support and respect, including: Jen Baker, Iowa Bauernfeind, Kallista Bernal,
Serena Bianchi, soon-to-be Dr. J-9 Chalk, Habiba Chirchir, Dr. Piz-aul Constantino,
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double-Dr. Rui Diogo, Du-Du Du, Dr. T Faith, soon-to-be Dr. Felicia Gomez, Dr. Deej
Green, Kevin Hatala, Dr. Amanda Henry, Dr. Griff, Dr. Lisa Nevell, Liz Renner, Kestopher
Schroer, Cheryl Stimpson, Dr. Robin Teague, Dr. Matt Skinner and Andrew Zipkin.
Thanks are due in particular to Amy, Kevin, and Tyler for their assistance in all things related to cognition (Amy) and statistical help (Kevin and Tyler), respectively. I am grateful to Deej, J-9, Amanda, and X-tyna for their help in avoiding holiday moments, and acknowledge that many sticky situations were avoided with the help of their proof reading and editing. J, you are clever and have lovely brown eyes.
Before entering the PhD program, I was a Masters student in the Anthropology department at GWU. A number of people made that experience easier and more enjoyable and I thank them for their help and support during that time. They include: Dr. Kitty Allen,
Dr. Robin Bernstein, Dr. Alison Brooks, Dr. Richard Grinker, and Dr. Stephen Lubkemann.
I would particularly like to thank Alison and Robin for their constant support and encouragement and for writing the references that helped me get into the PhD program.
Dr. Rick Potts and Jenny Clark at the Smithsonian National Museum of Natural History were also extremely kind and supportive while I worked towards my MA, and I appreciate all of their help, from the job (and air mattress) they gave me to their tolerance of my odd working hours.
Our program functions as well as it does in large part because we have a lot of support from the larger GWU community. Within the Anthropology department I am grateful to the entire administrative staff, particularly Jonathan Higman, Savannah
Fetterolf, Goby Mann, and Amanda Warner. Hom/Pal students also receive an additional helping hand from the CASHP Administrator, and I feel lucky to have received a great deal
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of help from the four that worked in this position while I was a student in the program:,
Kayla Jarvis, Gaby-Baby Mallozzi, Phillip Williams, and Sophie Thibodeau.
In the larger GWU community, I am grateful to Iva Beatty and Geri Rypkema for all of the administrative help they have given me over the years. I am also extremely grateful to Dr. Tara Wallace and the GW chapter of the Edward A. Bouchet Society
Graduate Honors Society for their support and for the funding which allowed me to complete my dissertation.
Beyond GWU, I have been supported and cared for by a large community of friends and family. They have provided friendship and support for many years now, and in general they make life more joyful. I am extremely grateful for my friends from Greenhills High
School, Grinnell College (even Jeff Brumfield), Memorial Christian Church, Ann Arbor, the GPF, NPR (particularly Jessica Goldstein, Chris Joyce, and Alison Richards) and my new community of friends that live just off of the aptly named One Wild Place on Portland
Street.
I was very fortunate to attend Grinnell College for my undergraduate education, where I majored in Anthropology. Grinnell’s Anthropology faculty made learning fun and exciting and they turned me on to the discipline that has come to mean so much to me. I hope one day I can become the type of teachers and mentors that they all are and I thank them for their early guidance. They include: Dr. Jon Andelson, Dr. Vicki Bentley-Condit,
Dr. Doug Caulkins, Dr. Katya Gibel Mevorach, and Sondi Burnell. I would particularly like to thank Dr. Kathy Kamp and Dr. John Whittaker. Together they have housed me, taught me, fed me, and encouraged over the last 10+ years, and their support continues to mean a great deal to me.
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I am also grateful to the organizations which funded my studies and research, including the Wenner-Gren Foundation’s Dissertation Fieldwork Grant (#7995), the
National Science Foundation’s Doctoral Dissertation Improvement Grant (# BCS-0903652) and Integrative Graduate Education and Research Traineeship (IGERT # DGE 9987590 and # DGE 0801634), The George Washington University’s chapter of Sigma Xi’s Grant in
Aid of Research, The George Washington University’s Research Enhancement Fund, and
The George Washington University’s Selective Excellence Fund.
Lastly, I would like to thank my large and loud family: Mom, Dad, Michael, the
Williams (in Chicago and Omaha), the Sprenkels (in Kansas and Pittsburgh), the Stovers, the Grahams, and the Broujous families.
Acknowledgments by chapter:
Chapter 2:
I wish to thank the knappers that flew in from around North America to participate in this study: Dr. Michael Bisson, Dr. Harold Dibble, Dr. Bruce Huckell, Dr. Grant McCall, Dr.
Dennis Sandgathe, and Merritt Sanders. Maria Pasquale and Susan Diekrager provided training and significant assistance with the Pliance system, for which I am grateful. I wish to thank Dr. Peter Lucas, who provided thoughtful comments regarding force and pressure on this chapter and chapters 4 and 5. I also wish to thank Craig Ratzat at Neolithics.com for his careful selection of raw materials for this project and all that followed. I am also grateful to Dr. Erin Vogel for her help with statistical analyses and suggestions.
Chapter 3.
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I am grateful to the students and volunteers that participated in the pilot studies I conducted, even those who mistook “knapping” for “napping” and thought they had signed up for an afternoon snooze, including: Dr. Jonah Choiniere, Matt Bukowski, Dr. Tyler
Faith, Nic Fourie, Dr. Nicole Griffin, Dr. Amanda Henry, Michael Frank, Nick Lonergan, and Dr. Christian Tryon. I wish to thank Dr. Can Kirmizibayrak for training me to use the
Vicon equipment and his invaluable help throughout my early data collection process. I am grateful to my co-authors, Dr. Brian Richmond and Dr. Adam Gordon, for helping me throughout this early project (and throughout all of the projects which comprise this dissertation) and helping me learn how to put together a manuscript for publication. I also wish to thank Dr. Alison Brooks for her helpful comments and insights on this project and throughout my dissertation.
Chapter 4
I wish to thank the knappers that took part in the experiments described in this chapter and in chapter 5: Dr. Michael Bisson, Dr. Harold Dibble, Dr. Grant McCall, Merritt Sanders,
Dr. Dennis Sandgathe, Steve Schwortz, Dr. John Shea, and Dr. Joanne Tactikos. I am very appreciative of the large chunk of time they all took out of their schedules in order to fly to
DC and participate in my dissertation experiments.
Chapter 5
The same eight knappers that participated in the experiments described in chapter 4 also took part in the experiments described in chapter 5, and again I am grateful for their participation. I am also grateful to Dr. Herzl Chai and Dr. Brian Lawn for all of the discussions we had regarding fracture mechanics and their help in applying their work to stone tool manufacture. I wish to thank Dr. Mark Reeves for his help and guidance in
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figuring out how to calculate strike force, and for allowing me to practice a portion of my defense on his unwitting physics class. Dr. Dave Braun and Wesley Flear helped with the lithic analysis, for which I am grateful and I look forward to future collaborations as we wade through the massive experimental collection together. Lastly, I would like to thank
Dr. Chet Sherwood and Amy Bauernfeind for their help with matters relating to cognition.
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Abstract of Dissertation
Biomechanical Strategies during Oldowan and Acheulean Stone Tool Production
Multiple hominin species used and/or produced stone tools (e.g., Australopithecus afarensis, Paranthropus robustus, Homo habilis), yet evidence suggests that only later
Homo (i.e., H. erectus sensu lato) intensified and developed the behavior. This difference has been attributed to later Homo’s ability to execute efficient tool production, to the exclusion of earlier hominin species. However, we lacked the data on upper limb motions needed to evaluate the biomechanical context of stone tool production. With this in mind, the goal of this dissertation was to investigate the kinematic strategies used by modern humans in the production of Early Stone Age stone tools in order to test the primary hypothesis that modern humans’ upper limb condition contributes to efficiency and accuracy during stone tool production.
My collaborators and I used high-speed 3-D motion capture technology and a high- speed manual pressure sensor system to capture some of the only quantitative data on knapping kinematics, and the only quantitative data on manual pressure distribution during stone tool production presently available. The data and conclusions produced during this dissertation document the upper limb motions employed during Oldowan and Acheulean stone tool production. In doing so, my collaborators and I have 1) provided evidence
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against hypotheses directly linking the derived pollical condition to stone tool manufacture;
2) demonstrated that knappers employ a common kinematic strategy that has proven to be energetically efficient in a variety of contemporary activities; 3) support the hypothesis that modern humans exploit the upper ranges of their wrist extension ranges during knapping and in doing so achieve greater accuracy and efficiency; and 4) provided evidence that large-scale motion sequences (e.g., sequence of force application) rather than small scale motion sequences (e.g., sequence of joint motions) contribute to greater right hemisphere activity during Acheulean handaxe manufacture compared with Oldowan flake production.
This dissertation and the data collected in its course represent another step towards understanding the manner in which modern humans produce stone tools and the relationship of our upper limb anatomy to this developmentally significant behavior.
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Table of Contents
Dedication iii
Acknowledgements iv
Abstract of Dissertation xii
List of Figures xvi
List of Tables xviii
Chapter 1: Introduction 1 The Oldowan and Acheulean stone tool industries 2 Early hominin upper limb anatomy and stone tool production 12 Dissertation goals and hypotheses 20
Chapter 2: Manual pressure distribution during Oldowan stone tool production 23 Abstract 23 Introduction 24 Methods 27 Results 32 Discussion 33
Chapter 3: Upper limb kinematics and the role of the wrist during stone tool production 56 Abstract 56 Introduction 58 Methods 63 Sample 63 Raw materials 63 Motion capture 63 Data analysis 65 Results 68 Upper limb motion patterns 68 Wrist extension/flexion and radial/ulnar deviation patterns 70 Work production 71 Discussion 71
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Conclusion 76
Chapter 4: Achieving accuracy and efficiency during Oldowan stone tool production 89 Abstract 89 Introduction 91 The proximal-to-distal joint sequence and muscular efficiency 92 Strike accuracy 94 Methods 95 Sample 95 Motion capture 96 Kinematics and lithic analysis 98 Results 101 The knapping swing 101 Joint angles 102 Joint motion initiations 102 Peak velocity 104 Timing of peak joint angular velocity 104 Braced joint angular velocity and knapping accuracy 105 Discussion 105 Muscular and interactive torque 108 Peak joint velocity 109 Contributions of wrist extension 110 Conclusion 113
Chapter 5: Acheulean and Oldowan knapping strategies 132 Abstract 132 Introduction 134 Upper limb kinematics 138 Methods 139 Sample 139 Motion capture 141 Kinematics analysis 142 Lithic analysis 146 Results 147 Standard swings 147 Trimming swings 150 Acheulean v. Oldowan knapping swings 152 Discussion 154 Conclusion 161
Chapter 6: Conclusion 186
Literature cited 193
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List of Figures
Chapter 2 figures 45 Figure 2.1, Oldowan bifacial chopper produced by Subject E 45 Figure 2.2, Relationship between angular acceleration (m/s2) at the wrist and peak normal force 46 Figure 2.3a, Peak normal force (N) 47 Figure 2.3b, Peak pressure (kPa) 48 Figure 2.3c, Normal force (N) at strike 49 Figure 2.3d, Pressure (kPa) at strike 50 Figure 2.4a, Impulse 51 Figure 2.4b, Pressure-time integral 52 Figure 2.5a&b, Peak normal force (N) and pressure (kPa) during up-swing 53 Figure 2.5c&d, Peak normal force (N) and pressure (kPa) during pre-strike down-swing 54 Figure 2.5, e&f, Peak normal force (N) and pressure (kPa) during post-strike down- swing 55
Chapter 3 figures 84 Figure 3.1, Placement of reflective markers on subjects’ dominant hand 84 Figure 3.2, Vertical position and vertical velocity of the olecranon process, radial styloid process, and second metacarpal head through a typical knapping cycle 85 Figure 3.3, Lateral and dorsal views of the forearm with a depicture of extension/flexion and radial/ulnar angles 86 Figure 3.4, Model of the forearm with the radial styloid process, the second metacarpal head, and their respective paths during a typical knapping cycle 87 Figure 3.5, Total extension-flexion and radial-ulnar excursions 88
Chapter 4 figures 125 Figure 4.1, Oldowan bifacial choppers produced under unbraced knapping circumstances 125 Figure 4.2, Points used to calculate angles at the elbow joint 126 Figure 4.3, Wrist angles through one knapping swing, moving through the dart thrower’s arc 127 Figure 4.4, 3-D model of the upper limb through one knapping swing 128 Figure 4.5, Change in wrist motion and vertical position of the RSP and MC II head at the end of up-swing through down-swing 129 Figure 4.6, Peak wrist flexion velocity and peak elbow extension velocity 130
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Figure 4.7, Elbow angle and angular velocity, and wrist angle and angular velocity through the knapping cycle 131
Chapter 5 figures 182 Figure 5.1, Experimental handaxes produced by Subjects A, B, C, and D 182 Figure 5.2, Wrist angles through one swing, moving through the dart-thrower’s arc 183 Figure 5.3, Oldowan and Acheulean strike forces 184 Figure 5.4, Strike force derived from lithic and kinematic data 185
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List of Tables
Chapter 2 tables 38 Table 2.1, Cumulative peak force (strike) in relation to peak angular acceleration 38 Table 2.2, Peak normal force (N) and pressure (kPa) 39 Table 2.3, Normal force (N) and pressure (kPa) at strike 40 Table 2.4, Impulse and kPa-time integral 41 Table 2.5, Peak normal force and pressure during up-swing 42 Table, 2.6, Peak normal force and pressure during pre-strike down-swing 43 Table, 2.7, Peak normal force and pressure during post-strike down-swing 44
Chapter 3 tables 78 Table 3.1, Wrist joint limits of Pan, Gorilla, Pongo, and Homo 78 Table 3.2, Timing of peak linear velocities (m/s) in the upper limb relative to the transition from up-swing to down-swing and strike 79 Table 3.3, Peak linear velocities (m/s) in the upper limb during down-swing 80 Table 3.4, Percent of total wrist excursion employed during knapping 81 Table 3.5, Timing of peak extension, angular velocity (m/s) and angular acceleration (m/s2) at the wrist 82 Table 3.6, Work production (J) at the second metacarpal head and the radial styloid process 83
Chapter 4 tables 115 Table 4.1, Muscular-induced maximum joint angles 115 Table 4.2, Strike accuracy 116 Table 4.3, Peak knapping angles (⁰) and total excursion ranges 117 Table 4.4, Correlation between wrist extension/radial deviation and flexion/ulnar deviation 118 Table 4.5, Initiation of elbow extension and wrist flexion relative to strike 119 Table 4.6, Peak segment endpoint linear velocities 120 Table 4.7, Peak angular velocity at the elbow and wrist 121 Table 4.8, Timing of peak linear velocities 122 Table 4.9, Timing of peak angular velocities relative to strike 123 Table 4.10, Flake production rate distribution between unbraced and braced knapping conditions 124
Chapter 5 tables 163
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Table 5.1, Upper limb masses (kg) and lengths (mm) 163 Table 5.2, Maximum handaxe length, width, and thickness (mm) 164 Table 5.3, Standard and trimming knapping swing count and associated flake count 165 Table 5.4, Timing of segment endpoint transition to down-swing motion direction 166 Table 5.5, Timing of initiation of elbow extension and wrist flexion relative to strike 167 Table 5.6, Correlation between wrist extension/radial deviation and flexion/ulnar deviation 168 Table 5.7, Peak knapping angles during standard and trimming Acheulean swings 169 Table 5.8, Post-strike wrist extension 172 Table 5.9, Peak linear velocities (mm/s) at segment endpoints 173 Table 5.10, Peak angular velocity (⁰/s) at the elbow and wrist 174 Table 5.11, Comparison between peak angular velocities (⁰/s) at the elbow and wrist between knapping positions 175 Table 5.12, Timing of peak angular velocities (⁰/s) at the elbow and wrist relative to strike 176 Table 5.13, Timing of the initiation of elbow extension and wrist flexion relative to the transition to down-swing 177 Table 5.14, Timing of peak angular velocity (⁰/s) at the wrist and elbow relative to the transition from up-swing to down-swing 178 Table 5.15, Timing of standard Oldowan and Acheulean joint initiations at the elbow and wrist 179 Table 5.16, Peak angular velocities (⁰/s) at the elbow and wrist during standard Oldowan and Acheulean knapping swings 180 Table 5.17, Comparison of the timing of peak angular velocity (⁰/s) at the wrist and elbow relative to the transition to down-swing 181
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Chapter 1: Introduction
No other species on the planet uses technology to the same extent as modern
humans. From processing our foods to cleaning those foods from our teeth; from
scribbling equations on a piece of paper to using mathematics to send people into space;
technology is an integral component of countless human activities across the world.
Despite its ubiquity in modern human cultures, our relationship with technology has rather humble beginnings. More than 2.6 million years ago our early human ancestors began creating sharp edged implements by knocking stones together, sometimes removing no more than 3 flakes from a core (Kimura, 1999). Thus begins technology in the archaeological record.
The simplicity of early stone tools belies their importance to early humans’ success. Stone tool behaviors (e.g., tool production and use) are widely regarded as a significant adaptation in the evolution of our species. These behaviors provided early hominins with significant advantages over contemporary competitors, such as increased access to high-quality foods and defensive implements (Schick and Toth, 1993; Plummer,
2004; Braun et al., 2010; McPherron et al., 2010). These advantages, in turn, may have accelerated a series of further adaptations that culminated in the emergence of our own genus, Homo (Aiello and Wheeler, 1995).
Numerous researchers have hypothesized that stone tool production was a major selective pressure inducing changes in upper limb anatomy from the primitive, ape-like condition to the derived condition of anatomically modern humans (AMH) (Napier,
1962a; Susman, 1994; Marzke, 1997; Tocheri et al., 2008). Yet virtually no data on hand
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and upper limb biomechanics during stone tool production are available for evaluating
these hypotheses. This dissertation is an investigation into the upper limb’s
biomechanical strategy used during stone tool production, undertaken to test hypotheses
relating hand and arm structure and function to stone tool production. It examines the upper limb motions used during the production of Oldowan and Acheulean stone tools through the lenses of functional anatomy, archaeology, and fracture mechanics to test the primary hypothesis that the modern upper limb anatomy found in later Homo plays a key
role in efficient stone tool production.
The remainder of this Introduction focuses on two main topics: 1) Oldowan and
Acheulean tools and their context and 2) the upper limb anatomy of Oldowan- and
Acheulean-era early hominins as it relates to stone tool manufacture. Next, the chapters that make up this dissertation are briefly described, as well as the specific hypotheses that will be tested in each chapter.
The Oldowan and Acheulean stone tool industries
The Oldowan and Acheulean stone tool industries make up the two primary industries of the Early Stone Age [2.6 – 0.3 mya (McBrearty and Brooks, 2000; Semaw,
2000)]. Although it is possible that hominins used and/or made stone implements prior to the known dates of the Oldowan (Panger et al., 2002), and recently discovered 3.4 million
year old cut marked bones from Dikika, Ethiopia may provide further support for this
hypothesis (McPherron et al., 2010, but see Dominguez-Rodrigo et al., 2010 for a
contrary interpretation of the specimens) , pre-2.6 mya tools have not yet been
discovered.
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The Oldowan Industrial Complex is named for Tanzania’s Olduvai Gorge, where
large cores, chopping tools, and flakes were excavated by Mary and Louis Leakey starting in the mid 1930s (Leakey, 1971). The earliest Oldowan assemblages, dated to ~
2.6 mya, occur in Ethiopia’s Awash Valley in the Gona River drainage area of the Hadar
Region (Semaw et al., 1997). The industry continued throughout East and Southern
Africa until 1.7 – 1.6 mya when the Developed Oldowan and Acheulean industries
became dominant (see Klein, 2009 for a thorough review of Oldowan characteristics and
chronology). Homo habilis [2.3 – 1.6 mya (Kimbel et al., 1997)] is widely regarded as having made Oldowan tools (Leakey et al., 1964). However Paranthropus boisei [2.3 –
1.4 mya (McHenry and Coffing, 2000)] and P. robustus [1.9 – 1.4 mya (McHenry and
Coffing, 2000)] were also contemporaneous with the Oldowan, and fossils from both species have been found in the same stratigraphic layers as Oldowan tools, making it difficult to rule out their contribution to Early Stone Age assemblages (Leakey et al.,
1964; Brain, 1988).
Though conceptions of the character of the Oldowan industry vary, with varying levels of complexity and cognitive ability attributed to assemblages and their makers
(Wynn, 1981; Roche et al., 1999; Semaw, 2000), its main tenets are commonly agreed upon. Adherence to a final flake or core form does not appear to have been a goal of
Oldowan tool-makers; artifacts are informal and generally lack standardization (Klein,
2000; Wynn, 2002). Despite this, broadly defined tool forms are consistently present in
Oldowan assemblages. Manuports (transported but non-modified stones), hammerstones
(stones used in percussive activities, mainly to remove flakes from other stone pieces),
and altered cores and flakes make up the classic Oldowan tool kit (Leakey, 1971; Isaac,
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1977). Mary Leakey developed one of two typologies commonly employed to describe the flake and core forms seen in Oldowan assemblages, categorizing them as scrapers, choppers, proto-bifaces, bifaces, discoids and spheroids (Leakey, 1971). Alternatively, cores and flakes may be categorized simply as whole, fragmented, retouched, or other non-functional terms to avoid the undue imposition of function onto form (Isaac, 1974).
Although Oldowan tools are frequently named and categorized according to function
(e.g., scrapers and choppers), experimental studies have demonstrated that these subjective categorizations are largely unwarranted (Toth and Schick, 2009). Instead, much of the variability that is found in Oldowan assemblages exists simply as an artifact of flake manufacturing processes (Toth, 1985).
The Oldowan is generally viewed as an opportunistic tool strategy in terms of reduction processes and raw material procurement. It is regarded as mainly static in terms of tool form and production complexity throughout the course of its history (Isaac and Harris, 1997; Kimura, 2002; Semaw et al., 2009). However, some researchers oppose the stasis hypothesis on the basis of assemblage variability, the diverse species of probable Oldowan tool-makers, and curation and production variability (Roche et al.,
1999; Delagnes and Roche, 2005). Still more researchers have demonstrated that a number of assemblages show evidence of the selective preference for higher quality raw materials. For instance, hominins at Gona, Ethiopia preferentially selected fine grained materials over those with a coarser grain (Semaw et al., 2003), the makers of the assemblages in the Koobi Fora Formation in Kenya avoided materials with internal flaws
(Toth, 1982) and hominins at Kanjera South, Kenya showed a preference for more durable materials (Braun et al., 2009).
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The primary production techniques utilized by Oldowan hominins were hard-
hammer percussion, bi-polar reduction, the anvil technique, and a simple throwing
method. Of the four, hard-hammer percussion was the most commonly relied upon
(Schick and Toth, 1993; Toth and Schick, 2009). During hard-hammer percussion a smaller, rounded stone is held in the hammer hand (i.e., dominant hand) and struck against a core to produce either a sharp edge on the core itself or sharp edged flakes that come off of the core. Trimming, in which small flakes are removed from the edge of a larger flake or core in order to shape the core or to set up a platform for the removal of a flake, was not as frequently applied until the Developed Oldowan or the Acheulean
(Clark, 1994).
Regardless of whether one adheres to the stasis hypothesis or believes that tool practices changed during the Oldowan, it is clear that the Oldowan marks a dramatic shift in hominin behavior which resulted in the earliest record of human material culture.
Stone tools recovered from Oldowan sites demonstrate that the makers held at least a
basic understanding of rather complex fracture mechanics. Mechanically, successful
flake production requires the application of a force sufficient to induce material failure in
the contact region and propagate the resulting crack through the material without causing
shattering or crushing. The nature of crack propagation, flake formation, and flake
morphology are governed by aspects of the nodule itself and the manner in which it is
struck (Dibble and Whittaker, 1981; Cotterell et al., 1985; Cotterell and Kamminga,
1987; Dibble and Pelcin, 1995; Dibble, 1997; Pelcin, 1997b; Pelcin, 1997a; Andrefsky,
1998). Oldowan-era hominins were able to exploit platform angles and platform depths
and apply an appropriate level of force to the core in order to remove a series of flakes; a
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feat which novice knappers can attest should not be underestimated. Though perhaps
unable to attain a specific flake size or shape, Oldowan hominins clearly understood the principles necessary to produce flaked implements.
It is highly likely that stone tool manufacture and/or use occurred to some degree
before 2.6 mya, particularly in light of stone tool use among extant chimpanzees, our
closest living relatives (Panger et al., 2002). Chimpanzees across Africa exhibit a variety
of tool behaviors, including the use of hammerstones to crack open nuts, and tool cultures differ between the populations (Whiten et al., 1999). Additionally, evidence of chimpanzee stone tool use dates to at least 4,300 years ago (Mercader et al., 2007). This date may well be extended in light of the emergence of a new sub-discipline focusing on primate archaeology. Given the existence of stone tool behaviors in both humans and chimpanzees, it is likely that the last common ancestor (LCA) of the two also participated in stone tool related behaviors (Haslam et al., 2009). However, there is no doubt that the
Oldowan at a minimum indicates the intensification of a series of novel behaviors in early humans’ adaptive regime, including the use of direct hard hammer percussion to fashion stones into tools, the long-distance transport of raw materials, and the inclusion of meat into the hominin diet (Toth and Schick, 2009). Stone tools became so prevalent and so significant to hominin subsistence strategies that it is reasonable to hypothesize that early humans would have shown morphological adaptations towards stone tool behaviors
(Napier, 1962a; Napier, 1962b; Susman and Creel, 1979; Susman, 1994; Marzke, 1997;
Richmond and Strait, 2000; Tocheri et al., 2007). Yet data on the biomechanical context of stone tool production has largely been lacking, making it difficult to evaluate the numerous hypotheses that have been erected. Using manual pressure pads and 3-D
6
motion capture technology, this dissertation research generated such data to investigate whether some of the modern anatomical features identified as adaptations towards stone tool manufacture in fact contribute to efficiency and/or accuracy during stone tool production.
The Oldowan was followed by the Acheulean Industrial Tradition, which spanned
1.9 – 0.3 mya (Asfaw et al., 1992; Deino and McBrearty, 2002). The Acheulean is found beyond the researches of the Oldowan in Europe, India, and arguably in East Asia, as
well (Klein, 1999; Yamei et al., 2000). The Acheulean is recognized as the industry of
Homo erectus sensu lato [1.9-0.5 mya (Swisher et al., 1994; Swisher et al., 1996)] and H.
heidelbergensis [600 – 250 kya (Clark et al., 1994; Parés et al., 2000)], and it marks the
dominance of the biface in archaeological assemblages. Acheulean-era bifaces include
picks, cleavers, and handaxes. All of these forms are implements commonly made on
large flakes, 30 cm or more in length. The tools themselves were frequently between 10
cm to 17 cm long (Ambrose, 2001). The ability to produce flakes of this size is regarded
as one of the primary characteristics distinguishing the Acheulean from the Oldowan
(Isaac, 1975; Klein, 1999).
The handaxe, a bifacial implement which has been flaked over all, or most, of the
surface on both sides, is the iconic tool of the Acheulean. The reduction technique used
to create handaxes produces a sharp edge around the entire perimeter of the tool.
Handaxes are generally classified in shape as ovates, teardrops, triangular, or sub-
triangular (Clark, 1954; Leakey, 1971; Clark, 2001). During the early Acheulean
handaxes tended to be crude and thick, distinguished from preceding Oldowan tools
mainly by the extent of flaking across both faces of the implement. However, by the end
7 of the tradition Acheulean handaxes were remarkably thin and flat and they had reached a high degree of sophistication and symmetry (Delson et al., 2000; Klein, 2000). At
Olduvai, Acheulean sites were further distinguished from Oldowan sites by a 40% minimum representation of bifaces among all of the tool types (Leakey, 1971).
In addition to the manufacture of large flakes for the production of bifaces, the
Acheulean, particularly the later stage, also ushered in the use of soft hammer percussion, platform preparation, and prepared core techniques such as Levallois and Kombewa
(Klein, 2000). As their name implies, soft hammers are implements that deform much more readily than the hammerstones commonly used in Oldowan stone tool production because of the materials from which they are made Platform preparation is the intentional shaping of the striking surface in order to facilitate removal of long and wide or thin flakes (Schick and Toth, 1993; Whittaker, 1994). Prepared core knapping techniques were used to produce a flake of a specific size and shape. In attaining this goal, a series of smaller flakes are systematically removed from the perimeter and both surfaces of a bifacial core. The final flake, removed after a striking platform is carefully prepared at one end, generally encompasses much or all of one surface of the core (Van
Peer, 1992).
Whereas Oldowan tools are frequently regarded as largely arbitrary in form,
Acheulean bifaces are thought to reflect preplanning and the use of high-level operational skills in order to obtain an end product of a specific size and shape (Wynn, 1979; Toth and Schick, 1993; Ambrose, 2001; de la Torre et al., 2003; Pelegrin, 2005). This, in combination with the increased cranial capacity and generally more modern body form of
H. erectus sensu lato (Wood and Collard, 1999a; Wood and Richmond, 2000) has lead
8 many researchers to assign greater cognitive abilities to Acheulean-era hominins. Others, however, view the assignment of increased cognitive capacity to Acheulean hominins on the basis of standardization and symmetry as premature (Silverman, 2002; Simão, 2002;
McNabb et al., 2004). Although it is difficult to determine whether Acheulean implements reflect a greater cognitive capacity per se, from a production point of view there are clear differences between the two industries.
Oldowan tools are frequently described as “crude,” “blocky,” or “irregular,”
(Leakey, 1971; Klein, 2000), whereas the Acheulean, particularly the later Acheulean, is described as “complex,” “standardized,” and “finely flaked,” (Clark, 1966; Wynn, 1979;
Ambrose, 2001; Noll and Petraglia, 2003). These descriptions seem warranted when one considers that bifacially flaked Oldowan implements frequently have no more than eight to eleven removals (Kimura, 1999; Roche et al., 1999), while Acheulean bifaces pass through three to four production stages, use hard and soft hammer percussion, and that it is not uncommon to remove 50 or more flakes in the production of a single tool (and this says nothing of the smaller flakes that are removed in the trimming and shaping process)
(Newcomer, 1971; Whittaker, 1994). Despite these clear differences, it remains difficult to determine the impetus for the Oldowan – Acheulean transition, particularly given the co-occurrence of the two industries at sites such as Olduvai Bed II and throughout the
Middle Awash (Leakey, 1971; Schick and Clark, 2003). Researchers have attributed the transition to invading populations of hominins, differences in activities, and raw material availability, among other reasons (Clark, 1978; Kimura, 1999; Schick and Clark, 2003).
Yet these explanations are somewhat unsatisfying because they fail to address the underlying reason for the transition. For example, if the shift is evidence of invading
9
hominin populations, did one population possess refined motor skills compared to the
other? If the two industries represent different activities, what were those activities and
how was one implement more appropriate than another for the task at hand?
Research recently conducted by Stout and Chaminade (2007) and Stout and
colleagues (2008) has begun elucidating some of the underlying causes behind the
transition. These researchers demonstrated that among modern human knappers the right
hemisphere of the brain was significantly more active during Acheulean tool production
compared with Oldowan tool production, including the right ventral premotor cortex, the right inferior prefrontal cortex, and the right supramarginal gyrus of the inferior parietal lobule. The authors attributed this greater right hemispheric activity to greater sensorimotor control, task-set switching, the inhibition of action and the regulation of complex action and action sequences. In the making of a bifacial handaxe, the success of
subsequent removals is determined by the patterns of flake scars created from the
preceding removals. This means that a misplaced flake removal has the ability to hinder
or arrest further reduction, potentially resulting in a failed attempt to produce a handaxe.
Consequently, the location, size, and shape of each flake must fit into the overall plan for
the entire tool and the knapper has to adjust his or her knapping strategy as the
morphology of the core changes.
Noting previous hypotheses linking language and tool use, Stout and colleagues
(2008) argued that their finding lent further support to these connections based on the right hemisphere’s contribution to the execution of hierarchical behavioral sequences and inhibition. The authors concluded that language skills and complex tool-making co-
10
evolved, and that their concurrent development reinforced the underlying processes of
both behaviors.
Faisal and colleagues (2010) compared hand postures and hand joint sequences in
the left hand between Oldowan and Acheulean tool manufacture, to determine whether
the increased right hemisphere activity simply reflected greater motor complexity in the
left hand. They found that no differences existed in the complexity of hand postures, and
concluded instead that greater right hemisphere activity during Acheulean stone tool
manufacture must reflect greater demands for cognitive control of action, task-set
switching, and action inhibition.
The control and inhibition of action sequences may refer to large scale actions,
such as alternating high force and low force hammerstone strikes, or small scale actions,
such as the sequence of joint activation in the course of executing a single knapping
swing. If Acheulean reduction sequences call for greater control of action and the
regulation of more complex action sequences on a large or small scale, these differences
may be evident in the upper limb kinematics. To investigate this issue, kinematics of the
dominant arm during Acheulean and Oldowan reduction sequences were captured and their actions and action sequences were compared using 3-D motion capture technology.
3-D motion capture records both large scale and more refined upper limb motions, making this technique particularly appropriate to investigate the issue of motion sequences and complexity.
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Early hominin upper limb anatomy and stone tool production
Since the recovery of the OH 7 (H. habilis) hand from Olduvai Gorge, we have known that stone tool manufacture did not require a modern hand (Napier, 1962b). There are now multiple lines of evidence demonstrating that the primitive upper limb anatomy was sufficient for stone tool production of Oldowan-like flakes and cores (Napier, 1962b;
Leakey et al., 1964; Marzke, 1997; Tocheri et al., 2003). For example, Kanzi, a male bonobo chimpanzee, has been making stone tools for more than two decades. The upper limb anatomy of the LCA is commonly hypothesized as closely approximating the condition of extant chimpanzees, making Kanzi an appropriate living analog for the tool- making capabilities of the LCA (Tocheri et al., 2008: and references within). Although the flakes and cores that Kanzi has produced are distinct from Oldowan tools, and his preferred style of knapping is quite different than that of modern humans, there is no doubt that Kanzi is capable of manufacturing stone implements (Toth et al., 1993; Schick et al., 1999). Similarly, Homo florensiensis [~ 74 – 14 kya (Brown et al., 2004)] made
Oldowan-like tools despite retaining a primitive wrist morphology (Morwood et al.,
2004; Tocheri et al., 2007).
However, there is a large gulf between mere ability and behavioral efficiency
(Marzke and Marzke, 2000), particularly in the context of competition among species.
This dissertation investigates that gulf, by testing whether aspects of the modern upper limb condition contribute to accuracy and efficiency during stone tool production.
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Manual pressure distribution during stone tool manufacture
We currently lack fossil evidence of the LCA between chimpanzees and humans1.
However, as stated above, the inferred condition of the LCA’s hand and wrist is quite similar to those of a modern chimpanzee. The LCA is hypothesized to have had a high finger to thumb length ratio, curved manual phalanges with strong flexor muscle markings, narrow apical tufts, and a gracile pollical metacarpal (Tocheri et al., 2008: and references within). It is likely that the LCA’s carpals were oriented similar to chimpanzees’ to better withstand the compressive loads experienced during locomotion.
Such loads are transmitted through MCs 2-4 to the carpal region and up into the forearm
(Tocheri et al., 2005; Wunderlich and Jungers, 2009). In contrast, modern humans’ carpals are oriented such that they are better able to withstand transverse loads that are transmitted across the MC I-trapezium joint through the scaphoid, such as occur during strong pinching activities (Momose et al., 1999). High compressive loads are also thought to occur across the first carpometacarpal joint during stone tool production and the modern configuration has been cited as an evolutionary method for withstanding wear in this region. The ability to resist loads along the pollex is viewed as so integral to a species’ ability to sustain stone tool behaviors that Susman (1994) proposed that the thumb’s structure was selected specifically to resist loads sustained during strong repetitive percussive activities such as stone tool manufacture. He further proposed that a single feature describing the thumb’s robusticity (the ratio of metacarpal I head width to metacarpal I length) is sufficient to detect human-like tool making abilities in fossil hominins.
1 In fact, despite a rich hominin fossil record, the chimpanzee fossil record consists of only three teeth from the Kapthurin formation in Kenya. McBrearty S, and Jablonski NG (2005) First fossil chimpanzee. Nature 437:105-8. 13
Two groups of researchers have indirectly investigated Susman’s (1994)
hypothesis by looking at pollical musculature recruitment during hard hammer
percussion2. Hamrick and colleagues (1998) reported high levels of muscle activity in
the flexor pollicis longus (FLP), the largest thumb muscle, during stone tool production
and use, indicating its important role in stabilizing the thumb and load resistance.
However, Marzke and colleagues (1998) captured data on a wider range of hand and
forearm muscles and reported varied FLP recruitment during their tool replication
experiments, with the lowest recruitment levels in the most experienced knapper. Noting
that the FLP is particularly susceptible to fatigue during strong pinching activities, the
authors hypothesized that Oldowan tool makers would have avoided those grips requiring
strong FLP activity to circumvent this problem. Surprisingly, Marzke and colleagues
(1998) reported that three of the five most heavily and consistently recruited muscles
were not directly related to thumb movement or stability, calling into question whether
the thumb does indeed experience larger forces compared to the other digits. However,
neither study directly tested whether the thumb is subjected to greater loads compared
with the other digits of the hand during stone tool manufacture.
Analyses of the carpal joint surfaces of fossil hominins have yielded further
surprising results regarding which hominins were anatomically committed to stone tool
production. H. habilis, the “handy man,” has long been regarded as a tool maker, and
was once touted as the very first (Napier, 1962b; Leakey et al., 1964; 2003). However,
2 Hamrick and Inouye (1995) also tested Susman’s (1994) hypothesis by expanding the hominoid sample set to include gorillas. In doing so they found that the range of pollical metacarpal head breaths recorded among gorillas overlaps those recorded for modern humans. Additionally, the ratio of pollical metacarpal head breadth to length recorded in mountain gorillas overlaps that of modern humans more than that of chimpanzees. The authors concluded that because all taxa possessing a wide pollical metacarpal head relative to length do not used a precision grip or make stone tools, the feature is not a reliable test for tool use in the fossil record. Hamrick MW, and Inouye SE (1995) Thumbs, tools, and early humans. Science 268:586-7; author reply 9. 14
Tocheri and colleagues (2003) demonstrated that the shape and orientation of the trapezium of A.L. 333-80 [Australopithecus afarensis, 4.18-3.0 mya (Kappleman et al.,
1996; Wood and Richmond, 2000)] is more similar to that of modern humans, while the trapezium of O.H. 7 is more similar to gorillas. The authors concluded that A. afarensis would have been capable of executing modern human-like pad-to-side and three-jaw chuck grips, and with greater force than chimpanzees. H. habilis, on the other hand, would have been required to knap in a manner distinct from modern humans. Yet neither species would have been able to resist pollical joint wear—which is assumed to arise from hard hammer percussion—as well as H. sapiens (Tocheri et al., 2007).
Despite studies indirectly testing whether the thumb is subject to high pressures during stone tool production and others that evaluated its ability to withstand this, the pressure distribution across the hand during such production is still unclear. This significant gap in our knowledge of tool production parameters has made it difficult to evaluate hypotheses relating pollical metacarpal and carpal shape and orientation to stone tool production. This dissertation addresses this issue by capturing manual pressure data during the production of Oldowan-like chopping tools. Pressure and normal force data were collected from six experienced stone tool producers at 200 Hz in order to test the hypothesis that during the production of Oldowan stone tools the thumb is subjected to significantly greater normal force and/or pressure compared to other regions of the hand, particularly along the remaining four digits.
15
The contributions of wrist extension to stone tool production
Cut marked bones recovered from Dikika, Ethiopia have been loosely associated with A. afarensis, making this early hominin the first that is in any way potentially associated with stone tool behaviors (McPherron et al., 2010). However, the stone tools themselves are currently lacking and the validity of the cut marks have been questioned
(Dominguez-Rodrigo et al., 2010). Whether the lack of associated tools is merely absence of evidence as opposed to evidence of absence is not yet clear. Researchers do, however, believe that if A. afarensis participated in stone tool behaviors, the species’ mixture of primitive and derived upper limb features would have restricted their knapping motions and rendered them perhaps less efficient compared to AMHs (Marzke, 1997).
In addition to a carpal orientation that is in some ways more modern than that of
H. habilis, A. afarensis also exhibited a more modern finger to thumb length ratio
(Marzke, 1983; Latimer, 1991; Alba et al., 2003). Many of the other features of the wrist and hand, however, remained primitive in form. A. afarensis retained curved proximal phalanges with strong flexor sheath markings (Bush et al., 1982; Stern and Susman,
1983; Susman et al., 1984; Haile-Selassie, 2001), narrow apical tufts (Bush et al., 1982), a gracile metacarpal I (Smith, 2000), and a projection on the dorsal distal radius which is also seen in extant Pan and Gorilla, as well as A. anamensis (Richmond and Strait, 2000;
Richmond et al., 2001). This combination of features indicates that A. afarensis lacked anatomical components that have been hypothesized to contribute to effective stone tool manufacture (Marzke, 1997; Tocheri et al., 2003). However, due to the nature of the primitive condition (e.g., short fingers compared to thumb length), it is difficult to mimic and model the effects of the primitive anatomy on stone tool production (Susman, 1998).
16
Consequently, it had not been possible to say with any certainty how A. afarensis would
have knapped. This difficulty is heightened by the variable representation of primitive
and derived features—i.e., unique anatomy—in each extinct hominin taxa. Thus,
although it appeared likely that A. afarensis would have manufactured stone tools in a
less efficient manner compared with modern humans, this hypothesis had not been
directly tested.
Oldowan-era hominins also showed a mixture of primitive and derived upper limb
features. However, it is difficult to determine the species designation of many of these
fossils due to the co-occurrence of multiple species within a given site. For example,
cranial and postcranial elements that may belong to either P. boisei or H. habilis have been recovered from Olduvai Gorge, Tanzania, although the OH 7 hand is generally regarded as H. habilis [(Napier, 1962b; Wood and Richmond, 2000) though see Moyà-
Sola and colleagues (2008) for an alternative classification]. Similarly, postcranial elements from Swartkrans, South Africa may belong to either P. robustus or H. cf. erectus. Susman (1988) argues that because 95% of the craniodental remains in Member
1 have been assigned to P. robustus, it is likely that the postcranial remains also belong to the same species. However the possibility that the fossils are actually Homo remains
(Trinkaus and Long, 1990).
Assuming the fossils have been correctly classified, features that may facilitate
stone tool behaviors that have been recorded among them include the broadening of
apical tufts in presumed H. habilis, P. robustus, and A. africanus specimens (Napier,
1962b; Ricklan, 1990; Smith, 2000), a reduction in curvature in the proximal phalanges,
and a more robust pollical metacarpal in P. robustus (Susman, 1988; Susman, 1994;
17
Tocheri et al., 2008), and a modern finger to thumb length ratio (Green and Gordon,
2007), a more modern carpal shape and orientation (Tocheri et al., 2003), and a
stabilizing styloid process at the MC III base (Ricklan, 1987) in A. africanus [3.5-2.4 mya
(Clarke and Tobias, 1995)]. Specimens of A. africanus and those presumed to belong to
P. robustus also exhibit flatter distal radii that is similar to the modern condition (Grine and Susman, 1991; Richmond and Strait, 2000). However, all of these species also retain a number of primitive features, including curved proximal phalanges in H. habilis and A. africanus (Susman and Creel, 1979; Ricklan, 1987), a primitive carpal arrangement in H. habilis (Tocheri et al., 2003), and an ape-like pollical breadth to length ratio in A. africanus (Green and Gordon, 2007). The few upper limb elements that have been described for the newly erected species A. sediba (Malapa, South Africa) are also primitive. Berger and colleagues (2010) report that sediba had long, curved phalanges with strong muscle markings for the flexor digitorum superficialis muscle.
As stated above, the functional significance of the majority of these features during stone tool manufacture is difficult to test directly. For instance, we cannot directly test the effects of a high finger to thumb length ratio or narrow apical tufts on one’s ability and efficiency in manufacturing stone tools. This difficulty is exacerbated by the fact that different species exhibit different combinations of primitive and derived features
(Susman, 1998). With Susman’s (1998) concern in mind, one of the goals of this dissertation was to investigate the upper limb kinematics of modern human knappers, as well as changes in their knapping kinematics across skill level and lithic tradition. The role of the wrist, and specifically the use of extension and flexion, is highlighted throughout the dissertation for three reasons. First, mobility in wrist extension to allow
18
greater rnages of wrist movements has been hypothesized to contribute to efficient stone
tool manufacture (Marzke, 1971; Ambrose, 2001; Richmond et al., 2001). Second,
among the fossils that currently make up the fossil record, the dorsal distal radial
projection differs greatly among taxa; A. anamensis and A. afarensis exhibited the
African ape-like condition, while P. robustus, A. africanus, H. neanderthalensis, and H.
sapiens did not (Grine and Susman, 1991; Richmond and Strait, 2000). And third, wrist extension may be non-invasively limited by using a simple extension-limiting wrist restraint, thereby mimicking what may have been the ancestral condition. Through this method it becomes possible to isolate wrist extension and examine its significance to aspects of stone tool production, such as accuracy and efficiency.
Following P. robustus, manual elements and carpals become scarce in the fossil record until H. neanderthalensis [300-30 kya (Arsuaga et al., 1997; Wood and Richmond,
2000)]. Those which exist have either yet to be made public or are of a fragmentary nature and consequently difficult to analyze. For example, upper limb elements from
Area 1A of the Koobi Fora Formation in Kenya (specimen KNM-ER 47000 date to 1.52
mya) resemble Pliocene Australopithecus (Richmond et al., 2011) and have been
cautiously assigned to P. boisei (Richmond et al., 2009). However, a full comparative
analysis on the fossils is currently unavailable. Additionally, one complete capitate and
11 fragmentary hand bones have been recovered from Gran Dolina, Spain which have
been assigned to H. antecessor (Lorenzo et al., 1999). Together the elements exhibit features which are derived with respect to Australopithecus and primitive with respect to modern humans. However their fragmentary nature makes a detailed analysis currently difficult. Manual elements and carpals are nearly absent from H. ergaster and H.
19 erectus, with the exception of a metacarpal from WT 15000 (Walker and Leakey, 1993), a lunate from Zhoukoudian (Weidenreich, 1941), and two distal phalanges from Dmanisi
(Lordkipanidze et al., 2007). Some of the hand elements from Swartkrans may belong to
H. erectus, however their species attribution remains unclear (Susman, 1988; Susman,
1989; Trinkaus and Long, 1990).
The lack of securely attributable H. erectus sensu lato hand and wrist fossils is particularly unfortunate given the association between H. erectus and the Acheulean
Industrial Tradition. Though clearly distinct and primitive with respect to H. sapiens, cranial dental and postcranial material overwhelmingly indicates that H. erectus sensu lato practiced an adaptive strategy with more similarities to H. sapiens than any previous hominin (Wood and Collard, 1999b; Wood and Collard, 1999a). It is regrettable that we cannot state with any degree of certainty the morphology of H. erectus sensu lato hands.
By the time manual elements are again abundant in the fossil record, with H. neanderthalensis, the upper limb was highly derived (Wood and Richmond, 2000;
Tocheri et al., 2008) and both species of Homo manufactured complex composite Middle and Upper Paleolithic stone tools, and arguably symbolic ornaments (d'Errico, 1998; d'Errico, 2003; Henshilwood et al., 2004).
Dissertation goals and hypotheses
The goal of this dissertation is to investigate the biomechanical strategies used by modern humans in the production of Early Stone Age stone tools in order to test the primary hypothesis that aspects of the modern upper limb condition contribute to
20
efficiency and accuracy during stone tool production. Specific sub-hypotheses will be
tested in four manuscripts presented as dissertation chapters as follows:
Chapter 2: Chapter 2 describes the investigation into the distribution of pressure and normal force across the hand during the production of Oldowan-like choppers. This experiments was undertaken to test the hypothesis that during stone tool production the thumb is subject to significantly greater pressure and/or normal force compared to other regions of the hand, particularly along the remaining four digits. Manual impulse and pressure-time integrals are also analyzed.
Chapter 3: Chapter 3 describes the knapping kinematics of four amateur knappers during simple flake production. Three sub-hypotheses were tested:
1) Across subjects, upper limb kinematics during stone tool production are grossly similar, but vary in the timing of specific events.
2) Upper limb joint kinematics will occur in a proximal-to-distal joint sequence (PDJS).
3) Wrist motions will significantly influence efficiency and accuracy.
Chapter 4: Chapter 4 describes of the knapping kinematics of eight experienced subjects during the production of Oldowan bifacial chopping implements. These data are compared to the knapping kinematics of the same eight subjects when their ability to extend at the wrist was limited to ~30⁰ – 35⁰ . The knapping swing is described in detail and the following sub-hypotheses are tested:
1) Skilled knappers will initiate joint motions in a proximal-to-distal sequence and utilize a full proximal-to-distal joint sequence in terms of peak linear segment endpoint velocities, peak joint angular velocities, and the onset of peak angular velocities.
21
2) Limiting wrist extension will result in significantly lower angular velocities at the wrist and in turn lower strike accelerations and forces.
3) Limiting wrist extension will significantly decrease target accuracy during stone tool production.
Chapter 5: Chapter 5 describes the kinematic strategy utilized by experienced knappers during the production of Acheulean bifacial handaxes. This strategy is compared to the kinematic strategy employed during the production of Oldowan bifacial choppers to test the hypothesis that Acheulean motion sequences exhibit greater complexity in upper limb action sequences.
Chapter 6: Chapter 6 is a summary of the four studies conducted in the course of this dissertation and a discussion of the implications of the findings in regard to stone tool production and upper limb adaptations as they related to stone tool production.
22
Chapter 2: Manual pressure distribution during Oldowan stone tool production
ABSTRACT
Modern humans possesses a highly derived thumb that is robust and long relative to the other digits, with enhanced pollical musculature compared to extant apes. Researchers have hypothesized that this anatomy was initially selected in early Homo in part to withstand high forces acting on the thumb during stone tool production’s hard hammer percussion; however, data were lacking on loads experienced during stone tool production and their distribution across the hand.
Here we report the first quantitative data on manual pressures (kPa) and normal forces (N) acting on the hand during Oldowan stone tool production, captured at 200 Hz.
Data were collected from six experienced subjects replicating Oldowan stone tools, the earliest tools in the archaeological record. Our data do not support hypotheses asserting that the thumb experiences greater pressure compared to other regions of the hand. Peak pressure, normal force, impulse, and the pressure/time integral were significantly greater on the 2nd and/or 3rd digits compared to the 1st in every subject. Our findings call into
question hypotheses linking modern human thumb anatomy specifically to load resistance
during stone tool production.
23
INTRODUCTION
Humankind’s ability to manipulate technology is commonly cited as an essential
component of what defines us as Homo sapiens. Consequently, the origin of our
relationship with technology is a subject of intense interest across a range of disciplines.
Stone tool manufacture, the earliest form of technology in the archaeological record
(Semaw, 2000), is regarded as a principal selective pressure that may have acted on some
aspects of early human development, such as cognition (Holloway Jr., 1969; Stout et al.,
2008; Faisal et al., 2010) and hand and wrist anatomy (Napier, 1965; Marzke, 1997;
Tocheri et al., 2007). In regard to the latter, anatomically modern H. sapiens possess a
number of derived features which enhance our ability to forcefully grasp and manipulate
small objects in a single hand, two important components of stone tool production
(Marzke and Shackley, 1986). These include short, relatively straight phalanges, broad
apical tufts on the distal phalanges, and carpometacarpal and metacarpolphalangeal shape
and articular orientation that allow simultaneous flexion and rotation of the 2nd and 5th
metacarpals, among others (Napier, 1962b; Lewis, 1977; Susman, 1979; Susman, 1994;
Panger et al., 2002). The thumb, in particular, has been directly linked to stone tool production and is hypothesized to have been selected to withstand the high repetitive
percussive forces experienced during stone tool production (Marzke, 1992; Susman,
1994; Hamrick et al., 1998; Tocheri et al., 2007). However, the data on manual pressure
during stone tool production that is needed to evaluate such hypotheses has not been
available until now. Here we present the first quantitative data on the manual pressures
and normal forces experienced during stone tool production, and their distribution across
24
the hand. Data were collected from six stone tool makers (i.e., knappers) experienced in
the replication of Oldowan stone tool industries, using a high speed manual pressure
sensor system and 3-D motion capture technology (both recording at 200 Hz). With these data we test the hypothesis that during the production of Oldowan stone tools, the
thumb is subject to significantly greater pressure and/or normal force compared with
other regions of the hand, particularly along the remaining four digits.
A number of features of the modern human thumb contribute to stability at the
first carpometacarpal joint and to our ability to forcefully oppose the thumb with the
finger digits. Both are considered important during stone tool manufacture. For
example, the modern human thumb is robust in build, particularly at the metacarpal base
and head, which helps reduce increased joint stress due to our enhanced thumb
musculature (Susman, 1994). We have a relatively flat first carpometacarpal joint
(Guthrie, 1991; Marzke et al., 2010) and the highest mean thumb length to index finger
length ratio of all extant primates (Napier, 1993). Together these assist in true thumb-to- finger opposition, necessary for precision handling. Our carpals are also oriented in a manner better able to withstand forces directed into the thumb—as has been hypothesized to occur during stone tool manufacture—compared with the primitive condition and that seen in extant African apes (Tocheri et al., 2003; Tocheri et al., 2007).
H. sapiens also possess a unique representation of pollical muscles, including two ventrally derived pollical muscles and one dorsally derived pollical muscle which cross the first metacarpophalangeal joint. Nearly all other primates lack these three muscles— the first volar interosseous of Henle, a true flexor pollicis longus (FPL) muscle, and an extensor pollicis brevis (Susman, 1994; Tocheri et al., 2008; Diogo and Abdala, 2010;
25
Diogo et al., 2010). The first muscle is a small intrinsic hand muscle associated with fine
manual motor control and more recently with grasping ability (e.g., holding a
hammerstone)(Marzke et al., 1998). The last muscle is an extrinsic hand muscle which
inserts into the first phalanx in humans to assist extensor pollicis brevis and abductor
pollicis brevis with thumb extension. The FPL is a large extrinsic thumb muscle,
comprising approximately 22% of the thumb’s total muscular physiological cross-
sectional area (Marzke, 1997). In humans the muscle belly of FPL is separate from that
of flexor digitorum profundus, and thus constitutes a separate and far stronger muscle
than the extrinsic pollical flexor in the former species, unlike the condition found in other
great apes (Straus, 1942).
FPL is widely hypothesized to be significant during stone tool manufacture
(Susman, 1988; Marzke, 1992; Susman, 1994; Hamrick et al., 1998; Wood and
Richmond, 2000). Indeed, just the presence of a large insertion site for the FLP is nearly sufficient to bestow tool-making capabilities on fossil hominins [(Napier, 1962b;
Susman, 1988; Ricklan, 1990; Susman, 1994) however see Marzke and colleagues (1998) and Tocheri and colleagues (2008) for an alternative interpretation of the volar
depression].
FLP’s hypothesized contribution to stone tool production is the primary role it
plays during strong pinching activities, allowing for forceful opposition of the thumb
against the other digits, which act in concert to stabilize objects held between the thumb
and finges. Researchers argue that this strong precision pinching ability helps ensure a
firm grip on the hammerstone and help resist hammerstone displacement that may occur
26 due to strong upward reaction forces at strike (Susman, 1994; Marzke, 1997; Hamrick et al., 1998; Susman, 1998).
METHODS
Normal force (N) and pressure (kPa) data were captured from six subjects
(Subjects A-F) during the production of Oldowan tools (n = 2 tools per subject, n = 148 swings). Associated kinematic data were captured from five of the six subjects (Subjects
A-E, n = 98 swings). Informed consent was obtained from each subject prior to experimentation. Data were captured in The George Washington University’s Motion
Capture and Analysis laboratory in Washington, DC.
Five males (Subjects A-C, E, and F) and one female (Subject D) participated in the study. All subjects were professional archaeologists or archaeological graduate students familiar with Oldowan tool typologies and proficient in the production of
Oldowan tools. All subjects were healthy adults free from muscular and/or osteological arm, forearm, and hand conditions which may have compromised their data. Subjects B-
F were right-hand dominant, Subject A was left-hand dominant.
All reduction sequences were conducted on nodules of cortex-free raw English flint (material toughness ≈ 1 Kc). Subjects were given two fist-sized flint “cobbles”, which were knobs of flint that been removed from larger nodules, and were requested to produce Oldowan bifacial chopper from each cobble with no more than 15 flake removals per chopper (Figure 2.1). Each subject was allowed to select a hammerstone of his or her choosing from among a group of hammerstone weighing < 0.75 kg each. The
27
sole stipulation placed on hammerstone selection was the knapper’s ability to wield the
hammerstone with digits I-III during stone tool production. Most employed a 3-jaw chuck grip or a variant of that grip. Previous knapping experiments have demonstrated that the 3-jaw chuck would have been particularly effective in Oldowan stone tool production, and modern hand morphology has been related specifically to this grip
(Marzke and Shackley, 1986; Marzke, 1992; Marzke, 1997). All knapping occurred with subjects seated in a wooden chair (seat height = 48.26 cm). Subjects all naturally assumed similar knapping positions, with the core nodule resting against one leg (i.e., the left leg for right-handed subjects, the right left for left-handed subjects) and the hammerstone held in their dominant hand. Subjects naturally employed a standard knapping swing (Williams et al., 2010) to remove flakes, swinging with their dominant hand and arm across their midline to the core, which rested on their contralateral leg. Up to three leather pads (5 mm thick) were laid across subjects’ core-leg to protect it from injury.
Dynamic normal force and pressure data were captured using a Novel Pliance pressure sensor system. The Pliance system is comprised of five sensor strips (100 x 10 mm2), each connected via a sensor cable to a pliance-x electronic analyser. Each sensor strip contains ten sensor elements (each 10 x 10 mm2, thickness: < 1.2 mm) arranged in a
single row and embedded between two thin sheets of rubber coating (thickness: 0.1 – 0.6
mm). A 1 cm extension of the rubber coating (i.e., the lead) extends off of the distal end
of each sensor strip. Sensors strips were connected to the palmar sides of subjects’ 1st -
3rd digits. The strips were held in place by wrapping the lead around the distal phalanx from the palmar side to dorsal side of the digit and taping the lead onto the dorsal side of
28 the digit. Care was taken to avoid placing any tape on the section of the strips containing sensor elements. Finger condoms were placed over each digit to further secure the sensor strip. The sensor strips traveled across the palm and wrist to connect to the sensor cable just proximal to the wrist. The trailing end of each sensor strip was held in place at the wrist using another small strip of Velcro. Within each strip, the sensor elements stopped mid palm while the sensor-free strip continued across the entire palm and wrist.
Proximal to the wrist, sensor cables were connected to the Pliance-x electronic analyser.
The Pliance-x electronic analyser was placed either in a utility belt worn by each subject or on a small table next to the subject. Both positions rendered it out of the knapping swing path.
The five subjects from whom kinematic data were collected also wore a tight, fingerless glove (rayon/cotton/rubber blend) fitted with five 10 mm diameter reflective markers to capture corresponding kinematics data. Each glove had separate holes for the thumb and index fingers, and a single large hole for the remaining digits. Reflective markers were placed at the following landmarks on the hand: the radial and ulnar styloid processes (RSP and USP, respectively), and metacarpal (MC) heads I, II and V.
Additional markers were placed on the olecranon process (OP) and the point of the shoulder (SH). Marker bases were taped to the subjects using double sided tape to further prevent their displacement.
The Vicon kinematics motion capture system operates using high speed infrared cameras that digitally record the position of the reflective markers afixed to each subject.
Up to eight cameras simultaneously recorded each subject’s motion from various angles to provide multiple views of the same movements. During data collection subjects and
29 markers move across a calibrated space allowing extraction of each landmarks’ coordinates, from which is it possible to derive landmark velocities and accelerations, joint angles, and joint angular velocities and accelerations. Coordinate data were compiled to create a 3-D model of the relevant anatomical region. A digital camera (60
Hz) was also used to capture digital images of the reduction sequence to verify subject behavior.
After each subject was fitted with the sensor strips and glove, the Pliance system was zeroed to factor out the pressure exerted by the attachment apparatus. Subjects then progressed through the production of two Oldowan bifacial choppers. Each removed simple flakes from the flint nodule by striking the hammerstone, held in a 3-jaw chuck, against the nodule as described above. Normal force, pressure, and kinematic data were captured at 200 Hz. Recording was paused after the production of each flake and each flake was retrieved.
Associated kinematic data was captured for Subjects A-D (n = 130 swings).
Those swings with missing data were removed from the sample set, resulting in 98 sets of associated normal force/pressure and kinematics data. Swings were sectioned into up- swing, pre-strike down-swing, and post-strike down-swing. The position of the radial styloid process was used as a proxy for the wrist’s position. Up-swing was defined as encompassing the instant of the wrists’ lowest vertical position immediately prior to the initiation of the wrists’ vertical ascent through the instant of the wrists’ highest vertical position immediately prior to beginning a downward trajectory. Pre-strike down-swing was defined as encompassing the instant immediately after the wrist reached its highest vertical position through the instant immediately prior to strike. Post-strike down-swing
30
was defined as encompassing the instant immediately after strike through the wrists’
lowest post-strike vertical position.
The coupling of kinematic and force data verified that strike occurred at the
instant of cumulative peak normal force across all three digits. Cumulative peak force, or
strike, is nearly coincident with peak angular acceleration at the wrist (extension to
flexion), occurring within 2 frames or 0.01 seconds of each other (Table 2.1, Figure 2.2).
The association between peak grip force and peak angular acceleration has previously
been reported by Werremeyer and Cole (1997). The present conclusion was strengthened
by findings that peak angular acceleration is nearly coincident with strike (occurring
0.032 seconds prior to strike when recorded at 50 Hz) (Williams et al., 2010).
Captured normal force and pressure data were analyzed on a per-swing basis for each subject. Peak normal forces and pressures acting on each digit were recorded for each swing, as well as forces and pressures occurring at strike. Peak pressures reported for each digit may occur anywhere along that digit, while peak normal force reports the total normal force acting across the entire digit. Peak normal forces and pressures occurring along an individual digit constitute a series of moments distinct from the instant of cumulative peak force across all digits (i.e., strike). Impulse and pressure-time integrals were also calculated for each digit during each swing. A nonparametric
Kruskal-Wallis was used to test for differences among the digits. P-values were determined using a post hoc pair-wise Mann-Whittney U test and treated with a standard
Bonferroni correction to determine significance: Mann-Whitney pairwise P-values were multiplied by the number of number of pairwise comparisons made and determined significant if they were < 0.05 (Zarr, 1996).
31
RESULTS
Across all six subjects, peak normal forces during knapping were consistently
significantly greater on the 2nd and/or 3rd digits compared to the 1st digit (p < 0.04, Table
2.2, Figure 2.3a). Peak pressures occurring anywhere along each digit at any point in the
knapping swing were similarly distributed; peak pressures were consistently significantly greater on the 2nd and/or 3rd digits compared to the 1st (p < 0.04, Table 2.2, Figure 2.3b).
Mean peak pressures acting on the 1st digit were greater compared to the means of the 2nd and/or 3rd digits in only one subject (Subject A), however this difference was insignificant.
Manual normal force distribution at strike was similar to the distribution of peak
normal force and pressure; normal forces acting on the 2nd and/or 3rd digits were
significantly greater than those acting on the 1st digit across all six subjects (p ≤ 0.03,
Table 2.3, Figure 2.3c). Pressures acting anywhere along a given digit at strike were
significantly greater on the 2nd and 3rd digits compared to the 1st digit in all six subjects (p
≤ 0.001, Table 2.3, Figure 2.3d). Mean normal force and pressure at strike were never greater, significantly or otherwise, on the 1st compared to either the 2nd or 3rd digits.
Impulse (i.e., force applied over time) and the pressure-time integral consider not only the load applied, but the duration that load is experienced. Results for both impulse and pressure-time integrals were significantly greater on the 2nd and/or 3rd digits
compared to the 1st for five of six subjects (Subjects A-C and E-F, p < 0.03 and p <
0.0001, Table 2.4, Figure 2.4a and 2.4b). Differences between the 1st and 2nd and 1st and
3rd digits were insignificant in Subject D.
32
Using corresponding kinematic data, normal forces and pressures were separated
into up-swing, pre-strike down-swing, and post-strike down-swing for five subjects (A-
E). During up-swing two of five subjects displayed significant differences between peak
normal forces (2nd and 3rd significantly greater than 1st, Subjects B and E). All other
differences were insignificant between the 1st and 2nd and 1st and 3rd (Table 2.5, Figure
2.5a). Peak pressures were significantly greater on both the 2nd and 3rd compared to the
1st in three of five subjects (B, C, and E) and significantly greater on the 3rd compared to
the 1st in Subject D (Table 2.5, Figure 2.5b). Subject A did not exhibit significant differences in pressure. During pre-strike and post-strike down-swing, peak normal forces were significantly greater on the 2nd and/or 3rd digits compared to the 1st digit in all
subjects (p < 0.02, Tables 2.6 and 2.7, Figure 2.5c, 2.5e). Peak pressures during the pre-
strike and post-strike phases were significantly greater on both the 2nd and 3rd digits
compared to the 1st in all subjects (p < 0.015 and p < 0.035, respectively, Tables 2.6 and
2.7, Figure 2.5d, 2.5f).
DISCUSSION
Results from the present study do not support the hypothesis that the thumb is subject to significantly greater forces and/or pressures compared to the other digits during
the production of Oldowan stone tools. Peak and strike normal forces and pressures, impulse, and pressure-time integrals were consistently significantly greater on the 2nd and/or 3rd digits compared to the 1st digit in all analyses. Normal force and pressure
acting on the thumb were never significantly greater compared to either of the other
33 digits, and were insignificantly greater in only Subject A in the analysis of peak normal forces and pressures (Table 2.2, Figure 2.3a, 2.3b). Corresponding kinematic data demonstrate that differences in normal force and pressure distribution across the three digits are not consistently present during up-swing. However, these differences are established prior to strike during the pre-strike phase of down-swing and continue to strike and through the post-strike down-swing phase (Tables 2.5 – 2.7, Figure 2.5).
Expectations regarding high forces acting on the thumb during stone tool production may lie in a misinterpretation of the hand’s orientation to the hammerstone and of the knapping swing. In regard to stone tool manufacture, the precision grip is traditionally described as a grip that secures the hammerstone between the volar aspect of the pollical distal phalanx and the volar pads of one or more of the remaining digits
(Napier, 1956; Napier, 1962a; Marzke, 1997). The palm is not employed in this grip. In depicting precision grips around hammerstones, Napier (Napier, 1956; Napier, 1965) displayed the thumb as strongly abducted, flexed, and rotated towards the 3rd and 4th digits so that it lay in full opposition to the fingers. If knappers were to position their hand around the hammerstone in this manner, they would also have to execute knapping swings with the forearm positioned midway between pronation and supination—similar to position used when hammering with a traditional hammer—in order to avoid crushing the thumb between the core and the hammerstone at strike. Thus, this implies that the knapping swing is similar to a hammering swing, with an emphasis on radial and ulnar deviation at the wrist (Leventhal et al., 2010) such that force is directed up into the pollical distal phalanx.
34
If one were to knap under these conditions, the thumb would likely experience
large loads as it acts to stabilize the hammerstone throughout down-swing, particularly at
strike. Further, FLP would be heavily recruited, as the pollical distal phalanx flexes to
resist hyperextension while acting to hold the hammerstone in place (Johnson and
Forrest, 1970; Hamrick et al., 1998).
In discussions of the knapping abilities of fossil hominins, multiple researchers have invoked a traditional hammering swing (Ricklan, 1987; Marzke et al., 1992).
Others, however, have regarded the two swings as constituting distinctly different motion
patterns (Hamrick et al., 1998). The results of our knapping kinematics study upheld this distinction (Williams et al., 2010). We found that the knapping swing relies primarily on wrist extension and flexion, with the forearm pronated and the palm facing the nodule, rather than on radial and ulnar deviation. This pattern of wrist motion and forearm orientation allows the knapper to execute a wrist snap immediately prior to strike and to aim from his or her most distal joint (i.e., the wrist), thereby increasing work production and potentially improving accuracy (Bernstein, 1967; Anderson and Sidaway, 1994; Hore et al., 1996; Chowdhary and Challis, 1999). The emphasis on wrist extension and flexion necessitates that the palm of the hand is oriented towards the nodule. This placed the hammerstone directly beneath the 2nd and 3rd metacarpal heads so that throughout swing and at strike force is directed up into the 2nd and 3rd metacarpal heads. These findings
have previously been qualitatively reported by Marzke and Shackley (Marzke and
Shackley, 1986), and support their hypothesis that the hand is not oriented in a traditional
precision grip in full opposition as described by Napier (Napier, 1956; Napier, 1965).
35
Results from the present study and our previous kinematics study (Williams et al.,
2010) suggest that knappers employ the thumb as a buttress against the side of the
hammerstone in a modified precision grip (Marzke and Shackley, 1986; Marzke, 1997),
rather than using a traditional, fully-opposed precision grip. When used as a buttress, and
in combination with the knapping swing (Williams et al., 2010), the thumb is necessarily
abducted, extended and rotated laterally, rather than abducted, flexed and rotated medially in true opposition. If the thumb were flexed and rotated medially, the knapper would either be required to employ a knapping swing that does not rely on extension/flexion and accordingly does not offer the accuracy and work production advantages previously reported, or risk catching his or her thumb between the
hammerstone and the nodule as the wrist quickly flexes through strike. EMG data of muscle recruitment during stone tool production indirectly supports this reinterpretation
of the thumb’s role during knapping (Marzke et al., 1998). Marzke and colleagues
(1998) found that FLP was not consistently strongly recruited in the dominant hand, suggesting that strong pinching activities were not occurring. Activity in the intrinsic thumb and index finger muscles did, however, display a consistent tendency to peak during the swing.
Results from the present study do not support the traditional interpretation of the
thumb’s role during stone tool production. When the entire swing was considered,
normal forces and pressures acting on the hand were consistently significantly greater at
the 2nd and/or 3rd digits compared to the 1st digit (Table 2.2, Figure 2.3a 2.3b). Normal
forces and pressures at strike were also significantly greater on the 2nd and/or 3rd digits compared to the 1st digits (Table 2.3, Figure 2.3c, 2.3d). The same was true for impulse
36
and pressure-time integrals—the 2nd and/or 3rd digits were significantly greater than those acting on the 1st digit across all subjects except Subject D for whom there was not a
significant difference between the 1st and 2nd digits nor a difference between the 1st and
3rd digits (Table 2.4, Figure 2.4). Normal force and pressure differentiation did not consistently occur during up-swing, however the distribution pattern described above was established prior to strike during pre-strike down-swing and maintained through the end
of post-strike down-swing (Tables 2.5 -2. 7, Figure 2.5). These findings directly support
Marzke and Schackley (1986) hypothesis that the thumb is used as a buttressing agent
against the side of the hammerstone during stone tool production, rather than as the
primary stabilizing fulcrum against the hammerstone while oriented in true opposition.
Further, the findings highlight the roles the index and middle fingers play in stabilizing
the hammerstone during down-swing and at strike.
These results call into question hypotheses directly linking modern human thumb
anatomy specifically to the resistance of high loads experienced during stone tool
production. It now appears that the thumb is unlikely to have played the role traditionally
assigned to it during the manufacture of the earliest stone tools, however this does not
mean that the thumb is not an integral component of stone tool behaviors. Preliminary
results on pressure distribution during stone tool use demonstrate that the thumb
experiences relatively higher loads compared with the other digits when gripping flakes
with a two-jaw pad-to-side grip (Marzke, 1997), as you would hold a key. These
preliminary results suggest that the derived human thumb anatomy may be selected to
withstand high loads during stone tool use, specifically those grips requiring a two-jaw
pad-to-side grip, rather than stone tool production.
37
Table 2.1. Cumulative peak force (strike) in relation to peak angular acceleration
Subject seconds ± CI A 0.008 ± 0.0096 B 0.002 ± 0.0009 C -0.0005 ± 0.0016 D -0.001 ± 0.0016 E 0.0005 ± 0.001 Negative results indicate peak force occurred before peak angular acceleration. Positive results indicate peak force occurred after peak angular acceleration.
38
Table 2.2. Peak normal force (N) and pressure (kPa)
Peak normal N Peak kPa
Subject A, n = 24 Subject A, n = 24 1st 2nd 3rd 1st 2nd 3rd Mean 27.652 26.87 28.859 Mean 191.09 168.478 156.63 1st - - - 1st - - - 2nd 0.860 - - 2nd 0.089 - - 3rd 0.036 0.000 - 3rd 0.031 1 - Subject B, n = 36 Subject B, n = 36 1st 2nd 3rd 1st 2nd 3rd Mean 45.046 59.239 51.398 Mean 166.25 222.841 213.977 1st - - - 1st - - - 2nd 0.001 - - 2nd 0.000 - - 3rd 0.055 0.004 - 3rd 0.107 0.228 - Subject C, n = 28 Subject C, n = 28 1st 2nd 3rd 1st 2nd 3rd Mean 28.313 46.031 47.609 Mean 93.281 173.594 213.594 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.004 0.160 - 3rd 0.000 1.000 - Subject D, n = 13 Subject D, n = 13 1st 2nd 3rd 1st 2nd 3rd Mean 20.519 39.058 21.539 Mean 60.385 190.192 114.808 1st - - - 1st - - - 2nd 0.001 - - 2nd 0.000 - - 3rd 1 0.001 - 3rd 0.001 0.005 - Subject E, n = 29 Subject E, n = 29 1st 2nd 3rd 1st 2nd 3rd Mean 3.917 50.286 18.583 Mean 15.119 145.357 57.976 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.000 0.000 - 3rd 0.000 0.000 - Subject F, n = 18 Subject F, n = 18 1st 2nd 3rd 1st 2nd 3rd Mean 5.885 34.442 16.231 Mean 27.308 110.962 39.808 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.000 0.000 - 3rd 0.018 0.000 - Mean peak normal force and pressure occurring along each digit at any point during the swing and p-values from post-hoc pairwise Mann-Whittney U tests. Bolded results indicate that the 2nd or 3rd digit is significantly greater than 1st.
39
Table 2.3. Normal force (N) and pressure (kPa) at strike
Peak normal N Peak kPa
Subject A, n = 24 Subject A, n = 24 1st 2nd 3rd 1st 2nd 3rd Mean 20.531 23.344 25.885 Mean 116.15 147.19 135.21 1st - - - 1st - - - 2nd 0.104 - - 2nd 0.001 - - 3rd 0.004 0.000 - 3rd 0.000 1 - Subject B, n = 36 Subject B, n = 36 1st 2nd 3rd 1st 2nd 3rd Mean 44.319 57.979 48.750 Mean 115.208 216.806 206.597 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.030 0.004 - 3rd 0.001 0.228 - Subject C, n = 28 Subject C, n = 28 1st 2nd 3rd 1st 2nd 3rd Mean 23.063 47.750 37.786 Mean 70.446 161.786 149.464 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.004 0.160 - 3rd 0.000 1.000 - Subject D, n = 13 Subject D, n = 13 1st 2nd 3rd 1st 2nd 3rd Mean 18.673 38.904 20.327 Mean 39.808 184.615 94.423 1st - - - 1st - - - 2nd 0.001 - - 2nd 0.000 - - 3rd 1 0.001 - 3rd 0.001 0.005 - Subject E, n = 29 Subject E, n = 29 Mean 2.638 49.724 16.638 Mean 10.517 133.534 48.707 1st 2nd 3rd 1st 2nd 3rd 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.000 0.000 - 3rd 0.000 0.000 - Subject F, n = 18 Subject F, n = 18 1st 2nd 3rd 1st 2nd 3rd Mean 2.611 29.486 15.667 Mean 7.778 87.500 33.889 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.000 0.000 - 3rd 0.000 0.000 -
Mean strike normal force and pressure occurring along each digit and p-values from post-hoc pairwise Mann-Whittney U tests. Bolded results indicate that the 2nd or 3rd digit is significantly greater than 1st.
40
Table 2.4. Impulse and kPa-time integral
Impulse kPa/time integral
Subject A, n = 24 Subject A, n = 24 1st 2nd 3rd 1st 2nd 3rd Mean 7.605 9.435 9.268 Mean 43.267 67.512 61.093 1st - - - 1st - - - 2nd 0.010 - - 2nd 0.000 - - 3rd 0.027 1.000 - 3rd 0.000 1.000 - Subject B, n = 36 Subject B, n = 36 1st 2nd 3rd 1st 2nd 3rd Mean 17.061 21.194 17.340 Mean 47.408 86.863 60.738 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 1 0.000 - 3rd 0.306 0.000 - Subject C, n = 28 Subject C, n = 28 1st 2nd 3rd 1st 2nd 3rd Mean 13.933 12.354 17.540 Mean 44.357 53.310 78.151 1st - - - 1st - - - 2nd 0.070 - - 2nd 0.352 - - 3rd 0.006 0.000 - 3rd 0.000 0.000 - Subject D, n = 13 Subject D, n = 13 1st 2nd 3rd 1st 2nd 3rd Mean 12.919 11.214 12.534 Mean 12.919 11.214 12.534 1st - - - 1st - - - 2nd 0.655 - - 2nd 0.655 - - 3rd 1 1.000 - 3rd 1 1.000 - Subject E, n = 29 Subject E, n = 29 1st 2nd 3rd 1st 2nd 3rd Mean 0.584 16.284 5.075 Mean 3.031 47.867 18.650 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.000 0.000 - 3rd 0.000 0.000 - Subject F, n = 18 Subject F, n = 18 Mean 1.876 9.106 8.103 Mean 6.322 27.948 15.592 1st 2nd 3rd 1st 2nd 3rd 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.000 0.375 - 3rd 0.000 0.000 - Mean impulse and kPa/time integrals and p-values from post-hoc pairwise Mann-Whittney U tests. Bolded results indicate that the 2nd or 3rd digit is significantly greater than 1st.
41
Table 2.5. Peak normal force and pressure during up-swing Peak normal N Peak kPa Subject A 1st 2nd 3rd 1st 2nd 3rd Mean 11.075 10.35 10.388 Mean 80.625 74.625 68 1st - - - 1st - - - 2nd 1 - - 2nd 0.444 - - 3rd 0.718 1 - 3rd 0.64 1 - Subject B 1st 2nd 3rd 1st 2nd 3rd Mean 18.264 27.564 21.4 Mean 51.786 108.643 86.071 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.004 0.000 - 3rd 0.000 0.02 - Subject C 1st 2nd 3rd 1st 2nd 3rd Mean 15.988 16.9 18.263 Mean 54.375 71.875 81.875 1st - - - 1st - - - 2nd 1 - - 2nd 1 - - 3rd 0.655 1 - 3rd 0.004 0.25 - Subject D 1st 2nd 3rd 1st 2nd 3rd Mean 14.5 15.577 15.212 Mean 35.385 75.385 82.5 1st - - - 1st - - - 2nd 1 - - 2nd 0.031 - - 3rd 1 1 - 3rd 0.000 1 - Subject E 1st 2nd 3rd 1st 2nd 3rd Mean 0.1 11.6 4.625 Mean 1 27.5 21.75 1st - - - 1st - - - 2nd 0.001 - - 2nd 0.001 - - 3rd 0.001 0.006 - 3rd 0.001 1 -
Mean peak normal force and pressure separated into knapping swing phases and p-values from post-hoc pairwise Mann-Whittney U tests. Bolded results indicate that the 2nd or 3rd digit is significantly greater than 1st.
42
Table 2.6. Peak normal force and pressure during pre-strike down-swing Peak normal N Peak kPa Subject A 1st 2nd 3rd 1st 2nd 3rd Mean 19.738 23.813 26.713 Mean 114.875 149.375 140.625 1st - - - 1st - - - 2nd 0.243 - - 2nd 0.009 - - 3rd 0.000 0.686 - 3rd 0.001 1 - Subject B 1st 2nd 3rd 1st 2nd 3rd Mean 41.214 57.643 47.586 Mean 145.286 212.357 186.143 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.001 0.004 - 3rd 0.003 0.023 - Subject C 1st 2nd 3rd 1st 2nd 3rd Mean 22.088 48.875 34.638 Mean 70.750 163.375 131.75 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.016 0.033 - 3rd 0.012 0.82 - Subject D 1st 2nd 3rd 1st 2nd 3rd Mean 18.731 32.039 20.039 Mean 48.077 148.462 106.538 1st - - - 1st - - - 2nd 0.001 - - 2nd 0.000 - - 3rd 1 0.008 - 3rd 0.000 0.082 - Subject E 1st 2nd 3rd 1st 2nd 3rd Mean 2 54.675 17.9 Mean 11.75 147.5 60.25 1st - - - 1st - - - 2nd 0.001 - - 2nd 0.001 - - 3rd 0.001 0.001 - 3rd 0.001 0.001 -
Mean peak normal force and pressure separated into knapping swing phases and p-values from post-hoc pairwise Mann-Whittney U tests. Bolded results indicate that the 2nd or 3rd digit is significantly greater than 1st.
43
Table 2.7. Peak normal force and pressure during post-strike down-swing Peak normal N Peak kPa Subject A 1st 2nd 3rd 1st 2nd 3rd Mean 21.813 22.838 25.825 Mean 123.75 146.75 146.875 1st - - - 1st - - - 2nd 0.702 - - 2nd 0.007 - - 3rd 0.014 1 - 3rd 0.001 1 - Subject B 1st 2nd 3rd 1st 2nd 3rd Mean 44.243 55.457 44.221 Mean 159.571 219.286 189.857 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.693 0.000 - 3rd 0.033 0.040 - Subject C 1st 2nd 3rd 1st 2nd 3rd Mean 23.188 47.7 36.025 Mean 73.875 163.375 138.625 1st - - - 1st - - - 2nd 0.000 - - 2nd 0.000 - - 3rd 0.015 0.088 - 3rd 0.014 1 - Subject D 1st 2nd 3rd 1st 2nd 3rd Mean 18.904 35.077 20.442 Mean 57.115 170.385 110.192 1st - - - 1st - - - 2nd 0.001 - - 2nd 0.000 - - 3rd 1 0.003 - 3rd 0.003 0.036 - Subject E 1st 2nd 3rd 1st 2nd 3rd Mean 1.400 53.7 18.75 Mean 7.5 140.75 54.75 1st - - - 1st - - - 2nd 0.001 - - 2nd 0.001 - - 3rd 0.001 0.001 - 3rd 0.001 0.002 -
Mean peak normal force and pressure separated into knapping swing phases and p-values from post-hoc pairwise Mann-Whittney U tests. Bolded results indicate that the 2nd or 3rd digit is significantly greater than 1st.
44
A B
Figure 2.1: Oldowan bifacial chopper produced by Subject E. The bifacial edge is shown from 2 sides.
45
6000 Force (N) Angular acceleration (m/s2) 1100 4000 1000
2000 900 ) 2 800
(m/s 0
wrist
700 (N)
‐2000 force 600 acceleration,
‐4000 400 Normal Angular 300 ‐6000
200
‐8000 100
‐10000 0 0 0.05 0.1 0.15 0.2 0.25 Seconds Figure 2.2: Relationship between angular acceleration (m/s2) at the wrist (blue) and peak normal force (red) averaged over 24 swings for Subject A. The black line indicates strike.
46
Figure 2.3, a 2nd significantly greater than 1st 108 3rd significantly greater than 1st 96
84
72
60 (N)
48 force
Peak 36
24
12
0 A BC D EF Subjects
Figure 2.3: Peak normal force and pressure occurring at any time during the knapping swing (a and b, respectively) and normal force and pressure occurring at strike (c and d) acting on digits I, II, and III.
47
Figure 2.3, b 2nd significantly greater 450 than 1st 3rd significantly greater st 400 than 1
350
300 (kPa)
250 Pressure 200 Peak 150
100
50
0 A B CD E F Subjects
Figure 2.3: Peak normal force and pressure occurring at any time during the knapping swing (a and b, respectively) and normal force and pressure occurring at strike (c and d) acting on digits I, II, and III.
48
Figure 2.3, c 2nd significantly greater than 1st 108 3rd significantly greater than 1st 96
84
72 (N)
force
60
Strike 48
36
24
12
0 A B CD E F Subjects
Figure 2.3: Peak normal force and pressure occurring at any time during the knapping swing (a and b, respectively) and normal force and pressure occurring at strike (c and d) acting on digits I, II, and III.
49
Figure 2.3, d 2nd significantly greater than 1st 320 3rd significantly greater than 1st
280
240 (kPa) 200
pressure 160
Strike 120
80
40
0 A B CD E F Subjects Figure 2.3: Peak normal force and pressure occurring at any time during the knapping swing (a and b, respectively) and normal force and pressure occurring at strike (c and d) acting on digits I, II, and III.
50
2nd significantly greater than 1st Figure 2.4,a 3rd significantly greater than 1st 32
28
24
20 (N)
16 Impulse
12
8
4
0 A B CD E F Subjects
Figure 2.4: Impulse (a) and pressure-time integral (b) acting on digits I, II, and III through the entirety of the knapping swing.
51
Figure 2.4,b 2nd significantly greater than 1st 3rd significantly st 112 greater than 1
96
80 integral
64
kPa/time 48
32
16
0 A B CD E F Subjects Figure 2.4: Impulse (a) and pressure-time integral (b) acting on digits I, II, and III through the entirety of the knapping swing.
52
2nd significantly Figure 2.5, a&b greater than 1st 3rd significantly a b greater than 1st 45 225 40 200 35 175 30 swing
‐ 150 swing up ‐ 25
up 125
(kPa)
(N), 20 100 force 15 75 pressure
Peak 10
Peak 50
5 25
0 0 A B C D E A B C D E
Figure 2.5: Peak normal force and pressure during each phase of the knapping swing, acting on digits I, II, and III during up-swing. a: peak normal force during up-swing; b: peak pressure during up-swing; c: peak normal force during pre-strike down-swing; d: peak pressure during pre-strike down-swing; e: peak normal force during post-strike down-swing; f: peak pressure during post-strike down-swing.
53
2nd significantly greater than 1st 3rd significantly greater than 1st Figure 2.5, c&d c d 360 108
swing 320 ‐ swing
‐ 96 280 down
down 84
strike 240 72 ‐ strike ‐ pre
pre 200 60 (kPa), (N),
160 48 force
A B CD E 120 pressure
36 normal
Peak 80 24 Peak 12 40
0 0 A B C D E A B C D E
Figure 2.5: Peak normal force and pressure during each phase of the knapping swing, acting on digits I, II, and III during up-swing. a: peak normal force during up-swing; b: peak pressure during up-swing; c: peak normal force during pre-strike down-swing; d: peak pressure during pre-strike down-swing; e: peak normal force during post-strike down-swing; f: peak pressure during post-strike down-swing.
54
2nd significantly Figure 2.5, e&f greater than 1st 3rd significantly e f greater than 1st
320 90 swing swing
‐ 280
80 ‐
down 70 down
240
strike 60 strike ‐ ‐ 200 post post
50 160 (N), 40 (kPa),
120 force 30
pressure 80
normal 20
Peak 40 Peak 10
0 0 A B C D E A B CD E Subjects
Figure 2.5: Peak normal force and pressure during each phase of the knapping swing, acting on digits I, II, and III during up-swing. a: peak normal force during up-swing; b: peak pressure during up-swing; c: peak normal force during pre-strike down-swing; d: peak pressure during pre-strike down-swing; e: peak normal force during post-strike down-swing; f: peak pressure during post-strike down-swing.
55
Chapter 3: Upper limb kinematics and the role of the wrist during stone tool production
ABSTRACT
Past studies have hypothesized that aspects of hominin upper limb morphology are linked
to the ability to produce stone tools. However, we lack the data on upper limb motions
needed to evaluate the biomechanical context of stone tool production. This study seeks
to better understand the biomechanics of stone tool-making by investigating upper limb
joint kinematics, focusing on the role of the wrist joint, during simple flake production.
We test the hypotheses, based on studies of other upper limb activities (e.g., throwing),
that upper limb movements will occur in a proximal-to-distal sequence, culminating in
rapid wrist flexion just prior to strike. Data were captured from four amateur knappers
during simple flake production using a VICON motion analysis system (50 Hz).
Results show that subjects utilized a proximal-to-distal joint sequence and disassociated the shoulder joint from the elbow and wrist joints, suggesting a shared strategy employed in other contexts (e.g., throwing) to increase target accuracy. The knapping strategy included moving the wrist into peak extension (subject peak grand mean = 47.3°) at the beginning of the down-swing phase which facilitated rapid wrist flexion and accelerated the hammerstone towards the nodule. This sequence resulted in the production of significantly more mechanical work, and by extension greater strike forces, than would otherwise be produced. Together these results represent a strategy for increasing knapping efficiency in Homo sapiens and point to aspects of skeletal anatomy
56 that might be examined to assess potential knapping ability and efficiency in fossil hominin taxa.
57
INTRODUCTION
Modern humans are the most eurytopic primate species the planet has ever hosted,
capable of inhabiting regions that our closest living relatives, the African apes, would
find inhospitable and even hostile to their survival. Our success has been attributed in
part to our elaborate relationship with technology, of which stone tools represent the
earliest evidence in the archaeological record (Stiner and Kuhn, 1992; Schick and Toth,
1993; Foley, 1995; Semaw et al., 1997; Wood and Brooks, 1999; Wrangham et al., 1999;
Ambrose, 2001; Lutz and Qiang, 2002; Wood and Strait, 2004; Wrangham, 2007).
Consequently, the ability to produce and use tools is recognized as a key adaptation in hominin evolution.
Hominin hands and wrists have undergone numerous alterations over the course of human evolution, many of which occurred soon after the origin of early stone tool
technologies (Tocheri et al., 2008). While major gaps remain, the fossil record
documents changes including broader apical tufts of the fingertips, a more robust thumb,
and a rearrangement of carpal and radiocarpal anatomy. Due in part to the temporal
proximity of these events, researchers have hypothesized that stone tool production was a
major selective pressure inducing some of these changes in upper limb anatomy (Napier,
1962b; Marzke and Shackley, 1986; Susman, 1988; Susman, 1994; Marzke and Marzke,
2000; Ambrose, 2001; Richmond et al., 2001; Panger et al., 2002; Tocheri et al., 2008).
With few exceptions (Marzke et al., 1998; Biryukova et al., 2005), we currently
lack the quantitative data on upper limb kinematics necessary for evaluating hypotheses
on the mechanical context of stone tool production. The current project was undertaken
58
to begin rectifying this issue by investigating upper limb kinematics during the
production of simple flakes. We examined upper limb motion patterns associated with
stone tool production as a means of testing basic assumptions underlying hypotheses
about the functional demands and selective pressures that may have been acting on the
upper limb during early hominin evolution. Susman (1998) noted that because experiments using modern humans cannot account for the mosaic combinations of primitive and derived features in fossil hominin upper limbs, their relevance to understanding the tool behaviors of early hominins is limited. While this critique may apply to some studies, it is not applicable here. The goal of the present study was to examine upper limb kinematics of knappers and the role of the wrist during stone tool production as currently practiced, enabling a more informed evaluation of some of the functional hypotheses linking the derived upper limb and wrist conditions to stone tool production. We concur with Lauder's (1995) and Marzke and Marzke and Marzke’s (2000) argument that functional data obtained through direct observation are necessary for evaluating functional hypotheses.
Compared with our closest living relatives, the African apes, human wrist anatomy differs in many respects (see Richmond et al, 2001; Tocheri et al, 2008 and references therein). Chimpanzees and gorillas possess a suite of traits related to knuckle- walking, which are thought to stabilize the wrist transversely and improve the resistance of compressive stresses experienced during the support phase of knuckle-walking (Tuttle,
1970; Jenkins and Fleagle, 1975; Richmond and Strait, 2000; Richmond et al., 2001;
Kivell and Schmitt, 2009). A component of this suite is a distally-projecting dorsal ridge
of the distal radius (Tuttle, 1967; Richmond and Strait, 2000). During the support phase,
59
as the wrist extends the dorsal portion of the distal radius reaches a close-packed position
with maximal articular congruence with the lunate and scaphoid that, in conjunction with
palmar ligaments, maintains a stable joint and prevents the wrist from further extension
(Tuttle, 1967; Jenkins and Fleagle, 1975; Richmond and Strait, 2000). In this manner,
the dorsal ridge of the radius plays a key role in minimizing the degree of maximum wrist
extension and contributing to a stable support column during knuckle-walking.
Empirically, average ranges of maximum wrist extension are greater in modern humans
[x = 70o (Almquist, 2001)] than in Gorilla [x = 58o (Tuttle, 1969)] and Pan [x = 34o
(Tuttle, 1967; Tuttle, 1969; Jenkins and Fleagle, 1975; Richmond, 2006)] (Table 3.1).
An African ape-like projecting dorsal ridge is present in Australopithecus anamensis and Au. afarensis, suggesting a lower range of wrist extension compared with later hominins (Richmond and Strait, 2000). Lovejoy et al. (2009) argue that the carpal
anatomy of Ardipithecus ramidus indicates that the wrist was capable of greater degrees
of extension than that observed in African apes. However, their analysis unfortunately
does not demonstrate a link between midcarpal morphology and ranges of motion in
extension, and lacks sufficient comparisons with primate taxa other than African apes and
humans to permit clear functional interpretations about wrist mobility. Although data
were not presented, Lovejoy et al (2009; see also White et al. 1994) imply that the distal
radius of Ar. ramidus has an African ape-like distally-projecting dorsal ridge. Therefore,
the functional significance of the Ar. ramidus wrist remains open to question and requires
further analysis. Furthermore, Ar. ramidus probably post-dates the Pan-Homo last
common ancestor (LCA) by over 1 million years (White et al., 2009). Without direct
evidence of the LCA, wrist morphology in the LCA will remain a matter of debate.
60
What can be concluded with certainty is that over the course of human evolution
the morphology of the wrist has undergone significant modifications, including a change
in the distal radius from an African ape-like morphology in Au. anamensis and Au.
afarensis to a relatively flat, modern human-like distal radius in later hominins
(Richmond and Strait, 2000; Richmond et al., 2001). Researchers have hypothesized that
such anatomical changes in wrist anatomy may have offered later hominins, including
Homo, increased wrist mobility that facilitated a variety of behaviors including throwing
and stone tool production (Marzke, 1971; Richmond and Strait, 2000; Ambrose, 2001;
Richmond et al., 2001). However, this hypothesis rests on the assumption that wrist mobility, particularly in extension, plays an important role in stone tool production.
In order to examine the role of the wrist in stone tool-making, this study tests several hypotheses. Previous research on upper limb activities such as writing, throwing, and piano playing has demonstrated that subjects maintained general kinematic uniformity (e.g., sequence of kinematic events) within a given task. Specific patterns
(e.g., kinematic values), however, varied among subjects and across competency levels
(Newell and Van Emmerik, 1989; Hore et al., 1996; Fleisig et al., 1999; Minetti et al.,
2007). We predict that during stone tool production knappers will similarly demonstrate
consistent gross upper limb motion patterns.
Biomechanics research on activities such as pitching (Debicki et al., 2004), dart throwing (McDonald et al., 1989; Jeansonne, 2003), hammering (Cote et al., 2005), and soccer kicking (Putnam, 1991) show that the consistent gross movement pattern of the
limb occurs in a proximal-to-distal sequence. This pattern allows for greater accuracy because movements of proximal joints have larger effects at the end of the limb than do
61
movements of distal joints (Hore et al., 1996). Therefore, the distal joints are thought to
act late in order to refine the final position before object release or strike. Distal joint
kinematics (e.g., position and velocity) have a demonstrated effect on accuracy in upper
limb activities such as pitching a baseball and striking a target (Southard, 1989; Hore et
al., 1996; Hirashima et al., 2007). In this study, we test the hypothesis that during
knapping upper limb movements will similarly occur in a proximal-to-distal sequence,
culminating in peak wrist extension followed by rapid wrist flexion just prior to strike.
Finally we predict, based on previous upper limb kinematics research, that wrist
motion will significantly influence knapping mechanical efficiency and strike accuracy.
In their EMG study of muscle recruitment during stone tool production, Marzke et al.
(1998) reported that all subjects maximally recruited their flexor carpi ulnaris (FCU)
muscles (the only forearm muscle monitored whose chief action is motion at the wrist
and/or midcarpal joint) during down-swing. FCU activity and wrist flexion may help
knappers reach higher joint velocities, thereby producing more mechanical work and greater strike forces than could be achieved with a rigid wrist (Bunn, 1955; Putnam,
1991). Here, we test the hypothesis that during knapping wrist movements significantly
increase mechanical work. We also test the hypothesis that wrist extension in particular
plays an important role in producing this increased mechanical work.
62
METHODS
Sample
Data were captured from four knappers; three males (Subjects A-C), and one
female (Subject D). Two of the subjects periodically participated in knapping (i.e., fewer
than five times per year, Subjects A and C), and two had limited-to-no prior knapping
experience (Subjects B and D). All subjects were healthy, right hand dominant adults
free from muscular and/or osteological conditions that may have compromised their
motion patterns.
Raw materials
Experiments were conducted in cortex-free raw Texas flint (material toughness =
1 Kc, unpublished data from Herzl Chai). The mass and dimensions of the nodules were
initially similar (n = 8, mean mass = 5.85 kg; stdev = 1.22; mean length = 40.5 cm, stdev
= 2.76; mean circumference = 34.1 cm, stdev = 6.65). Flint was obtained from
Neolithics.com. A single quartzite hammerstone was used for all flake production (0.765 kg).
Motion capture
Kinematics data were captured using the VICON motion capture analysis system
in The George Washington University’s Motion Capture and Analysis lab. The VICON
system uses high speed cameras to digitally record reflective markers applied to subjects as they move across a calibrated space, allowing extraction of motion data such as
63 landmark coordinates, joint angles, joint (e.g., wrist) angular velocity and acceleration, and segment (e.g., forearm) velocity and acceleration. Multiple cameras are linked and simultaneously record the subject’s motion from various angles, thereby providing multiple views of the same movement. Six to eight infrared cameras and one digital video recorder were used to capture each subject’s knapping motions, recorded at 50 Hz.
Each subjects’ dominant hand (i.e., hammer hand) was fitted with a tight, fingerless glove (rayon/cotton/rubber blend) with separate holes for the thumb and index finger, and a single large hole for the three remaining digits. Six 10-mm diameter reflective markers were affixed to the glove and arm at the following landmarks: the olecranon process (OP), the radial and ulnar styloid processes (RSP and USP, respectively), and metacarpal (MC) heads I, II and V (Figure 3.1). Each marker consisted of a base with a short rod projecting upward and a reflective globe. Globes can be screwed and unscrewed from the rods to secure or unsecure them from the bases.
Markers were affixed to the appropriate position on the glove by pushing the rod through the glove from the inside and screwing the globe back onto the rod such that the glove was sandwiched between the base and globe of each marker. Marker bases were taped to the subjects using double sided tape to further prevent their displacement. Markers had initially been taped directly to subjects without the glove; however their frequent displacement upon strike necessitated the alternative method described above.
Data capture occurred in two phases for each subject. Phase 1 consisted of recording subjects’ excursions at the wrist (extension, flexion, radial deviation, and ulnar deviation). Data were captured from a “neutral” position in which subjects held their dominant arm flexed at the shoulder joint with the arm, forearm, and palm parallel to the
64
floor, fully pronated with the palm facing the floor and fingers fully extended. Subjects sequentially moved their wrist in each direction to their natural excursion maximum and held the position for five seconds before releasing, returning to the neutral position, and proceeding to the next direction. We note that the wrist may be capable of greater ranges of movement than these natural excursion maxima due to accelerations during rapid movements or if subjected to an external force. However, these provide baseline excursion maxima based on each subject's voluntary muscle activity.
Phases 2 consisted of data capture during flake production. Subjects removed simple flakes from the nodule without regard to flake dimensions or mass by forcefully striking the hammerstone against the nodule, which subjects balanced on their left leg.
Recording was halted after the successful production of each flake and the flake was
retrieved and labeled to allow later association of each flake with its appropriate knapping trial and landmark coordinate data. Data for each subject were captured at two or more separate knapping sessions to prevent fatigue of the upper limb.
Data analysis
Captured coordinate data were sectioned into individual swings. Those swings with relevant missing data were removed from the sample set, resulting in 66 flake production knapping cycle swings (Table 3.1). When subjects did not flex or deviate in the ulnar direction past their neutral position reported flexion and ulnar deviation means represent degrees above the neutral position (i.e., extension or radial deviation). The position of the radial styloid process was used as a proxy for the wrist’s position to demarcate swing initiation and termination, vertical and lateral excursions, and the
65
transition from up-swing to down-swing. Swing initiation was demarcated by the lowest
vertical position of the wrist immediately prior to the initiation of the wrist’s vertical
ascent during up-swing. Swing termination was demarcated by the lowest vertical
position of the wrist immediately following the termination of the wrist’s vertical decent
during down-swing. The transition from up-swing to down-swing (TR) was demarcated by the highest vertical position of the wrist during each swing (Figure 3.2). Motion at the shoulder joint was estimated by tracking directional changes in the horizontal plane at the
OP in relation to its starting position (e.g., the transition from flexion to extension during the knapping cycle).
Wrist extension/flexion and radial/ulnar deviation angles were calculated using captured coordinate data from the OP, RSP, USP, and the MC II head, and evaluated through the course of each swing. Using the midpoint between the RSP and USP as the angle vertex (Point A), the following procedure was used to derive extension/flexion angles (ΦDV [dorsal/ventral], Figure 3.3a) and radial/ulnar deviation angles (ΦML
[medial/lateral], Figure 3.3b). Two planes were established: 1) Forearm Plane ML, representing 0° of extension/flexion or the neutral plane, defined by the RSP, USP and
OP, defined as Point B, and 2) Forearm Plane DV, the plane perpendicular to Forearm
Plane ML which runs along Line AB, representing 0° radial/ulnar deviation. The MC II
3 head , defined as Point C, was projected onto Forearm Plane DV (Point CDV) for
3 The ulnar deviation angles reported here, based on the MCII head, are likely to be systematically lower than those reported by studies that used the MC III head to calculate radial and ulnar deviation Youm Y, McMurthy RY, Flatt AE, and Gillespie TE (1978) Kinematics of the wrist. I. An experimental study of radial-ulnar deviation and flexion-extension. J Bone Joint Surg Am 60:423-31 Moritomo H, Goto A, Sato Y, Sugamoto K, Murase T, and Yoshikawa H (2003) The triquetrum-hamate joint: an anatomic and in vivo three-dimensional kinematic study. J Hand Surg Am 28:797-805 Murgia A, Kyberd PJ, Chappell PH, and Light CM (2004) Marker placement to describe the wrist movements during activities of daily living in cyclical tasks. Clin Biomech (Bristol, Avon) 19:248-54., but ranges of motion would be comparable. 66
calculation of ΦDV or onto Forearm Plane ML (Point CML) for calculation of ΦML.
Extension/flexion angles (ΦDV) were calculated as the angle supplementary to BACDV.
Radial/ulnar deviation angles (ΦML) were calculated as the angle supplementary to
BACML. All angles were calculated relative to each subject’s neutral position as recorded during Phase 1. For statistical analyses, all wrist excursion angles were measured in degrees and converted to radians to standardize measurements.
Vertical velocity (v, m/s) and acceleration (a, m/s2) were calculated through each
swing at time (t) and position (x) for each landmark using captured coordinate data
according to:
X(t+1) - X(t-1) v (t) = (Eq. 1) (t+1) - (t-1)
V - V a = (t +1) (t -1) (Eq. 2) (t) (t+1) - (t-1)
Angular velocity and angular acceleration at the wrist were calculated through the course
of each swing using ΦDV. These angles replaced x in Eq. 1. Wrist angles, velocity,
acceleration, angular velocity, and angular acceleration were derived using the R
statistical program language, versions 2.5 and 2.7 (Ihaka and Gentleman, 1996).
Total work per swing (Wtotal, measured in Joules) was defined as the sum work for the up-swing and down-swing phases for each swing:
Wtotal = Wup + Wdown (Eq. 3).
The coordinate position of the MC II head was used as a proxy for the hammerstone
position for all work production calculations, unless otherwise stated. Up-swing work
production was calculated according to:
Wup = FΔdup (Eq. 4), where
67
F = (9.8 m/s2)(m);
m = hammerstone mass (kg); and
Δdup = (maximum hammerstone vertical position – hammerstone vertical position at
swing initiation).
Down-swing work production was defined as the difference between maximum kinetic
energy of the hammerstone and maximum potential energy of the hammerstone:
1 2 2 Wdown = ( /2mv ) – (9.8m/s mΔddown) (Eq. 5), where
m = hammerstone mass (kg);
v = maximum hammerstone velocity (m/s); and
Δddown = (maximum hammerstone vertical position – hammerstone vertical position at
strike).
In order to maintain the error rate across multiple (k) comparisons, a modified
Bonferroni adjustment method was employed to determine significance (alpha: 0.05) for
all analyses: the k p-values were ordered and the smallest p-value compared was
compared to 0.05/k; if that was found to be significant, then the next smallest p-value was
compared to 0.05/(k-1), etc. (Holm, 1979).
RESULTS
Upper limb motion patterns
Flake production knapping cycles were divisible into two phases with distinct, consistent sets of motion: an up-swing and a down-swing phase. Up-swing was
characterized by upward limb motion, flexion of the shoulder and elbow joints, and
68
increasing wrist extension. Down-swing was characterized by downward limb motion,
extension at the shoulder joint, continued elbow flexion and wrist extension through peak
wrist extension, followed by rapid elbow extension and wrist flexion (Figure 3.4).
Following the termination of down-swing, motion patterns varied widely by subject and trial. Knapping was often halted between swings to adjust the core or hammerstone.
Subjects rarely maintained a fluid knapping rhythm, in which they proceeded directly into the next up-swing following down-swing termination.
Comparison of the timing and magnitude of peak linear velocities of limb segment endpoints was used to evaluate joint coordination through down-swing—the knapping phase that is responsible for positioning the hammerstone to strike the nodule.
All subjects began down-swing with velocity increasing in the negative direction (i.e., downwards) at the OP, RSP, and MC II head (Figure 3.2). The OP reached peak linear velocity first, then reduced velocity. The RSP and the MCII head continued to increase velocity inferiorly, until their velocities peaked just prior to strike. Post strike, linear velocities of all segment endpoints quickly decreased inferiorly, approaching zero velocity (i.e., all segment endpoints decelerated while continuing to travel downwards).
The temporal onset of upper limb joint peak linear velocities showed a partial proximal- to-distal relationship, with the OP peaking significantly before the RSP and the MCII head (p < 0.003 and p < 0.001, respectively). All temporal differences between the RSP and MCII head were insignificant (Table 3.2). Peak linear velocity at each segment endpoint proceeded in a complete proximal–to–distal fashion, with velocity significantly increasing from the OP to RSP (p < 0.0001), and from the RSP to MC II head (p < 0.05) for each subject (Table 3.3).
69
Wrist extension/flexion and radial/ulnar deviation patterns
Subjects’ peak extension range encompassed 30.6° - 70.1° (total Subject low and high values, respectively). Subjects’ peak knapping extension grand mean was 47.3°
(Table 3.1). Measured relative to their respective neutral positions, Subjects C and D utilized 58.7% and 56.4% of their muscular-induced extension range, respectively, while
Subjects A and B utilized 82.5% and 98.4%, respectively (Table 3.4). Negative values represent the extent to which subjects failed to flex past their neutral position. Subjects A and B passed their passive muscular-induced extension maxima (as recorded in Phase 1) in 17% and 30.8% of their trials, respectively. All subjects did not approach their passive muscular-induced flexion maxima, flexing on average 12.82° – 34.14° below their knapping extension peak and all failed to flex beyond their respective neutral positions except Subject C in 20% of his trials.
Radial deviation was emphasized over ulnar deviation. All subjects employed at least 35% of their radial deviation range, but only one (Subject B) deviated in the ulnar direction past his neutral position (Tables 3.1 and 3.4). All subjects employed significantly more absolute motion in the dorsal-ventral plane compared to the radial- ulnar plane (Bonferroni adjusted p < 0.02, Figure 3.5). However, each subject employed similar percentages of their respective total available range of motion in the dorsal- ventral and radial-ulnar planes (Table 3.4).
The timing of peak extension, angular velocity and angular acceleration were temporally constrained across all subjects in relation to both the transition to down-swing and to strike (Table 3.5).
70
Work production
Knapping work production was calculated at the MCII head as described above to generate a work production baseline. To isolate the contribution to work production gained through increased velocity at the MCII head, work production was calculated a second time by substituting velocity and vertical displacement at the MCII head with the
RSP. All subjects produced significantly more work employing the greater velocities achieved at the MC II head during both phases of flake production (p ≤ 0.006, Table 3.6).
DISCUSSION
The results of this study demonstrate that the wrist plays an important role in simple stone tool production, and supports the hypotheses set out in the introduction.
First, all knappers displayed broadly similar upper limb motion patterns. This was evident in the temporal consistency of significant events within each subject (i.e., segment endpoint velocity [Table 3.2]) and among all subjects (i.e., peak wrist extension, angular velocity, and angular acceleration [Table 3.5]) during the down-swing phase.
These similarities were expected given the task uniformity and subjects’ prior exposure to knapping demonstrations and theory.
The second hypothesis that during down-swing upper limb movements will occur in a proximal-to-distal sequence culminating in rapid wrist flexion just prior to strike was also supported. The shoulder, elbow, and wrist joints moved in a coordinated fashion through the up-swing phase. However, subjects replaced a rigid upper limb with mobile joints that act synergistically—a transition from simple to complex motion patterns
71
(Newell and Van Emmerik, 1989)—following the transition to down-swing. A mobile upper limb offers two advantages over maintenance of a rigid limb. One, subjects are
able to utilize a proximal-to-distal joint sequence in which the most proximal joint begins
forward motion before the distal joints, and the proximal joint reaches peak linear
velocity and begins to slow down prior to the distal joints reaching their respective peak
linear velocities (Figure 3.2; Tables 3.2) (Putnam, 1991). This motion sequence can
result in a velocity “summation effect” at the most distal joint due to torque interactions
among the preceding joints such that the distal joint experiences greater velocities than
could otherwise be achieved (Bunn, 1955; Southard, 1989; Putnam, 1991). The results of
this summation effect are evident in the knapping kinematics reported here; segment
endpoints’ linear velocities significantly increased from the OP to RSP and from the RSP
to MC II head during flake production knapping cycles (Table 3.3).
The second advantage of a mobile upper limb is the accuracy increase afforded by disassociating motion of the distal joint from the proximal joints (Bernstein, 1967;
Arutyunyan et al., 1968; Newell and Van Emmerik, 1989; Southard, 1989; Anderson and
Sidaway, 1994; Hore et al., 1996). Bernstein (1967) hypothesized that additional degrees
of freedom are liberated through joint disassociation, thus increasing the potential to
coordinate a greater number of degrees of freedom and increase motor control. However,
Hore et al. (1996) cautioned that the magnitude of the effects of joint kinematics on strike
accuracy increases in a proximal-to-distal fashion. Therefore, the wrist joint plays a
greater role than more proximal joints in determining strike accuracy by virtue of its
distal position. Joint disassociation was evident in the timing and magnitude of segment
endpoints’ peak linear velocities (Tables 3.2 and3. 3). Temporal disassociation between
72
the RSP and MC II head was not evident, which may be due to subjects’ relative inexperience with this behavior, resulting in reduced capability to achieve joint disassociation (Bernstein, 1967). Given its utility in other accuracy-seeking activities,
such as pistol shooting, writing, throwing, and kicking, we suggest that joint
disassociation is naturally similarly employed during stone tool production to increase
strike accuracy (Bernstein, 1967; Arutyunyan et al., 1968; Newell and Van Emmerik,
1989; Southard, 1989; Anderson and Sidaway, 1994; Hore et al., 1996). Ongoing kinematic studies are being conducted with skilled knappers to directly measure strike accuracy as it relates to joint disassociation.
The consistent proximal-to-distal sequence supports the hypothesis that the wrist
plays an important role in stone tool production, through greater mechanical work and
likely greater accuracy. The wrist reaches peak extension 0.05 seconds after the
transition to down-swing, setting up the wrist for flexion prior to strike (Table 3.5). The
degree of wrist extension (individual means of 36 °– 66o for subjects in this study) used is
likely a function of the manner in which the moment arms and tensile strengths of the
forearm’s flexor muscles change with the degree of wrist extension/flexion. The
relationship between the moment arms of two of the forearm’s flexors (FCU and flexor
carpi radialis) and wrist posture approximate positive second order polynomial
relationships (Pigeon et al., 1996), meaning that their mechanical advantage increases as
wrist extension increases. Thus, by utilizing greater degrees of wrist extension knappers are able to more fully exploit the forearm’s flexors.
The rapid release of peak wrist extension (i.e., burst of wrist flexion), illustrated by angular velocities and accelerations recorded at the wrist, peaked at 0.019 seconds and
73
0.032 seconds prior to strike, respectively (Table 3.5). In their EMG study of muscle
recruitment during stone tool production, Marzke et al. (1998) reported that FCU muscle
activity in the hammer hand peaked immediately prior to strike, inducing a wrist “flick” which further accelerated the hammerstone towards the nodule. The peak angular velocities and angular accelerations recorded here are likely due in part to strong FCU
recruitment following peak wrist extension.
The relationship between muscles’ mechanical advantage and the position of the
wrist also appears to influence the degree of flexion subjects utilized. All subjects
employed greater than 50% of their respective extension range, but subjects flexed
minimally out of peak extension, and typically avoided wrist flexion past their neutral
position (Tables 3.1 and 3.4). Maintaining the wrist in an extended position may reflect a
strategy to maintain hammerstone control against the strong reaction forces produced at
strike. According to Pigeon et al., (1996), the digital flexors are weaker when the wrist is
held in a flexed position compared to an extended position, which may render the
hammerstone more susceptible to displacement when the wrist is strongly flexed. By
avoiding excessive wrist flexion the knapper may be better able to maintain a tighter grip
on the hammerstone. The avoidance of strong flexion, particularly past Forearm Plane
ML, has also been reported in pitching activities (Debicki et al., 2004), during which
flexion-inducing interactive torques produced by the more proximal upper limb segments
and experienced at the distal joints were dampened by heightened forearm extensor
activity. The dampening effect on the part of the extensors allowed pitchers to better
control wrist flexion and ball release, thereby increasing target accuracy.
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The results of this study also support the final hypothesis that rapid wrist flexion from an extended position significantly contributes to the production of mechanical work
(and by extension to strike force given ; Table 3.6). This contribution is evident in comparing the significantly greater linear velocities attained at the MCII head and the RSP. Achieving significantly greater linear velocities and rapid angular velocities and accelerations as the wrist flicks the hammerstone towards the nodule may be dependent on reaching a minimum degree of wrist extension due to the mechanical relationship between the FCU and FCR and the degree of wrist extension (Pigeon et al.,
1996). The observation that the wrist and particularly wrist extension plays an important role in the biomechanics of stone tool-making is consistent with hypotheses suggesting that stone tool-making, as well as other upper limb dependent activities, may have influenced the evolution of the hominin wrist.
Kanzi, a male bonobo chimpanzee that has been practicing stone tool production since 1990, provides a ready contrast to the knapping strategies reported here and an illustration of the effects on stone tool production of upper limb anatomy that is primitive in some respects. Toth et al. (1993) reported that Kanzi preferred to knap using a “hard, rapid thrust” to throw cores against hard objects, rather than employing a hand-held hard hammer method. The authors stated that with a hard hammer, Kanzi’s applied force was often insufficient for fracturing rocks in a manner producing useful cores, flakes, or edges. Resulting cores were described as “simple,” with “non-invasive flake scars and steep, battered edges,” (Toth et al., 1993:85), markedly different from descriptions of the earliest Oldowan cores from Gona and Lokalalei (Roche et al., 1999; Semaw, 2000;
Semaw et al., 2003). The difference in preferred method may have been an effect of
75
Kanzi’s inability to employ effectively the hard hammer method due in part to Pan’s limited wrist extension, as well as more limited opposability between the thumb and ulnar fingers (Marzke, 1983; Guthrie, 1991), differences in thumb musculature and robusticity
(Susman, 1994; Marzke et al., 1999; Diogo and Wood, 2009) and carpal arrangement
(Lewis, 1977; Marzke, 1983; Tocheri et al., 2005; Tocheri et al., 2008; Marzke et al.,
2010).
However, Kanzi’s knapping ability and stone tool assemblages associated with multiple hominoid species provide strong evidence that more than one hominoid species practiced stone tool behaviors (Susman, 1988; Marzke, 1997; Morwood et al., 2004;
Mercader et al., 2007). Thus rather than adhering to what increasingly appears to be a false dichotomy between the ability or inability to fashion and use stone tools, it may prove more beneficial to investigate the factors involved in effective and efficient tool use and production, such as mechanical efficiency and strike accuracy.
CONCLUSIONS
Our analysis of upper limb motion during simple flake production shows that during down-swing the upper limb movements occur in a proximal-to-distal sequence, culminating in rapid wrist flexion just prior to strike, and that gross limb motion patterns are consistent (e.g., in order and timing of key events) within and among subjects. In this manner, knapping is similar to other complex upper limb activities involving striking or throwing, such as hammering, pitching, and dart-throwing. The results also show that during knapping use of a mobile versus rigid wrist significantly increases mechanical
76
work, and is also thought to be critical for strike accuracy. Wrist extension plays an
important role in this increased mechanical work by positioning the hand for effective flexor muscle recruitment and rapid flexion immediately prior to strike. These findings support the hypothesis that knapping, as well as other complex upper limb activities, was an important factor in the evolutionary reorganization of the human wrist. Further, the use of the higher degrees of wrist extension afforded to modern humans has a demonstrated impact on economic and effective stone tool production, providing a method to increase joint linear velocity, strike force, and potentially strike accuracy.
Future studies would benefit from an approach investigating the factors involved in effective and efficient tool use and production, such as mechanical efficiency and strike accuracy.
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Table 3.1. Wrist joint limits of Pan, Gorilla, Pongo and Homo
radial ulnar taxon n method extension flexion deviation deviation references A, B, C, Pan troglodytes 51 mixed 34 127 27 60 E* Gorilla 10 anesthetized 58 117 44 70 B Pongo pygmaeus 27 anesthetized 85 139 57 97 A Homo sapiens NA in vivo 70 80 20 30-40 D Subject A 1 muscular 58.79 49.8 20.53 17.68 Subject B 1 muscular 66.68 60.2 12.89 40.01 Subject C 1 muscular 62.05 67.14 24.01 25.89 Subject D 1 muscular 68.43 75.02 26.40 33.89 Subject A 12+ knapping 48.51 + 25.84 7.96 + 0.76 Subject B 26+ knapping 65.61 + 31.47 6.52 5.78 Subject C 10+ knapping 36.43 + 6.59 12.19 + 0.01 Subject D 18+ knapping 38.62 + 25.8 9.28 + 1.04 Knapping data are presented in degrees relative to each subject’s neutral position. The values for Pan represent the weighted means derived from reported range of motion values. n+ represents the number of knapping swing trials per subject. A: Tuttle (1967); B: Tuttle (1969); C: Jenkins and Fleagle (1975); D: Almquist (2001); E: Richmond (2006).
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Table 3.2. Timing of peak linear velocities (m/s) in the upper limb relative to the transition from up-swing to down-swing and strike Subject A, n = 12 Subject A, n = 12 Landmark OP RSP MC II Landmark OP RSP MC II Seconds Seconds 0.088 0.128 0.132 -0.062 -0.015 -0.012 relative to TR relative to ST Stdev 0.031 0.026 0.023 Stdev 0.025 0.012 0.013 OP to RSP p = 0.003, t = 3.394 OP to RSP p < 0.0001, t = 5.827 OP to MC II p = 0.001, t = 3.849 OP to MC II p < 0.0001, t = 6.147 RSP to MC II p = 0.745, t = 0.329 RSP to MC II p = 0.534, t = 0.632
Subject B, n = 26 Subject B, n = 26 Landmark OP RSP MC II Landmark OP RSP MC II Seconds Seconds 0.034 0.095 0.101 -0.055 -0.011 -0.006 relative to TR relative to ST Stdev 0.032 0.018 0.019 Stdev 0.061 0.022 0.018 OP to RSP p < 0.0001, t = 8.435 OP to RSP p = 0.002, t = 3.376 OP to MC II p <0.0001, t = 9.067 OP to MC II p = 0.001, t = 3.876 RSP to MC II p = 0.215, t = 1.091 RSP to MC II p = 0.343, t = 0.998
Subject C, n = 10 Subject C, n = 10 Landmark OP RSP MC II Landmark OP RSP MC II Seconds Seconds 0.032 0.096 0.092 -0.068 -0.004 -0.008 relative to TR relative to ST Stdev 0.010 0.026 0.023 Stdev 0.033 0.026 0.023 OP to RSP p < 0.0001, t = 7.155 OP to RSP p < 0.0002, t = 4.8 OP to MC II p < 0.0001, t = 7.398 OP to MC II p < 0.0003, t = 4.692 RSP to MC II p = 0.678, t = -0.359 RSP to MC II p = 0.798, t = -0.359
Subject D, n = 18 Subject D, n = 18 Landmark OP RSP MC II Landmark OP RSP MC II Seconds Seconds 0.057 0.103 0.102 -0.048 -0.004 -0.006 relative to TR relative to ST Stdev 0.016 0.016 0.015 Stdev 0.025 0.009 0.009 OP to RSP p < 0.0001, t = 8.907 OP to RSP p < 0.0001, t = 6.991 OP to MC II p < 0.0001, t = 8.848 OP to MC II p < 0.0001, t = 6.755 RSP to MC II p = 0.863, t = -0.216 RSP to MC II p = 0.67, t = -0.375 Olecranon process (OP), radial styloid process (RSP), second metacarpal head (MC II), the transition from up-swing to down-swing (TR), strike (ST). Positive values represent seconds after TR or ST, negative values are prior. Bolded results are significant.
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Table 3.3. Peak linear velocities (m/s) in the upper limb during down-swing Subject A Landmark OP RSP MC II Peak velocity -1.136 -3.321 -4.080 OP to RSP p < 0.0001, t = -13.28 RSP to MC II p = 0.004, t = -3.178 Subject B Landmark OP RSP MC II Peak velocity -0.124 -2.216 -3.148 OP to RSP p < 0.0001, t = -28.796 RSP to MC II p < 0.0001, t = -8.176 Subject C Landmark OP RSP MC II Peak velocity -0.392 -2.123 -2.967 OP to RSP p < 0.0001, t = -6.765 RSP to MC II p = 0.046, t = -2.112 Subject D Landmark OP RSP MC II Peak velocity -0.914 -3.001 -3.620 OP to RSP p < 0.0001, t = -14.176 RSP to MC II p = 0.007, t = -2.849 Olecranon process (OP), radial styloid process (RSP), second metacarpal head (MC II). All results are significant.
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Table 3.4. Percent of total wrist excursion employed during knapping Extension Flexion Radial Ulnar Extension-flexion Radial-ulnar Subject A 82.51 -51.89 38.77 -4.30 20.88 18.84 Subject B 98.40 -52.27 50.58 14.45 26.91 23.25 Subject C 58.71 -9.82 50.77 -0.03 23.10 24.41 Subject D 56.44 -34.39 35.15 -3.07 8.94 13.67 The percentage of motion in any one direction represents the motion utilized in that direction relative to each subject’s relevant neutral plane.
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Table 3.5. Timing of peak extension, angular velocity (m/s) and angular acceleration (m/s2) at the wrist
Subject total n = 66 time ± 95% CI peak extension TR + 0.053 ± 0.006 peak extension ST - 0.055 ± 0.008 angular velocity TR + 0.095 ± 0.008 angular velocity ST - 0.019 ± 0.006 angular acceleration TR + 0.078 ± 0.01 angular acceleration ST - 0.032 ± 0.007
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Table 3.6. Work production (J) at the second metacarpal head and the radial styloid process Subject A B C D n 12 26 10 18 Up-swing Landmark MCII RSP MCII RSP MCII RSP MCII RSP Mean 3.552 2.909 1.776 1.386 1.800 1.383 2.322 1.861 stdev 0.796 0.655 0.239 0.206 0.347 0.299 0.264 0.238 p 0.004 < 0.0001 0.006 < 0.0001 t -2.198 -6.293 -3.094 -5.581 Down-swing Landmark MCII RSP MCII RSP MCII RSP MCII RSP Mean 3.077 1.500 1.336 0.051 1.975 0.720 2.986 1.948 stdev 1.432 1.024 0.772 0.424 1.129 0.712 0.906 0.945 p 0.005 < 0.0001 0.005 0.002 t -3.144 -7.440 -3.214 -3.414 All results are significant.
83
fingerless
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blend ulnar
3:
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tight
a radial
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5:
reflective
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tape.
I
of
sided
metacarpal Placement
double
4:
3.1:
and
glove Figure process,
84
Up‐swing Down‐swing A
olecranon 310 process
300 (mm)
360 RSP
300
position
240
Vertical MC II head
350
250 B
6 olecranon
process 2
2 ‐
(m/s) 10 RSP
10 velocity
‐ 30 ‐
Vertical MC II head 10
20 ‐ 50 ‐
0 0.05 0.10Time 0.15 0.20 Seconds Figure 3.2: Vertical position (A, mm), and vertical velocity (B, m/s) of the olecranon process, radial styloid process, and the second metacarpal head through a typical knapping cycle, illustrating swing initiation (dashed vertical black line), the transition from up-swing to down-swing (solid vertical black line), peak wrist extension (black arrow), strike (dashed black arrow), and peak linear velocity (black stars). Note the scale differences on the left.
85
as
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of defined
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is angle
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(Figure the
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Plane
measure Plane
Plane styloid
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Point
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Depiction
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to
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deviation
Lateral
of
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3.3:
A.
DV Φ perpendicular Depiction Point radial/ulnar Figure
86
A B
C D
E F
Figure 3.4: Model of the forearm with the radial styloid process (RSP, orange), the second metacarpal head (MC II, blue), and their respective paths during a typical knapping cycle. A: up-swing initiation. B: mid up-swing. C: the apex of up-swing. D: 0.06 seconds after down-swing initiation at peak wrist extension. E: strike. F: end of down-swing. Up-swing (A-C) is characterized by: 1) upward limb motion, 2) flexion at the shoulder and elbow joints, and 3) increasing wrist extension. Down-swing (D-F) is characterized by: 1) downward limb motion, 2) extension at the shoulder joint, 3) flexion of the elbow through peak wrist extension, 4) extension of the forearm following peak wrist extension, and 5) increasing wrist flexion (0.05 seconds after the initiation of down-swing).
87
Individual excursions Composite excursions
p < 0.0001
54
48 p < 0.0001
42 p = 0.012
) p = 0.001 ° (
36
30 Degrees
24 p = 0.003
18
12
6
0 ADB C Figure 3.5: Total extension-flexion (grey) and radial-ulnar deviation excursions (white), measured in degrees. Box plots do not include outliers. Individual subject (left) and composite results (right) are reported.
88
Chapter 4: Achieving accuracy and efficiency during Oldowan stone tool production
ABSTRACT
Multiple hominin species used and/or produced stone tools, yet evidence suggests that
only later Homo intensified the behavior. This difference has been attributed to later
Homo’s ability to efficiently produce stone tools, to the exclusion of earlier hominin species. The current study evaluates whether modern human upper limb anatomy contributes to energetic efficiency and/or accuracy during knapping. Knapping kinematics were captured from eight experienced knappers using a VICON motion analysis system. Each subject produced a series of Oldowan bifacial choppers under two
conditions: with normal wrist mobility and while wearing a brace that reduced wrist
extension to ~30°- 35°, simulating one aspect of the hypothetical primitive hominin
condition. Under normal conditions subjects employed a variant on the common
proximal-to-distal joint sequence. Upper limb motion was initiated at the shoulder joint
and proceeded distally, and peak linear and angular velocities increased from the
shoulder to the elbow to the wrist. Wrist extension was emphasized over other motion
directions and subjects utilized the “dart-throwers arc,” which is the most stable
radiocarpal plane of motion. With an unrestrained wrist, subjects achieved significantly
greater angular velocities at the wrist and struck their targets with significantly greater
accuracy. Additionally, the risks of carpal and ligamentous damage due to
hyperextension were decreased by the wrist’s ability to reach high degrees of extension
(≥ 28.5) following strike. These results suggest that modern human upper limb anatomy
89 contributes to efficiency and accuracy to stone tool production and reduces the risk of injury during this repetitive high stress activity.
90
INTRODUCTION
Evidence from East and Southern African sites indicate that multiple hominin species made and/or used stone tools, including Homo habilis, H. erectus sensu lato, and possibly Australopithecus garhi and Paranthropus robustus (Leakey et al., 1964;
Susman, 1988; Semaw, 2000; Klein, 2009). Further, new evidence of 3.4 million year old cut marked bones from Dikika, Ethiopia suggests that Australopithecus afarensis may have practiced stone tool behaviors, as well, and ~ 0.8 million years earlier than was previously believed (McPherron et al., 2010 but see Dominguez-Rodrigo et al., 2010).
However, only later Homo intensified and developed stone tool behaviors. This has been attributed to later Homo’s ability to execute efficient tool production to the exclusion of earlier hominin species. This enhanced performance ability likely arose due to cognitive and/or morphological divergences between the species (Susman, 1988; Marzke, 1997;
Stout et al., 2008; Faisal et al., 2010; Williams et al., 2010). The current study focuses on the latter by examining the upper limb motions of experienced knappers during the production of Oldowan bifacial choppers in order to determine which anatomical features contribute to efficiency and accuracy, and how they do so. We begin by describing the knapping swing, placing it in the context of the biomechanics of modern upper limb activities, such as throwing a ball. This is followed by looking specifically at the effects of wrist extension on knapping by comparing upper limb motions of knappers under normal conditions to their knapping motions when their wrists were restrained to approximately 30° - 35° of extension. The role of the wrist was chosen for examination because multiple researchers have suggested that wrist extension in particular may
91
contribute to knapping efficiency and accuracy, as well as manual activities such as
throwing (Marzke, 1971; Ambrose, 2001; Richmond et al., 2001).
The proximal-to-distal joint sequence and muscular efficiency
Many accuracy-seeking upper and lower limb activities, such as playing the piano, kicking or throwing a ball, and throwing a javelin, rely on a proximal-to-distal
joint sequence (PDJS) (Putnam, 1991; Putnam, 1993; Bartlett et al., 1996; Furuya and
Kinioshita, 2007). In this sequence, motion commences at the most proximal joint (e.g.,
the shoulder joint) and proceeds in a distal direction down the limb (e.g., elbow to wrist
to finger joints). The production of muscular torque early in the sequence in proximal muscles induces beneficial interactive torques at the more distal joints. This in turn reduces the need for distal musculature contribution to the generation of muscular torque in order to move the arm in its primary direction (Hirashima et al., 2003a; Hirashima et al., 2007). Instead distal muscular activity is reserved to play the less demanding role of counteracting passive interactive motion (Dounskaia et al., 1998)4. Angular velocity also
occurs in a proximal-to-distal (PD) fashion; the most proximal joint peaks first, and then
begins to slow down before the next joint peaks and slows. This joint sequence results in
a angular velocity “summation effect” at the most distal joint, such that the most distal
joint is able to reach significantly greater angular velocities than other joint sequences
would yield (Bunn, 1955; Putnam, 1991; Putnam, 1993). In motion sequences executed
4 Considering that muscular torque is derived from both active and passive viscoelestic components, the reduction in muscular activity among the distal muscles through the PDJS may be even greater compared with other motion sequences Dounskaia NV, Swinnen SP, Walter CB, Spaepen AJ, and Verschueren SMP (1998) Hierarchical control of different elbow-wrist coordination patterns. Experimental Brain Research 121:239-54 Hirashima M, Ohgane K, Kudo K, Hase K, and Ohitsuki T (2003b) Counteractive relationship between the interaction torque and muscle torque at the wrist is predestined in ball-throwing. Journal of Neurophysiology 90:1449-63. 92
by skilled individuals, greater angular velocity at the distal joint enhances performance ability. For example, greater angular velocity at the wrist and fingers increases ball release velocity in pitching (Debicki et al., 2004).
Williams and colleagues (2010) demonstrated that during stone tool production
amateur knappers utilize a partial PDJS regarding the onset and decline of segment
endpoint peak linear velocities. Linear velocity at the shoulder joint peaked and slowed
significantly before velocity at the elbow and wrist peaked. Knappers did not, however,
disassociate the elbow and wrist, and instead moved these joints as a unit. Following
Bernstein’s (1967) and Newell and Ven Emmerik’s (1989) hypothesis that greater joint
disassociation is attained with increased skill and coordination, we proposed that as
knappers improved they would initiation joint motion in a proximal-to-distal progression
and advance to using a complete PDJS in the upper limb. We test this prediction in the
current study using skilled knappers replicating Oldowan bifacial choppers.
Although motion was not disassociated between the elbow and wrist among
amateur knappers (Williams et al., 2010), all subjects attained a complete PDJS in terms
of the peak segment endpoint velocity attained. The resulting rapid wrist flexion allowed
knappers to reach significantly greater velocities at the wrist. Given that the moment
arms of the flexor carpi ulnaris and flexor carpi radialis muscles increase as the wrist
extends (Pigeon et al., 1996), we concluded that utilization of higher degrees of wrist
extension (i.e., average subject peak extension = 43°) during knapping provides a
biomechanical advantage and facilitates the achievement of high velocities and strike
forces.
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However, the forearm model we used tested only differences between a normal forearm and wrist and a forearm without the contribution from the wrist (Williams et al.,
2010). It is possible that knappers are able to achieve similarly high wrist angular
velocities using lower degrees of wrist extension. Determining whether this is the case is
needed to test hypotheses linking efficient stone tool production to greater wrist mobility
(Marzke, 1971; Richmond and Strait, 2000; Ambrose, 2001). Additionally there is evidence that angular velocity is a more appropriate biomechanical variable compared with linear velocity in kinematic studies (Putnam, 1991). Here we test the impact of lower degrees of wrist extension on the angular velocity achieved at the wrist by physically restraining knapper’s wrists to ~ 30° - 35° during the production of Oldowan bifacial choppers. We predict that limiting wrist extension will result in significantly lower angular velocities at the wrist and hammerstone, and in turn lower accelerations and strike forces.
Strike accuracy
Target accuracy is also associated with the PDJS (Chowdhary and Challis, 1999), specifically with the disassociation of the distal joints from the proximal joints
(Bernstein, 1967). Such joint disassociation allows the individual to coordinate a greater number of degrees of freedom along the limb, thereby refining movement at the most distal joint. Control of the most distal joint is particularly important give the strong evidence demonstrating that target accuracy and performance ability are determined by motion at the most distal joint (Newell and Van Emmerik, 1989; Hore et al., 1996; Watts et al., 2004; Hore et al., 2005).
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In our investigation of the kinematics of amateur knappers (Williams et al., 2010),
we proposed that knappers are able to increase their strike accuracy by disassociating the
wrist from the more proximal joints and through the utilization of high degrees of wrist
extension. We directly test this hypothesis here by comparing the accuracy of knappers
under normal knapping conditions to that when their wrists are restrained to limit
extension.
METHODS
Sample
Knapping kinematic data were captured from eight subjects, six males (Subjects
A-F) and two females (Subjects G and H). All subjects were familiar with Oldowan
stone tool types. Subjects A-E were highly experienced knappers proficient in knapping
techniques spanning the Oldowan, Acheulean, Middle and Late Stone Ages. Subjects F-H
were proficient in making Oldowan and Acheulean tools, but not yet practiced in more
complex tool traditions. Subjects were all right-hand dominant and free from muscular
and/or osteological conditions.
For the analysis of kinematic data, each subject produced a total of four Oldowan
bifacial choppers using nodules of raw English flint that was largely cortex-free. Two
bifacial choppers were produced under normal conditions and two while subjects wore
the extension-limiting brace, resulting in a total of 32 choppers (Figure 4.1). The
reduction sequence for each bifacial chopper was started with a fist-sized nodule that had been removed from larger nodules. Subjects were requested to produce “rough” bifacial
95 choppers with no more than five flake removals per side in the creation of each chopping edge. A total of 517 swings were recorded—250 when subjects’ wrists were unbraced, and 267 when subjects’ wrists were braced. Swings missing relevant data were removed from the sample resulting in a total of 435 swings (237 unbraced, 198 braced). Braced data were not obtained from Subject C.
Subjects produced three additional bifacial choppers for accuracy analysis, two under normal conditions and one while braced. In total, each subject produced four choppers under normal conditions and three while wearing the extension-limiting brace.
Motion capture
Data were captured using the VICON motion capture analysis system in The
George Washington University’s Motion Capture and Analysis lab. Subjects sat in a wooden chair (seat height = 48 cm) in the center of a calibrated space, with six to eight infrared cameras positioned around them to enable capture of their motions from various angles. All data were captured at 200 Hz. A digital camera simultaneously recorded knapping motions to verify behavior during analysis.
Subjects wore a fingerless glove on their dominant hand (i.e., hammer hand), as previously described in Williams and colleagues (2010). Seven reflective markers were placed on subjects at the following landmarks: the point of the shoulder (SH), the olecranon process (OP), the radial and ulnar styloid processes (RSP and USP, respectively), and metacarpal heads (MC) I, II, and V. Hand and wrist markers were secured to the glove by inserting their plastic bases through the glove mesh and fastening
96
each marker back onto its base. The elbow and shoulder markers were taped directly to
the subjects’ skin and held in place using medical tape.
Data were captured in four phases for each subject. During Phases 1 and 3
subjects’ muscular-induced maximum excursions at the wrist were captured (Table 4.1).
Subjects wore the extension-limiting brace during Phase 3. Size-appropriate braces were
chosen for each subject, which restricted extension without impeding motion in other
directions. The braces consisted of a form-fitting cotton/rayon sleeve that covered the
hand to mid forearm and a stiff metal rod running along the posterior side of the hand and
forearm. A series of Velcro strips sewn onto the sleeve held the sleeve in place. An
additional Velcro strip passed from the dorsal surface of the sleeve to the palmar surface of the sleeve, passing between the first and second digits. The metal rod was placed into a pocket sewn into the sleeve to hold the rod stationary during knapping. Each metal rod was contoured at the distal end to allow the wrist ~ 30° - 35° of extension rotation.
Excursion ranges were captured from a “neutral” position in which subjects held their
right arm parallel to the floor with the shoulder joint flexed, elbow joint fully extended, and the forearm fully pronated so that the palm faced the floor. Subjects moved their wrist to their muscular-induced maximum extension, flexion, radial deviation, and ulnar deviation positions respectively and held each position for approximately five seconds before moving on to the next posture. All reported angles were calculated relative to each subjects’ neutral position.
Phases 2 and 4 consisted of knapping kinematic data capture under normal and braced conditions, respectively. In both phases subjects removed flakes from raw
English flint nodules using hard hammer percussion to produce simple Oldowan bifacial
97 choppers. Subjects were allowed to pick the hammerstone of their choice and switch hammerstones as frequently as they desired. All hammerstones were weighed prior to each knapping session to document any change in weight that may have occurred from use in previous sessions. The hammerstone used for each swing was recorded during data capture for inclusion in arm and hand mass calculations.
Subjects were instructed to remove no more than ten flakes per chopper, five per tool face. Not every swing produced a flake and most choppers required more than ten swings for completion. Subjects were instructed to refrain from preparing and/or grinding platforms. Prior to each swing, subjects drew an “X” on their intended point of percussion with a gold ink pen. After each flake was produced, recording was halted and the flake was retrieved and coded for later association with the swing.
Kinematics and lithic analysis
Data were divided into individual swings and further divided into the up-swing and down-swing phases. All phases and events in the knapping swing were defined with respect to the RSP marker as follows:
1. Swing initiation: the frame prior to the initiation of the ascent of the RSP
2. The transition from up-swing to down-swing: occurs at the frame immediately
following the apex of the RSP
3. Swing termination: the frame immediately following the lowest vertical
position of the RSP
4. Up-swing: the time encompassing swing initiation to the point when the RSP
reached its apex
98
5. Down-swing: the time encompassing the transition to down-swing through
swing termination
Analyses focused on the down-swing phase because all significant events occurred in this
phase—peak joint angles, peak angle and linear velocities, and strike. Unless otherwise stated, all analyses and discussions refer to the down-swing phase of the knapping swing.
Wrist and elbow joint angles were calculated through the duration of the up-swing and down-swing phases of each knapping swing. Wrist angles were calculated according to the method described by Williams and colleagues (2010). Elbow angles were measured as the angle created by points ABC as follows: the midpoint between the RSP and the USP was defined as Point A; the OP was defined as Point B; and the SH was defined as Point C (Figure 4.2).
Segment endpoint velocities and accelerations (horizontal and vertical) were
calculated through the course of each swing for each landmark. Angular velocities and
accelerations were calculated at the wrist joint (extension/flexion and radial/ulnar deviation) and elbow joint (extension/flexion) through the course of each swing. Linear motion at the OP was used as a proxy for motion at the shoulder joint. All angles, velocities, and accelerations were calculated using R statistical programming language
(Ihaka and Gentleman, 1996). In R, flexion angular velocities were coded as negative numbers and extension angular velocities were coded as positive numbers. The nature of upper limb motions during the down-swing is such that elbow and wrist angular velocities have opposite vector signs for the majority of the knapping phase.
Consequently, vector signs were controlled for statistical analyses but they are reported with the appropriate sign. Intra-subject averages for peak joint angles, peak joint angular
99 velocities, peak segment endpoint linear velocities and their associated timing in the knapping swing is reported for each subject.
Kinematic data were predominantly non-normally distributed. Data were analyzed using a Kruskal-Wallis test. All P-values were determined using a post-hoc pairwise Mann-Whitney U test and have been treated with a Bonferroni correction to determine significance: Mann-Whitney pairwise P-values were multiplied by the number of number of pairwise comparisons made and determined significant if they were < 0.05
(Zar, 1996).
Following production, all flakes were labeled for association with their appropriate knapping trial for accuracy measurement. The contact point between the hammerstone and the nodule (the point of percussion) remains visible on the struck flake or nodule, showing up as a small indentation that frequently is surrounded by rock dust and radial scars. Knapping accuracy was calculated for each subject under each knapping condition as the average distance between the intended point of percussion and actual point of percussion (Table 4.2). When the knapper was significantly off target, the resulting flake platform frequently did not incorporate the intended point of percussion, as indicated by the gold “X” drawn on prior to flake production. Consequently, each core was reconstructed in its entirety. This ensured the accurate association of intended and actual points of percussion in those cases in which the intended point of percussion remained on the core or adjacent flakes. Flakes with shattered platforms, which held no record of either the intended or actual point of percussion, were discarded from the analysis. Similarly, flakes on which one of the two points of interest was not visible on
100 either the flake or the reconstructed nodule were also discarded. After removing all such flakes, 369 flakes remained—183 unbraced and 186 braced.
RESULTS
The knapping swing
Experienced knapping subjects employed a basic knapping swing similar to the swing used by amateurs described in Williams and colleagues (2010). However, the higher recording frequency used in the current study (200 Hz) allowed the observation of many motions initially undetected when recording was set at 50 Hz. All swings were executed in one of two variations on a common knapping position, with the exception of one set of swings executed by Subject A. In all swings, subjects held the hammerstone in their right hand and the core in their left. The core was either braced against their left leg or held above the left leg, with the left arm resting along the left thigh. Subject A was the only subject to deviate from these two positions, and executed eight of 27 swings while bracing the core against his right thigh. In all other swings subjects executed swings by moving their dominant hand and arm across their midline to strike the core. When subjects used more than one knapping position, data were examined to determine whether the data sets were statistically different for each analysis. When differences were present, data sets were separated by swing posture for that particular analysis.
101
Joint angles
Among all subjects, extension was the dominant wrist motion employed during
knapping (Table 4.3). All subjects employed > 70% of their total muscular-induced
extension range, and seven of eight employed > 90% at some point in each knapping
swing. In contrast, all subjects did not once flex past their neutral position. Subjects also
employed relatively high degrees of radial deviation (with the exception of Subject H).
This was due to the tendency to deviate in the radial direction when extending and to deviate in the ulnar direction when flexing (Spearman’s R ≥ 0.888 between motion in the
dorsal-ventral plane (i.e., extension/flexion) and motion in the medio-lateral plane (i.e.,
radial/ulnar deviation) across Subjects A - G, Table 4.4, Figure 4.3. Reported
Spearman’s R values represent the average R for each subject. P-values represent the
highest P-value across all trials). The low correlation value in Subject H is attributable to
her avoidance of radial deviation.
Joint motion initiations
We previously reported that down-swing was characterized by downward limb
motion, extension at the shoulder joint, elbow flexion and wrist extension through peak
wrist extension, followed by elbow extension and rapid wrist flexion (Williams et al.,
2010). The higher frequency recording speed used in the current experiments revealed
that both knapping phases are more complicated than what was visible at 50 Hz and we
can now refine the description of each phase as follows.
Through most of up-swing, all segment endpoints followed an upward trajectory
as the shoulder and elbow joints flexed and the wrist joint extended (Figure 4.4A). In all
102 subjects, the OP began its downward trajectory at least 0.022 seconds prior to the transition from up-swing to down-swing, while the RSP and MC II head continued to travel upwards (Figure 4.4B). Thus, the shoulder joint was the first to shift from its primary up-swing motion direction (flexion) to its down-swing motion direction
(extension). After the transition from up-swing to down-swing had occurred (i.e., the
RSP had begin traveling in a downward direction), the MC II head also shifted to a downward motion trajectory. However, the elbow continued to flex and the wrist continued to extend (Figure 4.4C). The elbow shifted from flexion to extension 0.153 –
0.08 seconds prior to strike (range of timing of elbow joint transition across all subjects,
Figure 4.4D), while the wrist continued to extend until 0.077 – 0.048 seconds prior to strike (range of timing of wrist joint transition across all subjects, Figure 4.4E). At strike, all segment endpoints were traveling downward, the shoulder and elbow joints continued to extend and the wrist joint reached peak flexion. Immediately following strike, the wrist joint was propelled out of peak flexion and back into high degrees of extension
(Table 4.3), while the RSP and MC II head continued to travel downward (Figure 4.5).
The shoulder transitioned from flexion to extension at least 0.022 seconds prior to down-swing and the elbow transitioned from flexion to extension significantly before the wrist transitioned from extension to flexion (maximum subject p ≤ 0.0009, Table 4.5 and
Figure 4.4). Thus, in terms of joint initiations, subjects displayed a complete proximal- to-distal joint sequence.
103
Peak velocity
The shoulder reached significantly lower linear velocities than both the MC II
head and the RSP in all subjects (maximum subject p ≤ 0.028, Table 4.6). Among highly
experienced knappers (Subjects A-E) and one mid-skill level knapper (Subject H), peak
wrist flexion velocity was significantly greater compared with peak elbow extension
velocity (maximum p ≤ 0.04 across all highly experienced subjects, Table 4.7, Figure
4.6). Among the remaining mid-skill level knappers (Subjects F and G) peak angular
velocities were not significantly different between the elbow and the wrist.
Timing of peak joint angular velocity
The timing of peak joint angular velocities occurred in a partial proximal-to-distal
pattern. This is similar to the linear velocity pattern observed among amateur knappers,
despite the difference in knapping ability between the groups. Velocity at the shoulder
joint peaked significantly before velocity at the elbow and wrist joints in all subjects with
one exception (maximum subject p ≤ 0.028, Table 4.8. Linear velocity at the OP was
used as a proxy for angular shoulder angular velocity). In Subject D, velocity peaked at the OP prior to the RSP and MC II head, however these differences were insignificant.
Among all subjects wrist flexion angular velocity peaked significantly prior to elbow extension angular velocity (maximum subject p ≤ 0.0009, Table 4.9), and the peak angular velocities were higher at the wrist than at the elbow (Table 4.7).
104
Braced joint angular velocity and knapping accuracy
When subjects’ wrists were restrained, the wrist flexion velocity each achieved
was significantly lower than the wrist flexion velocity achieved when their wrist was not
restrained (maximum subject p ≤ 0.001, Table 4.7).
All of the seven subjects that produced both unbraced and braced bifacial
choppers had lower mean accuracy distances when braced, and in six of them the
differences were statistically significant (maximum significant subject p < 0.012, Table
4.2). To compare rates of flake production between the unbraced and braced knapping
conditions, the ratios of successful swings (i.e., those which produced a flake) to
unsuccessful swings (i.e., those which did not produce a flake) were compared between
the two conditions (Table 4.10), and a pairwise Mann-Whitney U test was used to
determine significance. The difference in the flake production rates between the two
knapping conditions was insignificant (p = 0.201).
DISCUSSION
This study was undertaken to refine our understanding of knapping motions and to investigate the contribution of modern upper limb anatomy towards stone tool production. The use of skilled subjects, a larger subject sample size, and a higher recording frequency refine out understanding of the knapping swing, a behavior that many researchers believe placed significant selective pressures on our upper limb anatomy. The results show that a) the knapping swing represents a variant on the
105
standard proximal-to-distal joint sequence and b) high degrees of wrist extension contribute to knapping accuracy and efficiency.
The proximal-to-distal joint sequence is frequently utilized in contemporary activities because it successfully exploits the biomechanical properties inherent in a
multi-joint system such as the upper limb (Bartlett et al., 1996; Crisco et al., 2005; Hore
et al., 2005; Gray et al., 2006; Wolfe et al., 2006; Furuya and Kinioshita, 2007; Furuya
and Kinoshita, 2008b). In using this sequence, the individual is able to a) reduce
muscular contribution at the most distal joint toward the generation of muscular torque
and b) fine tune movement at the most distal joints.
In this joint sequence, muscular torque generated at the proximal joint (e.g.,
shoulder or trunk) is recruited to accelerate the proximal joint which then generates
passive interactive torques at the more distal joints (Dounskaia et al., 1998; Hirashima et
al., 2003a; Hirashima et al., 2007). Thus, the individual is able to increase distal joint
interactive torques (which help move the arm in its primary motion direction) simply by
increasing muscular torque at the proximal joint (Hirashima et al., 2007). Distal
muscular torque plays the less demanding role of counteracting interactive torques in
order to refine motion, rather than contributing to the primary movement of the limb
(Hirashima et al., 2003b; Dounskaia, 2010).
In addition to reducing the need for muscular input in the distal portion of \the
arm, the initiation of motion at the proximal joint lays the foundation for a velocity
summation effect at the distal joints. The limb moves much like a whip, resulting in the
generation of significantly greater angular velocity at the most distal joint than could
otherwise be achieved by distal joint muscular contribution alone (Putnam, 1991). In
106
stone tool production, these higher angular velocities directly translate into higher strike forces. Distal joint muscles are thus relieved from the generation of high muscular torques in the effort to achieve high joint angular velocities and high strike forces.
As discussed above, skilled practitioners are able to reserve distal joint muscular torques primarily to counteract passive interactive torques through the proximal-to-distal joint sequence, which in turn refines distal joint movements (Hirashima et al., 2003a;
Debicki et al., 2004; Dounskaia, 2010). Motion is further refined through the joint disassociation that is inherent in a proximal-to-distal joint sequence. As joints become disassociated from one another, the arm replaces a columnar movement pattern with a more complex joint motion pattern in which newly liberated joints have the opportunity to act in concert with one another to accomplish a target-based task (Bernstein, 1967;
Chowdhary and Challis, 1999). Because proximal joints exert a disproportional mechanical influence on movement at the distal joint (Hore et al., 1996; Dounskaia,
2010), disassociation of the distal joint from the proximal joint is particularly important when seeking accuracy.
The knapping swing documented among experienced knappers involves a complex series of motions at each joint in the upper limb, in which joints rarely acted as a synchronized unit in terms of motion direction, event timing, and velocities. We previously demonstrated that amateur knappers utilize a complete proximal-to-distal joint sequence in regard to peak segment endpoint linear velocities and a partial proximal-to distal joint sequence in regard to the occurrence of peak linear velocities (i.e., the shoulder peaked first, but the wrist peaked before the elbow) (Williams et al., 2010). We hypothesized that more experienced knappers would command a full proximal-to-distal
107
joint sequence in terms of the onset of motion, peak linear and angular velocities, and the onset of angular velocities. We also hypothesized that use of higher degrees of wrist extension would result in significantly greater angular velocities at the wrist compared to knapping trials when the wrist was restrained. Finally, we hypothesized that when the wrist was unimpaired, knappers would be significantly more accurate in their strike placement.
Muscular and interactive torque
Our data demonstrated that subjects practiced a full proximal-to-distal joint sequence in terms of joint motion initiations (Table 4.5). Movement at the shoulder
(represented by movement at the OP) transitioned from flexion to extension and the upper arm began moving in a downward direction at least 0.022 seconds before the end of up-swing. The elbow initiated extension only after the forearm had begun moving in a downward direction, and significantly before the wrist initiated flexion (Table 4.5, Figure
4.4).
Early and strong shoulder muscle recruitment and high muscular torque alleviate the need for strong muscle recruitment further along the arm in the generation of high muscular torques. Among skilled baseball players, muscular activation begins at the trunk and shoulder and proceeds down the limb (Hirashima et al., 2002). In order to increase ball speed, they increase only shoulder muscular torque, to the exclusion of the elbow and wrist (Hirashima et al., 2007). Skilled pianists actually decrease muscular contribution at the elbow and wrist compared to amateurs and rely mainly on shoulder muscular torque for key depression (Furuya and Kinoshita, 2008a).
108
Peak joint velocity
Peak linear velocity proceeded in a complete PDJS (Table 4.6). Among highly skilled subjects and one mid-skill level subject (Subject F), peak angular velocities also proceeded in a PDJS (Table 4.7). In the remaining mid-skill level subjects, angular velocity occurred in a partial PDJS. However, all subjects retained a partial PDJS in regard to the onset of peak joint linear and peak angular velocities (Tables 4.8 and 4.10).
Differences in wrist angular velocity according to skill level have previously been
observed among baseball pitchers (Gray et al., 2006). Considered with Gray et al.
(2006), the current study provides more evidence that angular velocity increases with
skill. Additionally, our results provide further evidence that angular velocity is a more
appropriate variable for evaluating joint motion compared to linear velocity. This is
likely due to the fact that angular velocity directly addresses limb segment motion about a
given joint axis. Linear velocity, on the other hand, tracks vertical segment endpoint
motion and the linear velocity of the hammerstone governs the fracture mechanics of the
detachment of the flake from the core.
In regard to the onset of peak joint angular velocities, the maintenance of a partial
PDJS across a range of knapping skill levels suggests that it is the behavior itself that
utilizes a variant on the common PD motion strategy, rather than a lack of skill or
coordination among the subjects. Hammering motions also vary from the common PD
upper limb strategy. During experiments on contemporary hammering activities
conducted by Cote and colleagues (2005), the elbow reached significantly greater angular velocities compared to the wrist, foregoing the velocity summation effect of the PDJS.
Though further studies of high velocity/high impact upper limb behaviors are needed for
109
confirmation, together these studies suggest that subjects may alter the common PDJS
employed in other upper limb activities when faced with a task that involves the forceful
striking of a target and which results in high joint reaction forces. Further investigations
into the kinematics of other high-force activities, such as nut cracking and wood splitting, may reveal whether this is in fact the case.
Despite the fact that subjects exhibited a partial PDJS in regard to the onset and decline of joint velocities (Tables 4.8 and 4.9), all achieved significantly greater peak
segment endpoint linear velocities at the MC II head compared to the RSP and OP (Table
4.6). Further, highly skilled subjects exhibited a full PDJS in terms of angular velocity
(Table 4.7). Closer examination of the progression of elbow extension angular velocity
revealed that subjects experience a distinct secondary peak prior to the maximum peak
(Figure 4.7). The secondary peak occurred before the peak in wrist flexion angular
velocity. These results suggest that early momentum generated at the shoulder joint in
combination with the secondary peak in elbow angular velocity are sufficient to produce
sequentially greater linear and angular velocities among highly experienced knappers.
Additionally, if shoulder momentum plays a similar role in knapping as in other upper
limb activities, then knappers have converged on a motion strategy which reduces the
need for muscular-derived torques at the more distal joints without detracting from performance ability.
Contributions of wrist extension
We previously demonstrated that during the knapping swing the removal of the wrist’s contribution to the generation of velocity and vertical displacement results in
110 significantly lower velocities at the wrist (Williams et al., 2010). However, the possibility remained that utilization of lower degrees of wrist extension (rather than a rigid wrist) could have the same beneficial effect as higher degrees of extension and result in similar angular velocities. Our current results demonstrate that this is not the case—the velocity summation effect experienced at the distal joint is clearly aided by the ability to achieve higher degrees of wrist extension (Table 4.7). All subjects reached significantly greater angular velocities when their wrist extension was unimpaired. This is due to the mechanical advantage the wrist’s flexors experience as wrist extension increases, in combination with the whip-like motion pattern that is initiated from early shoulder movement. The use of lower degrees of extension was insufficient to reach the same velocity of wrist flexion, clearly demonstrating the importance of higher degrees of extension in achieving rapid wrist flexion.
The modern wrist’s ability to quickly reach high degrees of extension immediately following strike also decreases the stress placed on the joint at strike, and therefore the likelihood of damage to that region. After strike, the wrist was propelled back up out of peak flexion into 28.5° - 56° of extension due to the strong reaction forces experienced at strike (Table 4.3). If the ability to extend at the wrist was limited as it may have been in the primitive condition (Richmond and Strait, 2000), the degree of post strike extension knappers reached would constitute hyperextension. Injuries to the scaphoid, lunate, triquetral, and their associated ligaments, are frequently caused by hyperextension (Linscheid and Dobyns, 1985; Rettig, 2003). The knapping motion would particularly impact the lunotriquetral ligament, which is susceptible to tearing when the wrist experiences high loads when the forearm is pronated and held in
111
hyperextension/radial deviation, as occurs during the knapping swing (Rettig, 2003).
Modern humans’ ability to achieve high degrees of wrist extension potentially reduces the possibility of injury to the carpal region following strike.
We also hypothesized that wrist extension in particular would affect strike accuracy. This hypothesis is supported by the difference in accuracy between unbraced and braced knapping swings. Under normal knapping conditions, intra-subjects average peak extension was 48° - 72°. When braced, intra-subject average peak extension was
23° - 33.5° (Table 4.3). When able to reach high degrees of wrist extension, six of seven subjects were significantly more accurate than when their ability to extend at the wrist was impaired (Table 4.2).
The significance of wrist extension to accuracy may lie in the inherent stability
(Moritomo et al., 2004; Crisco et al., 2005) of the plane of motion known as ‘the dart thrower’s arc,’ (Palmer et al., 1985). This plane is defined by simultaneous extension/radial deviation and flexion/ulnar deviation and offers “a unique degree of
radiocarpal stability” that anatomical directions (i.e., extension-flexion and radial
deviation-ulnar deviation) do not (Crisco et al., 2005). Under normal knapping conditions, subjects displayed a high correlation between extension and radial deviation, adhering to the dart thrower’s arc (Table 4.4). Disruptions to this natural directional coupling are known to destabilize the wrist, thereby decreasing control (Wolfe et al.,
2006). The results of this destabilization are apparent in knappers’ significant reduction in accuracy when their ability to extend at the wrist was impaired.
Similarities in flake production rates (Table 4.10) were not surprising given the parameters placed on knappers. Subjects were instructed to produce no more than ten
112
flakes per end product, regardless of the nature of the flake removed. Subjects frequently
produced flakes they deemed as undesirable removals due to their size and/or shape.
However, even undesirable flakes were included in the flake counts.
CONCLUSION
The knapping swing documented among highly skilled and mid-skill level knappers involves a complex series of motions at the shoulder, elbow, and wrist joints.
These joints act together yet out of sync with one another in the production of high velocity, accurate hammerstone strikes against the nodule. The success of the knapping swing is dependent on the effective utilization of a variation of the proximal-to-distal joint sequence and the ability to reach high degrees of wrist extension prior to strike.
We have provided direct evidence that high degrees of wrist extension contribute to efficient and accurate stone tool production. When subjects’ wrist extension was unimpaired, they reached significantly greater wrist angular velocities and were significantly more accurate in striking their intended target. Additionally, the modern wrist condition enabled the wrist to reach high degrees of extension in recoil following strike, which could arguably reduce the likelihood of sustaining carpal and ligament damage in hyperextension. Through the utilization of motion initiation early in the sequence at the shoulder joint and high degrees of wrist extension knappers have converged on an efficient method for generating and sustaining the impact of high strike forces. The use of this variant of the proximal-to-distal motion sequence in stone tool
113 production may represent one of the earliest known behaviors to use this now common joint sequence.
114
Table 4.1. Muscular‐induced maximum joint angles A B C D E F G H Years knapping > 15 ~ 12.5 > 30 > 30 > 30 ~ 7 > 15 < 5 Unbraced Swings 27 35 41 28 32 22 22 30 Extension 76.58 58.23 50.6 60.9 56.2 68.85 64.58 52.31 Flexion 39.7 54 58.63 53 56.4 57.25 49.7 43.8 Radial deviation 19.49 15.79 25.55 21.69 18.32 13.93 9.57 15.26 Ulnar deviation 18.2 29.1 27.8 31.7 29.87 29.2 31.5 36.6 Subjects' years of knapping experience and muscular‐induced maximum joint ranges of motion at the wrist for unbraced and braced conditions. All joint angles are reported relative to subjects' individual neutral position.
115
32 UnBR 4.718 2.726
H
2.651 0.011 ‐
27 BR 6.997 3.699
18
UnBR 4.242 3.022
G
4.575 0.0001
‐ < 16 BR 4.287 10.129
unbraced.
30 UnBR 3.838 3.040
F
2.991 UnBR: 0.004
‐ 25 BR 6.455 3.383
braced,
22
UnBR 4.975 2.824 BR:
E
1.942 0.058 ‐ 28 BR 6.94 4.30
significant.
42 UnBR 2.755 2.160
are
D
4.418 0.0001
‐ < 43 BR 5.428 3.314 results
20 UnBR 3.525 1.820
Bolded B
4.413
0.0001
‐ < 24 BR mm.
6.943 3.228 in
19 accuracy
UnBR 3.373 2.429
A
reported
3.204
0.003 ‐ Strike 23 BR
are 5.638 2.087
4.2.
t p n sd mean Distances Table
116
leg
G 48.34 38.78 27.84 11.03 12.23 25.31 56.64 above +19.41 left neutral radial
the
leg
peak H
33.2 0.81 22.5 62.38 48.01 43.15 above +20.91 12.474 left and above
left F
leg 7.6 29.7 4.37 flexion 63.26 56.01 19.04 40.97
on +39.25 degrees
peak
and
leg
G to
48.34 38.78 27.84 11.03 12.23 25.31 56.64 above +19.41 left
direction
left
leg extension 9.1
9.83 +8.38 55.02 31.39 41.42 39.28 on that
E in
peak
23.08 leg
4.46 from +8.26 56.16 28.56 14.82 42.54 40.01 above
left position
range
leg
D 4.14 neutral 64.87 43.02 33.45 24.22 25.67 53.11 above
+35.63 left
her
excursion or left
C
32 leg NA 6.99 48.46 15.57 30.88 16.08 his
on +14.73
total
past
the left
ranges leg
60.6 4.71 16.39 22.42 46.99 on +28.78
move B
report
34.82 33.34 leg not
excursion 5.76
53.95 11.55 29.58 38.56 above +20.75 left did
total
deviation
right subject and
leg
6.25 4.12 72.05 29.54 27.52 + °) radial +41.18
on ( ‐
A to
‐
41.28 32.93
left
angles leg
ulnar 4.44 7.48
72.19 43.66 31.64 deviation. on +24.79
and
ulnar
knapping
flexion peak ‐ reported.
Peak to to
to
‐
ulnar
are
extension
deviation to type 4.3.
deviation
strike ‐ Table swing Unbraced extension Post extension Braced Flexion Radial Ulnar Extension flexion Radial deviation Extension deviation position
117
Table 4.4. Correlation between wrist extension/ radial deviation and flexion/ulnar deviation Swing Average Maximum Subject position Spearman's R p STonR 0.915 < 0.0001 A STonL 0.895 0.0005 STonL 0.965 < 0.0001 B SToffL 0.982 < 0.0001 C STonL 0.928 ≤ 0.001 D SToffL 0.976 < 0.0001 SToffL 0.88 0.05 E STonL 0.83 < 0.025 F STonL 0.888 0.013 G SToffL 0.899 < 0.0001 H STonL 0.366 0.079 STonR: strike occurred on the right leg. STonL: strike occurred on the left leg. SToffL: strike occurred with the non‐dominant arm over, but not on the left leg.
118
Table 4.5. Initiation of elbow extension and wrist flexion relative to strike Subject A Swing type on R on L Joint Elbow Wrist Elbow Wrist Time ‐0.104 ‐0.048 ‐0.096 ‐0.05 p 0.0009 < 0.0001 Subject B Swing type on L above L Joint Elbow Wrist Elbow Wrist Time ‐0.153 ‐0.051 ‐0.137 ‐0.051 p < 0.0001 < 0.0001 Subject C D Swing type on L above L Joint Elbow Wrist Elbow Wrist Time ‐0.08 ‐0.048 ‐0.118 ‐0.056 p < 0.0001 < 0.0001 Subject E Swing type on L above L Joint Elbow Wrist Elbow Wrist Time ‐0.134 ‐0.077 ‐0.14 ‐0.074 p < 0.0001 0.0001 Subject F G Swing type on L above L Joint Elbow Wrist Elbow Wrist Time ‐0.093 ‐0.057 ‐0.095 ‐0.076 p < 0.0001 < 0.0001 Subject H Swing type on L Joint Elbow Wrist Time ‐0.107 ‐0.062 p < 0.0001 Joint initiation times reported in seconds before strike. All results were significant. "on R": strike occurred on the right leg. "on L": strike occurred on the left leg. "above L": strike occurred with the core held above, but not on, the left leg.
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Table 4.6. Peak segment endpoint linear velocities Subject A Swing type on R on L Landmark OP RSP MC II OP RSP MC II Linear velocity ‐0.073 ‐0.984 ‐1.59 ‐0.121 ‐2.99 ‐4.336 OP to RSP p 0.0028 < 0.0001 OP to MC II p 0.0028 < 0.0001 Subject B Swing type on L above L Landmark OP RSP MC II OP RSP MC II Linear velocity ‐0.113 ‐0.757 ‐1.126 ‐0.234 ‐2.15 ‐3.448 OP to RSP p < 0.0001 < 0.0001 OP to MC II p < 0.0001 < 0.0001 Subject C D Swing type on L above L Landmark OP RSP MC II OP RSP MC II Linear velocity ‐0.033 ‐0.848 ‐1.456 ‐0.026 ‐1.265 ‐1.813 OP to RSP p < 0.0001 < 0.0001 OP to MC II p < 0.0001 < 0.0001 Subject E Swing type on L above L Landmark OP RSP MC II OP RSP MC II Linear velocity ‐0.551 ‐4.023 ‐4.768 ‐0.524 ‐3.861 ‐4.681 OP to RSP p < 0.0001 0.0002 OP to MC II p < 0.0001 0.0002 Subject F G Swing type on L above L Landmark OP RSP MC II OP RSP MC II Linear velocity ‐0.22 ‐1.437 ‐1.65 ‐0.705 ‐3.216 ‐4.074 OP to RSP p < 0.0001 < 0.0001 OP to MC II p < 0.0001 < 0.0001 Subject H Swing type on L Landmark OP RSP MC II Linear velocity ‐0.14 ‐0.889 ‐1.046 OP to RSP p < 0.0001 OP to MC II p < 0.0001 Olecranon process (OP), radial styloid process (RSP) and second metacarpal head (MC II). Velocities are reported in m/2. All results are significant. "on R": strike occurred on the right leg. "on L": strike occurred on the left leg. "above L": strike occurred with the core held above, but not on, the left leg.
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Table 4.7. Peak angular velocity at the elbow and wrist A B
Elbow Unbraced Braced Elbow Unbraced Braced wrist wrist wrist wrist Angular velocity 494.005 ‐915.416 ‐83.356 296.165 ‐656.418 ‐441.898 Unbraced elbow to < 0.0001 0.000 wrist p
Unbraced to braced < 0.0001 0.007 wrist p C D
Elbow Unbraced Braced Elbow Unbraced Braced wrist wrist wrist wrist Angular velocity 178.02 ‐264.53 ‐ 237.71 ‐527.97 ‐338.933 Unbraced elbow to < 0.0001 0.000 wrist p
Unbraced to braced NA 0.020 wrist p E F
Elbow Unbraced Braced Elbow Unbraced Braced wrist wrist wrist wrist Angular velocity 657.27 ‐716.445 ‐530.012 282.725 ‐295.645 ‐102.223 Unbraced elbow to 0.04 0.218 wrist p
Unbraced to braced < 0.0001 < 0.0001 wrist p G H
Elbow Unbraced Braced Elbow Unbraced Braced wrist wrist wrist wrist Angular velocity 626.493 ‐549.768 ‐325.899 142.263 ‐255.288 ‐80.163 Unbraced elbow to 0.113 0.007 wrist p
Unbraced to braced 0.000 < 0.0001 wrist p Bloded results are significant. Subject C did not knap while braced.
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Table 4.8. Timing of peak linear velocities Landmark OP RSP MC II OP RSP MC II A on R above L Sec before ST ‐0.094 0 0.01 ‐0.09 0 ‐0.01 OP to RSP 0.0028 < 0.0001 OP to MC II 0.0028 < 0.0001 RSP to MC II 0.0028 < 0.0001 B on L above L Sec before ST ‐0.08 ‐0.006 ‐0.01 ‐0.081 ‐0.002 ‐0.01 OP to RSP < 0.0001 < 0.0001 OP to MC II < 0.0001 < 0.0001 RSP to MC II 0.7054 < 0.0001 C D on L above L Sec before ST ‐0.084 0 ‐0.01 ‐0.046 ‐0.038 ‐0.01 OP to RSP < 0.0001 1 OP to MC II < 0.0001 0.7392 RSP to MC II < 0.0001 < 0.0001 E on L above L Sec before ST ‐0.081 0 0 ‐0.077 0 0 OP to RSP < 0.0001 0.0002 OP to MC II < 0.0001 0.0002 RSP to MC II 0.0024 1.0000 F G on L above L Sec before ST ‐0.081 0 0 ‐0.067 ‐0.003 ‐0.01 OP to RSP < 0.0001 < 0.0001 OP to MC II < 0.0001 < 0.0001 RSP to MC II 0.0001 0.0326 H on L Sec before ST ‐0.09 ‐0.027 ‐0.01 OP to RSP < 0.0001 OP to MC II < 0.0001 RSP to MC II < 0.0001 Time is reported in seconds before strike. Bolded results are significant. "on R": strike occurred on the right leg. "on L": strike occurred on the left leg. "above L": strike occurred with the core held above, but not on, the left leg.
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Table 4.9. Timing of peak angular velocity relative to strike A on R on L Elbow Wrist Elbow Wrist sec before ST 0 ‐0.017 0 ‐0.02 p 0.0009 < 0.0001 B on L above L Elbow Wrist Elbow Wrist sec before ST ‐0.002 ‐0.01 ‐0.001 ‐0.011 p < 0.0001 < 0.0001 C D on L above L Elbow Wrist Elbow Wrist sec before ST 0 ‐0.013 ‐0.014 ‐0.023 p < 0.0001 0.0002 E on L above L Elbow Wrist Elbow Wrist sec before ST 0 ‐0.015 0 ‐0.014 p < 0.0001 < 0.0001 F G on L above L Elbow Wrist Elbow Wrist sec before ST ‐0.004 ‐0.025 0 ‐0.028 p < 0.0001 < 0.0001 H on L Elbow Wrist sec before ST ‐0.003 ‐0.009 p < 0.0001 Time reported in seconds before strike. All results are significant. "on R": strike occurred on the right leg. "on L": strike occurred on the left leg. "above L": strike occurred with the core held above, but not on, the left leg.
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Table 4.10. Flake production rate distribution between unbraced and braced knapping conditions Flake? Subject Yes No Yes No Unbraced Braced A 29 47 26 34 B 35 42 31 34 D 42 25 29 37 E 31 9 23 13 F 43 9 20 21 G 30 12 33 17 H 43 19 42 38
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cts [Subject A A cts [Subject Figure 4.1. Oldowan bifacial choppers produced under normal knapping circumstances (i.e., unbraced) by highly experienced subje experienced by highly unbraced) (i.e., circumstances knapping normal under produced choppers bifacial Oldowan 4.1. Figure (a and b), SubjectC (c andd)] and mid skill levelsubjects [Subject F (e and f) andSubject G (g andh)]. abc d ef g h
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Figure 4.2 Points used to calculate angles at the elbow joint. A: the midpoint between the radial styloid process and the ulnar styloid process. B: the olecranon process. C: the point of the shoulder.
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Figure 4.3. Wrist angles through one swing, moving through the dart‐thrower’s arc. Radial deviation/ulnar deviation are depicted in the top figure. Extension/flexion are depicted in the bottom figure.
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Figure 4.4. 3‐D model of the upper limb during up‐swing and down‐swing, a: point of the shoulder (SH); b: olecranon process (OP); c: radial styloid process (RSP); d: second metacarpal head (MC II head). Red arrows indicate the motion direction of the associated anatomical landmark. Black arrows indicate the motion direction of the associated joint. The location of the red right angle is held constant throughout the figures to illustrate the landmarks’ change in position relative to the starting point. A: mid up‐swing, the shoulder and elbow joints are flexing and the wrist joint is extending. All landmarks are moving upward. B: end of up‐swing, the OP begins is downward trajectory and the shoulder joint shifts from flexion to extension (≤ 0.022 seconds prior to the transition from up‐swing to down‐swing). The elbow continues to flex and the wrist continues to extend. C: the transition from up‐swing to down‐ swing, the elbow continues to flex and the wrist continues to extend. However, the RSP and the MC II head begin traveling in a downward direction. D: the elbow shifts from flexion to extension 0.153‐0.08 seconds prior to strike (range of timing of elbow joint transition across all subjects), the wrist continues to extend. E: the wrist is the last joint to transition, shifting from extension to flexion 0.077 – 0.048 seconds prior to strike (range of timing of wrist joint transition across all subjects). F: strike, all landmarks are moving downward. The shoulder and elbow continue to extend and the wrist reaches its lowest point of flexion.
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Figure 4.5. Change in wrist angle (A) and the vertical position of the RSP and MC II head (B and C, respectively) at the end of up‐swing and through the end of down‐swing. The wrist is propelled back into extension following strike, while the RSP and MC II head continue to travel downward. The solid black line denotes the transition from up‐swing to down‐swing. The dashed black line denotes strike.
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-200
-400
-600 /s) ⁰ (
-800
-1000 velocity
-1200 Angular -1400
-1600 Elbow -1800 Wrist -2000 A B C D E F G H Subjects Figure. 4.6. Peak wrist flexion velocity (grey) and peak elbow extension velocity (white) during down‐swing across all subjects. Differences in wrist and elbow angular velocity were significant among all highly‐skilled Subjects and in Subject H.
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Figure 4.7. Elbow angle (A) and angular velocity (B), and wrist angle (C) and angular velocity (D) through the knapping swing. Both joints transition from their up‐swing motion direction to their down‐swing motion direction after the commencement of down‐swing (as defined by motion at the RSP). Wrist angular velocity peaks significantly before elbow angular velocity (D and B, respectively, black stars). However, the elbow experiences a secondary peak prior to the peak in wrist angular velocity (B, open star). The solid black line denotes the transition from up‐swing to down‐swing. The dashed black lines denote each joint’s motion direction transition. The dashed/dotted line denotes strike.
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Chapter 5: Acheulean and Oldowan knapping strategies
ABSTRACT
The impetus for the Oldowan-Acheulean transition is difficult to pinpoint in part because it was likely driven by changes in cognition and anatomy, as well as their intersection— motor control processes. However, recent research has addressed this elusive issue by demonstrating that motor-related portions of the brain’s right hemisphere are more active during Acheulean stone tool production compared with Oldowan stone tool production
(Stout et al., 2008). Here we apply high-speed 3-D motion capture technology to investigate the upper limb motion sequences used during both traditions to test the hypothesis put forth by Stout and colleagues, that Acheulean reduction sequences require greater control of motion and more complex motion sequences compared with Oldowan reduction sequences. Data were collected from eight experienced subjects replicating mid-to-late Acheulean handaxes. These data were compared with Oldowan knapping data described in Williams et al. (2011). We found no evidence of differences in small- scale (i.e., joint) upper limb kinematics between the two traditions. However, trimming swings employed in shaping during Acheulean tool production were executed with a unique set of joint motions compared with those used to execute standard knapping swings during both tool traditions. Additionally, trimming swings were interspersed with standard swings throughout the reduction sequence. These results demonstrate Oldowan and Acheulean reduction sequences share a common upper limb joint kinematic strategy,
132 and suggest that the use of trimming swings may contribute to differences observed in brain activity during the two tool traditions.
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INTRODUCTION
The Acheulean industry [1.9 ~0.3 MA (Asfaw et al., 1992; McBrearty and
Brooks, 2000; Deino and McBrearty, 2002)] is the later of the two stone tool traditions that make up the Early Stone Age. It is widely known for its iconic handaxes—bifacially flaked stone implements frequently made on large flakes. The handaxes themselves were commonly approximately 10 – 17 cm in maximum length (although European handaxes were frequently considerably more diminutive) (Delson et al., 2000; Ambrose, 2001). In the later Acheulean, handaxes reached a high degree of standardization, with flaking across the entire surface of both faces which resulted in thin, symmetrical bifaces (Schick and Toth, 1993; Clark, 1994; Ambrose, 2001). The cognitive capacity of the hominins responsible for these implements is a contentious issue in paleoanthropology. Some researchers assert that a high level of cognitive ability would be required to produce
Acheulean tools (Holloway Jr., 1969; Wynn, 1985; Toth and Schick, 1993; Pelegrin,
2005). As evidence, they argue that sophisticated operational skills and a high degree of planning are necessary for biface production, as well as advanced technical skills in order to achieve the desired shape. Others, however, contend that the designation of increased cognitive capacity on the basis of tool-form standardization and symmetry is premature
(Silverman, 2002; Simão, 2002; McNabb et al., 2004).
Acheulean biface manufacture certainly does differ in a number of readily apparent ways from Oldowan tool manufacture, as do the tools themselves. Oldowan tools were likely produced using mainly hard hammer percussion, whereas Acheulean tools show evidence of hard and soft hammer knapping (Schick and Toth, 1993; Toth and
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Schick, 2009). Oldowan tools generally lack a standard shape and many had no more
than eight to ten flake removals (Kimura, 1999; Roche et al., 1999), whereas Acheulean
tools show increasing standardization through time and require 50 or more flake removals
in the production of a single Acheulean biface (Newcomer, 1971; Wynn, 1979; Ambrose,
2001; Schick and Clark, 2003). Further, bifaces pass through three to four reduction stages in the course of attaining a common shape (Whittaker, 1994; Andrefsky, 1998). In
the first stage a single large flake generally 30 cm or more in maximum length (Isaac,
1975), is removed and used as the core [Stage 0 in Whittaker (1994), Stage 1 in
Andrefsky (1998)]. In the second stage, the core attains a rough handaxe shape by
systematically removing flakes from both surfaces. In doing so, a sharp edge is produced
around the entire perimeter of the tool1. In the third stage, the tool is further thinned,
likely through the use of prepared platforms which facilitate the removal of broad, thin
flakes from both surface (Whittaker, 1994). It is unclear whether hominins used this
reduction method in order to produce bifacially flaked implements, or if they sought the
flakes that came off of those implements (Clark, 1975; Jelinek, 1977; Jones, 1980; Kohn
and Mitchen, 1999). Or perhaps both strategies were used. Yet despite the clear
differences between Oldowan and Acheulean tools and their respective reduction
sequences, the impetus for the transition in tool behaviors continues to elude
anthropologists. Is the transition attributable to a cognitive shift or to an anatomical
shift? Or perhaps the reason lies at the intersection of the two, in motor skills and motion
control.
5 Andrefsky (1998) separates this stage into 2 distinct phases—bifacial edging and primary bifacial thinning. 135
Recent research by Stout and colleagues (2007, 2008) addresses this issue by
imaging brain activity during the production of Oldowan and Acheulean tools using
positron emission tomography scanning. The authors demonstrated that the right
hemisphere of the brain was significantly more active during late Acheulean handaxe
production compared with Oldowan flake production. Based on this, they argued that
greater right hemisphere activity reflected increased demands placed on the left hand during Acheulean core reduction, or increased demands for the control of action
(including the inhibition of actions and task-set switching) and the regulation of complex action sequences, or perhaps both. The authors linked this to the extended reduction process inherent in handaxe production, and the need to anticipate the final product form throughout the production process. Noting that one of the more active right hemisphere areas (Brodmann area 45) corresponds to Broca’s area in the left hemisphere and citing the right hemisphere’s roles during language processing, the authors concluded that complex tool-making may have co-developed with language skills. Further, their concurrent development likely reinforced the underlying processes of both behaviors.
To clarify the cause of Stout and colleagues (2007, 2008) findings, Faisal and colleagues (2010) compared hand postures and hand joint sequences in the left hand between the two tool traditions. They found that no differences existed in the complexity of hand postures, and concluded that Acheulean tool production imposed novel cognitive demands on the right hemisphere for the control of action and the regulation of complex action sequences.
In language production and processing, “action sequences” may be broadly or narrowly defined. For example, it may refer to lexicon order on a larger scale or
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phoneme order on a smaller scale. Similarly, in regard to motor tasks, an action sequence
may mean the sequence of gross movements, such as a fast pitch followed by a slow
pitch, or it may refer to motion on a smaller scale, such as the sequence of joint or
muscular activation that occurs during a fast or slow pitch (Dounskaia, 2010). Both large and small scale language processes occur in Wernicke’s area and Broca’s area (Démonet et al., 1992, 1994). Thus following Stout and colleague’s (2008) logic, if Brodmann area
45 (the right hemisphere homolog of Broca’s area) plays a similar role during stone tool production as Broca’s area does during language processing, then its activation during handaxe manufacture may contribute to the control of small scale action sequences (e.g., joint motions) as well as or rather than higher-order, large scale action sequences (e.g., sequence of force applications and order of operations). While it is clear that different
regions of the brain are differentially active during Oldowan and Acheulean reduction
sequences, the reasons for these differences remain unclear. Here we apply high-speed 3-
D motion capture technology to investigate action sequences during Oldowan and
Acheulean stone tool production to test 1) whether differences in action sequences are
evident between Acheulean stone tool manufacture and if so, 2) whether sequence
differences are small scale, large scale, or both.
This study is the first quantitative kinematic investigation of Acheulean knapping
strategies. It documents the production of 15 Acheulean bifaces by eight experienced
flint knappers from reduction Stage 0 through Stage 2 (Whittaker, 1994). We begin by
describing the Acheulean knapping kinematics captured from eight experienced
knappers. These are compared to the knapping kinematics of the same eight experienced
knappers producing Oldowan style choppers, previously reported in Williams and
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colleagues (2011). This study seeks to further clarify the significance of increased right
hemisphere activity during Acheulean stone tool production, specifically addressing the
types of action sequencing which may occur.
Upper limb kinematics
During the production of Oldowan bifacial choppers, experienced knappers
employed the stable dart-thrower’s arc of wrist motion and utilized a full proximal-to-
distal joint sequence (PDJS) in terms of joint motion initiations, peak linear velocity, and
peak angular velocity. Knappers modified the common PDJS in regard to the onset of peak joint angular velocities such that angular velocity peaked at the wrist before the
elbow (Williams et al., 2011). Investigations of contemporary hammering activities also
demonstrated that individuals altered the common PDJS, though in a different manner; subjects reached significantly greater angular velocity at the elbow compared with the
wrist (Cote et al., 2005). Such alterations to the common PDJS may be an adjustment
made in anticipation of the imminent high-acceleration strike that is the culmination of
both activities.
The PDJS is widely utilized in contemporary activities. Among skilled
individuals, it is the kinematic strategy used for pitching a baseball, fly fishing, kicking a
soccer ball, throwing a javelin, and even playing the piano (Putnam, 1991; Bartlett et al.,
1996; Debicki et al., 2004; Wolfe et al., 2006; Furuya and Kinioshita, 2007). The
benefits of this motion sequence are well established in motion analysis literature. The
sequence’s early recruitment of large proximal muscles generates sufficiently high
muscular torque that the need for contribution from the distal muscles toward further
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muscular torque generation is reduced (Hirashima et al., 2003a; Hirashima et al., 2007).
Thus recruitment of smaller distal muscles is reduced and they are able to stay off fatigue
(Furuya and Kinoshita, 2008a), yet high distal joint angular velocities are still achieved
(Putnam, 1991; Debicki et al., 2004). Through the joint separation that is inherent in the proximal to distal sequence, subjects are also able to aim from the more distal joint, thereby increasing their target accuracy (Bernstein, 1967; Hore et al., 1996; Chowdhary and Challis, 1999; Williams et al., 2011).
METHODS
Sample
Knapping kinematic data were captured from eight subjects, six males (Subjects
A-F) and two females (Subjects G and H). All subjects had previously participated in the
Oldowan stone tool replication experiments described in Williams and colleagues (2011)
which employed a similar protocol. All subjects were archaeologists or archaeology
graduate students familiar with Acheulean stone tool types. Subjects A-E were highly
experienced knappers proficient in knapping techniques spanning the Early, Middle, and
Late Stone Age tool traditions. Subjects F-H were proficient in Oldowan and Acheulean
technologies, but not yet skilled in more complex tool traditions. Subjects were all right-
hand dominant and did not have any known muscular and/or osteological conditions.
Hand, forearm, and arm masses were determined from volumes recorded for each
subject using a water displacement method (Table 5.1). Forearm length and metacarpal
(MC) II length there recorded for each subject. The forearm was measured along the
139 posterior surface from the olecranon process (OP) to the midpoint between the radial styloid process (RSP) and the ulnar styloid process (USP). The MC II was measured along the palmar surface of the hand from the head of the MC II to the midpoint between the RSP and the USP (Table 5.1).
All subjects produced two Acheulean handaxes using cortex-free raw English flint except Subject E, who produced one. Subjects were requested to produce handaxes that approximated those seen in the mid to late Acheulean. This was further explained to mean implements that were flaked across both surfaces, symmetrical, and thin, without
Stage 3 and 4 shaping and refining (Whittaker, 1994). The reduction sequence was started at Stage 0, or the blank stage (Whittaker, 1994), and continued through Stage 2 or until they were forced to abandon the effort (Table 5.2, Figure 5.1).
During data collection sessions (i.e., prior to analysis), those swings in which the knapper chose a specific striking target with the intention of producing a flake of a specific size and shape were designated as “standard swings.” Short, rapid swings which were executed with the intention of shaping or retouching the core or preparing a platform were designated as “trimming” swings. These swings were not intended to produce a single flake of a specific size and shape and the resulting flakes were not collected. Subjects specified prior to motion initiation whether they were performing a standard or a trimming swing. Flakes produced from standard swings were collected after their production and labeled with the appropriate knapping trial number. A total of
1288 swings was recorded, with 744 standard swings and 544 trimming swings. After swings missing relevant data were removed from the sample, 808 swings remained: 615 standard swings and 193 trimming swings (Table 5.3).
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Some swings did not produce a flake and some flakes were excluded from the
lithic analysis (e.g., broken or split flakes and those missing the platform). Consequently,
some subjects have more swings than flakes. Alternatively, some swings were excluded
from analysis due to missing data, but the associated flakes were retained in the analysis.
A total of 393 unbroken flakes with intact platforms were produced.
Motion capture
Data were captured at 200 Hz using the VICON motion capture analysis system in The George Washington University’s Motion Capture and Analysis lab. Six infrared cameras were positioned around the subject to enable capture of their motions from various angles. A digital camera simultaneously recorded motions to verify behavior during analysis. Subjects were seated in a wooden chair (seat height = 48.26 cm) in the center of a calibrated space.
On their dominant hand, subjects wore a fingerless glove (rayon/cotton/rubber blend) with separate holes for the thumb and index finger, and a single large hole for the
three remaining digits. Seven reflective markers were placed on subjects at the following
landmarks: the point of the shoulder (SH), the olecranon process (OP), the radial and
ulnar styloid processes (RSP and USP, respectively), and metacarpal heads (MC) I, II,
and V. Hand and wrist markers were secured to the glove by pushing their plastic base
through the glove mesh and fastening the marker back onto the base. Marker bases were
also taped in place to the individual. The elbow and shoulder markers were taped directly
to the subjects’ skin and further secured in place using medical tape.
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All knapping took place using only hard hammer percussion. Subjects were allowed to pick the hammerstone of their choice and switch hammerstones as frequently as they desired. All hammerstones were weighed prior to each knapping session to document weight changes that occurred from use in previous sessions. The hammerstone used for each swing was recorded during data capture for inclusion in arm mass calculations. Subjects were not asked to refrain from trimming or platform preparation as they had been during Oldowan stone tool production (Williams et al., 2011), nor was a flake removal limit placed on them.
Kinematics analysis
Data were examined to verify the validity of their separation into “standard” and
“trimming” swings. It became clear that the swing types have distinct kinematic patterns and that the separation was warranted (see results section). Data were divided into individual swings and further divided into the up-swing and down-swing phases. Up- swing, down-swing, swing initiation and termination, and the transition from up-swing to down-swing were defined with respect to the RSP in both types of knapping. In standard swings these events were defined as follows:
1. Swing initiation: the frame prior to the initiation of the ascent of the RSP
2. The transition from up-swing to down-swing: the frame immediately following
the RSP’s apex
3. Swing termination: the frame immediately following the lowest vertical
position of the RSP
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4. Up-swing: the time between swing initiation and the RSP’s apex
5. Down-swing: the time between the transition to down-swing and swing
termination
Due to the nature of trimming swings, some events within the knapping swing
necessitated a revised definition. When trimming, knappers executed a series of small
swings in rapid succession, progressing out of one swing directly into the next. They did
not halt their motion after each strike in order to reposition their arm for the next strike,
as they had during standard swings. Consequently, there was no clear swing initiation for swings following the first trimming swing and no clear termination for swings preceding the last trimming swing. Additionally, upon examination of the data, it became apparent that strike is not clearly discernable in trimming swings. This was likely due to significantly lower striking forces. For these reasons, trimming swing initiations and terminations were defined as follows:
1. Swing initiation (for all swings following the first swing in the series): the
frame immediately following swing termination of the previous swing (i.e.,
following the lowest vertical position of the RSP of the previous swing)
2. Swing termination (for all swings preceding the final swing in the series): the
frame of the lowest vertical position of the RSP during down-swing
The definitions of up-swing, down-swing, and the transition from up-swing to down-swing were the same for standard and trimming swings.
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Analyses focused on the down-swing phase because all significant kinematic
events occurred in this phase—peak joint angles, peak angular and linear velocities, and
strike. Unless otherwise stated, all analyses and discussions refer to the down-swing
phase of the knapping swing.
Wrist and elbow joint angles were calculated throughout the duration of the up-
swing and down-swing phases of each knapping swing. Wrist angles were calculated as
the angle created by the MCII head, the midpoint between the RSP and the USP, and the
OP according to the method described in Williams et al., (2010). Elbow angles were
calculated as the angle created by the midpoint between the RSP and the USP, the OP, and the SH according to the method described in Williams et al. (2011).
Angular velocities and accelerations were calculated at the wrist joint
(extension/flexion and radial/ulnar deviation) and elbow joint (extension/flexion) through
the course of each swing. Linear motion at the OP was used as a proxy for motion at the
shoulder joint. All angles, velocities, and accelerations were derived using the R statistical programming language, version 1.19.4.7 (Ihaka and Gentleman, 1996). In R, flexion angular velocities were coded as negative numbers and extension angular velocities coded as positive numbers. Due to the nature of upper limb motions during the down-swing phase, elbow and wrist angular velocities have opposite vector signs for the majority of the knapping swing. Vector signs were controlled for statistical analyses but appear in the tables with the appropriate sign. We report the intra-subject averages for peak joint angles, peak joint angular velocities, peak segment endpoint linear velocities
and their associated times in the knapping swing for each subject.
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Strike force was calculated for each standard swing in Oldowan and Acheulean
reduction sequence. For the derivation of strike, the forearm was considered a rigid body
with a force applied to the distal end (i.e., angular velocity at the wrist) and the hand and
tool were considered as joined objects making a single sphere. The parallel-axis theorem,
which relates the moment of inertia of an object about its central axis to the moment of
inertia of the object about a secondary and parallel axis, was applied to the moment of
inertia of the sphere in order to calculate the moment of inertia of the hand + tool unit
about the wrist. Given these parameters, strike force was derived as:
ି ,( ೌ (Eq. 1 ௧ሺଶାೌሻ
where La is the angular momentum of the arm about the elbow axis and Lh is the angular
momentum of the hand about the wrist axis, and t is time.
2 La = Ihela ωa + Iarmωa (Eq. 2), where Ihe is the moment of inertia of the hand + tool unit about the elbow axis such that
2 2 Ihe = /5(hand mass + tool mass)la (Eq. 3),
where la is the length of the forearm, measured along the posterior side of the forearm
from the OP to the midpoint between the RSP and the USP,
ωa is the angular velocity of the wrist
and Iarm is the moment of inertia of the arm about the elbow such that
2 Iarm = 1/3(arm mass – (hand mass + tool mass)la (Eq. 4).
Lh = Ihwωh (Eq. 5),
where Ihw is the moment of inertia of the hand + tool unit about the wrist axis such that
1 2 Ihw = Icm + (hand mass + tool mass)( /2r) (Eq. 6),
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where Icm is the moment of inertia of the hand + tool unit about the central axis of the
hand + tool unit such that
2 2 Icm = /5(hand mass + tool mass)r (Eq. 7),
and r = ½(length of MC II),
1 2 (hand mass + tool mass)( /2r) is the application of the parallel-axis theorem
and ωh is the angular velocity of the MC II head.
Kinematic data were analyzed intra-knapper and intra-swing-position (see results
for descriptions of the swing positions used). In order to allow comparison between
trimming and standard swings the timing of events (e.g., peak joint angles and peak joint
angular velocities) was calculated relative to the transition from up-swing to down-swing
because strike was not consistently discernable in trimming swings. In standard swings,
events were calculated relative to strike, to allow comparison with Oldowan knapping kinematics (Williams et al., 2011). Kinematic data were predominantly non-normally distributed. Data were analyzed using Kruskal-Wallis tests. All p-values were determined using a post-hoc pairwise Mann-Whitney U test and have been treated with a
Bonferroni correction.
Lithic analysis
Platform depth, symbolized in Equation 8 as h, was recorded for each flake with
complete associated kinematic data. Dibble and Pelcin’s (1995:430) definition of
platform depth was utilized, which reads, “the distance from the interior edge to the
exterior edge along the platform surface”. The maximum load to flake removal (PF) was derived according to the equation presented in Chai and Lawn (2007b) that models PF as
146
a function of the fracture toughness of the material (Kc), h, and β, which is a geometrical coefficient:
(2/3) 2 PF = Kcβh (Eq. 8, Chai and Lawn, 2007b)
Only those flakes with associated kinematic data, complete platforms, and recordable external platform angles were included in the analysis.
RESULTS
Standard swings
Subjects held the hammerstone in their right hand and the core in their left for all
swings. One or more of the following knapping positions was assumed during their
reduction sequences. Knapping occurred with the core braced against the leg or held
over the leg. Subjects B – H knapped across their midline and placed the core onto or
above their left leg. Subject A knapped on his right leg.
The up-swing and down-swing motion sequences during standard Acheulean
swings followed the pattern previously described in Williams and colleagues (2011). At
the initiation of up-swing, all segment endpoints were moving upward, the shoulder and
elbow were flexing, and the wrist was extending. Across all subjects, the shoulder
6 The equation we used to calculate PF assumes a 90⁰ external platform angle and the application of a load normal to the striking surface. This equation was used due to the notoriously difficult, if not currently impossible, nature of accurately recording external platform angles and angle of indentation, with the understanding that the resulting PF would be an overestimate of what would actually be necessary to produce the flake. Even with this expectation, the difference between PF and kinematically derived Oldowan and Acheulean forces are far greater than anticipated (see results). Further, the real difference exceeds what we have reported here. Chai and Lawn (2007a) developed a supplementary equation which calculated PF for off-angle external platform angles and non-normal loads. Further experiments are underway that investigates the application of the off-angle equation to stone tool production.
Chai H, and Lawn B (2007a) Edge chipping of brittle materials: effect of side-wall inclination and loading angle. International Journal of Fracture 145:159-65. 147
shifted to extension, its primary down-swing motion direction, at least 0.022 seconds
prior to the transition to down-swing during on-leg swings and at least 0.006 seconds prior to the transition during above-leg swings (Table 5.4). At the time of the shoulder’s transition to its primary down-swing motion direction, the elbow was still flexing, the wrist was still extending, and the arm was still being cocked in preparation for down- swing. The elbow and wrist did not shift to their respective down-swing motion directions (i.e., elbow extension and wrist flexion) until after the transition to down- swing had occurred. In both on- and above-leg standard swings, the RSP shifted to a downward direction next, followed by the MC II head. After the RSP had changed directions, the elbow transitioned to extension, its primary down-swing motion direction.
This shift took place 0.084 – 0.147 seconds prior to strike (range of timing of elbow transition across all subjects, Table 5.5). The wrist was the last joint to shift to its down- swing motion direction, flexion, at 0.048 – 0.92 seconds prior to strike. Across all subjects, the initiation of down-swing segment endpoint motion and joint angular motion occurred in a complete proximal to distal fashion.
Subjects utilized the “dart-throwers arc,” (Palmer et al., 1985) in which the wrist moves from a position of extension and radial deviation through to flexion and ulnar deviation (range of average Spearman’s R correlation between extension/flexion and radial/ulnar deviation across all subjects, 0.787 – 0.977, Table 5.6, Figure 5.2).
Extension/radial deviation was emphasized over flexion/ulnar deviation. All subjects reached at least 46.5° of extension and utilized a minimum of 76% of their extension range when knapping on their leg. When executing swings with the core held above the leg, subjects reached at least 50° of extension and utilized a minimum of 58% of their
148 extension range. However, subjects failed to flex past their neutral position in both knapping positions, with one exception (Subject F, above-leg swings, 2.16° flexion,
Table 5.7). Both Subjects E and G exhibited lower average correlations between extension/flexion and radial deviation/ulnar deviation than the other subjects. In the case of Subject E, this was likely due to his avoidance of radial deviation. Subject G avoided radial deviation when resting the core on her left leg, and avoided ulnar deviation when holding the core above her left leg, both of which likely contributed to the low correlation she exhibited. Following strike, subjects’ wrists were propelled out of flexion and back into extension. Across all subjects, post-strike extension reached a minimum of 26° of during on-leg swings and 6.4° during above-leg swings (Table 5.8).
The peak angular velocity attained at each joint also occurred in a complete PDJS, with one exception. Peak velocity at the shoulder was significantly lower than peak velocity at both the RSP and MC II head across all subjects (maximum OP:RSP subject p
≤ 0.0001, maximum OP:MC II p ≤ 0.0001, Table 5.9). Peak extension angular velocity at the elbow was significantly lower than peak flexion angular velocity at the wrist with one exception (maximum p for Subjects A-F and H ≤ 0.003, Table 5.10). Subject G executed strikes with the core braced against her left leg and while it was held above her left leg. During both types of swings the extension angular velocity attained at her elbow was greater than the flexion angular velocity attained at her wrist. This difference was significant when the core was held above over her leg (p = 0.008, Table 5.10).
Subjects B, F and G executed on- and above-leg standard swings. In each case the angular velocity achieved at the elbow and wrist were significantly greater in above- leg swings compared with on-leg swings (Table 5.11). However, there were no
149
consistent patterns in either the peak extension and flexion angles these subjects exhibited
or in the duration of down-swing that would uniformly explain the differences in peak
angular velocities exhibited by all three subjects.
Subjects maintained the modified PDJS used during Oldowan knapping swings in
regard to the onset of peak angular velocities (Williams et al., 2011). Velocity at the
shoulder peaked and declined significantly prior to the RSP and MC II head (maximum
subject p < 0.009 and p ≤ 0.0001, respectively). Angular velocity at the wrist peaked
0.01 – 0.028 seconds prior to strike (total range of timing of peak wrist angular velocity
across all subjects, Table 5.12). Across all subjects, this was significantly prior to the onset of peak angular velocity at the elbow, which occurred 0 – 0.013 seconds prior to strike (total range of timing of the onset of peak angular velocity at the elbow across all subjects, maximum subject p ≤ 0.0005). Among the three subjects who executed both on- and above-leg swings (Subjects B, F, and G), there were no consistent patterns in the onset of peak angular velocities between the two swing positions. For example, wrist angular velocity in Subject B peaked significantly earlier during on-leg compared with off-leg swings. However, in Subjects F and G there were no significant differences in timing.
Trimming swings
Trimming swings were more variable than standard swings in regard to the sequence of motions, the use of the dart-thrower’s arc, and the onset of peak angular velocities. Six subjects executed trimming swings during biface reduction sequences
(Subjects A – D, F, and H). All six subjects initiated trimming up-swing in the same
150 manner; all segment endpoints moved in an upward direction, the shoulder and elbow were flexing, and the wrist was extending. After this point, subjects utilized various strategies. Subjects C and F followed the same sequence of motions used during standard swings through both phases of trimming. The SH began moving downward at least 0.78 seconds prior to the transition to down-swing (Table 5.4). At the transition to down- swing as the RSP began moving in a downward direction, the MC II continued to travel upward, and the elbow and wrist continued to flex and extend, respectively. The elbow transitioned to extension significantly before the wrist transitioned to flexion, at 0.006 –
0.008 seconds after the transition to down-swing (maximum subject p ≤ 0.0001, range of timing of elbow transition across Subjects C and F, Table 5.13). At the time of the elbow’s transition, the MC II head continued to move upwards, and the wrist continued to extend. At least 0.015 seconds after the transition to down-swing, the MC II head began moving downward. The wrist was the last joint to transition to its down-swing motion direction, 0.035 – 0.05 seconds after the transition (range of timing of wrist transition across Subjects C and H).
Subjects A, B, C, and D varied from the above pattern. Subject A did not transition to a downward direction at the OP until the RSP had done the same at the transition to down-swing. Subjects B, C, and D began moving their MC II head downward before the transition to down-swing (Table 5.4). Despite these differences in the initiation of segment endpoint motion, all subjects initiated joint angular motion in a complete PDJS (calculated relative to the transition to down-swing, Tables 5.4 and 5.13).
All subjects reached significantly lower degrees of extension during trimming swings compared to standard swings. Conversely, there was a general trend to reach
151
significantly greater degrees of wrist flexion during trimming swings (Table 5.7). This
did not, however, correspond to a similar increase in the utilization of ulnar deviation, as
would be expected when using the dart-thrower’s arc. All subjects exhibited lower average correlation values between extension/radial deviation and flexion/ulnar deviation
during trimming swings compared to standard swings (range of average Spearman’s R
correlation between extension/flexion and radial/ulnar deviation across all subjects 0.476
– 0.833, Table 5.6). The reason for the lower correlation varied across subjects. For
instance, Subject A tended to radially deviate during both extension and flexion, whereas
Subject D deviated only minimally in both the radial and ulnar directions.
Angular velocities occurred in a complete PDJS. Velocity at the shoulder (OP) was significantly lower than velocity at RSP and MC II head (Table 5.9), and angular velocity at the elbow was significantly lower than angular velocity at the wrist joint across all subjects (maximum p across all subjects ≤ 0.0001, Table 5.10). All angular velocities attained at the elbow and wrist during trimming swings were significantly lower than those reached during standard swings (Table 5.11).
Subjects did not consistently utilize the modified PDJS in regard to the onset of peak angular velocities that was seen in standard swings. Of the six subjects who employed trimming swings, only two exhibited elbow and wrist separation (Subjects A and F, Table 5.14). Subject F exhibited joint separation only during above-leg swings.
Acheulean v. Oldowan knapping swings
The motions of standard knapping swings utilized in Acheulean biface production
were remarkably similar to the kinematics captured during Oldowan chopper production
152
(Williams et al., 2011). At the most basic level of analysis, all subjects held the
hammerstone in the right hand and the core in their left. Subjects B-H held the core
braced against or held above their left leg. Subject A held the core braced against his right leg.
Looking closer at specific details of the knapping kinematics of both traditions, the swings remained very similar. The dart-thrower’s arc was used by all subjects to a similar degree in both knapping traditions, with one exception [Table 5.6, and Table 4.5 in Williams et al. (2011)]. Subject H exhibited a higher average correlation between extension/radial deviation and flexion/ulnar deviation during Acheulean reduction sequences than she had during Oldowan reduction sequences. This was likely due to the difference in the degree to which she radially deviated between the two traditions.
During Oldowan reduction sequences, Subject H failed to radially deviate past her neutral position, whereas she used an average maximum of 6.5° of radial deviation during
Acheulean reduction sequences.
The sequence of motions at the segment endpoints and at the joints did not change between the two tool traditions. The PDJS in segment endpoint motion initiations and joint angular motion initiations that was recorded during Oldowan stone tool production
was maintained during Acheulean stone tool production. Some subjects exhibited significant differences in the absolute timing of joint initiations between the two traditions. However, the manner of these differences was inconsistent when considered across all of the subjects. For example, Subject A initiated elbow flexion significantly
earlier during Acheulean swings but there was no difference in his timing of the initiation
of wrist extension. Alternatively, Subject C initiated motion at both joints earlier during
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Acheulean swings, whereas there were no differences at either joint in Subject E (Table
5.15).
The PDJS in peak angular velocities was also maintained between the two stone tool traditions. Yet again, no consistent patterns emerged regarding the relationship of peak angular velocities achieved at the elbow and wrist between Oldowan and Acheulean swings (Table 5.16).
Subjects also exhibited the same modified PDJS in regard to the onset of peak angular velocities during both Acheulean (Table 5.12) and Oldowan [Table 4.9, Williams et al. (2011)] stone tool production. As was the case with joint angular motion initiations and peak angular velocities, there was not a consistent pattern between the two in terms of the absolute timing that peak angular velocities occurred.
Average strike force was significantly greater during the production of Oldowan bifacial choppers compared to Acheulean handaxe in five of eight subjects (Subjects A,
B, D, G, and H, Figure 5.3). Conversely, Subject E used significantly greater force at strike during Acheulean tool production compared with Oldowan tool production, and differences in Subjects C and F were insignificant.
DISCUSSION
High-speed 3-D motion capture provides an expedient method to systematically document and quantify knappers’ approaches to the stone tool making process as well as the kinematic decisions they make throughout that process. We used this method to investigate upper limb motions and action sequences during Oldowan and Acheulean
154
stone tool production to test the hypothesis that Acheulean reduction sequences call for
greater control of motion and more complex action sequences compared with Oldowan
reduction sequences.
Our data demonstrate that the standard knapping swings used to make Acheulean
and Oldowan tools are remarkably similar to one another in terms of wrist motions, and
the sequences of upper limb segment endpoint motions, joint angular motion initiations,
peak segment endpoint linear velocities, peak angular velocities, and the onset of peak
angular velocities. We did not find small-scale kinematic evidence (i.e., joint motions) of
more complex or even different joint action sequences between standard swings from the
two tool traditions.
During standard swings from both tool traditions, subjects moved their wrist
through the dart-thrower’s arc of motion, in which the wrist moves from a position of
extension/radial deviation to flexion/ulnar deviation (Palmer et al., 1985). Average
correlation values between extension/radial deviation and flexion/ulnar deviation were
similar for all subjects during Oldowan and Acheulean swings, demonstrating that there
was no difference between the traditions in regard to the wrists’ arc of motion [Table 5.6,
and Table 4.5 in Williams et al. (2011)]. This wrist motion sequence offers the most
stable plane of motion, more so than pure anatomical directions (i.e., pure extension or flexion). This stability is due to the minimal rotation experienced at the scaphoid and lunate as the wrist moves through the arc (Werner et al., 2004). Use of the dart-thrower’s
plane of motion has been documented in a range of modern upper limb activities in which
accuracy is the primary goal, including pitching, batting, and golfing (Wolfe et al., 2006).
Consequently, it is not surprising to find that it is used during knapping, as well. In fact,
155 stone tool production may constitute one of the earliest hominin behaviors that relied upon this now commonly used plane of motion (Wolfe et al., 2006).
Subjects applied the same modified PDJS that was documented during Oldowan knapping swing (Williams et al., 2011) to Acheulean knapping swings. During the standard swings of both tool traditions, segment endpoint initiations, joint angular motion initiations and peak angular velocities occurred in a complete PDJS. Peak velocity occurred at the shoulder first, followed by the wrist and then elbow, in a modified PD sequence. Although between the two traditions there were some specific differences in the absolute velocity attained and absolute timing of events within each subject, consistent patterns across subjects were not present. For example, at the elbow two subjects (A and C) initiated motion significantly earlier during Acheulean swings compared with Oldowan swings. Two other subjects (D and H) initiated motion significantly later during Acheulean swings. Subjects E and F did not exhibit significant differences, and Subjects B and G initiated motion significantly earlier when executing off-leg swings, but this was not the case during on-leg swings (Table 5.15).
It is presently unknown whether the central nervous system controls movement through combinations of motor primitives (Mussa-Ivaldi et al., 1994; Thoroughman and
Shadmehr, 2000), from memories of previously executed similar movements (Schmidt,
1975), through the exploitation of biomechanical properties inherent in a multi-joint system (Dounskaia, 2010), or by some other means entirely. Regardless of the underlying processes, researchers agree that the PDJS offers a muscularly efficient way to execute high-velocity, multi-joint behaviors while also achieving accuracy during activities such as pitching a ball, throwing a javelin, or making stone tools (Putnam,
156
1991; Putnam, 1993; Chowdhary and Challis, 1999; Hirashima et al., 2002; Hirashima et al., 2007; Williams et al., 2011). The use of the PDJS across a wide variety of behaviors, particularly sporting activities, speaks to its versatility. Adhering to parsimony, it is reasonable to expect that the same upper limb joint kinematic pattern would be applied to different types of stone tool production sequences, regardless of the tradition or end product. If greater activity in the right hemisphere during Acheulean handaxe production compared with Oldowan tool production is evidence of increased control of action and more complex action sequences (Faisal et al., 2010), it is not manifested as small-scale differences in upper limb movements or the sequences of joint motions used during
Oldowan and Acheulean stone tool production.
However, the sequence of muscular actions during the two traditions remains to be determined. Generalized kinematic patterns may arise from different muscular action patterns and different muscular control sequences (Gandolfo et al., 1996; Conditt et al.,
1997; Fagg et al., 2002). It is possible that Acheulean handaxe production recruits a
pattern of muscle activation that is unique from Oldowan stone tool reduction sequences.
If that is the case, these differences may not necessarily be detected at the level of gross upper limb movements. Though given the similarities in joint sequences presented here, as well as the similarities in the actions themselves, this possibility seems unlikely but the issue warrants further investigation before any conclusions can be reached.
Alternatively, the regulation of complex action sequences and action inhibition can be viewed on a larger scale in terms of gross upper limb movements, rather than individual joint (or muscular) motions. Acheulean handaxes are widely believed to require abundant pre-planning and a high degree of operational intelligence on the part of
157
the maker (Wynn, 1979; Schick and Toth, 1993; Ambrose, 2001; Pelegrin, 2005), and the
nature of the biface reduction sequence is such that the success of subsequent removals is
determined by the flake scars left behind from the preceding removals. A mistake made
at any point along the reduction process may inhibit or halt the successful production of
the tool. This emphasis on pre-planning and order of operations may be reflected in the
significantly lower strike forces recoded among five of the eight subjects during
Acheulean reduction sequences, and may indicate their increased effort to produce flakes
of a specific size and shape in order to attain an ultimate bifacial shape (Figure 5.3).
Given the simplicity of the Oldowan choppers produced during our Oldowan replication
experiments (Williams et al., 2011)—each with no more than 10 flake removals—
subjects were not placed under similar constraints. It appears that the constraints placed
on subjects during Acheulean reduction sequences may have been manifested as force
regulation and inhibition.
However, only five of eight subjects displayed the difference in strike force
application described above. Further, the force used during both Oldowan and Acheulean
reduction subsequences was significantly greater than that required based on the lengths
of the platform depths from the associated lithics. Using the equation developed by Chai
and Lawn (2007b) and the associated lithic data, we calculated the predicted PF for each lithic produced. Both Oldowan and Acheulean strike forces greatly exceeded the predicted PF (Figure 5.4). Although further investigation into this issue is necessary, these results may indicate that subjects are unable to accurately evaluate the current knapping parameters in order to apply the minimal force needed to produce a flake of a given size and shape. Additionally, it is not possible to determine whether the knappers
158
that participated in the studies conducted by Stout and colleagues (2007, 2008) controlled
their force application in a similar manner as those that participated in the current study.
Thus we cannot say with any certainty whether the difference in strike force between
Oldowan and Acheulean reduction sequences contributed to differences in hemisphere
activity. However, this pattern is noteworthy in light of the relationship between force
and flake morphology (Chai and Lawn, 2007b; Chai and Lawn, 2007a; Dibble and
Rezek, 2009) and the increased need to produce flakes of specific dimensions during
Acheulean handaxe production.
A third possibility is that the kinematic strategy documented in the execution of
trimming swings may constitute a novel or more complex action sequence compared with
the kinematic strategy used during Oldowan stone tool production. Trimming is a
knapping technique, used to prepare striking platforms, which was more intensively
applied during the Developed Oldowan and early Acheulean (Clark, 1994). In their study
of grips and hand movements across early stone tool industries, Marzke and Shackley
(1986) suggested that the trimming swings knappers used to retouch tool margins during
production of Developed Oldowan tools may require greater skill than all techniques used
during their production of Oldowan tools.
The picture that emerges from our data on trimming swings suggests that their
kinematics are more simplistic from a biomechanical standpoint and less uniform across
knappers than the kinematics of standard knapping swings. Subjects variably employed
the stable dart-thrower’s arc of motion, and in general exhibited lower correlation values
between extension/radial deviation and flexion/ulnar deviation. As previously stated, the
reasons for the lower correlations varied across subjects. The sequence of segment
159
endpoint motions during both knapping phases also varied among the six subjects who
executed trimming swings. Subject A did not separate motion at the OP and RSP, and
both segment endpoints transition to down-swing together. Alternatively, the MC II head
began moving downward during the up-swing phase in Subjects B, C, and D.
During trimming, subjects retained a PDJS in terms of joint angular motion
initiations (with the exception of Subject B), which occurred earlier at the elbow and
wrist than during standard swings (Table 5.17). Subjects also retained a PDJS in terms of
peak velocities (Table 5.10), which were significantly lower at each joint compared to
standard swings (Table 5.11). However, only two of the six subjects who used trimming swings exhibited separation between the elbow and wrist in terms of the onset of peak angular velocity. This indicates that these subjects moved their forearm and wrist as a unit, freezing out degrees of freedom that become available with more mobile joints.
Despite Marzke and Shackley’s (1986) report that trimming swings required greater precision and skill, our data suggest otherwise. Subjects exhibited high variability in their execution of trimming swings and maintained more rigid upper limb joints. Joint freezing is associated with a lack of skill and detracts from accuracy (Bernstein, 1967;
Newell and Van Emmerik, 1989; Chowdhary and Challis, 1999; Gray et al., 2006). In combination with the potential for greater instability at the wrist due to subjects’ reduced use of the dart-thrower’s arc, trimming swings were likely less precise than standard
swings. Thus, it is not presently clear whether the decreased joint excursions, reduction
in the use of the dart-thrower’s arc, or the maintenance of a more rigid upper limb during trimming swings call for greater control of motion or constitute a more complex motion sequence than standard swings.
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Trimming swings were interspersed with standard swings throughout the duration of the handaxe reduction process. Given the difference in intention and motions between standard and trimming swings, this interspersal constitutes task-set switching and results in a more complex motor sequence compared with that applied to Oldowan stone tool production. If subjects in Stout and Chaminade (2007) and Stout and colleagues (2008) similarly executed trimming swings and employed them similarly during their reduction sequences, this may have contributed to right hemisphere activation in Broadman area 45.
CONCLUSIONS
It is unlikely that differences in small-scale upper limb kinematics utilized during
Acheulean stone tool reduction contributed to the different levels of hemispheric activity
Stout and colleagues (2007, 2008) reported when comparing Acheulean with Oldowan stone tool production. In those instances when Acheulean upper limb kinematics differed from Oldowan upper limb kinematics, it was merely a difference of degree and not of kind. The PDJS that knappers utilized during both traditions is a highly versatile kinematic strategy. It has been documented in the execution of a wide variety of accuracy-seeking upper and lower limb activities. Given its versatility, it is not surprising to find that two activities that are grossly similar, Oldowan and Acheulean stone tool production, rely on the same kinematic strategy.
However, the difference in force output between the two traditions may be evidence of long-term planning in order to attain a specific goal, as knappers exercised greater control of the force they applied to the core in order to remove a flake of a
161
specific size and/or shape. If force was similarly regulated in experiments conducted by
Stout and colleagues (2007, 2008), this may have contributed to the differences in right hemisphere activity, although these differences are not displayed in the joint kinematics.
A third possibility is that trimming swings executed during Acheulean stone tool production constitute novel action sequences in terms of upper limb joint motions and the sequence of upper limb motions. Though mechanically simpler and executed with greater variability than standard swings, they do represent a kinematic strategy that is unique from that recorded during standard swings. Considered with their interspersal with standard swings through the reduction process, the use of trimming swings may have placed novel demands for control of action and action sequences which contributed to the activity observed in the right hemisphere.
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Table 5.1. Upper limb mass (kg) and lengths (mm) Subject A B C D E F G H hand 0.46 0.5 0.5 0.5 0.4 0.35 0.3 0.23 hand & 1.82 1.95 2.3 2.4 1.7 1.11 1.05 1 forearm
Upper limb 3.98 4.5 4.75 4.85 4 3.16 2.5 2.45 forearm 270 263 260 268 320 265 248 250 MC II 107 105 110 101 87 75.2 98.83 81
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Table 5.2. Maximum handaxe length, width, and thickness (mm) handaxe 1 handaxe 2 Subject length width thickness length width thickness A 163.49 115.11 37.97 125.05 101.35 41.28 B 101.49 72.18 33.14 119.8 91.66 33.8 C 157.02 87.23 52.14 142.32 125.66 52.98 D 136.38 81.15 34.15 162.16 83.45 45.97 E 221.37 137.92 77 NA F 105.22 82.89 29.89 132.4 112.4 30.75 G 128.96 95.95 41.99 176.8 85.74 73.3 H 116.87 55.75 33.81 131.88 77.66 31.57 Bolded measurements indicate that the reduction effort was abandoned before the subject was completely satisfied with the end product.
164
‐ 2 8
33 20 35 22 22 29 Biface
Flakes
1 40 20 29 31 40 29 13 22 Biface
left
0 0 0 0 0 0 27 26 swings
above
left 0 0 0 0
19 37 38 23 trimming on
counts
0 0 0 0 0 0 0 right flake 23
Complete on
left
0 0 0 0 0 24 23 15 associated swings
above
and
left 0
41 78 29 53 20 90 151 standard
on
swings
0 0 0 0 0 0 0 right 91
Complete on
knapping ‐
2 48 27 83 43 49 39 57
Biface
1 67 44 79 41 41 43 44 39 swings
Biface trimming
2 and 15 16 16 35 17 15 17 26
standard
Chopper
Total
1 13 18 13 19 17 18 14 19 Standard
Chopper
5.3.
F E C B A D G H Subject Table
165
was OP OP
0.165 0.022 ‐ ‐
core
0 0 E RSP RSP STonL
TRaboveL onR: II II
leg. 0.003 MC MC
0.005 ‐
swing; left
the OP OP OP OP
0.067 0.145 0.194 0.078 ‐ ‐ ‐ ‐
trimming
above
0 0 0 0
RSP RSP RSP RSP TR:
B TRonL TRonL held STaboveL
TRaboveL
II II II II
swing; was
0.01 0.004 0.006 MC MC MC MC 0.018 ‐ ‐
core D H
direction
standard OP OP OP OP
0.068 0.142 0.122 0.103 ‐ ‐ ‐ ‐ ST: aboveL:
motion
0 0 0 0 leg;
RSP RSP RSP RSP STonL STonL STonL TRonL swing. ‐ left
swing ‐
II II II II
the
F down
MC MC MC MC 0.003 0.006 0.015 0.005 down
to
to
against
0 OP OP OP OP
0.120 0.139 0.006 ‐ ‐ ‐
transtion
braced
0 0 0 0 RSP RSP RSP RSP transitions the
TRonL TRonR
was
STaboveL STaboveL to II II II II
core MC MC MC MC 0.005 0.032 0.012 0.002
C A G
endpoint
relative
onL:
OP OP OP OP 0.140 0.165 0.214 0.032 ‐ ‐ ‐ ‐ leg;
segment seconds
0 0 0 0
RSP RSP RSP RSP right of
in StonL STonL STonL
STonR
II II II II
the
0.01 MC MC MC MC 0.007 0.009 0.005 Timing
presented
against
5.4.
is
Time Table Subject Position Segment endpoint Time Subject Position Segment endpoint Time Subject Position Segment endpoint Time Subject Position Segment endpoint Time braced
166
Table 5.5. Timing of initiation of elbow extension and wrist flexion relative to strike Subject A B Position STonR STonL STaboveL Joint elbow wrist elbow wrist elbow wrist Time ‐0.088 ‐0.048 ‐0.147 ‐0.06 ‐0.132 ‐0.054 p < 0.0001 < 0.0001 0.000 Subject C D E Position STonL STonL STonL Joint elbow wrist elbow wrist elbow wrist Time ‐0.084 ‐0.054 ‐0.105 ‐0.062 ‐0.131 ‐0.076 p < 0.0001 < 0.0001 < 0.0001 Subject F Position STonL STaboveL Joint elbow wrist elbow wrist Time ‐0.096 ‐0.059 ‐0.081 ‐0.044 p < 0.0001 < 0.0001 Subject G H Position STonL STaboveL STonL Joint elbow wrist elbow wrist elbow wrist Time ‐0.095 ‐0.073 ‐0.11 ‐0.092 ‐0.085 ‐0.066 p < 0.0001 0.001 < 0.0001 Time is presented in seconds relative to strike. All results are significant. ST: standard swing; TR: trimming swing; onR: core was braced against the right leg; onL: core was braced against the left leg; aboveL: core was held above the left leg.
167
Table 5.6. Correlation between wrist extension/radial deviation and flexion/ulnar deviation Standard swings Swing Average Maximum Subject position Spearman's R p A STonR 0.966 < 0.0001 STonL 0.938 0.031 B STaboveL 0.957 < 0.0001 C STonL 0.952 < 0.0001 D STonL 0.977 < 0.0001 E STonL 0.612 0.024 STonL 0.958 < 0.0001 F STaboveL 0.962 0.0042 STonL 0.843 0.0042 G STaboveL 0.787 0.0129 H STonL 0.976 0.0002 Trimming swings A TRonR 0.495 0.05 B TRaboveL 0.476 0.016 C TRonL 0.822 0.0002 D TRonL 0.592 0.016 TRonL 0.833 0.036 F TRaboveL 0.683 0.018 H TRonL 0.77 0.001 ST: standard swing; TR: trimming swing; onR: core was braced on the right leg; onL: core was braced on the left leg; aboveL: core was held above the left leg
168
Table 5.7. Peak knapping angles during standard and trimming Acheulean swings A B C Wrist extension Wrist extension Wrist extension STonR TRonR STonL STaboveL TRaboveL STonL TRonL 66.56 57.92 63.23 57.55 33.05 46.84 16.67 STonR ‐ ‐ STonL ‐ ‐ ‐ STonL ‐ ‐ TRonR < 0.0001 ‐ STaboveL < 0.0001 ‐ ‐ TRonL < 0.0001 ‐ TRaboveL < 0.0001 < 0.0001 ‐ Wrist flexion Wrist flexion Wrist flexion STonR TRonR STonL STaboveL TRaboveL STonL TRonL +44.31 +33.66 +31.38 +19.02 +17.72 +11.94 4.03 STonR ‐ ‐ STonL ‐ ‐ ‐ STonL ‐ ‐ TRonR < 0.0001 ‐ STaboveL 0.015 ‐ ‐ TRonL < 0.0001 ‐ TRaboveL < 0.0001 0.938 ‐ Radial deviation Radial deviation Radial deviation STonR TRonR STonL STaboveL TRaboveL STonL TRonL 10.09 7.33 16.08 15.71 7.08 13.35 +3.76 STonR ‐ ‐ STonL ‐ ‐ ‐ STonL ‐ ‐ TRonR 0.032 ‐ STaboveL 1 ‐ ‐ TRonL < 0.0001 ‐ TRaboveL < 0.0001 < 0.0001 ‐ Ulnar deviation Ulnar deviation Ulnar deviation STonR TRonR STonL STaboveL TRaboveL STonL TRonL +2.13 +3.76 2.71 2.98 +2.52 7.14 13 STonR ‐ ‐ STonL ‐ ‐ ‐ STonL ‐ ‐ TRonR 0.014 ‐ STaboveL 1 ‐ ‐ TRonL < 0.0001 ‐ TRaboveL < 0.0001 0.0001 ‐ Elbow extension Elbow extension Elbow extension STonR TRonR STonL STaboveL TRaboveL STonL TRonL 73.33 79.34 95.69 95.74 93.78 82.66 79.88 STonR ‐ ‐ STonL ‐ ‐ ‐ STonL ‐ ‐ TRonR < 0.0001 ‐ STaboveL 1 ‐ ‐ TRonL < 0.0001 ‐ TRaboveL 0.020 1 ‐ Elbow flexion Elbow flexion Elbow flexion STonR TRonR STonL STaboveL TRaboveL STonL TRonL 57.08 70.62 72.12 69.68 89.36 63.8 73.88 STonR ‐ ‐ STonL ‐ ‐ ‐ STonL ‐ ‐ TRonR < 0.0001 ‐ STaboveL 0.185 ‐ ‐ TRonL < 0.0001 ‐ TRaboveL < 0.0001 < 0.0001 ‐ All angles are presented in degrees. + signs indicate that the subject did not move past their neutral plane in that direction. Bolded results are significant
169
Table 5.7 continued D F Wrist extension Wrist extension STonL TRonL STonL SToffL TRaboveL TRonL 61.98 24.99 52.3 50.13 41.52 36.3 STonL ‐ ‐ STonL ‐ ‐ ‐ ‐ TRonL < 0.0001 ‐ STaboveL 0.303 ‐ ‐ ‐ TRaboveL < 0.0001 < 0.0001 ‐ ‐ TRonL < 0.0001 < 0.0001 < 0.0001 ‐ Wrist flexion Wrist flexion STonL TRonL STonL SToffL TRaboveL TRonL +38.69 +20.56 +24.26 2.16 0.82 +15.09 STonL ‐ ‐ STonL ‐ ‐ ‐ ‐ TRonL < 0.0001 ‐ STaboveL < 0.0001 ‐ ‐ ‐ TRaboveL < 0.0001 1 ‐ ‐ TRonL 0.025 < 0.0001 < 0.0001 ‐ Radial deviation Radial deviation STonL TRonL STonL SToffL TRaboveL TRonL 16.74 7.65 8.2 7.3 4.46 3.89 STonL ‐ ‐ STonL ‐ ‐ ‐ ‐ TRonL < 0.0001 ‐ STaboveL 1 ‐ ‐ ‐ TRaboveL 0.016 0.014 ‐ ‐ TRonL 0.001 0.010 1 ‐
Ulnar deviation Ulnar deviation STonL TRonL STonL SToffL TRaboveL TRonL 1.28 +4.4 2.35 7.65 5.48 4.67 STonL ‐ ‐ STonL ‐ ‐ ‐ ‐ TRonL < 0.0001 ‐ STaboveL 0.009 ‐ ‐ ‐ TRaboveL 1 0.039 ‐ ‐ TRonL 1 0.049 1 ‐ Elbow extension Elbow extension STonL TRonL STonL SToffL TRaboveL TRonL 89.15 61.98 79.81 90.22 82.55 89.66 STonL ‐ ‐ STonL ‐ ‐ ‐ ‐ TRonL < 0.0001 ‐ STaboveL 0.024 ‐ ‐ ‐ TRaboveL 0.018 1 ‐ ‐ TRonL < 0.0001 1 < 0.0001 ‐ Elbow flexion Elbow flexion STonL TRonL STonL SToffL TRaboveL TRonL 71.3 85.3 59.66 56.14 64.92 79.74 STonL ‐ ‐ STonL ‐ ‐ ‐ ‐ TRonL < 0.0001 ‐ STaboveL < 0.0001 ‐ ‐ ‐ TRonL < 0.0001 ‐ TRaboveL < 0.0001 < 0.0001 ‐ ‐ TRonL < 0.0001 < 0.0001 < 0.0001 ‐ All angles are presented in degrees. + signs indicate that the subject did not move past their neutral plane in that direction. Bolded results are significant
170
Table 5.7 continued G H Wrist extension Wrist extension STonL SToffL STonL TRonL 50.84 50.36 66.62 53.27 STonL ‐ ‐ STonL ‐ ‐ STaboveL 1 ‐ TRonL < 0.0001 ‐
Wrist flexion Wrist flexion STonL SToffL STonL TRonL +30.58 +38.98 +43.87 +40.69 STonL ‐ ‐ STonL ‐ ‐ STaboveL 0.012 ‐ TRonL 0.698 ‐
Radial deviation Radial deviation STonL SToffL STonL TRonL 2 10.54 6.5 +1.79 STonL ‐ ‐ STonL ‐ ‐ STaboveL < 0.0001 ‐ TRonL < 0.0001 ‐
Ulnar deviation Ulnar deviation STonL SToffL STonL TRonL 9.26 +1.77 5.85 4.52 STonL ‐ ‐ STonL ‐ ‐ STaboveL 0.000 ‐ TRonL 0.034 ‐
Elbow extension Elbow extension STonL SToffL STonL TRonL 81.85 74.87 94.11 100.23 STonL ‐ ‐ STonL ‐ ‐ STaboveL 0.036 ‐ TRonL < 0.0001 ‐
Elbow flexion Elbow flexion STonL SToffL STonL TRonL 58.1 46.18 69.5 96.56 STonL ‐ ‐ STonL ‐ ‐ STaboveL < 0.0001 ‐ TRonL < 0.0001 ‐
All angles are presented in degrees. + signs indicate that the subject did not move past their neutral plane in that direction. Bolded results are significant
171
H 90 49.9 STonL
20 45.04 STonL
G
15 42.11 STaboveL
53 43.3 STonL
F
23 6.40 STaboveL
E 29 23.28 STonL
D 78 45.40 STonL
C 151 26.62 STonL
41 (°)
41.18 STonL
B
24 extension
29.40 STaboveL
wrist
A 91 54.92 STonR strike ‐
Post
type
strike n
5.8.
Subject extension Post Swing Table
172
II II
the 0.22 1.35
‐ ‐ MC MC
E against 0.11 1.13 RSP RSP
0.0001 0.0001 0.0001 0.0001 ‐ ‐ STonL
< < < < TRaboveL
braced OP OP
0.03 0.19 ‐ ‐
was
II II II II
core 0.91 0.18 0.59 0.10
‐ ‐ ‐ ‐ MC MC MC MC
onL:
0.51 0.14 0.29 0.07 RSP RSP RSP RSP B 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 ‐ ‐ ‐ ‐ leg; TRonL TRonL TRonL
< < < < < < < < STaboveL
right
OP OP OP OP 0.04 0.03 0.06 0.02 ‐ ‐ ‐ ‐ the
D H II II II II
0.09 0.57 0.33 0.81 ‐ ‐ ‐ ‐ MC MC MC MC against
braced
0.59 0.37 0.16 0.63 RSP RSP RSP RSP 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 ‐ ‐ ‐ ‐ STonL STonL STonL
< < < < < < < < TRaboveL was
core
OP OP OP OP 0.09 0.06 0.03 0.07 ‐ ‐ ‐ ‐
F onR:
II II II II
0.18 0.36 1.87 0.75
‐ ‐ ‐ ‐ MC MC MC MC
swing;
0.09 0.11 1.17 0.60 RSP RSP RSP RSP ‐ ‐ ‐ ‐ 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 TRonL
TRonR 0.0001 0.0001 endpoints < < < < < < STaboveL STaboveL
trimming
TR:
leg. OP OP OP OP 0.02 0.02 0.05 0.06 ‐ ‐ ‐ ‐
C A G segment left
swing; II II II II at
the 0.59 1.26 1.19 1.05 ‐ ‐ ‐ ‐ MC MC MC MC
(m/s)
above standard
0.36 0.67 0.79 0.76 RSP RSP RSP RSP 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 STonL STonL STonL ‐ ‐ ‐ ‐ STonR ST: held < < < < < < < <
velocities
was OP OP OP OP 0.06 0.72 0.06 0.12 ‐ ‐ ‐ ‐ core linear
significant.
II II II II
Peak are
aboveL: velocity velocity velocity velocity
5.9.
OP:RSP OP:RSP OP:RSP OP:RSP Subject Subject Subject Subject Position Position Position Position Segment OP:MC Segment OP:MC Segment OP:MC Segment OP:MC leg; results
Linear Linear Linear Linear Table All left
173
wrist wrist core 103.28 833.16
‐ ‐
E
0.0001
0.000
onL: STonL
< TRaboveL leg;
86.84 wrist wrist
20.16 144.17 elbow elbow ‐ 673.59 ‐ right
0.0001 0.0001 TRonL TRonL
< < the
27.82 wrist wrist 41.16 15.09 665.98 elbow elbow
‐ ‐
H B against
0.0001
0.001 TRonL
< STaboveL
38.3 wrist wrist
213.79 212.91 elbow elbow braced 246.27 ‐ ‐
D
was 0.0001 0.0001
STonL
< < TRaboveL core
wrist wrist 61.55 123.6
313.26 159.25 elbow elbow ‐ ‐ onR:
0.0001 0.0001
leg.
STonL STonL
< <
F left
swing;
786.9 wrist wrist 70.77 227.33 elbow elbow
‐
113.25 ‐ the
wrist
0.003 0.008
STaboveL STaboveL above trimming
and
wrist wrist 153.73 175.59 elbow elbow 453.75 465.62 TR: ‐ ‐
held G
elbow 0.0001 0.0001
TRonL
TRonR
was
< < swing;
the
22.6 wrist wrist core
35.96
at 242.37 137.46 elbow elbow
‐ ‐
C A
(°/s)
0.0001 standard 0.298
STonL STonL
< aboveL:
ST:
wrist wrist 215.72 283.55 elbow elbow 143.41 148.76 ‐ ‐ leg; velocity
0.0001 0.0001
left STonL
STonR
< <
the significant. 86.9
elbow elbow angular 138.43
are
against
Peak
ar ar ar ar l l l l
results p p p p
5.10.
Joint Joint Joint Joint braced Subject Subject Subject Subject angu angu angu angu velocity velocity velocity velocity Position Position Position Position Table was Bolded
174
‐ ‐ ‐ ‐
38.3 27.82
TRonL TRonL ‐
flexion D
‐ ‐
extension
0.0001 0.0001 70.77 STonL STonL 159.25
‐ < <
Wrist
Elbow
‐ ‐ ‐ ‐ 86.84 15.09 TRonL TRonL ‐
STonL TRonL STonL TRonL
‐ ‐ ‐ ‐ ‐ ‐
22.6 flexion 0.0001 0.0001 H 123.6
STonL STonL 175.59 212.91 TRonL TRonL
extension ‐ ‐
< <
ons
Wrist
iti Elbow
‐ ‐ pos flexion C
extension 0.0001 0.0001 STonL STonL 283.55
138.43 ‐ ng < < i STonL TRonL STonL TRonL Wrist
Elbow
napp k
‐ ‐ ‐ ‐
227.33 465.62 ‐ ween STaboveL STaboveL
t e STonL TRonL STonL TRonL
b
t
s
i
wr
flexion ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐
G
extension d
20.16 0.002 0.029 STonL STonL 103.28 137.46 148.76 ‐ ‐ an TRaboveL TRaboveL
Wrist Elbow ow
lb
e
e ‐ ‐ ‐ ‐
th
0.0001 0.0001 665.98
t 246.27 ‐ a < <
STaboveL STaboveL ) flexion B s
STonL STaboveL STonL STaboveL extension
(°/
es Wrist ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ Elbow iti 0.0001 0.0001 0.0001 0.0001 41.16 STonL STonL 313.26 144.17 oc TRonL TRonL
113.25 l ‐ ‐ < < < < ve
ar l
eL v
‐ ‐ ‐ ‐ ‐ ‐ angu 0.0001 0.0001
61.55 213.79
‐ k < < TRaboveL TRaboveL
STonL STaboveL TRabo STonL STaboveL TRaboveL
pea
‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ flexion F
extension
ween 786.9 0.0001 0.0001 0.0001 0.0001 35.96 153.73 t
‐ TRonR TRonR 453.75 ‐
e < < < < STaboveL STaboveL b
Wrist
Elbow
son i
significant.
‐ ‐ ‐ ‐
flexion A
86.9 extension 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.605 STonL STonL 215.72 242.37
STonR STonR 143.41 are ‐ ‐ < < < < < < <
ompar C Wrist
. Elbow
11 results . eL
v
5
e bl a T STonR TRonR STonR TRonR STonL STaboveL TRaboveL TRonL STonL STaboveL TRabo TRonL Bolded 175
Table 5.12. Timing of peak angular velocity (°/s) at the elbow and wrist relative to strike Subject A B Position STonR STonL STaboveL Joint elbow wrist elbow wrist elbow wrist Time 0 ‐0.013 ‐0.008 ‐0.011 0 ‐0.01 p < 0.0001 0.0002 0.0005 Inter‐swing STonL wrist to STaboveL wrist p = 0.0002 comparison STonL elbow to STaboveL elbow p = 0.2209 Subject C D E Position STonL STonL STonL Joint elbow wrist elbow wrist elbow wrist Time 0 ‐0.011 ‐0.013 ‐0.021 0 ‐0.016 p < 0.0001 < 0.0001 < 0.0001 Subject F Position STonL STaboveL Joint elbow wrist elbow wrist Time ‐0.002 ‐0.02 ‐0.003 ‐0.019 p < 0.0001 0.0001 Inter‐swing STonL wrist to STaboveL wrist p = 0.6436 comparison STonL elbow to STaboveL elbow p = 0.7286 Subject G H Position STonL STaboveL STonL Joint elbow wrist elbow wrist elbow wrist Time 0 ‐0.017 ‐0.007 ‐0.028 ‐0.003 ‐0.01 p < 0.0001 0.0002 < 0.0001 STonL wrist to STaboveL wrist p = 0.0643 Inter‐swing STonL elbow to STaboveL elbow p = comparison 0.02445 Time presented in seconds relative to strike. Bolded results are significant. ST: standard swings. onR: core braced on the right leg. onL: core braced on the left leg. aboveL: core braced above the left leg.
176
Table 5.13. Timing of the initiation of elbow extension and wrist flexion relative to the transition to down‐swing Subject A B Swing type STonR TRonR STonL STaboveL TRaboveL Joint elbow wrist elbow wrist elbow wrist elbow wrist elbow wrist Time 0.006 0.042 0.005 0.014 0.025 0.112 0.016 0.094 0.009 0.02 p < 0.0001 0.328 < 0.0001 0.000 0.036 Subject C D E Swing type STonL TRonL STonL TRonL STonL Joint elbow wrist elbow wrist elbow wrist elbow wrist elbow wrist Time 0.012 0.042 0.008 0.05 0.006 0.05 0.006 0.048 0.055 0.12 p < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 Subject F Swing type STonL STaboveL TRonL TRaboveL Joint elbow wrist elbow wrist elbow wrist elbow wrist Time 0.008 0.045 0.006 0.039 0.007 0.035 0.006 0.038 p < 0.0001 < 0.0001 < 0.0001 Subject G H Swing type STonL STaboveL STonL TRaboveL Joint elbow wrist elbow wrist elbow wrist elbow wrist Time 0.029 0.052 0.014 0.033 0.016 0.044 0.005 0.026 p < 0.0001 0.000 < 0.0001 < 0.0001 Time is presented in seconds relative to the transition to down‐swing. Bolded results are significant. ST: standard swings. onR: core braced on the right leg. onL: core braced on the left leg. aboveL: core braced above the left leg.
177
Table 5.14. Timing of peak angular velocity (°/s) at the wrist and elbow relative to the transition from up‐swing to down‐swing Subject A B Position STonR TRonR STonL STaboveL TRaboveL Joint elbow wrist elbow wrist elbow wrist elbow wrist elbow wrist Time 0.092 0.08 0.09 0.061 0.159 0.155 0.145 0.136 0.059 0.046 p < 0.0001 < 0.0001 0.108 0.249 0.198 Subject C D Position STonL TRonL STonL TRonL STonL Joint elbow wrist elbow wrist elbow wrist elbow wrist elbow wrist Time 0.09 0.08 0.098 0.09 0.095 0.085 0.088 0.096 0.173 0.164 p < 0.0001 0.215 < 0.0001 0.12 0.019 Subject F Position STonL STaboveL TRaboveL TRonL Joint elbow wrist elbow wrist elbow wrist elbow wrist Time 0.094 0.079 0.089 0.073 0.101 0.081 0.075 0.075 p < 0.0001 0.002 < 0.0001 0.634 Subject G H Position STonL STaboveL STonL TRonL Joint elbow wrist elbow wrist elbow wrist elbow wrist Time 0.12 0.104 0.112 0.091 0.101 0.096 0.075 0.069 p 0.000 0.019 < 0.0001 0.386 Time presented in seconds relative to the transition to down‐swing. Bolded results are significant. ST: standard swings. onR: core braced on the right leg. onL: core braced on the left leg. aboveL: core braced above the left leg.
178
L L
‐ ‐ ‐ ‐ on on
0.087 0.066 A A ‐ ‐
L L
H
‐ ‐ Wrist on on Elbow
0.01 0.107 0.062 0.0001
O O ‐ ‐ <
L L L L
L L
‐ ‐ ‐ ‐ on on on on
on on
0.105 0.062 A O
A A ‐ ‐ O A
D L L
Wrist L L Elbow
‐ ‐ ‐ ‐ ‐ ‐ on on
0.118 0.056 0.110 0.092 0.0001 0.0001 above above
O O ‐ ‐ ‐ ‐
< < A A swings.
L L
L L L L
‐ ‐ on on
on on on on 0.095 0.073
0.005
A A ‐ ‐ 0.0001
O A O A
standard L L
G
L L Wrist
Elbow
‐ ‐ ‐ ‐ ‐ ‐ 1 1 on on
0.084 0.054 0.095 0.076 above above 0.001 0.003 A A ‐ ‐ ‐ ‐
O O Acheulean L L L
L
A: L
L L
on
A
‐ ‐ on on L C above above
0.080 0.048 0.0001
O O ‐ Wrist ‐ 0.0002 A Elbow O above on above <
swings;
O A A
wrist
L L
and
‐ ‐ ‐ ‐ ‐ ‐
L L L L
standard above above
on on on on
A A
O A O A elbow
L L
L L
the
Oldowan
‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐
on on at
0.132 0.054 0.096 0.059 O: above above A A ‐ ‐ ‐ ‐
F A A wrist Elbow
L L L L
‐ ‐ ‐ ‐ ‐ ‐ on on on on initiations
0.88 0.147 0.060 0.093 0.057
0.584 0.033 0.037 A A O O ‐ ‐ ‐ ‐ significant.
L L L
B
L
joint
are
L L Wrist
Elbow
on on
L L ‐ ‐
A
O on on
above 0.153 0.051
0.872 0.707 0.009 O O ‐ ‐ on 0.0005 on above
A results
O A A
Acheulean L L
L L
L L ‐ ‐ ‐ ‐ ‐ ‐ L L
Bolded
on on and
0.131 0.076 A A ‐ ‐ on on on above on above
O A A O A A
L L strike.
R R
Oldowan
‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 1 1 on on
0.104 0.048 0.140 0.074 above above ‐ ‐ ‐ ‐ A A
before
O O
E
standard
Wrist
Elbow L L R R
of
seconds
‐ ‐ ‐ ‐ 1 1 A on on on on
in 0.088 0.048 0.134 0.077 0.0001 Wrist
0.053 0.907 0.656 Elbow
O O ‐ ‐ ‐ ‐ O O < Timing
L
L L L
L L R R R R 5.15.
presented on on
on above on on on on on A O
above A O O A A
O O Table Time O
179
L L
‐ ‐ ‐ ‐ on on
123.6 212.91 A A ‐ L L L L
‐ ‐ ‐ ‐ ‐ ‐ on on on on
H 70.77 0.755 0.017 159.25 255.29 A A O O 180.95 ‐ ‐ Wrist Eblow D L L
Wrist swings Elbow
‐ ‐ on on L L L L
0.0001 0.0001 527.97
O O 237.71 ‐ on on < < on on
O A O A L L
knapping
L L L L ‐ ‐ ‐ ‐
above above 227.33 on on
465.62
on on
‐
A A O A O A L L L L
Acheulean ‐ ‐ ‐ ‐ ‐ ‐
on on on on
0.005 0.087 283.55 137.46 A A A A 138.43 148.76 ‐ ‐ and
G L L
L L Wrist Elbow
‐ ‐ ‐ ‐ on on
C 0.0001 0.0001 0.0001 0.0001 0.0001 above above 0.054 264.53 549.77
O O
178.02 626.49 Wrist ‐ ‐ Elbow < < < < < Oldowan O O
L L L L
L L L L L L
swings.
on on above above standard on on on above on above
O A O A O A A O A A L L L L
standard
during ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐
786.9 above above above above 665.98 ‐
246.27 453.75
‐ A A A A wrist
Acheulean L L L L
and
‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ A:
on on on on
0.0001 0.0001 0.004 0.002 313.26 242.37 A A A A
143.41 113.25 ‐ ‐ < < F elbow B
swings; wrist
Wrist Elbow Elbow L L L L
the
‐ ‐ ‐ ‐ 1 on on on on
at 0.36
0.0001 0.0001 0.0001 0.273 0.387 0.016 656.42 295.65
O O O O 296.17 282.73 ‐ ‐ < < < standard
(°/s)
L L L L
L L L L L L L L
Oldowan on on on on
on above on above on on above above
A O A A O O A A A A O A O:
velocities
L L R R
‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ on on on on
86.9 215.72 833.16 A A A A 673.59 ‐ ‐ angular
significant.
L L
R R
are
E A ‐ ‐ ‐ ‐ Peak on on
Wrist Wrist Elbow Elbow 0.0001 0.0001 above above 0.382 716.45 915.42
O O 494.01 657.59 0.0007 ‐ ‐ < < O O results
5.16.
R R L L R R L L
on on on on on on on on
Table Bolded O A O A O A O A
180
‐ ‐ ‐ ‐ 0.088 0.096 TRonL TRonL swing ‐ flexion ‐ ‐ ‐ ‐ ‐ ‐ D
extension
down
0.069 0.002 0.075 0.205 0.085 0.095 STonL STonL TRonL TRonL flexion to H
extension Wrist
Eblow ‐ ‐ Wrist 0.0001 0.0001 0.101 0.096 STonL STonL
Elbow < < STonL TRonL STonL TRonL transition
‐ ‐ ‐ ‐ 0.09 0.098 the TRonL TRonL
STonL TRonL STonL TRonL to
flexion C
‐ ‐ ‐ ‐ ‐ ‐ extension
0.09 0.08 0.112 0.091 0.001 0.002 STonL STonL STaboveL STaboveL relative
Wrist Elbow ‐ ‐ flexion G
extension
elbow 0.120 0.294 0.104 0.069 STonL STonL
STonL TRonL STonL TRonL Wrist and
Eblow ‐ ‐ ‐ ‐ ‐ ‐ 0.059 0.046 wrist
TRaboveL TRaboveL STonL STaboveL STonL STaboveL the
at
‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 0.0001 0.0001 0.145 0.136 0.075 0.075 TRonL TRonL
(°/s)
< < STaboveL STaboveL flexion B
extension
velocity
Wrist ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ Elbow 0.0001 0.0001 0.0001 0.0001 0.159 0.155 0.101 0.081 0.034 0.002 STonL STonL
< < < < TRaboveL TRaboveL angular
‐ ‐ ‐ ‐ peak
0.089 0.198 0.073 0.137 0.016 0.019 of
STaboveL STaboveL flexion STonL STaboveL TRaboveL STonL STaboveL TRaboveL F
extension
timing
Wrist ‐ ‐ ‐ ‐ ‐ ‐ Elbow the 0.09 0.23 0.0001
0.155 0.079 0.141 0.224 0.094 0.044 0.001 STonL STonL
TRonR TRonR < of
flexion A significant
extension ‐ ‐
0.08 0.0001 0.092 0.208 STonR STonR are
< Wrist Comparison
Elbow TRonL STonL STaboveL TRaboveL TRonL STonL STaboveL TRaboveL results
5.17.
Table Bolded STonR TRonR STonR TRonR
181
a
b
c
d
Figure 5.1. Experimental handaxes produced by Subject A (a), Subject B(b), Subject C (c), and Subject D (d). Both surfaces and the lateral view of each handaxe is shown.
182
183
184
Kinematics derived force Lithics derived force
700007000 Subject C 10000010000 600006000 Subject B Subject D 600006000 900009000 500005000 800008000 500005000 700007000 400004000 400004000 600006000 (N) 300003000 500005000 300003000 Force 400004000 200002000 200002000 300003000
200002000 100001000 100001000 100001000
0 0 00 00 1 11213141 1 112131415161 1 1121314151617 Knapping trial Knapping trial Knapping trial Figure. 5.4. Examples of the difference between strike forces derived from lithic data (yellow) and kinematic data (green) for subjects B, C, and D.
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Chapter 6: Conclusion
“The primary means of evaluating the accuracy of inferences of function from structure should be a comparison of functional predictions based on form to measured functions in living taxa. Only in a situation where we can measure actual movements, actions, and experimentally evaluate alternative functional hypotheses can we test predictions from structure,” (Lauder, 1995)
In Napier’s (1962b) discussion of the hand bones that make up OH 7, he wrote of the functional abilities of the hand based on the forms of the elements he analyzed.
Homo habilis, the ‘handy man,’ was determined to be capable of exercising a strong power grip and an imperfect precision grip. Moreover, he stated that Oldowan stone tool manufacture did not require a modern hand, or a precision grip. Since then, investigations into the tool-making abilities of fossil hominins have largely focused on whether they showed adaptations towards the execution of a modern precision grip. Such investigations presuppose that precision grips are integral in the production of stone tools.
However, quantitative data demonstrating that this is the case were not available until recently, rendering the foundation of such studies unstable. As Lauder (1995) argued, without observational data on the function of a structure, functional hypotheses are circular.
With this in mind, the goal of this dissertation was to investigate the kinematic strategies used by modern humans in the production of Early Stone Age stone tools in order to test the primary hypothesis that modern humans’ upper limb condition contributes to efficiency and accuracy during stone tool production. My collaborators and I used high-speed 3-D motion capture technology and a high-speed manual pressure
186 sensor system to capture some of the only quantitative data on knapping kinematics, and the only quantitative data on manual pressure distribution during stone tool production presently available. These data 1) do not support hypotheses directly linking modern human thumb anatomy to stone tool manufacture, 2) document that knappers employ a common kinematic strategy in the production of stone tools, a variant of which is widely used in high-velocity upper limb activities, 3) support hypotheses that wrist extension significantly contributes to effective stone tool manufacture, specifically efficiency and accuracy, and 4) provide evidence that large-scale motion sequences (e.g., changes in force application) rather than small scale motion sequences (e.g., sequence of joint motions) may contribute to greater right hemisphere activity during Acheulean handaxe manufacture compared with Oldowan flake production.
Our robust, relatively long thumbs and derived carpal orientation have lead many researchers to link the derived pollical condition to stone tool production, specifically core and hammerstone manipulation and force resistance (Napier, 1962b; Susman, 1994;
Marzke, 1997; Tocheri et al., 2007; Marzke et al., 2010). However, data generated during the course of this dissertation on pressure and normal force distributed across the hand during Oldowan stone tool manufacture do not support these hypotheses. The thumb did not receive higher pressure or normal force compared to the other digits in any of the analyses we conducted (i.e., peak pressure and normal force throughout the knapping swing, pressure and normal force at strike, impulse, and pressure-time integral).
Instead, peak pressure and normal force are experienced along the second and third metacarpals (Chapter 2). This pressure distribution is to be expected, in light of the knapping swing described in Chapters 3, 4, and 5. Rather than emphasizing radial and
187 ulnar deviation, as occurs during modern hammer use, the knapping swing emphasizes extension and flexion at the wrist. This necessitates a hand-hammerstone orientation that keeps the thumb from being caught between the hammerstone and the nodule at strike.
Thus, the thumb cannot be held in full opposition to the other digits around the hammerstone. Instead it is laterally rotated and used as a buttress. The hammerstone is held against the second and third metacarpals, which together bear the brunt of the force experienced throughout the knapping swing and at strike.
During stone tool manufacture, knappers across different skill levels conform to the same upper limb kinematic strategy. They employ a variant on the common proximal-to-distal joint sequence (PDJS), which has been documented during pitching, fly fishing, javelin throwing, piano playing (Bartlett et al., 1996; Hore et al., 2005; Wolfe et al., 2006; Furuya and Kinioshita, 2007; Furuya and Kinoshita, 2008b). During the modified PDJS used in stone tool production, joint motion initiations, peak linear velocities, and peak angular velocities, occur in a complete PDJS and the onset of peak angular velocities occurs in a modified PDJS. Velocity peaks at the shoulder first, followed by the wrist and then at the elbow joint last. This modification may arise from the anticipation of the forthcoming high-impact strike; however, additional experiments are needed to support or refute this hypothesis.
The PDJS offers three advantages over other motion sequences which contribute to efficiency and accuracy. Early proximal muscular and joint activation generates strong proximal muscular torques which produces beneficial interactive torques among distal muscles. This reduces the need for input towards muscular torque from small distal muscles that are prone to fatigue (Hirashima et al., 2003a; Hirashima et al., 2007). Early
188 proximal joint motion also lays the foundation for a velocity summation effect, which results in the generation of significantly greater angular velocity at the most distal joint than can be achieved using other joint sequences (Bunn, 1955; Putnam, 1991). In pitching, increased angular velocity contributes to an increase in ball release speed
(Debicki et al., 2004). In stone tool production, increased angular velocity contributes to increased strike forces (Eq. 1, 2, and 5 in Chapter 5).
Third, motion at the distal joint is refined through the PDJS in two manners.
First, distal muscular torque is reserved for the counteraction of passive interactive torques, which increases control at the distal joint (Hirashima et al., 2003a; Dounskaia,
2010). Second, the inherent joint separation of the PDJS increases the opportunity to control degrees of freedom along the limb (Bernstein, 1967). Thus subjects are able to aim from their distal element rather than their proximal element, thereby increasing accuracy (Chowdhary and Challis, 1999). This ability is particularly important when seeking accuracy given the disproportional influence proximal elements exert on motion compared with distal elements (Hore et al., 1996; Dounskaia, 2010).
The wrist is particularly important in the employment of a PDJS, and during stone tool production. By utilizing the upper range of wrist extension knappers 1) attain significantly greater linear and angular velocities at the wrist than they otherwise could
(Chapters 2 and 4); 2) reduce the risk of hyperextending at the wrist, which can lead to injury at the carpal region (Chapter 4) (Linscheid and Dobyns, 1985; Rettig, 2003); and
3) increase their strike accuracy by a) disassociating the wrist from the proximal upper limb elements (Bernstein, 1967; Chowdhary and Challis, 1999) and b) utilizing the most
189 stable plane of motion available at the wrist, known as ‘the dart thrower’s arc,’ (Chapters
4 and 5) (Palmer et al., 1985; Wolfe et al., 2006; Calfee et al., 2008).
After discussing OH 7’s physical attributes and their functional implications,
Napier (1962a) stated one of the issues that continues to confound investigations into the tool-making abilities of fossil hominins; namely that in tool manufacture there are both physical requirements and intellectual requirements of the maker. While it is difficult to investigate the primitive physical form and its function, this difficulty does not match that of investigating the primitive intellect and its capabilities and functions.
Napier’s (1962a) observation regarding the interplay between cognitive and physical ability in the production of stone tools is at the heart of discussions regarding the
Oldowan-Acheulean transition. The impetus for this transition is difficult to directly investigate given the nature of the evidence we have to work with. However, the differences in hemispheric activity observed during the production of Oldowan and
Acheulean tools addresses both variables (Stout and Chaminade, 2007; Stout et al.,
2008). The activation in Brodmann area 45—the right hemisphere homolog of Broca’s area—suggests that Acheulean handaxe production requires greater control of motion and motion sequences, task-set switching, and action inhibition. Yet, given that Broca’s area contributes to the control of both large scale language sequences (e.g., lexicon order) and small scale language sequences (e.g., phoneme sequences) (Démonet et al., 1992;
Démonet et al., 1994), it was unclear whether the activation of Brodmann area 45 referred to greater control of motion sequences on a large scale (e.g., the sequence of force application) and/or on a small scale (e.g., the sequence of joint motions).
190
Knapping kinematics captured during Oldowan bifacial chopper production and
Acheulean handaxe production indicate that different tool traditions do not necessitate
different kinematic strategies. The same modified PDJS is applied during both Oldowan
and Acheulean reduction sequences. This is to be expected in light of the myriad of other
activities that employ this upper limb sequence (Bartlett et al., 1996; Hore et al., 2005;
Wolfe et al., 2006; Furuya and Kinioshita, 2007; Furuya and Kinoshita, 2008b). Thus,
more complex and/or greater control of small-scale motion sequences are unlikely to
contribute to the observation of increased activity in the right hemisphere during
Acheulean handaxe production compared with Oldowan flake production (Chapter 5).
However, during handaxe production five of eight knappers applied significantly
lower strike forces compared with Oldowan bifacial chopper production (Chapter 5).
This may reflect 1) the increased need to produce flakes of a specific size and shape in
order to impose a pre-determined form on the core and 2) greater control of large scale
action sequences (Wynn, 1979; Wynn, 1985; Ambrose, 2001; Stout et al., 2008; Toth and
Schick, 2009). Thus, trimming swings and their interspersal with standard knapping swings may contribute to right hemisphere activity. The use of trimming in order to shape the core became a common practice during the Acheulean but has not been frequently recorded among Oldowan assemblages (Clark, 1994).
The data and conclusions produced during this dissertation begin clarifying the upper limb motions employed during Oldowan and Acheulean stone tool production. In doing so, my collaborators and I have 1) provided evidence against hypotheses directly linking the derived pollical condition to stone tool manufacture; 2) demonstrated that knappers employ a common kinematic strategy that has proven to be energetically
191
efficient in contemporary activities; 3) provided support that modern humans exploit the
upper ranges of their wrist extension ranges during knapping and in doing so achieve
greater accuracy and efficiency; and 4) provided evidence that large-scale motion
sequences (e.g., sequence of force application) rather than small scale motion sequences
(e.g., sequence of joint motions) contribute to greater right hemisphere activity during
Acheulean handaxe manufacture compared with Oldowan flake production.
This dissertation and the data collected in its course represent another step towards understanding the manner in which modern humans approach stone tool production. They also provide an illustration of why it is important to adhere to Lauder’s
(1995) recommendation to inform functional studies with direct observation of the behavior in question (whenever possible). My hope is that this work proves to be of use to future scientists investigating upper limb anatomy, stone tool production, and human evolution in general.
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