Force Measures at the Hand-Stick Interface During Ice Hockey Slap

Home , Ice hockey stick

Force Measures at the Hand-Stick Interface during Ice

Hockey Slap and Wrist Shots

Lisa Zane

Department of Physical Education and Kinesiology

McGill University

475 Pine Avenue West

Montreal, Quebec, Canada

H2W 1S4

May 2012

A thesis submitted to McGill University in partial fulfillment of the

requirements of the degree of Master of Science

Acknowledgements

I would like to thank a number of individuals who helped me kick-start this project when it seemed impossible, and see it to its end. First and foremost, I would like to thank my supervisor, Dr. David Pearsall, whose optimism, guidance, extensive knowledge, and encouragement were instrumental in the birth and progression of this project. I would also like to thank my “co-supervisor”, Dr. Rene Turcotte, for his input on this project, and for his impeccable shooting accuracy. Yannick Michaud-Paquette was an invaluable resource, as his technical expertise, creativity, patience, and ability to teach his many skills helped me on countless occasions when I saw no end in sight. Ryan Ouckama helped greatly in the development of the force sensing system used for this project and was always full of great ideas. I would also like to thank the rest of the Bmech Boys for the shooting competitions, the territorial lunch time hockey arguments, and also for their support and insight along the way. A big thank you goes to the McGill Martlets and Redmen, and every one of my participants, who were all very cooperative and expressed sincere interest in this study. I greatly appreciate the work of Lainie Smith, for her tenacity and willingness to help with the intricacies of this project. I must acknowledge

Bauer Hockey, Inc. for providing test sticks and financial support. Most

importantly, I would like to thank my family for giving me the freedom to pursue this degree, and for always having my back, even from across the country. I am so happy to have had the opportunity to belong to the Ice

Hockey Research Group – to me, the epitome of cool research.

Abstract

The purpose of this study was to quantify the contact forces between the lower and upper hands to the stick shaft during the ice hockey slap and wrist shot. Four cohorts (male/female x high/low calibre; HC, LC) of ice hockey players were tested using three sticks of different shaft bending stiffnesses (77, 87, 102). Forty-one subjects (21 male, 20 female) performed seven slap and seven wrist shots with each of the three stick types. Force at the stick-hand interface was recorded at 1000 Hz using 32 piezoresistive sensors about the shaft at the upper and lower-hand grip locations. The results demonstrated the feasibility for direct measurement of forces at the hand-stick interface while executing shooting tasks in ice hockey. As anticipated, peak forces acquired during both the slap and wrist shot differed by calibre and by gender, with males exhibiting higher forces than females, and HC players demonstrating higher forces than LC players, within each gender; however, stick type was not a significant factor. Notably, each player displayed unique, repeatable “force signatures”. In general, for both slap and wrist shots, grip force patterns demonstrated typical bimanual coordination patterns pertinent to understanding the mechanical dynamic control of the stick for effective performance.

Resumé

Le but de cette étude était d'examiner les forces de contact entre les mains et le baton de hockey pendant l’exécution de lancers frappés et de tirs du poignet. Quatre cohortes (homme/femme x haut / bas calibre;

HC, LC) de joueurs de hockey sur glace ont été testés en utilisant trois bâtons de rigidités différentes (77, 87, 102). Quarante et un sujets (21 hommes, 20 femmes) ont effectué sept lancers frappés et sept tirs du poignet avec chacun des trois types de bâton. La force de pression a été enregistrée à 1000 Hz en utilisant 32 capteurs piézorésistifs aux interfaces entre la main et le bâton. Les résultats ont démontré la faisabilité de la mesure directe des forces à l'interface main-bâton lors de l'exécution de tirs au hockey sur glace. Les forces de pointe atteintes au cours de lancers frappés et de tirs du poignet différaient en fonction du calibre et du sexe, les hommes présentant des forces supérieures comparativement aux femmes. De plus, les joueurs de HC ont démontré des forces supérieures par rapport aux joueurs LC. Par contre, le type de bâton ne représentait pas un facteurs significatif pour la production de force qui plus est, chaque joueur a affiché une ‘signature’ de force reproductible et constante. En général, les modèles de forces pour les lancers frappés et les tirs des poignets ont démontré des pattrons de

coordination bimanuelle à la compréhension du contrôle mécanique dynamique du bâton et possiblement a l’évaluation de la performance.

Table of Contents

Abstract...... 4

Resume...... 5

List of Tables...... 10

List of Figures...... 11

Chapter 1: Introduction...... 13

1.0 Introduction...... 13

1.1 Objectives and Hypotheses of Proposed Research...... 14

1.2 Limitations and Delimitations of this Study...... 15

1.2.1 Limitations...... 15

1.2.2 Delimitations...... 16

1.3 Operational Definitions...... 17

1.4 Contribution to the Field...... 23

Chapter 2: Review of Literature...... 24

2.0 Review of Literature...... 24

2.1 Hockey Stick Design...... 24

2.2 Stick Skill Classification...... 27

2.3 Ice Hockey Shots...... 28

2.4 Function of the Human Hand...... 35

2.5 Hand Grip...... 36

2.6 Grip Size and Force...... 39

2.7 Position/Posture of the Upper Limb...... 41

2.8 Instrumentation to Measure Grip Forces...... 43

2.9 Application in Sports...... 46

Chapter 3: Methods...... 50

3.1 Subjects...... 50

3.2 Measurement Equipment...... 51

3.2.1 Force Sensors...... 51

3.2.2 Sensor Map Configuration...... 52

3.2.3 Sensor Calibration...... 55

3.2.4 Shooting Test Surface...... 56

3.3 Task Protocol...... 57

3.4 Data Processing and Analysis...... 61

3.4.1 Data Processing...... 61

3.4.2 Research Design...... 63

Chapter 4: Results...... 66

4.1 Descriptive Statistics...... 67

4.2 Grip Strength Results...... 69

4.3 Results for Slap Shots...... 69

4.4 Results for Wrist Shots...... 81

Chapter 5: Discussion...... 93

5.0 General Discussion...... 93

5.1 Slap Shots...... 95

5.2 Wrist Shots...... 105

Chapter 6: Conclusions...... 109

References...... 113

Appendix I...... 126

Appendix II……………………………………………………………………………………...127

Appendix III...... 129

List of Tables

Table 1: Summary of puck velocities (km/h) reported by various studies (Adapted from Pearsall et al., 2000)...... 29 Table 2: Independent variables with respective levels...... 65 Table 3: Descriptive statistics based on calibre of player (x̄ ± SD)...... 65 Table 4: Male descriptive statistics based on calibre of player (x̄ ± SD)...... 68 Table 5: Female descriptive statistics based on calibre of player (x̄ ± SD)...... 68 Table 6: Tukey HSD comparisons for peak forces (N) at the upper and lower hands during the slap shot (group means and associated p-values)...... 75 Table 7: Tukey HSD comparisons for peak forces (N) at the upper and lower hands during the wrist shot (group means and associated p-values)...... 87 Table 8: Analysis of covariance summary (male slap shot)...... 129 Table 9: Analysis of covariance summary (female slap shot)...... 129 Table 10: Analysis of covariance summary (male wrist shot)...... 129 Table 11: Analysis of covariance summary (female wrist shot)...... 129

List of Figures

Figure 1: Subject positioning for grip strength test using an electronic hand dynamometer in the supinated hand position...... 18 Figure 2: The phases of the slap shot include the backswing (a), downswing, (b) preloading, loading (c), release and follow through (d)...... 19 Figure 3: The phases of the wrist shot includes draw back (optional), blade positioning (a), loading, pushing (b), and follow through (c)...... 20 Figure 4: Upper (a) and lower (b) hands grasping the stick shaft...... 20 Figure 5: Stick surface nomenclature...... 21 Figure 6: The basic components of a hockey stick (Adapted from Magee, 2009)...... 25 Figure 7: Ice hockey skill classification, focusing on stick skills (Magee, 2009) adapted from (Pearsall et al., 2000)...... 28 Figure 8: Sleeve constructed from shrink wrap and Velcro® straps…………...... 53 Figure 9: Sensor configuration layout for both the mid-shaft and butt-end region of the stick...... 53 Figure 10: Sensor configuration for lower hand (left) and upper hand (right) on stick...... 54 Figure 11: Fully-instrumented stick with mid-points of each sensing region visibly marked…………………………………………………………………………………………....54 Figure 12: Sample sensor calibration (force vs. voltage) for one sensor with line and equation of best regression fit...... 56 Figure 13: Experimental Setup on Synthetic Ice Surface (floor plan)...... 57 Figure 14: Subject positioning for grip strength test using an electronic hand dynamometer in the supinated hand position...... 58 Figure 15: Subjects were asked to align the second finger with the mid-point line on the mid-shaft sensing region (left to right: lower hand initial wrap around shaft; second finger aligned to mid-sensor region; full grip)...... 59 Figure 16: Testing set-up with subject holding instrumented stick (with cabling feeding into backpack to the amplifier, then exiting to DAQ board and PC)...... 60 Figure 17: Each sleeve of sensing arrays was able to slide off of one stick model in order to be inserted onto the next...... 61 Figure 18: Example of data visualization interface in LabView™. Top left panel shows views of sensors 1-16; top right panel shows views of sensors 17-32, real-time………...62

Figure 19: Format of experimental design………………………………………..……..……64 Figure 20: Male grip strength vs. slap shot velocity, with calibre of player indicated…….71 Figure 21: Female grip strength vs. slap shot velocity, with calibre of player indicated....71 Figure 22: Peak total force* means for slap shots, separated into calibre and gender, for the upper hand (a) and lower hand (b)………………………………………………………..73 Figure 23: Colour map representing fastest male slap shot trial (118 km/h) (a) and slowest male slap shot trial (65 km/h) (b)...... ……………………………………..77 Figure 24: Colour map representing fastest female slap shot trial (100 km/h) (a) and slowest female slap shot trial (36 km/h) (b)...... 78 Figure 25: (a) An overlay of 7 slap shot trials (SS87) of one subject...... 80 Figure 26: Male grip strength vs. wrist shot velocity, with calibre of player indicated……82 Figure 27: Female grip strength vs. wrist shot velocity, with calibre of player indicated...82 Figure 28: Peak total force* means for wrist shots, separated into calibre and gender, for the upper hand (a) and lower hand (b)...... 84 Figure 29: Colour map representing fastest male wrist shot trial (105 km/h) (a) and slowest male wrist shot trial (45 km/h) (b)…………………………………………………….89 Figure 30: Colour map representing fastest female wrist shot trial (79 km/h) (a) and slowest female wrist shot trial (29 km/h) (b)………………………………………………….90 Figure 31: An overlay of 7 wrist shot trials (WS77) of one subject………………...... 92 Figure 32: Lower hand in contact with the shaft. Typically, the large thenar muscles of the thumb were located on the toe up surface with the hand palm and hypothenar muscles pressing on the lagging surface. Unknown are the forces at corners (dashed arrows)....96 Figure 33: Top view stick position as seen by player in (a) neutral, and (b) closed-blade due to forearm pronation………………………………………………………………………..97 Figure 34: Typical HC slap shot: hand and stick side forces and relative timing…………99 Figure 35: Different views of upper and lower hand and stick side “cupping” forces at (a.) ice contact and (b.) stick recoil time window boxes (from Fig 33 above)………………..100 Figure 36: Upper and lower hand forces (total) vs. Time. Colour map representing differences in magnitude and timing coordination in 3 male (a) HC subjects vs. (b) LC subjects………………………………………………………………………………………….101 Figure 37: Interpretation of forces about the stick (1. upper hand, 2. lower hand and 3. blade)………... …………………………………………………………………………………104 Figure 38: Representative trials of four subjects with similar wrist shot timing parameters and force magnitudes in the upper toe down and lower toe up regions (see arrows)….107 Figure 39: Male slap shot velocity frequency distribution………………………………….126

Chapter 1: Introduction

1.0 Introduction

Hockey has developed from a grassroots recreational activity of the first nations to the technical sport that it is today via grand changes in equipment, facilities, rules and the way modern athletes train (Hannon,

2010). During the 2010-2011 season, 486 594 males and 85 827 females were registered in Canadian hockey leagues (Hockey Canada, 2011).

According to the International Ice Hockey Federation (2011), 1 549 975 players worldwide are registered to play in hockey leagues. Despite these numbers, a limited amount of research has focused on a biomechanical analysis of ice hockey (Magee, 2009).

In particular, biomechanical studies of hockey sticks have not been widely published, despite the number of tasks the hockey stick is used for

(Pearsall et al., 2000). The hockey stick is a tool used to extend one’s arms that, with respect to shooting skills, affords greater leverage to in turn generate greater distal end swing velocities to ultimately project the puck

(Pearsall et al., 2000). It is a bimanual tool that, in addition to shooting, is used to control the puck’s position while conducting a variety of tasks (i.e. skating, passing, etc.). In sports where implements are used, the point(s) of contact between it and the participant’s hand(s) is mediated by the grip

(Komi et al., 2008). Often taken for granted, exactly how hockey players grip the hockey stick is an aspect of the game that has not been investigated specifically. Insight from other research fields may be gleaned, for examples, on different facets of grip for ergonomic purposes

(object circumference, posture of the upper limb, etc.), the effectiveness of different types of instrumentation to measure grip force, and the gripping of instruments used in sports such as tennis, baseball and golf. By and large, grip properties are crucial in determining the effective manipulation of the implement. Hence, the ultimate goal of this study was to better understand the nature of the hand(s) grip on the hockey stick with specific respect to the interface forces that occur during wrist and slap shots.

1.1 Objectives and Hypotheses of Proposed Research

The objective of this study was to identify both the magnitudes and regions of forces that occur between the hands and the stick during the wrist shot and slap shot. In addition to shot type (slap or wrist), several factors that could potentially modify force profiles were examined; player calibre (high and low), gender (male and female), and stick shaft stiffness (77, 87, 102 flex). Hypotheses related to this study are as follows:

Force measures will be:

1. Greater for HC vs. LC players, within each gender

2. Greater for males vs. females, within each calibre

3. Positively correlated with shot velocity

Within each shot type (slap and wrist), force magnitudes by hand region and stick surface will be:

1. different between gender and calibre;

2. the same between stick types;

1.2 Limitations and Delimitations of this Study

1.2.1 Limitations

• The study was conducted under laboratory conditions (i.e. not in a

game-setting)

• Grip force was recorded for slap and wrist shots (no other tasks

were executed with the stick, i.e. stickhandling)

• Shots were taken on an artificial ice surface consisting of low

friction polyethylene sheets

• The laboratory experiments were conducted at room temperature

(22-24°C)

• Sensor area resolution (i.e. the sensors did not cover the entire

surface of the stick)

• Subjects used sticks that they did not necessarily use on a regular

basis (i.e. different blade pattern, flex, and/or brand of stick)

• The sticks used in the study were instrumented (i.e. increased

weight, wiring, etc.)

• Lower and upper hand position were fixed during the actual

shooting task (i.e. the hands will not be able to slide up or down the

shaft during the execution of a shot)

• The forces recorded occurred solely at the stick-hand interface (a

combination of grip, hand closure, and the forces of the stick

against the hand) during each shooting task

• The timing of interface forces was not measured simultaneously

with an accelerometer, high speed video, or motion capture system

(making it impossible to definitively identify discrete phases of the

shot)

1.2.2 Delimitations

Subjects:

• were between 18-41 years of age

• used sticks of fixed length (166 cm from end of handle to blade tip)

• were between 167 cm and 187.5 cm tall (to match stick length)

• wore their own hockey skates

• did not wear hockey gloves, or any other protective equipment

(gloves were not used due to inability to standardize across

subjects and monitor effect modifications)

• only executed stationary shots (i.e. not while skating in stride)

• executed shots from a distance of 3.5 m at a 90° angle from the

center of the net

• used one stick model (Bauer Vapor X60)

• used 3 stick types within the Bauer Vapor X60 model, each with a

different stiffness (77,87,102 flex)

1.3 Operational Definitions

Grip strength Amount of force measured by an electronic hand

dynamometer (Logger Lite™, Vernier Software and

Technology, Beaverton, OR) from the lower shooting hand

(i.e. if the subject used a left-handed stick, the grip strength

was taken from the left hand) in a supinated position with

the elbow in full extension (in a position to mimic and

approximate the stick posture) (Figure 1).

Figure 1: Subject positioning for grip strength test using an electronic hand dynamometer in the supinated hand position.

Skill level Subjects were divided into “high calibre” (HC) and “low

calibre” (LC). Skill level was stratified post-hoc according to

slap shot velocity. Males with slap shot velocities of 95 km/h

or greater were defined as HC; under 95 km/h were defined

as LC. Females with slap shot velocities 70 km/h or greater

were defined as HC and under 70 km/h were defined as LC

(Appendix I).

Slap Shot A method of shooting the puck that involves grasping the

stick with both hands approximately 40-60 cm apart and

guiding the stick through a rapid perpendicular path from

side to side (Pearsall et al., 1999). This skill may be divided

into six distinct phases: backswing, downswing, preloading,

loading, release, and follow-through (Pearsall et al., 1999).

The slap shot is used for maximum velocity (Figure 2).

Figure 2: The phases of the slap shot include the backswing (a), downswing, (b) pre-loading, loading (c), release and follow through (d).

Wrist Shot A method of puck shooting that involves less swing than the

slap shot. The hands are held 0.15-0.30 m apart on the stick,

and the stick blade contacts the puck while the stick is

swung forward in a snapping or pushing motion to propel the

puck. The wrist shot is used for maximum accuracy (Wu et

al., 2003). The wrist shot may be divided into 3-4 phases:

draw back (optional), blade positioning, loading, pushing,

and follow through (Wu et al., 2003) (Figure 3).

Figure 3: The phases of the wrist shot includes draw back (optional), blade positioning (a), loading, pushing (b), and follow through (c).

Upper hand The hand placed near the top (butt-end) region of the stick

shaft (Figure 4).

Lower hand The hand placed near the mid region of the stick shaft

(Figure 4).

Figure 4: Upper (a) and lower (b) hands grasping the stick shaft.

Stick Surfaces (faces) (Figure 5):

Leading (Lead) Side of the front (concave) face of the stick blade.

Lagging (Lag) Side of the back (convex) face of the stick blade.

Toe up (TU) Side of the toe of the blade.

Toe down (TD) Side of the heel of the blade.

Figure 5: Stick surface nomenclature (transverse plane).

Force Profile The force distribution across the four stick

surfaces (faces): leading, lagging, toe up and

toe down, over time.

Upper Total Force Summation of forces from upper leading, upper

lagging, upper toe-down and upper toe-up stick

surfaces for the entire shot duration.

Lower Total Force Summation of forces from lower leading, lower

lagging, lower toe-down and lower toe-up stick

surfaces for the entire shot duration.

Mean Force (Slap Shot) Mean force obtained during a 100 ms window

(50 ms before and 50 ms after lower toe-up

maximum force value). This time frame was

chosen based on the nature of the skill and

based on pilot testing.

Mean Force (Wrist Shot) Mean force obtained during a 200 ms window

(100 ms before and 100 ms after lower toe-up

maximum force value). This time frame was

chosen based on the nature of the skill and

based on pilot testing.

Hand Length Distance from the metacarpophalangeal wrist

crease to the most forwardly projecting point on

the middle finger.

Hand Breadth Distance from the radial side of the second

metacarpophalangeal joint to the ulnar side of

the fifth metacarpophalangeal joint.

Wrist Shot Grip Distance Distance between the mid-points of each

sensing region during the wrist shot.

Slap Shot Grip Distance Distance between the mid-points of each

sensing region during the slap shot.

Playing experience Number of years playing ice hockey.

1.4 Contribution to the Field

Though shooting and puck control are obvious fundamental skills in the game of ice hockey, the specific nature of interactions between the stick and hands has not been rigorously examined. These power and accuracy skills often improve with trial-and-error practice, though the explicit motor control strategies for these bimanual tasks are not known, nor how they adapt to the dynamic environment context (e.g. relative distance and angle to net; approach speed, goalie position to net, etc.) . Observations from this study will provide insight pertaining to the nature of bimanual force application during these power tasks in a wide range of subjects (ranging in calibre and gender), and, in turn, may help identify potential ergonomic changes in equipment used (i.e. stick, glove properties) and cues for stick manipulation, possibly to help to improve grip behaviour to induce faster and more accurate shooting.

Chapter 2: Review of Literature

2.0 Review of Literature

Given the study’s goal to examine grip forces during hockey shooting tasks, an examination of background literature addressing the fundamental aspects involved in this task is merited. In this review chapter, the topics to be presented will include the:

• evolution, design and material construction of the ice hockey stick,

and mechanics of ice hockey shooting skills;

• ergonomics of hand grip and tool use; and,

• hand grip biomechanics in sports.

2.1 Hockey Stick Design

The materials and construction of hockey sticks have evolved considerably since the 1880’s (Pearsall et al., 2007). Hockey sticks were initially produced in one-piece from hardwood, then later from softwood two- and three-part designs in the 1920’s. Subsequently, other materials were adopted, such as laminates and fiberglass composites between the

1960’s and 1970’s, aluminum shafts in the 1980’s, and carbon fiber composites in the 1990’s (Pearsall et al., 2007). Currently, official rules stipulate the specific dimensions the shaft and blade must fall within, with both the National Hockey League (NHL) and International Ice Hockey

Federation (IIHF) stating that sticks shall be made of wood or other material approved by the governing league (NHL.com, 2011; IIHF, 2010).

The hockey stick is composed of the following features (Figure 6): shaft, blade (including toe, heel), hosel (junction of blade and shaft), lie angle (between blade and shaft) and butt (top of shaft). The blade was originally flat but since the 1960’s has different forehand curve patterns, thereby creating either left or right hand sticks (Pearsall et al., 2007).

Figure 6: The basic components of a hockey stick (Adapted from Magee,

2009).

Characteristics of the hockey stick include shaft’s length, cross- section dimensions (minor and major axes), blade pattern (length, thickness, curvature, lie angle), as well as stick center of mass and mechanical bending and torsional stiffness and elastic properties (Pearsall et al., 2000). Various sticks have been produced with different shaft characteristics (i.e. rounded corners, concave shape of the shaft face), thought to improve the “feel” of the stick; however, no research exists to substantiate these performance claims (Anderson, 2008).

The hockey stick has to match the player’s size, strength, preferred carrying (and shooting) side and playing style (Pearsall et al., 2007).

Marino (1998) evaluated characteristics of various types of hockey stick shafts that contribute to the feel and performance capacity of the stick. It was surmised that the selection and use of a stick depends on several factors such as cost, appearance, feel, performance and durability. For elite players specifically, feel and performance were predicted to be the most significant factors. At first glance, this seemingly simple implement may not appear to offer a lot in terms of scientific investigation; however, the many variables involved in stick construction and use make it a prime topic for ice hockey research. In general, optimal upright stick length while wearing skates extends up to the player’s chin (Pearsall et al., 2007), though players often have slightly longer or shorter preferred stick lengths depending on their playing position; for example, some defensemen prefer longer sticks to increase their poke-checking ability.

The IIHF outlines the allowed stick dimensions in the most current rulebook, valid for 2010-2014. Stick shafts can be a maximum length of

163 cm, maximum width and thickness of 3 and 2.5 cm, respectively. The blade must be 32 cm or smaller and between 5.0 to 7.5 cm wide. The curvature of the blade must not exceed 1.5 cm (IIHF, 2010). The NHL has

similar guidelines, citing a maximum stick length of 63” (though exceptions for players over 6’6” is allowed), and blade dimensions of 2 to 3 inches in width and maximum curvature of 0.75 inches (NHL.com, 2011)

A variety of blade patterns currently exist defined by their curve, face, toe, length, and lie specifications. Again, players will have their own blade pattern preferences, though in terms of slap shot skills difference in these patterns have shown to have minimal effect on shot velocity

(Lomond et al., 2007).

2.2 Stick Skill Classification

The execution of many different skills executed both individually and simultaneously is necessary in ice hockey. The required skills can be categorized into skating, checking and shooting (Pearsall et al., 2000).

The ice hockey stick is used primarily for shooting and passing, checking and face offs (Marino, 1998). The function of the hockey stick and the skills associated with it are of great importance to the overall success of a player or team (Pearsall et al., 2007). A survey of over 900 NHL scouting reports ranked three skills associated with the stick (shooting/scoring, puck control, and passing) in its top ten list of skills/attributes in both forwards and defensive players (Renger, 1994). Figure 7 provides a categorical breakdown of hockey skills, with a focus on stick skills.

Figure 7: Ice hockey skill classification, focusing on stick skills (Magee,

2009) adapted from (Pearsall et al., 2000).

Despite the large number of tasks executed using the hockey stick, the majority of existing literature has focused solely on shooting—there has been virtually no research published concerning puck control, and this field remains a viable area for future investigation.

2.3 Ice Hockey Shots

The slap and wrist shots are two of the primary shots performed in ice hockey. The slap shot is generally used to produce maximum puck velocity, while the wrist shot is used for higher accuracy (Wu et al., 2003).

A summary of reported shooting velocities is presented in Table 1. 28

Differences in measurement techniques, subject age and skill level, shooting protocol, shooting surface, and stick instrumentation potentially accounted for the variation of reported shot velocity values.

Table 1: Summary of puck velocities (km/h) reported by various studies (Adapted from Pearsall et al., 2000)

Studies Method Velocity Age Slap Wrist

Skate Stand Skate Stand

Alexander et al. 1963 Ballistic Impact Adult 127 111 117 97

Alexander et al. 1964 Ballistic Impact Varsity 121 114

Cotton 1966 Adult 100 90 90 81

Furlong 1968 Stop watch Avg Pro’s 175 163

Chau et al. 1973 Cine Instant Adult 132 110 143 132

Roy et al. 1974 Cine Avg Junior B 89 92 81 64

Roy and Doré 1976 Sound Avg Pee-wee 69

Adult 96

Doré and Roy 1976 Sound Avg Adult 104 97

Simm and Chau 1978 Cine Max High school 150

Adult 200

Rothsching 1997 Radar Max Varsity 108

Wu et al. 2003 Radar Max Varsity (rec and 76 52 elite, male and

female)

Lomond et al. 2007 Radar Avg Varsity (male rec 70 and elite)

Hannon 2010 Motion Max Varsity (male rec 89 80 capture and elite)

The slap shot is a skill that involves the use of the ice hockey stick and a hard-rubber cylindrical puck (thickness= 2.54 cm; diameter= 7.62 cm; mass= 156-170 g) (Pearsall et al.,1999). This shot involves grasping the stick with both hands 0.40 to 0.60m apart, initiated by raising of the 29

stick backwards and then a swing forwards with maximum effort to make blade contact with the puck at upwards of 100 km/h (Wu et al., 2003). The phases of the slap shot include the backswing, downswing, pre-loading, loading, release and follow through (Wu et al., 2003). This swing is comparable to the golf club swing but without the same duration or length of backswing, with the puck being contacted only momentarily to propel it forward (Hoerner, 1989).

Unlike the golf drive, where an impact occurs exclusively between the club’s head and ball, in the ice hockey slap shot the stick makes contact with the ice first, storing elastic bend energy in the shaft that may be returned to the puck during the stick’s recoil. During the preloading phase, the blade of the stick makes contact with the ice surface (creating the slap sound) and precedes puck contact (by approximately 0.15 to 0.30 m), and stick shaft bending is initiated by the coupled loading from the ice

(ground) reaction force and the downward pressing of the lower hand on the shaft (Villaseñor et al., 2006). The stick bending may be modeled as a cantilever beam with three point loads: the upper hand and ice acting as the outside constraints, while the lower hand applies the central load

(Bigford and Smith, 2009).

Roy and colleagues (1974) studied Junior B calibre hockey players and found that 40-50% of the shot velocity was attributable to the deflection of the shaft. Higher velocities could be expected due to improvements in the construction of hockey sticks, coaching, and hockey- specific training programs (Pearsall et al., 2000).

The wrist shot does not involve as substantial a swing as the slap shot, and the hands are placed closer together (0.15-0.30 m apart) on the stick (Wu et al., 2003). The blade of the stick starts in contact with the puck and is pushed rapidly forward, ending with a quick wrist “snap” and follow-through for maximum velocity of the shot (Hoerner, 1989) upwards of 70 km/h (Wu et al., 2003). In comparison to the slap shot, Roy et al.

(1974) stated 25-34% of the puck velocity was due to shaft deflection.

Several factors are commonly thought to influence the outcome of the shots such as skill level, body strength, stick material type, and ice surface condition (Wu et al., 2003). Pearsall et al. (1999) examined the influence of stick stiffness on the performance of slap shots. Six elite male ice hockey players performed six slap shot trials with each of four sticks.

Stick stiffness was found to have a minimal effect on shot velocity. The results suggested that the subjects’ behavior is more important in

determining shot velocity than the range of stick flex properties commercially available.

Expanding on the work of Pearsall et al. (1999), Wu and colleagues

(2003) studied the effects of stick construction and player skill in the performance of the slap and wrist shots with a large sample size (20 males and 20 females). In general, they found that shot velocity depended more on subject skill level than on the actual construction of the stick. For instance, a skilled player produced the equivalent maximum puck velocity using different stick stiffnesses. Villaseñor et al. (2006)’s high speed video study of stick bend behavior supported these observations that human experience / skill accounted more for shot speed than stick flex properties.

Furthermore, Worobets et al. (2006) observed similar trends for the slap shot in so far that how the athlete loaded the stick had as much influence on puck speed as stick construction. Selection of sticks by stiffness may be more important in terms of complementing a player’s maturity and / or muscular strength as reported by Roy & Dore (1976)’s high speed cinematography study of 11-12 year-old boys in hockey, and similarly proposed by Milne and Davis (1992) for golf drives.

The above findings were in part explained by Woo and colleagues’

(2004) investigation of body kinematics during stationary slap shots.

Whereas recreational players tended to utilize only pendular movements, elite players combined both rotational and translational stick motions, yielding a net higher puck velocity; plus, elite players showed distinct proximal-to-distal segment temporal movement ordering. The latter is consistent with general movement theory for optimal throwing and racquet/club skills (Tinmark, 2007; Zheng et al., 2008; Joris et al., 1985;

Pan et al.,1998; Lacroix et al.,1998). Furthermore, more skilled players are thought to better take advantage to the elastic and “kick-point” properties of the stick by means of strategic hand placement (Milne &

Davis, 1992; Anderson, 2008; Bigford and Smith, 2009;). For example,

Villaseñor and colleagues (2006) speculated that elite players’ greater shot velocity must be related to optimal force placement along the shaft distributed between top and bottom hands, so as to best “tune” the stick’s recoil response. This concept was supported by studies using dynamic shaft strain measurements slap shots (Roy and Doré, 1975; Hannon et al.,

2011) in that higher calibre players were able to generate both greater shaft bend and recoil strains due to precise “force signatures” on the stick shaft.

With regards to wrist shots, Michaud-Paquette et al. (2008) used

3D kinematic tracking of the ice hockey stick of good and poor shooters,

and identified specific movement traits that corresponded to shot target accuracy: key stick release orientation and velocity, shaft bending, and change in blade orientations. Michaud-Paquette et al. (2011) expanded on this, conducting the first comprehensive study focusing on whole-body kinematics of the wrist shot and how these might influence resulting accuracy outcomes. The results of this study indicated that there was not one specific predictor of wrist shot accuracy—instead, there were a number of characteristics that appeared to indicate accuracy. In particular, more dynamic use of the lead arm (especially at the wrist and shoulder joints) was associated with greater accuracy. Evaluating exactly how players shoot accurately in ice hockey is a complicated matter that leaves ample room for future study.

These studies have pointed researchers in the direction of evaluating facets of player skill more in-depth. Exactly what is happening at the points of force application on the stick by way of the hands is an area that may yield more insight as to exactly how these skills are being executed. Output variables such as vertical force, stick bending, and puck velocity have been examined but the magnitude and timing of input forces exerted by the hands are currently unknown.

2.4 Function of the Human Hand

Since the only point of contact between the player and the hockey stick are the hands, it is important to examine their anatomy. The hand is a complex anatomical system of dynamic and static structures (Moran,

1989). The human hand has 27 bones and 15 joints.

In total, the fingers have 21 Degrees of Freedom (DoF; Lin et al.,

2000). The rotational and translational motion of the palm contribute 3 DoF each, resulting in the hand having a total of 27 DoF (Lin et al., 2000).

Although the human hand is highly articulated, it has many constraints

(dependencies among fingers and joints) (i.e. constraints of joints within the same finger, constraints of joints between fingers, and the maximum range of finger motions) (Lin et al., 2000). For instance, it is natural to most people to bend (flex/extend) their fingers such that both PIP and DIP joints flex/extend. Also, there is only a certain range of angles that the hand joints can naturally assume (Pavlovic et al., 1997).

The complex apparatus of the human hand is used both to grasp objects of all shapes and sizes through the linked action of multiple digits and to perform the skilled, individual finger movements needed for a large variety of creative and practical endeavors, such as handwriting, painting, sculpting, and playing a musical instrument (Schieber & Santello, 2004). In

addition to these tasks, the hands are also used to communicate, such as using hand gestures (i.e. sign language), and to manipulate implements in work and sports settings. Although complex, the numerous structures are arranged to maximize function (Moran, 1989).

The structure of the hands has played a key role in tool design throughout history. Tool designers often work on the assumption that ‘one size fits all’, when in fact there is a large variation in hand anthropometry over the working population (Buchholz et al., 1992). The hand is used dynamically, and humans seek to optimize its geometry continuously

(Nikonovas et al., 2004). The interaction of handle size and shape with the kinematics and anthropometry of the hand have a great effect on hand posture and grip strength (Buchholz et al., 1992).

2.5 Hand Grip

In order to exploit the hand to greatest effect, it is necessary to understand the types and magnitudes of forces that it is capable of generating, and for what duration these are sustainable. Such ergonomic considerations can aid the design of tools or other products with which hands interact

(Nikonovas et al., 2004).

In any grip action, a balance must be found between the force used to secure the object in the hand and wrist range of motion, as the two are

inversely related (i.e. an increase in grip force produces a decrease in wrist range of motion) (Komi et al., 2008). Grip pressure is developed by activating the muscles in the lower arms through the tendons in the wrist

(Budney, 1979). Gripping actions require simultaneous external forces to act upon two or more segments of each of the fingers as they cooperate to produce the desired function (Amis, 1987). Napier (1956) investigated human hand movements as a whole and was the first to apply the now commonly used classifications of “power” and “precision grip” to gripping actions. In the power grip, the ring and little fingers, assisted by the long and index fingers as necessary, form a mobile jaw to squeeze objects against the palm of the hand and the thenar eminence, with the thumb and index finger providing any necessary precision on the grip (Hazelton et al.,

1975). Precision grip is executed between the terminal digital pad of the opposed thumb and the pads of the fingertips, and is employed when delicacy of handling and accuracy of instrumentation are essential and power is a secondary consideration. Large objects held in this way involve all the digits, but smaller objects require only the thumb, the index, and the middle digits (Napier, 1993). These two types of grip are not mutually exclusive, and overlap can occur during specific tasks. There are an

almost infinite number of phases within these two grip complexes (Napier,

1956).

Effective prehension requires that we stabilize the object within our grasp as we move the object or use it as a tool (Johansson & Cole, 1994).

We first adopt an appropriate grip configuration (i.e. power or precision)

(Napier, 1956) based on the properties of the object and nature of the task the object will be used for. In order to prevent slips and accidental loss of the object, we must maintain adequately large grip forces in relation to destabilizing load forces and the frictional conditions at the skin— object contact patches (Johansson & Cole, 1994). Meanwhile, exceedingly large forces must be avoided as they may cause unnecessary fatigue, may crush fragile objects, and impede further manipulation (Johansson & Cole,

1994).

Although hand kinematics has proved to be a very useful avenue for studying hand control and its underlying neural mechanisms, further progress will be made by combining complementary research techniques, in particular digit force measurements, to fully understand the relation(s) between hand configuration and forces generated for skilled object manipulation (Lukos et al., 2008). Though there is a large body of research concerning different facets of hand grip from an ergonomics

perspective, there has been limited insight regarding the gripping of rectangular objects. In particular, there is virtually no literature with respect to gripping the ice hockey stick.

2.6 Grip Size and Force

Grip strength is an index of the power the hand can exert, and maximum grip strength is an important predictor for hand function (Roman-Liu,

2003). There is strong evidence that grip strength is correlated with body weight, age, height, and gender (Thorngren & Werner, 1979; Koley et al.,

2008; Ager et al., 1984, Newman et al., 1984; Leyk et al., 2007; Enemark-

Miller et al., 2009, Kong & Lowe, 2005, Frederiksen et al., 2006).

With respect to gender, in particular, studies have consistently shown that females cannot exert as much hand grip force as males.

Having a smaller physiologic cross-sectional area (PSCA) of the muscle than males, females will theoretically generate smaller amount of force than males (Morse et al., 2006). In addition, women tend to have a lower portion of their lean body mass located in the upper body (Miller et al.,

1993). Female hand grip force has been reported to be between 50-74% that of male hand grip force (Kong & Lowe, 2005; Mathiowetz et al, 1985;

Hallbeck and McMullin, 1993).

Leyk et al. (2007) investigated hand grip force in untrained men and women, and in a group of highly trained female athletes from sports known to require high hand grip strength (judo and handball) and found that 90% of females in the study produced less force than 90% of males. The results suggest that the differences in hand grip strength between men and women are larger than previously reported, and that the strength level attainable for women via extremely high training will rarely surpass the 50th percentile of untrained men. In addition, mean maximal hand grip force differed substantially—by more than 200 N.

Many investigations have focused on determining the most effective cylindrical circumference for gripping. For instance, Gutierrez & Castillo

(2002) evaluated measures for optimal circumference of a hand-grip dynamometer in order to obtain maximal grip strength and developed an algorithm to determine optimal grip span (y=0.1997x + 1.4798, where y= optimal grip span and x= hand size). Kong & Lowe (2005) tested maximum grip force on cylindrical aluminum handles differing in diameter to evaluate relationships between handle diameter, perceived comfort, finger and phalange force distribution, and EMG efficiency of finger flexor and extensor muscle activity and found that the optimal handle was 19.7% of the user’s hand length. The general trend in optimal cylindrical grip

strength is represented as an inverted-U relationship, with optimal grip falling in the middle ranges of cylindrical diameter (3.8 cm). This relationship has been demonstrated by Pheasant & O’Neill (1975), Edgren et al. (2004), Kong & Lowe (2005), and Blackwell et al. (1999). Reported optimal cylindrical diameters for gripping tasks appear to range from 3-5 cm (Pheasant & O’Neill, 1975; Ayoub & Presti, 1971; Amis, 1987; Edgren et al., 2004).

Pheasant & O’Neill (1975) suggested that grip is weaker on smaller handles due to inadequate contact, and weaker on greater than optimal diameters due to reduction in contact area, and it is probable that in this range, the muscles themselves limit performance. Kong & Lowe (2005) supported this, finding higher muscle efficiencies with smaller diameter handles (i.e. participants exerted greater total finger force with the same amount of EMG activity) than with larger diameter handles. This information can be used to support handheld implement designs of ideal size; however, it remains unclear what the implications are for objects of different shapes (i.e. rectangular).

2.7 Position/Posture of the Upper Limb

The position and posture of the upper limb is a factor that plays an important role in grip strength (Roman-Liu, 2003). Hazelton et al. (1975)

investigated the effects of radial and ulnar deviation on maximal force exertion, and suggests that varying wrist position causes changes only in the magnitude of the total force produced; however, the percentage distribution of the total force produced by the finger flexors to each individual finger maintains a constant relationship, regardless of wrist position. Morse et al. (2006) found wrist angle to be the most significant variable for grip force and wrist torque, recording mean maximal grip forces for males and females occurring at the wrist position of 70° flexion.

Mathiowetz et al. (1985) found that grip strength was significantly stronger with the elbow flexed at 90° than fully extended. Marley & Wehrman

(1992) reported a significant decrease in maximum grip strength with forearm pronation. When the shoulder was at 45° horizontal abduction,

180° arm flexion, 45° medial rotation, 135° elbow flexion, 60° forearm pronation or 30°, 60°, and 70° supination, 0° wrist abduction/adduction, and 15° to 0° wrist extension, Roman-Liu (2007) reported the greatest handgrip force. Hallbeck (1994) examined flexion and extension forces generated by wrist-dedicated muscles over the range of motion in sixty subjects (30 male, 30 female). It was found that gender, wrist position, direction of force exerted, and the wrist position interaction with direction were significant in terms of their effects on maximal force exertion. On

average, females had 76.3% of the mean flexion force of males, and

72.4% for extension. Extension forces were found to be 83.4%, on average, of those generated by flexing the wrist-dedicated muscles. It appears as though the upper limb position that offers the greatest grip strength involves abduction, flexion and medial rotation of the shoulder, flexion of the elbow, and flexion of the wrist.

2.8 Instrumentation to Measure Grip Forces

In order to exploit the hand to greatest effect, it is necessary to understand the types and magnitudes of forces that it is capable of generating, and for what duration these are sustainable (Nikonovas et al., 2004). Practical finger and hand sensors are needed for hand biomechanics research

(Jensen et al., 1991) and techniques to assess hand function have been essential in quantifying hand biomechanics. The most widely used tool for assessing grip strength is the Jamar™ dynamometer, which was introduced in the 1950s (Nikonovas et al., 2004). Leyk et al., (2007) employed a simple hand grip-ergometer to measure hand grip strength, while Morse et al. (2006) used an isokinetic wrist dynamometer to assess grip force and wrist torque while controlling wrist angle and angular velocity.

Grip pressure is both subjective and difficult to measure (Budney,

1979). Budney (1979) instrumented a steel-shafted golf club with three transducers that were designed to respond to grip pressure applied. Exact location of the transducers was determined by measurement of hand locations for numerous professional golfers as well as amateurs. Similarly,

Eggeman & Noble (1985) developed a strain-gage transducer to measure the grip and unbalanced forces on a baseball bat during the swing. Keller et al., (2000) developed a novel dynamic grasping assessment system

(DGAS) to measure the wrist angle simultaneously to the grasping force of each finger and thumb.

Nikonovas et al. (2004) developed a system for measuring unobtrusively the forces developed at the human hand surface. The system has many sensors located over much of the gripping surface of the hand, which do not significantly affect the manner in which the subjects use their hands— light, thin, and flexible Tekscan FlexiForce™ sensors

(Tekscan Inc., Boston, MA) were judged to be the most sufficient.

Attached directly to the subject’s hands using Tegaderm Transparent

Dressing™ film (3M, St. Paul, MN), this method was found to be less obtrusive than fixing the sensors inside a glove worn by the subject. The authors conducted a case study involving one subject hitting a golf ball

with a club. Twenty sensors were placed on each hand and obtained results that were in agreement with the work of Budney (1979).

Kong & Lowe (2005) employed FlexiForce™ sensors (Tekscan Inc.,

Boston, MA) to evaluate total grip force and individual finger/phalange force by placing these thin, conductive polymer pressure sensors over the pulpy region of each phalange and each metacarpal head using a glove.

To avoid the effect of glove size on the force exertion, six different glove sizes were used, depending on the hand size of the subject. The glove- based sensors measure phalangeal segment forces but these do not equal the total hand force when summed, due to the fact that the handle could make contact with other areas of the hand that do not have sensors.

In addition, the authors suggested that frictional conditions of the glove- handle may alter the grip distribution and magnitude. Despite these shortcomings, the small sensors allowed easy positioning and simultaneous measurement of all fingers and phalanges.

Schmidt et al. (2006) applied Tekscan FlexiForce™ sensors (Tekscan

Inc. Boston, MA) that they had evaluated for accuracy, repeatability, hysteresis, and drift errors during static and dynamic conditions to the grip of a standard golf driver in a preliminary study designed to understand the role of grip force during a golf shot. Following this study, Komi and

colleagues (2008) attached 31 Tekscan FlexiForce™ sensors (Tekscan

Inc. Boston, MA) to strategic locations on two gloves. Additionally,

Tekscan 9811™ matrix sensors (Tekscan Inc. Boston, MA) were attached directly to the golf club to measure total grip force. Measurement techniques have recently undergone considerable evolution and have yielded numerous applications in real-world settings, such as in sport.

2.9 Application in Sports

There are many sports in which an instrument (bat, racket, club, stick, etc.) is used as an extension of the arm in order to strike or maneuver an object

(ball, puck, etc.). In sports such as golf, tennis, cricket and baseball where a striking device is used, the grip is the only point of contact between the player and that implement (Schmidt et al., 2006). Only subtle differences in grip forces and wrist positions can affect shot distance and accuracy and may define the difference between a beginner and a seasoned player or a high-level amateur or professional (Hume et al., 2005).

In terms of grip strength measured specifically in hockey players,

Reed et al. (n.d.) examined upper body strength and handedness (left or right hand dominance) and the shooting characteristics of junior and professional hockey players and found that right grip strength was greater than left grip strength within both professional and junior groups, even if

players were left-handed. Wu et al. (2003) studied the performance of the slap and wrist shots in ice hockey as affected by different stick types across players of different skill level and body strengths, and found grip strength to be higher for skilled vs. unskilled women (M = 40.3 N, SD = 3.5

N vs. M = 33.5 N, SD = 3.9 N, respectively) and also higher for skilled vs. unskilled males (M = 59.0 N, SD = 11.6 N vs. M = 57.5 N vs. SD = 9.1 N).

In the sport of baseball, Eggeman & Noble (1985) confirmed the

“push-pull theory” by measuring grip forces exerted on the bat during the swing cycle, concluding that the top hand of the batter is pushing the bat forward toward the ball, while the bottom hand guides the orientation of the bat just before the ball is struck. The grip is firm and then releases slightly just before hitting the ball.

In tennis, Hatze (1976) conducted grip research in the realm of tennis and found that a tight grip increases both the impulse imparted on the ball, consequently increasing the power of the stroke. In addition, a tight grip also increases the vibrational shocks absorbed by the hand.

Choppin et al. (2010) varied the impact velocity, impact angle, impact position, and restorative torque on a tennis racket for a total of 900 impacts to examine how these variables affect the outbound ball velocity and trajectory. The authors created a bespoke apparatus which allowed

for simulated handle grip of at three levels –a light grip (0 Nm), firm grip

(7.5 Nm), and extremely tight grip (15 Nm). It was found that by gripping the handle with a torque above 0 Nm resulted in an outbound ball angle which was reduced by as much as 2°.

In the sport of golf, substantial research has been done to analyze the golf swing. Hume et al. (2005) conducted a review to analyze the role of biomechanics in maximizing distance and accuracy of golf shots. The set-up of the golf swing is outlined, discussing the proper hand positioning for a “strong grip” and a “weak grip”. A strong grip increases the ability of the player to release the hands during the downswing and impact phases, producing more speed but also more risk of miss-hits and off-line shots, while a weak grip decreases the amount of hand speed contributed to the swing but allow more club-face control. Providing the most extensive experimental study on grip force in golf to date, Komi et al. (2008) measured the grip profiles of 20 golfers (male and female, of varying skill levels) using FlexiForce™ sensors (Tekscan Inc., Boston, MA) sensors.

Each golfer, regardless of skill level, was found to have a unique grip force

“signature” (a very repeatable grip force trace). For the individual finger force data, it was found that several groups of golfers used their left hand in a similar fashion—applying an early force with their left hand that is

approximately maintained until impact, with another peak just after impact, followed by a decrease in force until the end of their follow-through. In general there was great variation between golfers; however, some trends include a dominant peak before impact produced by the thumb or ring and little fingers of the left hand, more even distribution of force over the left hand, while with the right hand, golfers tended to use their middle and ring fingers to control the club during part of the take-away and backswing. In addition, many golfers had peaks just after impact using one or more of the index, middle, and ring fingers. For every golfer, grip force measures were greater for the left hand than the right hand for the majority of the shot. The authors surmised that this study could aid in future grip design, the creation of training aids, injury evaluation and prevention, and answering existing questions about how elite players actually grip the club.

The ice hockey stick is a tool that has undergone numerous design changes in order to optimize its potential. Research has focused on the effect of specific stick characteristics, and the interaction between the player and the implement. Although hand grip has been studied extensively, the majority of research in this area has focused on the gripping of cylindrical objects, rather than rectangular ones. A variety of techniques exist to quantify hand grip, with a trend towards developing

less cumbersome and more accurate measurement technologies. The complexity of the tasks executed with the hockey stick leaves ample room for future investigation

Chapter 3: Methods

3.1 Subjects

For this study, 41 subjects (21 men and 20 women) aged 18-41, participated in the following protocol. Based on power analyses during pilot testing, this was a sufficient sample size for the given variables.

Subjects varied in skill level from high to low calibre, from varsity hockey players and those with junior hockey experience, to recreational players and those who solely played in intramural leagues. Subjects were all healthy and had no current upper body injuries. Subjects were recruited from both the McGill men’s and women’s varsity hockey teams, as well as from the university and greater Montreal area hockey community. Both left and right handed shooters of all playing positions (with the exception of goalies) were recruited. Prior to each testing session, subjects read and signed a consent form in accordance with the Tri-Council Policy Statement on Ethical Conduct for Research Involving Humans. Ethics were approved by the McGill University Research Ethics Board (REB #129-10110).

Subjects received no financial compensation for their participation. 50

3.2 Measurement Equipment

3.2.1 Force Sensors

Piezoresistive force sensors (FSA, ISS-O; Vista Medical, Winnipeg,

Manitoba) were selected for measurement of grip forces. These sensors were thin (2mm), flexible and lightweight (1 gram), and had the ability to measure forces with high response times (0.001 sec). Each sensor measured 1.8 cm x 1.8 cm with an active sensing area of 1.2 cm x 1.1 cm.

As well, these Teflon® coated sensors were sufficiently durable to avoid tearing at the hand-stick interface during the shooting tasks. Thin, flexible sensor wires leads were tethered to the shaft, extending upwards to the top of the stick, allowing for unencumbered movement of the stick by the subjects. In turn, these leads were connected to a ribbon cable (UL Style

2651 300 Volt Max, Phalo Corporation, Manchester, NH) then a 32- channel amplifier. This signal amplifier was designed using PCB Artist software (Advanced Circuits, Aurora, CO), assembled using two custom printed circuit boards manufactured by Advanced Circuits (Aurora, CO) and surface mounted electronics to minimize size and weight. The amplifier was then connected to a data acquisition device (cDAQ-9174,

National Instruments, Austin, TX) sampling at 1000 Hz linked via USB cable to the computer using LabVIEW™ Version 2010 (National

Instruments, Austin, Texas) software to record the voltage of each sensor. The amplifier and DAQ board were driven by 5V DC. The sensor cables were 40 cm and the ribbon cable feeding into the amplifier was 340 cm. The ribbon cable connecting the amplifier to the DAQ board was 320 cm.

3.2.2 Sensor Map Configuration

Sixteen sensors were placed in both of the mid-shaft region and the butt- end region (corresponding to the lower and upper hand placement regions, respectively). The sensors themselves covered a region of approximately 136 cm2 (68 cm2 for each hand region) on the shaft of the stick. This was the optimal configuration based on pilot tests. The butt-end and mid-shaft sensors were mounted onto a sleeve around the shaft. The mid-shaft sensor sleeve could be slid to match each subject’s lower hand placement location during slap and wrist shots. These sleeves were constructed out of black polymer plastic shrink wrap (Shenbo Electronics

Co. Ltd.) with Velcro® straps that could tighten each end (to prevent sliding during the actual shots) (Figure 8). A diagram showing a flattened sensor layout can be seen in Figure 9, in addition to how the sensors were attached to the constructed sleeve (Figure 10). The sensors were then covered with a layer of vinylidene chloride polymer (Saran™ plastic wrap)

to prevent hand moisture affecting the sensors, followed by a final layer of masking tape. The mid-point region for both the mid-shaft and butt-end sensors was identified and visibly marked (Figure 11) to ensure consistent hand placement between trials.

Figure 8: Sleeve constructed from shrink wrap and Velcro® straps.

Figure 9: Sensor configuration layout for both the mid-shaft and butt-end region of the stick.

Figure 10: Sensor configuration for lower hand (left) and upper hand (right) on stick.

Figure 11: Fully-instrumented stick with mid-points of each sensing region visibly marked.

3.2.3 Sensor Calibration

Each sensor was calibrated individually using a quasi-static stepwise calibration using a force plate (4060-10, Bertec, Columbus, OH). Force was applied to the entire sensing area using a square plastic “puck” underneath a wooden platform on top of which 1 kg weights were placed.

In order to account for material deformation causing creep, each sensor was loaded with increasing weight, but unloaded between each incremental increase. The relationship between voltage and force was determined for each sensor individually (Figure 12). In addition, the

RMSE was calculated for each sensor individually. The average RMSE of the 32 sensors was 1.86 ± 0.77 N.

Figure 12: Sample sensor calibration (force vs. voltage) for one sensor with line and equation of best regression fit.

3.2.4 Shooting Test Surface

An artificial ice surface (Viking, Toronto, Canada) made of polyethylene and lubricated with a silicone spray was used to simulate real ice conditions in the McGill University Biomechanics Laboratory. The surface was installed over top of a concrete floor and levelled (Stidwill et al.,

2010). The coefficient of friction of the artificial ice surface was reported to be 0.27 (Viking Ice, 2009). The complete surface covered an area of 61.97 m2, but the testing area measured 9.5 m by 5.7 m (14.15 m2) (Figure 13).

Figure 13: Experimental Setup on Synthetic Ice Surface (floor plan).

3.3 Task Protocol

After obtaining subject’s informed consent, descriptive data were obtained

(age, mass, height, hand length, hand breadth, years of hockey playing experience and type of hockey playing experience). Subjects then performed three maximum grip strength trials using an electronic hand dynamometer (Logger Lite™, Vernier Software and Technology,

Beaverton, OR). Grip strength was measured from the lower shooting hand of each subject (i.e. if the subject used a left-handed stick, the grip

strength was taken from the left hand; for a subject using a right-handed stick, the grip strength was taken from the right hand) in a supinated forearm position with the elbow in full extension (in a position to mimic and approximate the stick posture) (Figure 14).

Figure 14: Subject positioning for grip strength test using an electronic hand dynamometer in the supinated hand position.

Three trials were recorded, with each subject’s representative grip strength measure being the highest peak force (N) obtained during the three trials. One minute of rest was given between grip strength trials to reduce fatigue (Heyward, 2006).

Each subject then put on their own skates to wear while on the artificial ice surface, and were given a backpack to wear (housing the amplifiers), in addition to the instrumented stick to grasp. Right and left- handed carbon-fibre sticks (X60 model) of three shaft flex stiffness (77, 87 and 102) were supplied by Bauer Hockey, Corp. The order each stick was used during testing was randomized across subjects. 58

Each subject was given a brief warm-up period to become familiarized with the testing environment and the stick instrumentation before each shot type. Subjects were instructed to grasp the upper and lower sensing regions of the stick. For the lower hand, their second finger was aligned with the midpoint of the sensing region at all times (Figure

15). During this warm-up period, subjects were asked to determine the most comfortable distance between their hands for each shot type.

Corresponding adjustments to the position of the lower hand sensor region were made. The upper-lower hands’ distances were measured and were maintained for the entire duration of shots for each shot type. The distance was defined as the distance between the marked midpoint regions of the mid-shaft and butt-end sensor arrays.

Figure 15: Subjects were asked to align the second finger with the midpoint line on the mid-shaft sensing region (left to right: lower hand initial wrap around shaft; second finger aligned to mid-sensor region; full grip)

When the subject indicated they felt comfortable and ready, seven stationary wrist shots and seven acceptable stationary slap shots were taken using the instrumented stick at a standing distance 3.5m and a 90° orientation centered to net, as seen in Figure 16. During each shot, a radar gun was used to measure shot velocity.

Figure 16: Testing set-up with subject holding instrumented stick (with cabling feeding into backpack to the amplifier, then exiting to DAQ board and PC).

This entire shooting protocol was repeated using the other flex variations of the stick model (77, 87, and 102 flex, resulting in a total of 21 acceptable slap shots and 21 acceptable wrist shots being taken). The transition between stick models was done by removing the sensing arrays and placing them onto the next stick model (Figure 17).

Figure 17: Each sleeve of sensing arrays was able to slide off of one stick model in order to be inserted onto the next.

3.4 Data Processing and Analysis

3.4.1 Data Processing

LabVIEW™ Version 2010 (National Instruments, Austin, Texas) software was used to collect all 32 sensors’ responses over the duration of each task within the protocol. Real time data capture display confirmed successful data capture at the end of each trial (Figure 18) and stored the data to ASCII file format. MATLAB (Ver 7.10.0, R2010a, MathWorks, Inc.,

Massachusetts, U.S.A.) software was used to post-process the data. The data was processed in 6 steps using developed modules. The first module zeroed the data based on baseline zero measurements of voltage obtained from all 32 sensors prior to each testing session. The second module converted the obtained voltage measures to estimated force

values (N). The third module filtered the data using a first order

Butterworth filter with a cutoff frequency of 50 Hz at a sampling rate of

1000 Hz. The fourth module added stick face variables (leading, lagging, toe up and toe down) for each hand region (lower and upper) by calculating the sum of the group of 4 sensors at each aforementioned location for the entire duration of each shooting trial. The fifth module aligned the data, while the sixth marked events and partitioned the data accordingly.

Figure 18: Example of data visualization interface in LabView™. Top left panel shows views of sensors 1-16; top right panel shows views of sensors 17-32, real-time.

3.4.2 Research Design

This study was divided into two separate and distinct parts, one slap shot study and one wrist shot study. Each study employed a 2x2x3 (gender x calibre x stick type) research design (Figure 19). The independent variables examined in each can be seen in Table 2.

Figure 19: Format of experimental design.

Table 2: Independent variables with respective levels Independent Variable Levels Gender Male Female Calibre of player High Low Shot type Wrist shot Slap shot Stick Stiffness High flexibility (77 flex) Medium flexibility (87 flex) Low flexibility (102 flex)

The following descriptive variables were examined for each gender:

Table 3: Descriptive statistics based on calibre of player Variable HC LC Age mean ± SD mean ± SD Years of Experience mean ± SD mean ± SD Height (cm) mean ± SD mean ± SD Mass (kg) mean ± SD mean ± SD Left Hand Length (cm) mean ± SD mean ± SD Right Hand Length (cm) mean ± SD mean ± SD Left Hand Breadth (cm) mean ± SD mean ± SD Right Hand Breadth (cm) mean ± SD mean ± SD Wrist Shot Grip Distance (cm) mean ± SD mean ± SD Slap Shot Grip Distance (cm) mean ± SD mean ± SD Grip Strength (Lower Hand Supinated) (N) mean ± SD mean ± SD Slap Shot Velocity (km/h) mean ± SD mean ± SD Wrist Shot Velocity (km/h) mean ± SD mean ± SD

Mean and peak forces were measured for each stick face of the lower and upper-hand regions, in addition to the total mean force and total peak force for each region. The following dependent variables within each gender, caliber, and stick type were of interest:

• Lower lagging mean and peak force (N)

• Lower leading mean and peak force (N)

• Lower toe down mean and peak force (N)

• Lower toe up mean and peak force (N)

• Lower total mean and peak force (N)

• Upper lagging mean and peak force (N)

• Upper leading mean and peak force (N)

• Upper toe down mean and peak force (N)

• Upper toe up mean and peak force (N)

• Upper total mean and peak force (N)

Chapter 4: Results

Descriptive statistics from all subjects (n = 41), grip force analyses, as well as stick-hand interface force results obtained from wrist shot and slap shot trials will be presented. SPSS Statistics (Ver 17.0, SPSS Inc., Chicago,

Illinois, U.S.A.) was used to perform statistical analyses of dependant variables extracted from the force-time data.

4.1 Descriptive Statistics

Subjects within each gender were divided into two groups (HC and LC) based on their ranking in slap shot velocities. Males who exhibited average slap shot velocities of 95 km/h or greater were defined as HC, while males who exhibited average slap shot velocities of under 95 km/h were defined as LC. Likewise, females who exhibited slap shot velocities of 70km/h or greater were defined as HC, while females who exhibited slap shot velocities of under 70 km/h were defined as LC. Descriptive information was collected from each subject during the testing session

(Tables 4 and 5). There were no significant differences in age, height, or mass for HC vs. LC groups within each gender, making the subject pools viable for comparisons. For males, significant differences were found between HC and LC groups for years of experience, grip strength, slap shot and wrist shot velocity. For females, significant differences were found between HC and LC groups for years of experience, right hand length, slap shot grip distance, grip strength, and slap shot and wrist shot velocity.

Table 4: Male descriptive statistics based on calibre of player (x̄ ± SD)

Variable HC LC F p

Age 24.3 ± 2.8 25.2 ± 5.9 0.150 0.703

Years of Experience 19.0 ± 4.1 13.8 ± 5.6 5.539 0.030*

Height (cm) 178.8 ± 3.9 179.3 ± 5.3 0.062 0.806

Mass (kg) 81.8 ± 5.9 84.3 ± 12.8 0.306 0.586

Left Hand Length (cm) 19.0 ± 1.0 19.1 ± 0.9 0.001 0.974

Right Hand Length (cm) 19.0 ± 1.0 19.1 ± 0.9 0.070 0.795

Left Hand Breadth (cm) 8.8 ± 0.4 8.8 ± 0.4 0.106 0.748

Right Hand Breadth (cm) 8.8 ± 0.5 8.9 ± 0.4 0.338 0.568

Wrist Shot Grip Distance (cm) 53.5 ± 6.8 55.1 ± 4.8 0.383 0.543

Slap Shot Grip Distance (cm) 73.1 ± 6.3 71.0 ± 6.8 0.564 0.462

Grip Strength (Lower Hand Supinated) (N) 437.9 ± 54.4 386.5 ± 51.5 4.905 0.039*

Slap Shot Velocity (km/h) 106.6 ± 4.8 82.4 ± 7.1 77.760 0.000*

Wrist Shot Velocity (km/h) 85.9 ± 5.9 69.2 ± 7.1 32.818 0.000* * p < 0.05.

Table 5: Female descriptive statistics based on calibre of player (x̄ ± SD)

Variable HC LC F p

Age 25.1 ± 6.4 22.1 ± 3.7 1.40 0.252

Years of Experience 17.9 ± 5.6 12.4 ± 5.7 4.63 0.045*

Height (cm) 171.6 ± 3.7 173.9 ± 3.8 1.81 0.195

Mass (kg) 73.2 ± 9.2 67.9 ± 6.6 1.96 0.178

Left Hand Length (cm) 18.3 ± 0.7 17.6 ± 0.8 4.39 0.051

Right Hand Length (cm) 18.3 ± 0.7 17.7 ± 0.8 4.46 0.049*

Left Hand Breadth (cm) 8.0 ± 0.2 7.7 ± 0.3 4.26 0.054

Right Hand Breadth (cm) 8.0 ± 0.3 7.8 ± 0.3 2.16 0.159

Wrist Shot Grip Distance (cm) 53.4 ± 4.4 56.5 ± 7.7 1.33 0.263

Slap Shot Grip Distance (cm) 71.9 ± 5.9 66.0 ± 6.4 4.50 0.048*

Grip Strength (Lower Hand Supinated) (N) 299.1 ± 62.1 223.9 ± 21.1 10.7 0.004*

Slap Shot Velocity (km/h) 78.9 ± 8.0 58.5 ± 7.3 33.3 0.000*

Wrist Shot Velocity (km/h) 63.0 ± 6.1 45.8 ± 4.3 46.7 0.000*

* p < 0.05. 68

4.2 Grip Strength Results

An ANOVA was performed in order to determine if there was a significant difference in grip strength (lower hand, supinated position) based on gender or calibre in the following comparisons:

• Male HC vs. Male LC

• Female HC vs. Female LC

A significant difference in grip strength was found between HC and LC males F (1, 19) = 4.91, p = 0.039, and also between HC and LC females F

(1, 18) = 10.72, p = 0.004.

4.3 Results for Slap Shots

An ANOVA was performed for each of the following comparisons to determine if slap shot velocities differed based on calibre, gender, and stick type:

• Male HC vs. Male LC

• Female HC vs. Female LC

• Male HC Stick 1 vs. Male HC Stick 2 vs. Male HC Stick 3

• Female HC Stick 1 vs. Female HC Stick 2 vs. Female HC Stick 3

There was a significant difference between slap shot velocities in

HC vs. LC males, F (1, 19) = 77.76, p = 0.000, and also in HC vs. LC

females F (1, 18) = 33.34, p = 0.000. No significant differences were found between each of the three stick models for males or females.

In order to examine if a relationship between slap shot velocity and grip strength existed, Pearson’s correlation analyses between the respective male and female slap shot velocity and grip strength data were performed. In males, a moderate positive correlation occurred between slap shot velocity and grip strength r (19) = 0.49, p < 0.05 (one-tailed,

Figure 20). An apparent stratification can be seen in the HC vs. LC player clusters, as these two groups are divided around a shot velocity of 96 km/h. In females, a strong positive correlation was seen, r (18) = 0.75, p <

0.01 (Figure 21). A calibre stratification can also be seen in females; however, female subjects with the highest shot velocities also demonstrated high grip strength.

Figure 20: Male grip strength vs. slap shot velocity, with calibre of player indicated.

Figure 21: Female grip strength vs. slap shot velocity, with calibre of player indicated. 71

The results of the regression indicated that grip strength explained a significant proportion (24.3%) of the variance in male slap shot velocity,

R2 = 0.24, F (1, 19) = 6.11, p < 0.05. In females, grip strength explained

55.8% of the variance in slap shot velocity, R2 = 0.56, F (1, 18) = 22.73, p

< 0.001. Refer to regression equations in Figures 20 and 21.

From the grip force instruments on each stick surface region, both mean and peak forces were calculated for slap shot trials. The corresponding mean and peak values for each were highly correlated, (r =

0.926 to 0.977). Due to the strength of this relationship, only peak force estimates will be reported below.

In terms of main effects, MANOVA results indicated that gender, calibre, and the interaction of gender and calibre had significant main effects (p < 0.05) at peak total force for both the upper and lower hands

(Figure 22). No significant differences were seen across stick type.

Figure 22: Peak total force* means for slap shots, separated into calibre and gender, for the upper hand (a) and lower hand (b).

*Note Peak total force was the sum of all four sides (Lag, Lead, TU, TD).

Upper and lower hand post-hoc comparisons (Tukey) of the group means for peak force for the four groups (female LC, female HC, male LC, male HC) are shown in Table 6. For the lower hand, comparisons indicated there were no significant differences (p < 0.05) between the female HC and male LC groups on all four surfaces of the stick (lower leading, lower lagging, lower toe up and lower toe down) in terms of peak force. The male HC group was significantly different than every other group on each surface of the stick, with the exception of lower toe down, where the peak force was significantly different than every group except male LC. In addition, the female HC and LC groups only showed a significant difference for the lower leading and lower lagging peak force.

Upper hand post-hoc tests (Tukey) found significant differences in peak force at the upper toe down and upper toe up surfaces between the male HC group and every other group.

Table 6: Tukey HSD comparisons for peak forces (N) at the upper and lower hands during the slap shot (group means and associated p-values)

Stick F- F- M- M- F-LC F-LC F-LC F-HC F-HC M-LC Surface LC HC LC HC – – – – – –

F-HC M-LC M-HC M-LC M-HC M-HC

Lower 10.5 15.2 14.4 20.8 0.024* 0.085 0.000* 0.945 0.004* 0.001* Leading

Lower 7.0 12.5 12.7 20.9 0.002* 0.001* 0.000* 0.998 0.000* 0.000* Lagging

Lower 35.8 38.4 44.1 61.9 0.862 0.054 0.000* 0.192 0.000* 0.000* Toe Up

Lower 7.6 9.8 11.9 14.6 0.593 0.083 0.002* 0.578 0.035* 0.401 Toe Down

Upper 9.8 12.1 16.0 20.8 0.646 0.014* 0.000* 0.154 0.000* 0.066 Leading

Upper 7.1 8.3 13.2 16.4 1.00 0.060 0.002* 0.113 0.004* 0.499 Lagging

Upper 3.8 6.8 9.2 20.2 0.419 0.032* 0.000* 0.507 0.000* 0.000* Toe Up

Upper 9.6 14.8 20.3 33.3 0.219 0.001* 0.000* 0.105 0.000* 0.000* Toe Down

(F-LC = Female LC; F-HC = Female HC; M-LC = Male LC; M-HC = Male HC). * p < 0.05.

With respect to grip forces over the duration of the shot, colour intensity maps were produced to compare sample trials from the fastest and slowest slap shots performed. From the male subjects, the fastest slap shot (118 km/h) exhibited visibly higher peak grip forces than by the slowest slap shot (65 km/h) (Figure 23). A similar trend was observed for females, with the fastest slap shot (100 km/h) also exhibiting visibly high forces compared to the slowest slap shot (36 km/h) (Figure 24). In addition, the relative forces applied to respective stick surface regions by the upper and lower hands show distinctive regional loading patterns. For example, in the selected trials, the lower toe up and upper toe down surfaces appear to be experiencing forces at relatively the same time

(assumed to be at time of stick impacting ice). Immediately after this event, the lower toe down and upper toe up surfaces are experiencing forces with a paired timing sequence, suggesting a clamping mechanism at impact and immediately after impact for the slap shot. This effect appears to be more evident in males than in females, perhaps due to the higher force range of male subjects.

Figure 23: Colour map representing fastest male slap shot trial (118 km/h)

(a) and slowest male slap shot trial (65 km/h) (b). Force scale ranges from

0 N (dark blue) to 90 N (dark red). Red arrow indicates approximate ice contact time.

Figure 24: Colour map representing fastest female slap shot trial (100 km/h) (a) and slowest female slap shot trial (36 km/h) (b). Force scale ranges from 0 N (dark blue) to 50 N (dark red). Red arrow indicates approximate ice contact time.

Alternative to the colour maps, force-time plots of shot trials can provide insight into grip dynamic behavior. Qualitatively, it can be seen that from trial to trial, there was little variation in the forces at the stick- hand interface within each subject and condition. For example, the force profile of one subject within one stick type appears to follow the same pattern. In addition, the force profile of one subject across all 3 stick types appears to be consistent with this pattern (Figure 25).

Figure 25: (a) An overlay of 7 slap shot trials (SS87) of one subject.

4.4 Results for Wrist Shots

An ANOVA was performed for each of the following comparisons to determine if wrist shot velocities differed based on calibre, gender, and stick type:

• Male HC vs. Male LC

• Female HC vs. Female LC

• Male HC Stick 1 vs. Male HC Stick 2 vs. Male HC Stick 3

• Female HC Stick 1 vs. Female HC Stick 2 vs. Female HC Stick 3

A significant difference was found between wrist shot velocities in

HC vs. LC males F (1, 19) = 32.82, p = 0.000, and also in HC vs. LC females F (1, 18) = 46.79, p = 0.000. No significant differences were found between each of the three stick models for males or females.

For males, a moderate positive correlation occurred between wrist shot velocity and grip strength r (19) = 0.51, p < 0.05 (one-tailed, Figure

26). An apparent stratification can be seen in the HC vs. LC player clusters, as these two groups are divided around a shot velocity of 80 km/h.

Likewise, a strong positive correlation was seen in females r (18) =

0.69, p < 0.01 (Figure 27). A calibre stratification can also be seen in

females; however, female subjects with the highest shot velocities also demonstrated high grip strength.

Figure 26: Male grip strength vs. wrist shot velocity, with calibre of player indicated.

Figure 27: Female grip strength vs. wrist shot velocity, with calibre of player indicated. 82

The results of the regression indicated that in males, grip strength explained a significant proportion (26.3%) of the variance in slap shot velocity, R2 = 0.263, F (1, 19) = 6.770, p = 0.001. For females, grip strength explained 47.9% of the variance in slap shot velocity, R2 = 0.479,

F (1, 18) = 16.569, p = 0.001.

From the grip force instruments on each stick surface region, both mean and peak forces were calculated for wrist shot trials. The corresponding mean and peak values for each were highly correlated (r =

0.901- 0.975). Due to the strength of this relationship, only peak force estimates will be reported below.

In terms of main effects, MANOVA results indicated that gender, calibre, and the interaction of gender and calibre had significant main effects (p < 0.05) for peak total force at the lower hand (Figure 28). For the upper hand, gender and calibre showed main effects (p < 0.05) at peak total force; however, the interaction between gender and calibre was not significant. Lastly, no significant differences were seen across stick type.

Figure 28: Peak total force* means for wrist shots, separated into calibre and gender, for the upper hand (a) and lower hand (b).

*Note Peak total force was the sum of all four sides (Lag, Lead, TU, TD). 84

Upper and lower hand post-hoc comparisons (Tukey) of the four groups (female LC, female HC, male LC, male HC) are shown in Table 7.

For the lower hand, there were no significant differences between the female HC and male LC groups on 3 of the 4 stick surfaces (lower leading, lower lagging, and lower toe up)—there was a significant difference (p <

0.05) between female HC and male LC groups on the lower toe down surface. The lower toe down surface showed a significant difference between the male LC group and every other group. The lower leading and lagging surfaces showed the same comparison results—there was no significant difference between female HC vs. LC groups, and no significant difference between male HC vs. LC groups; however, there were significant differences between female LC and male HC and LC groups, and also between female HC and male HC groups. Finally, all males (male

LC and HC) were significantly different than all females (female HC and

LC) on the lower toe up surface, but within each gender, calibre was not significant (i.e. male LC was not significantly different than male HC; female LC was not significantly different than female HC).

Upper hand post-hoc tests (Tukey) found that the male HC group was significantly different than every other group on the upper lagging, upper toe down and upper toe up surfaces. There were no significant

differences between female HC and male LC groups for both the upper lagging and upper toe up surfaces. Both LC and HC males were significantly different than LC and HC females for the upper leading and upper toe down surfaces. For the upper leading surface, there were no significant differences between LC and HC males, and also between LC and HC females. For the upper toe down surface, there was a significant difference between male LC and HC groups.

Table 7: Tukey HSD comparisons for peak forces (N) at the upper and lower hands during the wrist shot (group means and associated p-values)

Stick F- F- M- M- F-LC F-LC F-LC F-HC F-HC M-LC Surface LC HC LC HC – – – – – –

F-HC M-LC M-HC M-LC M-HC M-HC

Lower 10.6 12.5 15.5 17.6 0.476 0.002* 0.000* 0.059 0.001* 0.348 Leading

Lower 8.2 13.1 15.1 18.8 0.072 0.005* 0.000* 0.700 0.022* 0.227 Lagging

Lower 29.0 26.9 37.3 42.7 0.880 0.022* 0.000* 0.001* 0.000* 0.214 Toe Up

Lower 6.8 7.0 12.5 7.8 0.999 0.003* 0.927 0.001* 0.951 0.014* Toe Down

Upper 6.2 9.4 14.7 17.6 0.062 0.000* 0.000* 0.000* 0.000* 0.096 Leading

Upper 4.0 7.0 8.6 13.0 0.143 0.006* 0.000* 0.542 0.000* 0.008* Lagging

Upper 2.9 8.4 7.7 12.7 0.000* 0.001* 0.000* 0.914 0.003* 0.000* Toe Up

Upper 8.8 15.0 22.9 32.3 0.066 0.000* 0.000* 0.004* 0.000* 0.001* Toe Down

(F-LC = Female LC; F-HC = Female HC; M-LC = Male LC; M-HC = Male HC). * p < 0.05.

With respect to grip forces over the duration of the shot, colour intensity maps were produced to compare sample trials from the fastest and slowest wrist shots performed. From the male subject, the fastest wrist shot (105 km/h) exhibited visibly higher peak grip forces than by the slowest wrist shot (45 km/h) (Figure 29). A similar trend was observed for females, with the fastest wrist shot (79 km/h) also exhibiting visibly high forces compared to the slowest wrist shot (29 km/h) (Figure 30). In addition, the relative forces applied to the respective stick surface regions by the upper and lower hands show distinctive regional leading patterns.

For example, the lower toe up, lower lagging and upper toe down and upper toe up stick surfaces appear to be working in synchrony during the approximate puck release phase. The lower leading surface appears to be experiencing a gradual increase in force before puck release. In general, forces exerted during the wrist shot appear to be more gradual than was the case for the slap shot.

Figure 29: Colour map representing fastest male wrist shot trial (105 km/h)

(a) and slowest male wrist shot trial (45 km/h) (b). Force scale ranges from

0 N (dark blue) to 70 N (dark red). Red arrow indicates approximate time of puck release. 89

Figure 30: Colour map representing fastest female wrist shot trial (79 km/h) (a) and slowest female wrist shot trial (29 km/h) (b). Force scale ranges from 0 N (dark blue) to 40 N (dark red). Red arrow indicates approximate time of puck release. 90

Figure 31: An overlay of 7 wrist shot trials (WS77) of one subject.

Chapter 5: Discussion

5.0 General Discussion

The proficient execution of ballistic human movement tasks such as slap and wrist shots require coordinated and sufficiently large forces exerted in a closed loop through both hands so as to manipulate the stick’s motion and in turn the puck’s projection direction and velocity. Despite this, little is known with precision about the stick – hand mechanical interaction.

Hence, the motivation for this study was to identify both the grips’ magnitude and regional force distributions during these fundamental shooting tasks. The instrumentation used was successful in capturing grip force patterns with a high temporal resolution. The observed grip forces of the upper and lower hands about the stick represented the sum total of both active flexor muscles recruited to compress the digits and palm about the shaft as well as forces transmitted by the upper limbs downward from the body AND the reactive forces upwards from the blade during ice contact.

Four cohorts of subjects (male HC, male LC, female HC and female

LC) performed these two shot types with three sticks of different stiffness

(77, 87 and 102 flex). The latter sticks represented the spectrum of shaft flexion stiffness commercially available. Measured shot velocities were

consistent with previously reported values for stationary shots (Pearsall et al., 1999; Wu et al, 2003; Roy et al., 1974; Roy and Doré, 1976; Alexander et al., 1963; Cotton, 1966; Rothsching, 1997) as was the observed shot velocity differences between gender and calibre groups. Corresponding grip strength group differences were measured. These observations were similar to previously reported population sample measures (Wu et al.,

2003; Kong & Lowe, 2005; Mathiowetz et al, 1985; Hallbeck and McMullin,

1993; Leyk et al., 2007). In this study, grip strength was found to be positively correlated with shot velocity (slap r = .49 to .75; wrist r =.51 to

.69) as expected and in general agreement with Wu et al. (2003) and Roy and Doré (1976). Within each gender group a visible clustering of all HC and LC players was evident in the shot velocity – grip strength scatter plots (Figures 20, 21, 26 and 27). However, when the grip strength factor was considered as a covariate, significant gender and calibre effects still persisted (Appendix III). Hence, grip strength alone cannot account for the inter-group shot velocity differences found. Other learned technique differences executed by HC subjects therefore must account for the greater net shot velocity. Such technique differences would include hand- stick grip kinetics.

In general, peak grip force measures were greater in HC than in LC players as well as greater in males than females. During slap shots, grip forces were affected by interactions of gender and calibre (HC male > HC female; LC male > LC female) for both the upper and lower hands’ grips.

Similarly interactions were seen with the grips during wrist shots.

However, stick flexion stiffness did not influence the peak forces at the hand-stick interface for either the slap shot or the wrist shot. This null effect of shaft stiffness counters common perceptions among players but is consistent with prior findings reported by Pearsall et al. (1999), Wu et al.

(2003), and Hannon (2010) with the exception of Worobets et al. (2006).

Given these results, a more detailed interpretation of grip forces with respect to shaft region and timing is provided in the following sections, first with regards to the slap shot.

5.1 Slap Shots

Within subjects, consistent force “signature” patterns were observed (as shown in Figure 25 agreeing with previously reported by Roy and Doré

(1975) and, with respect to golf, by Komi et al. (2008)). Greatest lower hand forces on the stick were expected on the lagging side at ice contact; however, in general substantial concurrent forces on the toe-up AND lagging sides were consistently seen. How much force was sustained at

the shaft’s toe-up: lagging corner was unknown (Figure 32). Due to the rectangular shaft cross-section in combination with the hand’s palm and thumb placement, this force “cupping” of the shaft pushed the blade down, forward AND inward to the ground with respect to the subject, effectively bending the shaft along both major AND minor axes. One may speculate that these forces may well increase the blade’s normal force to the ice, increasing its drag friction, momentarily holding and / or slowing the blade position to prevent it from skipping uncontrolled over the ice. Furthermore, this force cupping may contribute to achieving a typical “closed-blade” position (Figure 33).

Figure 32: Lower hand in contact with the shaft. Typically, the large thenar muscles of the thumb were located on the toe up surface with the hand palm and hypothenar muscles pressing on the lagging surface. Unknown are the forces at corners (dashed arrows).

Figure 33: Top view stick position as seen by player in (a) neutral, and (b) closed-blade due to forearm pronation.. Note lower hand’s thumb and palm wrap around lagging and toe up sides of shaft. Pushing down vertically on the shaft from position (b) would translate into greater pressure on these two sides.

In general, grip forces at the lower hand were greatest during downswing then peaking at ice impact, whereas at the upper hand grip forces first peaked at impact and then were sustained during follow through. Given that the upper and lower hands and stick shaft formed a closed mechanical loop, the function of the hands would be to a certain extent interdependent. This was evident in the force-time plots of hands’ and their respective stick side forces, particular in HC subjects during stick-ice contact wherein the ground reaction force generated a force 97

coupled response in lower and upper hands; that is, initially between the lower hand lagging / toe up coupled with the upper hand leading / toe down sides, immediately followed between the lower hand leading / toe down and upper hand lagging / toe up sides (Figure 34). In mechanical terms, the former coupling permitted bending (loading) of the stick whereas the latter coupling “clamped” the stick during unbending (recoil)

(Figure 35). The mechanical actions of both hands exerting forces in opposite directions could allow for greater leverage in transitioning from a two-point force application to a three-point cantilever bend. In contrast, the

LC subjects did not show this distinct coupling suggesting that they did not effectively take advantage of the stick elastic flexion properties (Figure 36 shows upper and lower hand coordination differences between 3 male HC subjects vs. 3 male LC subjects). This would be consistent with the technique differences between HC vs. LC as observed Villaseñor et al

(2006).

Figure 34: Typical HC slap shot: hand and stick side forces and relative timing. “Cupping” force groups shown for lower hand lagging / toe up AND upper hand leading / toe down surfaces in time window box (a.) corresponding to stick-ice contact; and lower hand leading / toe down surfaces; AND upper hand lagging / toe up surfaces in time window box (b.) corresponding to stick recoil.

Figure 35: Different views of upper and lower hand and stick side

“cupping” forces at (a.) ice contact and (b.) stick recoil time window boxes

(from Fig 33 above). In A. Top view a. and b. note how the upper and lower hand forces act in equal but opposite directions. In B. Side view and

C. Front view, note how at ice contact (a.) a 3 point bending of the shaft with force 2 (lower hand) acting against forces 1. and 3. (upper hand and ice reaction). At stick recoil (b.), forces 1. and 2. both hold (clamp) the stick and resist stick recoil. 100

Figure 36: Upper and lower hand forces (total) vs. Time. Colour map representing differences in magnitude and timing coordination in 3 male (a) HC subjects vs. (b) LC subjects. Force scale ranges from 0 N (dark blue) to 160 N (dark red). HC players typically exerted large total grip force first at the lower hand then followed by the upper hand. LC players displayed no evident lower-upper hands’ time coordination.

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These grip force patterns are complementary to the observations by

Hannon et al (2010) that used strain gages to quantify stick shaft major axis bend dynamics during slap shots. They speculated that coordination of bimanual forces and ground reaction force would afford optimal shaft flexion (bend) and recoil to achieve maximal horizontal force translated to the puck. Current results support this concept with regard to the shaft’s major axis; however, the unanticipated large forces seen at both the toe up and toe down surfaces would not directly translate into forward projection of the puck. As noted early though, these latter forces may function to augment blade friction to hold it in place, thereby allowing longer and more substantial bending about the major axis. Further, these large toe up and toe down forces at ice contact may well be interpreted as the necessary clamping force to prevent the stick from being dislodged from the hands (Figure 37).

Lastly, the larger peak hand forces generated by HC versus LC players were consistent with the observed ground reaction forces presented by Woo et al. (2004). In their study, skilled players created substantially greater peak ground reaction force than unskilled (123 N vs.

73 N respectively) that, in turn, corresponded to greater stick bend deflection (15° vs. 11°) and shot velocity (24 m/s vs. 18 m/s). Further, Wu

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speculated that the HC performance outcomes were achieved by more substantive use of their body core muscles as well as optimal proximal- distal upper limbs’ segment movements; ultimately, transmitting greater forces to the last segment – the stick.

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Figure 37: Interpretation of forces about the stick (1. upper hand, 2. lower hand and 3. blade). Forces 1 and 2 act as a force couple above the stick’s center of mass (yellow circle) creating counter-clockwise torques (as viewed in diagram) that is opposed by the ground reaction force 3’s induced clockwise torque. From the grip force data, the upper and lower hand forces may be interpreted as a force couple that would 1) allow kinetic (angular velocity generation) during downswing, and 2) create a torque pressing the blade into the ice. These forces create bending about

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both the minor (a) and major (b) axes and create a three-point bending configuration of the stick shaft (beam).

5.2 Wrist Shots

The observed peak total force for the upper and lower hands agreed with the original hypotheses; that is, significant gender and calibre main and interaction effects were found (Figure 28). In particular, on the lower hand’s toe up surface, males exerted more force than females, irrespective of calibre. For the upper hand, the male HC groups’ forces were significantly greater than the other groups for toe up, toe down and lagging surfaces. Notable differences in regional loading patterns were observed among subjects; for example, some subjects showed large forces at the lower hand lagging surface, while others did not. As was the case for the slap shot, a highly repeatable pattern was observed within each subject (as shown in Figure 31). This enabled the force “signature” of each player to emerge, and agreed with the findings of Roy and Doré

(1975) for hockey wrist shots and, in the case of golf driving, by Komi et al.

(2008).

The most consistent finding across subjects (similar to as shown in slap shots) was that the lower hand’s toe up region exhibited high forces, synchronized with high forces in the upper hand’s toe down region. Figure

105

38 demonstrates four subjects’ representative trials showing this timing coordination approximately at stick bending. In terms of force magnitudes, the male HC group demonstrated the highest peak forces on the lower toe up (M = 42.7 N) and upper toe down (M = 32.3 N) regions. Unlike the slap shot, substantial lead / lag regional were not observed.

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Figure 38: Representative trials of four subjects with similar wrist shot timing parameters and force magnitudes in the upper toe down and lower toe up regions (see arrows). Note other regional patterns varied by magnitude and timing. Force scale ranges from 0 N (dark blue) to 60 N (dark red).

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The large inter-subjects grip profile variations may be attributed to several factors. Differences in anthropometrics (e.g. hand breath, finger length), body positioning (e.g. feet pointed towards or perpendicular to the net), dynamic base of support control (e.g. stable / unstable) and upper limbs segments’ coordination could affect end result regional grip force patterns. The delimited capture area of the sensors about the shaft may well be attributed to inter-subject grip variances. Hence, aliasing of the true grip force patterns could occur, as slight variations in finger placement by 1 cm could lead non-detection of contact. An augmented sensor count and detection resolution (< 1 cm), including coverage at the shafts’ corners, for the hand placement areas are needed in future studies.

Nonetheless, given the average peak forces reported provide reasonable estimate of magnitude order needed to generate the observed stick bend during ice contact. For instance, comparing the values reported by Wu et al (2003) for wrist shot peak vertical ground reactions forces (37 to 51 N), with this study’s observed lower hand’s peak forces on the toe up (29 to

42 N) and lagging (8 to 19 N) demonstrate comparable values. Within the

3 point shaft bend configuration, these regional loading magnitudes are realistic.

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Chapter 6: Conclusions

This study demonstrated the feasibility to directly measure forces at the hand-stick interface while executing shooting tasks with the ice hockey stick. With these sensor maps on the stick shaft, force measures during both the slap shot and wrist shot were observed to vary by calibre and by gender. In general males had higher forces than females, and HC players had higher forces than LC. Stick type did not influence measured forces, or shot velocity for either the slap or wrist shots.

The detailed spatial and temporal mapping of the grip force mechanics about the shafts’ outer cross-sectional perimeter of both upper and lower hands provides insight into how the stick is being manipulated in order to project the puck towards the net. For instance, the distinct force amplitude and relative timing of shaft faces illustrated the specific manner of bimanual coordination, particularly during ice contact, needed for stick movement and bending. In general, the more extenuated force coupling and force cupping behavior specific to toe up/down and lead/lag side combinations were detected in HC players. Mechanically, interpretation of these may be related to the required need for 1) sufficient “clamping” forces about the shaft to prevent aberrant stick movement or loss of hold onto the stick, and 2) generation of off center forces to create torques on

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the stick (for momentarily storage of elastic bend energy to be release coincident with puck projection). Furthermore, the observation of cupped forces that in turn coincided with both major and minor axes bending deflection, particularly in the ice contact phase of the slap shot, was unanticipated. Manufacturers of sticks may well want to consider this latter bi-axial loading phenomenon in product development and testing, as well as other stick geometric design to enhance grip properties for a range of anthropometric hand dimensions, from children to adults, male to female.

The quality of one’s grip has a strong predictive relationship in terms of performance outcome (in this case, shot velocity). One’s grip strength, for instance, was positively correlated to shot velocity. However, this alone could not account for all performance differences; that is, player skill or calibre was as a strong factor. Hence, a player’s dynamic “grip force signature” during these shots may provide quantitative feed-back relevant to player and coaches alike. Caution in over generalizing these is warranted though, as different “signatures” can lead to the same results; for example, two HC players had distinctive grip force patterns yet yielded the same shot velocity. In the future, the system could be brought from the laboratory setting onto the ice to examine not only the forces at the stick-

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hand interface during slap and wrist shots, but also for stick handling tasks e.g. passing (delivery and reception), and a variety of other stick-related skills performed while skating.

In addition to improved spatial mapping resolution and potential identification of shear (friction) forces, a more detailed analysis of the force-time waveforms (Komi et al., 2008) may be pursued. Possible consideration may be extended to understanding hand haptics, the role of dominant / non-dominant hands, and how grip force transmission is modified while wearing actual ice hockey gloves should be considered. By understanding where high loads are occurring, glove design may be altered by using different materials on the palm to improve contact or prevent early deterioration, including padding or adhesive-type materials.

Other potential grip studies may consider synchronized with motion capture systems such as the Vicon MX® (Oxford, UK) in order to examine the motion of the body in relation to the forces between the hands and stick as well as recording of discrete motions of the hands by linking with kinematic gloves (e.g. Measurand ShapeHand™, Measurand, Inc.,

Fredericton, NB; AcceleGlove Motion Capture Glove, AnthroTronix Inc.,

Silver Spring, MD; PhaseSpace IMPULSE Glove, PhaseSpace Inc., San

Leandro, CA; and 5DT Dataglove, Fifth Dimension Technologies, Irvine, 111

CA). Alternatively, future grip studies may consider linking to strain gauge systems (e.g. Hannon et al., 2010) and force plates (e.g. Wu et al., 2003) in order to directly ascertain how the input forces at the hands and correspond to stick bend deformation and ground reaction forces (only speculated here).

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Appendix I

For calibre stratification, average shot velocity was recorded for each subject for both the slap and wrist shot. The velocity values were categorized by frequency in a histogram (Figure 39). For both males and females, the calibre cutoff velocity was chosen based on the mid-way point in the distribution.

Male Average Slap Shot Velocity 5

2 Frequency

Frequency 1

0 70 75 80 85 90 95 100 105 110 115 More -1 Bin

Figure 39: Male slap shot velocity frequency distribution (95 km/h chosen as cutoff for HC/LC stratification).

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Appendix II

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Appendix III

Table 8: Analysis of covariance summary (male slap shot) Source Sum of Squares df Mean Square F p

Grip 35.861 1 35.861 0.919 0.350

Calibre 2143.366 1 2143.366 54.916 0.000* Error 702.541 18 39.030 *p < 0.05

Table 9: Analysis of covariance summary (female slap shot) Source Sum of Squares df Mean F p Grip 318.131 1 318.131 7.117 0.016*1

Calibre 598.938 1 598.938 13.400 0.002* Error 759.875 17 44.699 *p < 0.05

Table 10: Analysis of covariance summary (male wrist shot) Source Sum of Squares df Mean F p Grip 65.792 1 65.792 1.551 0.229

Calibre 903.948 1 903.948 21.313 0.000* Error 763.424 18 42.412 *p < 0.05

Table 11: Analysis of covariance summary (female wrist shot) Source Sum of Squares df Mean F p Grip 93.766 1 93.766 3.532 0.077

Calibre 570.263 1 570.263 21.482 0.000* Error 451.290 17 26.546 *p < 0.05

1 Refer to Figure 21 (female grip strength vs. slap shot velocity)

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