The Biomechanics and Evolution of High-Speed Throwing The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Roach, Neil. 2012. The Biomechanics and Evolution of High-Speed Throwing. Doctoral dissertation, Harvard University. Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:9822375 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA © 2012 Neil Thomas Roach All rights reserved. Advisor: Daniel Lieberman Neil Thomas Roach The Biomechanics and Evolution of High-Speed Throwing Abstract Throwing with power and accuracy is a uniquely human behavior and a potentially important mode of early hunting. Chimpanzees, our closest living relatives, do occasionally throw, although with much less velocity. At some point in our evolutionary history, hominins developed the ability to produce high performance throws. The anatomical changes that enable increased throwing ability are poorly understood and the antiquity of this behavior is unknown. In this thesis, I examine how anatomical shifts in the upper body known to occur during human evolution affect throwing performance. I propose a new biomechanical model for how humans amplify power during high-speed throwing using elastic energy stored and released in the throwing shoulder. I also propose and experimentally test a series of functional hypotheses regarding how four key shifts in upper body anatomy affect throwing performance: increased torso rotational mobility, laterally oriented shoulders, lower humeral torsion, and increased wrist hyperextensability. iii These hypotheses are tested by collecting 3D body motion data during throws performed by human subjects in whom I varied anatomical parameters using restrictive braces to examine their effects on throwing kinematics. These data are broken down using inverse dynamics analysis into the individual motions, velocities, and forces acting around each joint axis. I compare performance at each joint across experimental conditions to test hypotheses regarding the relationship between skeletal features and throwing performance. I also developed and tested a method for predicting humeral torsion using range of motion data, allowing me to calculate torsion in my subjects and determine its effect on throwing performance. My results strongly support an important role for elastic energy storage in powering humans’ uniquely rapid throwing motion. I also found strong performance effects related to anatomical shifts in the torso, shoulder, and arm. When used to interpret the hominin fossil record, my data suggest high-speed throwing ability arose in a mosaic-like fashion, with all relevant features first present in Homo erectus. What drove the evolution of these anatomical shifts is unknown, but as a result the ability to produce high-speed throws was available for early hunting and likely provided an adaptive advantage in this context. iv Table of Contents Abstract………………………………………………………………………………………………...... iii List of Figures………………………………………………………………………………………….. vii List of Tables…………………………………………………………………………………………… viii Acknowledgements…………………………………………………………………………………. ix Chapter 1 – An Introduction………………………………………………………………….. 1 1.1. Why throwing?......................................................................................................... 1 1.2. Thesis Overview...................................................................................................... 1 1.3. Literature Cited....................................................................................................... 6 Chapter 2 – The Effect of Humeral Torsion on Rotational Range of Motion in the Shoulder and Throwing Performance…………………….. 7 2.1. Chapter Summary……………………………………………………………………… 7 2.2. Introduction……………………………………………………………………………… 9 2.3. Model………………...……………………………………………………………………… 13 2.4. Materials and Methods…….………………………………………………………… 17 2.5. Results…………………………….………………………………………………………… 22 2.6. Discussion……………………….………………………………………………………… 25 2.7. Acknowledgements………….………………………………………………………… 31 2.8. Literature Cited……………….………………………………………………………… 32 Chapter 3 – Elastic Energy Storage in the Shoulder and the Evolution of High-Speed Throwing in Homo………………………………………………….. 37 3.1. Chapter Summary……………………………………………………………………… 37 3.2. Introduction……………………………………………………………………………… 38 3.3. Model………………...……………………………………………………………………… 39 3.4. Materials and Methods…….………………………………………………………… 40 3.5. Results…………………………….………………………………………………………… 44 3.6. Discussion……………………….………………………………………………………… 48 3.7. Acknowledgements………….………………………………………………………… 50 3.8. Literature Cited……………….………………………………………………………… 51 Chapter 4 – The Biomechanics of Power Generation during Rapid, Overhand Throwing in Humans…………………………………………………..… 54 4.1. Chapter Summary……………………………………………………………………… 54 4.2. Introduction……………………………………………………………………………… 55 4.3. Model………………...……………………………………………………………………… 58 4.4. Hypotheses……………….…….………………………………………………………… 60 4.5. Materials and Methods……….……………………………………………………… 65 4.6. Results…………………………….………………………………………………………… 71 4.7. Discussion……………………….………………………………………………………… 80 v 4.8. Acknowledgements………….………………………………………………………… 89 4.9. Literature Cited……………….………………………………………………………… 90 Chapter 5 – Throwing and the Evolution of the Hominin Forelimb……… 95 5.1. Chapter Summary……………………………………………………………………… 95 5.2. Introduction……………………………………………………………………………… 96 5.3. Who throws?……...……………………………………………………………………… 99 5.4. How do we throw?…………..………………………………………………………… 104 5.5. Who threw first?.............……….……………………………………………………… 110 5.6. Why do we throw?…….…….………………………………………………………… 135 5.7. What questions remain?…………………….………………………………………. 142 5.8. Acknowledgements………….………………………………………………………… 144 5.9. Literature Cited……………….………………………………………………………… 145 Appendix 1 – Supplemental Data for Chapter 2..................................................... 168 Appendix 1 – Supplemental Material for Chapter 3............................................ 169 vi List of Figures Figure 2.1: Humeral torsion vs. retroversion……………………………………………. 10 Figure 2.2: Predicting humeral torsion from rotational range of motion…… 15 Figure 2.3: Measuring humeral torsion using computed tomography............. 19 Figure 2.4: Measuring range of motion at the shoulder…….………………………. 20 Figure 2.5: Torsion prediction regressions………………...…….………………………. 24 Figure 3.1: Model of throwing differences between Pan and Homo…...………. 41 Figure 3.2: Shoulder and elbow power during throwing……………...…...………. 45 Figure 3.3: Brace-restricted power during throwing…………………...…...………. 47 Figure 3.4: Humeral torsion and throwing performance……………...…...………. 49 Figure 4.1: Free-Body diagram of the arm during cocking……….…...…...………. 59 Figure 4.2: Critical upper Body motions during throwing.…………...…...………. 61 Figure 4.3: Reflective markers and kinetic model…………......………...…...………. 66 Figure 4.4: Arm-cocking and acceleration phases…………......………...…...………. 70 Figure 4.5: Kinematic and kinetic data for 4 critical motions..……...…...………. 72 Figure 5.1: Chimpanzee throwing…………………………………….....……...…...………. 100 Figure 5.2: Arm-cocking and acceleration phases…………......………...…...………. 103 Figure 5.3: Critical upper Body motions during throwing.…………...…...………. 106 Figure 5.4: Free-Body diagram of the arm during cocking……….…...…...………. 108 Figure 5.4: Humeral torsion………………………………………….……….…...…...………. 125 Supplemental Figure 3.1: Reflective markers…...………….……….…...…...………. 170 Supplemental Figure 3.2: Kinetic model……...…...………….……….…...…...………. 174 Supplemental Figure 3.3: Residual analysis – full trial…...………….……….….... 177 Supplemental Figure 3.4: Residual analysis – critical period…......……….….... 177 Supplemental Figure 3.5: Phase duration………………………….…......……….….... 183 Supplemental Figure 3.6: Maximum Ball speed………………….…......……….….... 188 Supplemental Figure 3.7: Shoulder sham power……….……….…......……….….... 189 Supplemental Figure 3.8: Free-Body diagram of the arm during cocking…. 196 Supplemental Figure 3.9: Static optimization…………………………………………. 197 vii List of Tables Table 4.1: Segment definitions…………………..……………………………………………. 67 Table 4.2: Torso restriction data………………..……………………………………………. 73 Table 4.3: Clavicle restriction data………………..…………………………………………. 75 Table 4.4: Shoulder restriction data………………..………….……………………………. 77 Table 4.5: Wrist restriction data…….…………..……………………………………………. 79 Supplemental Table 2.1: Measurement reliability data…………………………… 168 Supplemental Table 3.1: Segment definitions…………….…………………………… 172 Supplemental Table 3.2: Joint definitions………………….…………………………… 175 Supplemental Table 3.3: Cutoff frequency data………….…………………………… 179 Supplemental Table 3.4: Shoulder muscle volumes…...…………………………… 185 Supplemental Table 3.5: Modeled power comparison….….……………………… 186 Supplemental Table 3.6: Chimpanzee throwing velocity….……………………… 191 viii Acknowledgements The process of completing a PhD is long, at times difficult, and could not be done alone. I owe a great debt of gratitude to many friends, colleagues, and mentors for their support and encouragement during the past seven years. You have made my time in graduate school interesting and enjoyable. Thank you! First and foremost, I would like to thank my advisor, Dan Lieberman. Over the last seven years Dan has been my strongest advocate, provided good