The influence of variations in shoe midsole density on the impact force and kinematics of landing in female volleyball players

Transcription

1 The University of Toledo The University of Toledo Digital Repository Theses and Dissertations 2004 The influence of variations in shoe midsole density on the impact force and kinematics of landing in female volleyball players Karen J. Nolan The University of Toledo Follow this and additional works at: Recommended Citation Nolan, Karen J., "The influence of variations in shoe midsole density on the impact force and kinematics of landing in female volleyball players" (2004). Theses and Dissertations. Paper This Dissertation is brought to you for free and open access by The University of Toledo Digital Repository. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of The University of Toledo Digital Repository. For more information, please see the repository's About page.

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3 Copyright 2004 This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.

4 An Abstract of The influence of variations in shoe midsole density on the impact force and kinematics of landing in female volleyball players Karen J. Nolan Submitted as partial fulfillment of the requirements for the Doctor of Philosophy in Exercise Science The University of Toledo May 2004 The purpose of this study was to examine the effect of changing the midsole density of athletic shoes on impact forces upon landing during a nonrhythmic athletic activity. Previous studies showed that changing the density of the midsole had neither a positive or negative effects on running, which is a repetitive rhythmic athletic activity. This investigation examined the influence of variations in athletic shoe midsole density on vertical ground reaction forces, loading rates, peak joint moments, and examined which specific kinematic variables were affected upon landing after a non-rhythmic vertical jump. Subjects included 20 female, NCAA volleyball athletes (21.1 ± 2.84 years). Each subject was tested in three different athletic shoe conditions: control midsole, soft iii

5 midsole, and hard midsole. For each of the athletic shoe midsole conditions, the subjects performed 10 volleyball approaches and spike jumps; landing onto two force platforms to measure impact forces. Kinematic data was collected simultaneously with the kinetic data using a six camera Motion Analysis system. Data was collected for each subject for a total of 30 trials (10 trials X 3 midsole conditions). A one-way repeated measures analysis of variance was used to compare the three different shoe conditions (significance at α =.05). Results indicated that variations in midsole density do not significantly affect impact forces or loading rates upon landing. Kinematic variables failed to sufficiently explain this result. It is possible that athletes may use neuromuscular adaptations to account for changes in midsole density during impact. More research is needed to determine if changes in muscle activity are used as a possible strategy during landing to affect impact forces. Further research is needed on the effects of athletic shoe midsole density during landings from non-rhythmic athletic activities. iv

6 Dedication This dissertation is dedicated to all those who helped me along the way to achieve my goals. v

7 Acknowledgements The completion of this dissertation would not have been possible without the support, assistance and guidance of many people. I would like to first thank my dissertation committee: Charles W. Armstrong, Ph.D., Richard A. Yeasting, Ph.D., and Phillip A. Gribble, Ph.D., ATC-L. I am extremely grateful that Charles W. Armstrong, Ph.D., has been my advisor during my entire course of study at the University of Toledo. I sincerely appreciate all of the challenges and opportunities he provided. I would like to thank him for his optimistic support and confidence in my abilities. I am most grateful to have had Dr Armstrong as a mentor. I would like to thank Richard A. Yeasting, Ph.D., for serving on my dissertation committee and being a supportive reviewer of my research. I would like to further thank him for providing me a thorough knowledge of anatomy and challenging me to think beyond the obvious. I consider myself lucky to have had such a competent professor in my minor field of study, anatomy. I would like to thank Phillip A. Gribble, Ph.D., ATC-L for serving on my committee and being an interested and enthusiastic reviewer of my dissertation. He was generous with his time, knowledge, and provided a careful analysis of my study. I would like to acknowledge the generous support of Fila USA for providing the prototype athletic shoes designed for this investigation. I would like to extend a very special thank you to Craig Wojcieszak, Director of Advanced vi

8 Research and Product Testing, Fila USA, who showed enough interest in my initial research to allow this project to get started. I would further like to thank him for his dedication to this project and his contributions along the way. I would to thank Bruce Kwiatkowski, M.A. for the generous use of his time providing technical support. I would also like to thank Donald B. White, Ph.D. for all of his assistance and statistical support. I appreciate the cooperation of Coach Kent Miller and the Women s Volleyball Team from the University of Toledo, and Coach Kim Berrington and the Women s Volleyball Team from Eastern Michigan University. The athletes from both teams provided me with excellent and cooperative subjects for this investigation. It was a pleasure to work with both teams. I am especially grateful for the never ending support and encouragement from my family. I would especially like to acknowledge my husband John for his constant support while I pursued my degree. vii

9 Table of Contents Abstract... iii Dedication... v Acknowledgments... vi Table of Contents...viii List of Tables... ix List of Figures... xii I. Introduction... 1 II. Review of Literature... 9 III. Methodology...50 IV. Results...66 V. Discussion VI. References VII. Appendix A VIII. Appendix B IX. Appendix C X. Appendix D XI. Appendix E XII. Appendix F XIII. Appendix G XIV. Appendix H XV. Appendix I XVI. Appendix J

10 List of Tables Table 1. Anthropometric Data of Subjects 58 Page Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. Means and Standard Deviations: Peak Vertical Ground Reaction Forces 71 Statistical Summary of Athletic Shoe Midsole Density on Left and Right Peak Vertical Ground Reaction Force 71 Means and Standard Deviations: Total Peak Vertical Ground Reaction Forces 72 Statistical Summary of Athletic Shoe Midsole Density on Total Peak Vertical Ground Reaction Force 72 Means and Standard Deviations: Left and Right Peak Vertical Ground Reaction Force 73 Means and Standard Deviations: Total Peak Vertical Ground Reaction Forces 74 Table 8. Means and Standard Deviations: Loading Rate 75 Table 9. Statistical Summary of Athletic Shoe Midsole Density on Left and Right Loading Rate 75 Table 10. Means and Standard Deviations: Peak Ankle Joint Moments 76 Table 11. Statistical Summary of Athletic Shoe Midsole Density on Left and Right Peak Ankle Joint Moments 76 Table 12. Means and Standard Deviations: Peak Knee Joint Moments 77 Table 13. Statistical Summary of Athletic Shoe Midsole Density on Left and Right Peak Knee Joint Moments 77 Table 14. Means and Standard Deviations: Peak Hip Joint Moments 78 Table 15. Table 16. Statistical Summary of Athletic Shoe Midsole Density on Left and Right Peak Hip Joint Moments 78 Means and Standard Deviations: Ankle Position at Initial Contact with the Ground 84 ix

11 Table 17. Table 18. Table 19. Table 20. Table 21. Statistical Summary of Athletic Shoe Midsole Density on Ankle Position at Initial Contact with the Ground 84 Means and Standard Deviations: Ankle Position at Peak Vertical Ground Reaction Force 85 Statistical Summary of Athletic Shoe Midsole Density on Ankle Position at Peak Vertical Ground Reaction Force 85 Means and Standard Deviations: Maximum Angular Displacement of the Ankle 86 Statistical Summary of Athletic Shoe Midsole Density on Maximum Angular Displacement of the Ankle 86 Table 22. Means and Standard Deviations: Left Ankle Range of Motion 87 Table 23. Table 24. Table 25. Table 26. Table 27. Table 28. Table 29. Means and Standard Deviations: Right Ankle Range of Motion 88 Means and Standard Deviations: Knee Position at Initial Contact with the Ground 89 Statistical Summary of Athletic Shoe Midsole Density on Knee Position at Initial Contact with the Ground 89 Means and Standard Deviations: Knee Position at Peak Vertical Ground Reaction Force 90 Statistical Summary of Athletic Shoe Midsole Density on Knee Position at Peak Vertical Ground Reaction Force 90 Means and Standard Deviations: Maximum Angular Displacement of the Knee 91 Statistical Summary of Athletic Shoe Midsole Density on Maximum Angular Displacement of the Knee 91 Table 30. Means and Standard Deviations: Left Knee Range of Motion 92 Table 31. Table 32. Means and Standard Deviations: Right Knee Range of Motion 93 Means and Standard Deviations: Hip Position at Initial Contact with the Ground 94 x

12 Table 33. Table 34. Table 35. Table 36. Table 37. Statistical Summary of Athletic Shoe Midsole Density on Hip Position at Initial Contact with the Ground 94 Means and Standard Deviations: Hip Position at Peak Vertical Ground Reaction Force 95 Statistical Summary of Athletic Shoe Midsole Density on Hip Position at Peak Vertical Ground Reaction Force 95 Means and Standard Deviations: Maximum Angular Displacement of the Hip 96 Statistical Summary of Athletic Shoe Midsole Density on Maximum Angular Displacement of the Hip 96 Table 38. Means and Standard Deviations: Hip Range of Motion 97 Table 39. Means and Standard Deviations: Vertical Hip Displacement 98 Table 40. Statistical Summary of Athletic Shoe Midsole Density on Vertical Hip Displacement 98 Table 41. Means and Standard Deviations: Vertical Hip Position 99 Table 42. Table 43. Table 44. Table 45. Table 46. Table 47. Means and Standard Deviations: Time to Maximum Flexion Angle of the Ankle 101 Statistical Summary of Athletic Shoe Midsole Density on Time to Maximum Flexion Angle of the Ankle 101 Means and Standard Deviations: Time to Maximum Flexion Angle of the Knee 102 Statistical Summary of Athletic Shoe Midsole Density on Time to Maximum Flexion Angle of the Knee 102 Means and Standard Deviations: Time to Maximum Flexion Angle of the Hip 103 Statistical Summary of Athletic Shoe Midsole Density on Time to Maximum Flexion Angle of the Hip 103 Table 48. Post Participation Questionnaire Results 104 xi

13 List of Figures Page Figure 1. Typical Athletic Shoe Construction 11 Figure 2. Anatomy of an Athletic Shoe 13 Figure 3. Fila Athletic Shoes used for data collection, soft midsole, Control midsole, and hard midsole 53 Figure 4. Position of 6 cameras, frontal, sagittal and transverse views 56 Figure 5. Alignment of the force plates 56 Figure 6. Frontal View of Experimental Set Up, testing area, position of six cameras, force plates and Crush It volleyball 57 Figure 7. Retroflective Marker and Sacral Wand 60 Figure 8. Position of 31 retroflective markers during static trial 61 Figure 9. Figure 10. Peak Vertical Ground Reaction Forces: After Initial Contact with the Ground 71 Total Peak Vertical Ground Reaction Force: After Initial Contact with the Ground 72 Figure 11. Left Peak Vertical Ground Reaction Force 73 Figure 12. Right Peak Vertical Ground Reaction Force 73 Figure 13. Total Peak Vertical Ground Reaction Force 74 Figure 14. Loading Rate: Time to Peak Vertical Ground Reaction Force 75 Figure 15. Peak Ankle Joint Moments 76 Figure 16. Peak Knee Joint Moments 77 Figure 17. Peak Hip Joint Moments 78 Figure 18. Ankle Position at Initial Contact with the Ground 84 Figure 19. Ankle Position at Peak Vertical Ground Reaction Force 85 Figure 20. Maximum Angular Displacement of the Ankle 86 xii

14 Figure 21. Left Ankle Range of Motion: From Initial Foot Contact to 0.75 sec Post Impact 87 Figure 22. Right Ankle Range of Motion: From Initial Foot Contact to 0.75 sec Post Impact 88 Figure 23. Knee Position at Initial Contact with the Ground 89 Figure 24. Knee Position at Peak Vertical Ground Reaction Force 90 Figure 25. Maximum Angular Displacement of the Knee 91 Figure 26. Left Knee Range of Motion: From Initial Foot Contact to 0.75 sec Post Impact 92 Figure 27. Right Knee Range of Motion: From Initial Foot Contact to 0.75 sec Post Impact 93 Figure 28. Hip Position at Initial Contact with the Ground 94 Figure 29. Hip Position at Peak Vertical Ground Reaction Force 95 Figure 30. Maximum Angular Displacement of the Hip 96 Figure 31. Hip Range of Motion: From Initial Foot Contact to 0.75 sec Post Impact 97 Figure 32. Vertical Hip Displacement (a measurement of jump height) 98 Figure 33. Vertical Hip Position: Maximum Vertical Hip Position during the Jumping Phase and Hip Position in Static Stance 99 Figure 34. Time to Maximum Flexion Angle of the Ankle 101 Figure 35. Time to Maximum Flexion Angle of the Knee 102 Figure 36. Time to Maximum Flexion Angle of the Hip 103 xiii

15 Chapter One INTRODUCTION The function of athletic shoes is to protect the feet from the stresses of athletic activity while permitting the athlete to achieve their maximum potential. During physical activity a large amount of force passes through the foot and lower extremities every time an athletic shoe strikes the ground. 25 A basketball player landing after jumping for a rebound can experience a landing force that exceeds five times their body weight. 67 Athletic shoes are designed to protect the athlete, and prevent injury. The midsole of an athletic shoe, in particular, can be modified to help control the amount of force attenuated during impact with the ground. 25 The degree of shock absorption provided by an athletic shoe is determined by the material characteristics and construction of its midsole. The differences in design and variations in material, weight, lacing characteristics and other factors engineered into athletic shoes are meant to protect the areas of the feet that encounter the most stress. 25 The typical construction of an athletic shoe consists of an insole, a midsole and an outsole. The insole is in direct contact with the foot, and is usually made of compressible foam that conforms to the foot to improve comfort and provide a minimal amount of shock absorption. 12, 25, 52 The midsole is located between the insole and the outsole, it provides cushioning and support. 25 The outsole is the treaded layer 1

16 2 6, 21, that is in direct contact with the ground; it provides traction and resists wear. 56 The midsole design of athletic shoes has evolved over the past 25 years from shoes that were relatively flat, for example Converse All Stars, to the current designs that utilize a significantly increased heel thickness. This increased heel thickness is used to provide better shock absorption and cushioning, especially 40, 50 during jumping and pounding activities. The midsole of an athletic shoe requires a delicate balance of support and cushioning. The support or stability provided by the midsole helps control excessive foot movements. Athletic shoes may contain different densities of foam or more rigid devices in specific areas of the midsole to aid in controlling abnormal foot motion. 4, 26 The primary purpose of cushioning in athletic shoe midsoles is to protect the body from the consequences of repeated impacts between the foot and the ground. Various materials have been used in athletic shoe midsoles to improve cushioning. 61 The cushioning provides an interface for shock absorption by spreading out the force of impact so it is not transferred directly to the feet and legs of the athlete. 25 Athletic shoe midsoles attempt to attenuate impact force by inserting a soft material at the foot-ground interface. The additional deformation of the midsole should act to reduce the stiffness of the impacting system, therefore, reducing impact force. 38 The modeling research by Gerritsen, van den Bogert and Nigg supported these presumptions. 28

17 3 The attenuation of impact forces during landing is important because the initial contact of the foot with the ground results in significant impact force. High impact forces and impact loading rates have been related to cartilage degeneration, fatigue fractures, shin splints, Achilles tendon problems, and hematological problems. It has been demonstrated that many of these injuries occur due to the excessively high forces acting on the body, rather than insufficient structural properties of the human body. It is the job of a well designed athletic shoe to reduce this impact force for the athlete without 40, 61 interfering with performance. Impact force can be defined as the force generated by a collision between two objects. The shock absorbing capability of an athletic shoe midsole should attenuate the impact forces between the athlete s foot and the ground. 25 There is general consensus among the researchers that cushioning is needed in athletic shoes for shock absorption and comfort. 15, 25, 38-40, 46, 50, 59-61, 71, 77 The ability of an athletic shoe midsole to attenuate impact forces is important for the prevention of pain and the development of musculoskeletal overuse injuries related to repetitive impacts. 10, 29 Schwellnus, Jordon and Noakes 71 indicated that improved shock absorption in athletic footwear could reduce the incidence of injury. Conversely, lack of cushioning in shoes was implicated as a cause of athletic injuries. 71 There is controversy, however, regarding how much cushioning should be in the midsole of an athletic shoe. Nigg and Segesser 61 suggested that changes in cushioning properties relate more to comfort than injury prevention. Robbins

18 4 and Gouw 69 caution that increased cushioning can actually lead to injury by reducing the sensory feedback coming from the plantar surface of the foot. Their research indicated that increased cushioning in an athletic shoe increased impact force with the ground upon landing. 69 Midsole cushioning is important in reducing impact loads, however, too much cushioning has been associated with instability and greater likelihood of injury. 15 Nigg, Bahlsen, Denoth, Luethi and Stacoff 52 found that, in studies of athletic shoes that were identical except for the midsole density, ground reaction forces were not what was expected, when compared to material tests. They found no significant difference in ground reaction forces during running; attributing the similarities in ground reaction forces to kinematic adaptations of the athletes during their running gait. They suggested that athletes were able to increase knee flexion while wearing the harder midsoles, to compensate for the changes in impact force. Their research indicates that athletes are able to compensate for variations in midsole density during rhythmic athletic activities, such as running. 52 Further evidence that knee activity changes with changes in midsole density comes from Frederick, Clarke and Larsen 23. They found a correlation between knee flexion velocity, caused by midsole hardness, and oxygen consumption. When the knee is more flexed the muscles have to work harder, requiring additional effort to consistently use knee flexion to attenuate impact forces. 23 Increased knee flexion initially appears to reduce impact peaks when running, but these increased knee flexion angles are not as effective as the

19 5 muscles fatigue. 23, 38 Therefore, the protective functions of an athletic shoe 38, 73 midsole might vary, depending on the level of fatigue. Sharkey, Ferris, Smith and Matthews also suggested that muscle fatigue can contribute to an increase in injuries because of the muscles decreased ability to attenuate forces. They indicated that repetitive impact forces were less detrimental than the increase in peak strain imposed by tired and uncoordinated muscles. 73 This further reinforces the need for an external protective mechanism, such as a sufficiently cushioned athletic shoe. An appropriately cushioned athletic shoe midsole should provide external force attenuation that is resistant to fatigue and may be better able to consistently reduce the transmission of impact shocks upon landing. 38 Extensive research has been conducted on the effects of constructional changes in the midsole of running shoes. 2, 15, 38, 50, 51, 53, 59, 61, 74, 86 Evaluations of athletic shoe cushioning, however, have produced inconclusive results. Most locomotor studies have shown improved cushioning of shod over barefoot conditions but they fail to detect differences between athletic shoes with different midsole properties during running trials. 13, 15, 37, 38, 53, 61 Anticipated reductions in force rate or magnitude through changes in midsole density has rarely be demonstrated. 2, 15, 38, 50, 51, 53, 59, 61, 74, 86 Kinematic adaptations have been used as 15, 28, 53 a possible explanation of these results. Most of the research on athletic shoes and midsole cushioning has focused on running. 4 Running is a very rhythmic activity. When running, the ground reaction forces can be changed or maintained through kinematic

20 6 adjustments made by the runner because the runner knows the next step should be similar to the last step. 26 In running, shock induced acceleration at the level of the tibia is between 5 g and 15 g. In contrast, a basketball player landing from a jump may experience up to 20 g s, generating shock waves at a level that has been implicated in athletic injuries. 4 Landing after a dynamic activity that is nonrhythmic produces tibial shock accelerations up to four times those seen when running. The role of the athletic shoe midsole cushioning may be crucial in athletic activities that are not rhythmic, but rather are practiced sequenced activities. There is limited data on the role of athletic shoe midsoles in jump landing sequences which are practiced but non-rhythmic skills. When landing from a vertical jump, athletes rely primarily on the lengthening of active muscle during joint flexion to attenuate the forces experienced during initial contact with the ground. 45, 49 If the midsole of an athletic shoe is appropriately cushioned, it should help delay the onset of peak impact force, increase impact duration, attenuate shock induced acceleration and redistribute pressure beneath the feet. The present study is designed to explore the effect of variations in athletic shoe midsole density on attenuating impact forces while athletes perform a jump landing sequence. Volleyball jump landing sequences were selected for study because of the continuous non-repetitive landing forces that occur during volleyball play. Volleyball players also frequently experience lower extremity injuries as a result of repetitive high impact peak landing forces experienced 9, 16 during play.

21 7 Statement of the Problem Running is a repetitive activity that allows athletes to get into a rhythm. When the midsole density of an athletic shoe is altered, a runner subconsciously responds to these differences by making kinematic adaptations after a series of repetitive and rhythmic gait cycles. As a result research has indicated that impact forces during running remain constant regardless of the density of the midsole. There is limited research determining what effect variations in midsole density will have on impact forces during a non-rhythmic jump landing sequence that involves an approach. Further, there is limited evidence in the literature demonstrating that athletes performing non-rhythmic jump landings will adapt kinematically to differences in athletic shoe midsole density. Purpose of the Study To determine the influence of variations in athletic shoe midsole density on impact forces, loading rates, and peak joint moments after landing from a volleyball spike approach jump. To determine what specific kinematic variables are affected when landing after a jump with athletic shoes of varying midsole densities Hypotheses H 1 : It is hypothesized that vertical ground reaction forces will be significantly different when landing in athletic shoes with different midsole densities. H 2 : It is hypothesized that loading rates will be significantly different when landing in athletic shoes with different midsole densities.

22 8 H 3 : It is hypothesized that peak joint moments will be significantly different when landing in athletic shoes with different midsole densities. H 4 : It is hypothesized that specific kinematic variables will be significantly different when landing in athletic shoes with different midsole densities.

23 Chapter Two REVIEW OF THE LITERATURE The following review of literature is divided into 4 sections: athletic shoe anatomy; impact forces; landing mechanics; and midsole cushioning. A review of the basic construction of athletic shoes is provided, followed by a discussion of the purpose and function of the athletic shoe midsole. The various components of an athletic shoe that provide comfort and protection are discussed with specific attention to the cushioning effects of the athletic shoe midsole. The midsole s effect on performance and gender differences are also briefly discussed. To understand the effect athletic shoes can have on impact force, it is important, first, to understand impact forces and attenuation of these forces. A general description of the components of impact forces are provided, followed by methods of attenuating impact forces. The effect athletic shoes have on attenuating impact forces enlarges on this basic discussion. The third section of this review will examine landing forces. It is important to fully understand the biomechanical principals related to landing forces and the possible strategies for dissipating impact upon landing. Landing mechanics and their specificity to volleyball athletes enlarge on this basic discussion. Current research on landing mechanics, athletic shoe midsoles, and volleyball are also examined. 9

24 10 The final section of this literature review focuses on midsole cushioning in an athletic shoe. There is a controversy in the literature regarding the potential positive or negative effects of midsole cushioning in athletic shoes. Some studies demonstrate the benefits of midsole cushioning, while others have shown a negative association between midsole cushioning, performance and safety. The controversy surrounding the benefits and drawbacks of cushioning the midsole of athletic shoes is currently unresolved and reviewed in detail. There are, however, aspects of this controversy that have yet to be examined. This review of the literature will examine some of these areas more fully and open a discussion regarding the effect of midsole cushioning in volleyball, where the athlete is subjected to repetitive, non-rhythmic landing forces.

25 11 Anatomy of an Athletic Shoe Shoe Construction Athletic shoes are constructed with different features and made of various materials that are commonly assumed to improve comfort and performance and to reduce the frequency of overuse injuries. 25, 58, 61 Typical construction of an athletic shoe consists on an insole, midsole and outsole (Figure 1). The foot shoe complex forms the dynamic base upon which an athlete functions. What happens at the foot shoe interface affects the total functional mechanism of the athletic shoe. 6 Figure 1. Typical Shoe Construction The upper is the part of an athletic shoe that holds the foot to the sole of the shoe and is generally the most aesthetic part of the shoe. It typically is very

26 12 colorful and contains the company logo. The upper is designed to enclose and help stabilize the foot on the sole. In traditional shoe production terms the part of the upper covering the forefoot is called the vamp. 12 The vamp is the area on the shoe where the laces are found. The lacing pattern can affect how the shoe fits. A good lacing system allows the upper to apply uniform pressure over the entire forefoot. Many athletes experience impingement of nerves and tendons on the top of their foot if the laces are not in the proper place. There are many ways to lace up an athletic shoe to relieve pressure points and still prevent heel slipping during activity. The tongue is another method of relieving impingement from laces. It is designed to protect the top of the foot from the pressure of the laces. The insole is the bottom inside portion of the shoe that is in direct contact with the foot. The first insole was developed out of cork in the 18 th century in Germany. 52 Throughout the evolution of insoles various materials have been used such as leather, cork, wood, metals and plastics. In athletic shoes today, plastics, in different combinations, are primarily used to create the athletic shoe insole. Most shoes have removable insoles. Good insoles are made from compressible foam that will mold to the contours of an athlete s foot. The insole is constructed to improve comfort and provide a minimal amount of shock absorption. It is important to realize that insoles tend to wear out faster then many other parts of an athletic shoe and should be replaced often to maintain 12, 25, 52 their effect. The midsole is the layer between the insole and outsole. Most of the recent advances in athletic shoe production have been made in midsole design

27 13 and materials. Support and cushioning, two of the most important elements found in athletic shoes, are based on the construct of the midsole. 25 The outsole is the treaded layer of the shoe which is in direct contact with the ground; it provides traction and resists wear. The amount of friction generated between the shoe and the athletic surface can influence injuries and the rate at which they occur. 21, 56 The traction pattern and materials of the outsole are usually developed specific to the playing surface. For example, a court shoe will usually be flat and flexible with a lot of traction, while a trail or hiking shoe will have a more rigid outsole with a more aggressive traction pattern. Outsole materials, however, have been found to have more influence on traction than the pattern on the bottom of the shoe. 21 The correct use of the term sole actually refers to the combination of the insole, midsole, and outsole. Figure 2. Anatomy of an Athletic Shoe The toebox or toe wrap is located in the front of the shoe and encloses the toes (Figure 2). It is important to have adequate room in the toebox to prevent

28 14 friction and irritation related injuries. Some toeboxes are made especially wide or narrow to accommodate various foot types. 12 The heel counter surrounds the back of the heel and prevents excessive rearfoot motion. It stabilizes the heel and aids in motion control. It is usually a 3, 52, 75 reinforced plastic cup within the upper portion of the athletic shoe. An important aspect of shoe design is the last of the shoe (type of last) and how the shoe is lasted (how the upper of the shoe is attached to the sole). The word last originates from the Old English laesk, meaning sole, footprint, or track; it refers to the shape of the shoe. 52 The first lasts were chiseled out of stone and later whittled out of wood. In 1969, the plastic last was developed by the Sterling Last Corporation, U.S.A., and today the shoe industry primarily uses plastic lasts. 52 The last is the mold or form on which the shoes are built, either a straight, curved, or semi-curved last. Shoe manufacturers construct shoes over a last, or model, that resembles a generic or average foot. Unfortunately, each person s foot is slightly different and the generic last may not exactly reflect the shape of all feet. It is possible to determine how a shoe is lasted by removing the insole to reveal the type of lasting used in the construction. There are three general methods of lasting, slip, board, and combination. In a slip lasted shoe the upper is sewn together at the bottom and then glued to the sole. This allows the shoe to be light and flexible. In a board lasted shoe the upper is sewn to a board, similar to cardboard and then attached to the sole. This creates a more rigid shoe with more support. A combination lasted shoe has a slip lasted front and a board

29 15 lasted back; this combination allows good flexion and cushioning while maintaining rearfoot control. 12 Athletic Shoe Midsole As mentioned previously the primary purpose of an athletic shoe midsole is to provide cushioning and support. Cushioning provides an interface for shock absorption, by spreading out the force of impact so it is not transferred directly to the feet and legs of the athlete. The midsole additionally provides support and stability to help control excessive foot movements. 25 Support is an important element in the athletic shoe midsole; it directly affects the health and comfort of the feet. Athletic shoes may contain different densities of foam or more rigid devices in specific areas of the midsole to aid in controlling the motion of the foot. 4 The midsoles affect on motion is brought about by a change in the effective lever arm of the ground reaction forces. Softer and more flexible shoes have a smaller lever arm resulting from different deformation of the midsole causing less pronation than shoes that are harder. 26, 30 The shoe hits at an angle to the ground, and the lateral edge of the shoe can either bend or compress so the effective lever arm of the ground reaction force is shifted medially. A smaller lever arm results in a smaller moment; this causes the subtalar joint to pronate more slowly than in an athletic shoe with a harder midsole. The lever arm affects the speed of pronation more then the total amount of pronation. Shoe manufacturers have addressed this problem by making shoes with dual density midsoles. 26 One example of this is the medial post, where a firmer density of midsole material is added to the inner side of the midsole of an

30 16 athletic shoe. This medial post is designed to reduce overpronation. The medial post has also been called the Footbridge (Nike), Support Bridge (Reebok), Diagonal Rollbar (Brooks) and Graphite Rollbar (New Balance). Although the shoe is considered a powerful manipulator of human movement, an extreme overpronator will always force the weakest material to yield. Therefore, footwear 4, 56, 74 design may not solve overpronation or correct bad running styles. The construction of the athletic shoe midsole is very important and requires a delicate balance between cushioning and support. A conflict of requirements exists when considering midsole material. A soft shoe designed for maximum cushioning may deform when loaded, resulting in increased rearfoot motion, whereas a thin firm midsole may minimize rearfoot motion but transmit 4, 15, 22 high impact forces. The primary purpose of cushioning in an athletic shoe midsole is to protect the body from the consequences of repeated impacts between the foot and the ground by providing an interface. The shock absorbing and attenuating properties of an athletic shoe are mainly determined by the type of material that is inserted into the midsole. 25 The density or hardness of a midsole is measured in durometers. A durometer is the international standard for measuring the hardness of rubber, foam rubber, plastic and most nonmetallic materials. Durometer hardness can be measured on different scales, typically the lower the number the softer the material, the higher the number the harder or more rigid the material. 26 Inserting any type of cushioning in an athletic shoe represents an

31 17 attempt to attenuate impact forces by inserting a material at the foot ground interface. 38 Athletic shoe midsoles function to reduce impact force by delaying the timing of the impact force peak through the use of cushioning. 59 By inserting a soft material at the foot-ground interface the additional deformation of the midsole should act to reduce the impacting system. 28, 38 In theory, a thicker sole will deform more than a thinner one causing more attenuation of impact forces. 4 The additional deformation of the midsole acts to reduce the stiffness of the impacting system, upon contact of the shoe to the ground. 38 In simpler terms, the midsole cushioning is compressed causing a delay in landing force or vertical impact force peak at footstrike. When all other factors are equal, collisions that involve greater deformations are generally characterized by lower peak forces and slower rates of loading. 2 According to this theory, the more cushioning, the more impact is attenuated. A number of different adaptations have been made to athletic shoes to improve midsole cushioning. 59 As a result, the midsole of athletic shoes have evolved over the past 25 years from relatively flat (for example, Converse All Stars) to the current designs that utilize a significantly increased heel thickness. This increased heel thickness is created by increased cushioning in the midsole which is believed to provide better shock absorption and cushioning, especially 40, during jumping and pounding activities. Shoe manufacturers must carefully consider the type and thickness of cushioning placed in an athletic shoe. In order for the midsole to assist in force

32 18 attenuation, shoe midsole materials need to be sufficiently stiff and retain their spring characteristics to prevent the midsole from bottoming out during impact. 4, 21, 52 When midsole cushioning is too soft it leads to maximum compression of the 52, 59 material, which can result in less support and loss of attenuation. Athletic shoe midsoles are usually manufactured from a combination of two basic materials, ethyl vinyl acetate (EVA) and polyurethane. These two materials have very different characteristics; EVA is light, has excellent cushioning properties, and can be manufactured at different densities. Polyurethane is denser and heavier but is more durable than EVA. Both of these materials have also been used to encapsulate other cushioning materials such as air (Nike), gel (Asics), silicone (Brooks), honeycomb pads (Reebok and Puma) 25, 61 and EVA (New Balance). One of the most well known forms of cushioning is the air midsole. Nike first introduced this concept in 1979, using encapsulated air pockets in the midsole to enhance cushioning. 25 Other companies have further implemented the concept of using air to cushion the midsole with some variations in types of air and placement within the shoe. Ambient air has been used by Etonic and freon was used by Nike. 25 In today s athletic shoes various types of midsole cushioning can be found in the heel, forefoot, or both, depending on the demands of the activity or the athlete. 25 This allows manufacturers to produce athletic shoes that can accommodate the different characteristics of various athletic movement patterns.

33 19 In the early 1960 s, several sports medicine clinics and research centers started projects to study the connection between sports activities, the occurrence of sports injuries, and the influence of footwear on movement and load characteristics. 72 Many of the changes in shoe construction over the last 30 years, however, have been purely cosmetic and simply a response to everchanging fashion trends with little concern for athletic function. 40 Performance and Athletic Shoes From an orthopedic point of view, with no regard to performance related criteria, athletic shoes should be constructed to: 1) support the function of the foot, 2) adapt to the physiological ranges of the foot, 3) avoid excessive rotational movements due to excessive moment arms, and 4) attenuate excessive forces. 54, 61 Shoes constructed according to these criteria are assumed to restrict motion and to avoid excessive movements in the joints. Consequently, the internal structures should be less strained and stressed, and the frequency injury should be reduced. 54, 61 These theoretical considerations, however, ignore athletic performance related criteria such as improvement in performance. Stacoff, Steger, Stussi and Reinschmidt 78 found that ignoring performance related variables in athletic shoes can be detrimental to athletic performance. They found that higher cut athletic shoes can reduce the risk of ankle sprain injuries but only at the expense of full ankle mobility and overall athletic performance. 78 Athletes may choose to wear lower cut athletic shoes, compromising protection for improved athletic performance. 78 Injury prevention in athletic shoe design is an

34 20 important criteria but success on the playing field is what is most important to the athlete. Many concepts have been studied with regard to increasing athletic performance and athletic shoes. Two specific concepts presented by Stefanyshun and Nigg 79 are return of energy and reduction of loss of energy. For each joint, there are phases where energy is absorbed and phases where energy is produced. If the absorbed energy is dissipated and not stored for later re-use, it could be speculated that a reduction of such energy absorption might lead to an increase in performance. 80 Return of energy to improve athletic performance has been studied for both sport surfaces and athletic shoes. 61 In theory, a system must be able to return the energy at the right time, location, and frequency. Sport surfaces can be appropriately tuned to return energy; surfaces for track and field 61, 79 gymnastics and are successful examples of energy return by sport surfaces. Return of energy by athletic shoes has been attempted several times, but has never been completed successfully. 82 The main reasons that these attempts were unsuccessful are that the materials associated with cushioning are not good energy return materials and the location of maximal possible energy storage (the rear foot) is not the location where effective use can be made of returned energy. 79 Possible shoe-related factors affecting energy conservation or reduction in loss of energy include: a) work against gravity (weight of shoe); b) work for acceleration; c) work due to cushioning; and d) work to stabilize joints. 61 The concept of reducing the loss of

35 21 energy has been unsuccessful and theoretical attempts to use this approach are limited in the literature. 79 Gender Differences in Shoe Design A very important factor in athletic shoe design is the anatomical differences between the male and female foot. The foot of the female tends to have a narrower heel in relation to the forefoot, a smaller Achilles tendon, and is narrower overall than a man s foot relative to length. Athletic shoes are currently being designed for men and women through research studies involving only men. 24 Females have different needs from athletic shoes than males. In many cases, athletic shoes are built on a scaled down version of a man s shoe rather than based on the specific anatomy of the female foot. The result is an athletic shoe that improperly supports the foot of a female. 61 Another gender difference is that the female athlete completes the heel-totoe gait cycle faster than the male athlete, and females almost always have a shorter leg length. 24 Therefore, female athletes take more steps to cover the same distance causing the female athlete s foot to strike the ground more often than male athletes. These increased impacts for the female athlete may require increased cushioning in a female athletic shoe. Another problem in athletic shoes design for women is that most of the studies on impact forces and performance are being performed with male subjects. These results cannot simply be translated and applied to women because the foot and ankle in females are structurally and biomechanically

36 22 different from males. 24 More research with female athletes is needed in the area of athletic shoe design. Impact Forces One of the first things to understand about impact forces is how force is transmitted from the ground to the foot. Newton s third law, For every action there is an equal and opposite reaction, is essential to understand forces acting on the body. In static stance, ground reaction forces are equal and opposite to the pull of gravity acting on the body and the shoes. The force from the shoe acting on the foot is equal and opposite to the weight of the body. 26 The force from the femur acting on the tibia is equal and opposite to the weight of the femur plus the rest of the body above the femur. Therefore, force acting on any particular body part in static stance can be determined by the weight of the rest of the body above it. 26 Forces are not as easily explained when they involve a fall from a height. Running and landing activities have been compared to falls from a height. 26 When an object is moving, it has momentum, equal to mass times velocity. An impulse, equal to force times time, is required to reduce the speed and the momentum of a body part to zero. A high amount of force, times a short amount of time, can produce an impulse equal to a lower amount of force, times a longer amount of time. In running and landing activities not all body parts stop their vertical motion at the same time. When the trunk takes a longer time to stop, the force required to stop it is less, lowering the effect of impact on the feet. 26

37 23 The previous explanation of impact forces mentioned the effect of ground reaction forces on different joints of the body. There are also forces at the joints generated by the muscles. When the soleus fires, it pulls the calcaneus upward and, at the same time, pulls the tibia downward, increasing the compressive forces at the subtalar and ankle joints. These types of forces are known as internal forces; they are usually calculated and cannot be measured directly like ground reaction forces. The compressive force at the knee from the quadriceps has been calculated as high as four times body weight at footstrike during 26, 61 running. Research indicates that there is a range of possible impact force accommodation strategies. 5, 34 Bates 5 suggested three specific categories of responses to impact forces: Newtonian, biomechanical, and neuromuscular. In a Newtonian response an increase in impact force is linearly related to the increase in the applied stressor and occurs at an equal rate. The impact force is determined by mechanical events and no biological accommodation occurs. 5, 34 A biomechanical response results when an increase in impact force occurs at a rate less than the rate of increase of the applied stressor. Partial biological accommodation occurs, but the response is mechanically driven. 5, 34 In a neuromuscular response, total biological accommodation occurs and an increase in the magnitude of the stressor does not change the magnitude of the impact force. 5, 34 Impact forces during human locomotion are the forces that result from the foot colliding with the ground. 55 A recent review article by Whittle, 85 indicated that

38 24 upon landing, ground reaction forces produce internal loading to the lower extremities and cause transient stress waves to travel up throughout the skeletal structures of the body. 85 Research suggests that stress waves (high frequency contacts) are major factors in the development of degenerative osteoarthritis, and cartilage breakdown. 29, 48 These high loading frequencies have been related to cartilage injury due to the viscoelastic behavior of the cartilage. High frequency vertical contact forces within joints prevent fluid flow within the cartilage which leads to radial displacements and high tensile forces in the collagen fibers. 63 High impact forces or impact loading have also been related to fatigue fractures, shin splints, Achilles tendon problems, and hematological problems. It has been demonstrated that many of these injuries occur due to excessive forces rather 29, 40, 48, 61 than insufficient structural properties of the human body. In order to reduce impact force three materials are thought to cushion the heel during landing: the midsole material of the shoe; the material of the running surface; and the soft tissue of the heel. 50 Cushioning is defined as any attempt to reduce the amplitude of the vertical ground reaction force during impact. 55, 61 This definition includes any method resulting in reduced impact forces, such as changes in shoe construction or movement. 61 The midsole of the athletic shoe is the easiest to change, by altering the cushioning properties of the shoe, but this is not the only method of reducing vertical impact force during footstrikes or landing. One of the goals of athletic shoes is to attenuate impact forces and accelerations that cause overloading of the musculoskeletal system and injury. It

39 25 has been suggested that injuries can be reduced by reducing the transmission of impact force upon landing through the use of cushioning. 29 An athletic shoe s cushioning or shock absorbing system, protects the body from potentially injurious repeated impact, by modifying the properties of the material used in the midsole. The effect of the midsole is to delay the onset of the peak impact force, increase impact duration, attenuate higher frequencies of the shock, and redistribute pressure beneath the foot. Improved shock absorption by athletic shoe midsoles should reduce the incidence of overuse injuries associated with loading (excessive forces and repetitive forces) but not alter the incidence of acute injuries or injuries where excessive shock wave transmission is not implicated. 71 Schwellnus, Jordan and Noakes 71 found a reduced incidence of tibial stress syndrome, and stress fractures, in a group of military recruits wearing shock absorbing insoles. This study demonstrates that with increased cushioning it is possible to reduce impact force and reduce the incidence of injury. It is the job of the athletic shoe to reduce impact force for the athlete without interfering 40, 61 with performance. The only interface between the ground and soft tissues of the foot are athletic shoes. 86 Many research studies have focused on the cushioning of athletic shoes to decrease this impact force. 15, 25, 38-40, 46, 50, 59-61, 71, 77 The midsole is one characteristic of athletic shoes that can be easily modified to control the amount of impact force absorbed. 25 The amount of shock absorption found in an