Characteristic Plantar Pressure Distribution Patterns During Soccer-Specific Movements

DOCTYPE = ARTICLE Characteristic Plantar Pressure Distribution Patterns During Soccer-Specific Movements Eric Eils, Markus Streyl, Stefan Linnenbecker, Lothar Thorwesten, Klaus Völker, and Dieter Rosenbaum From the Funktionsbereich Bewegungsanalytik (Movement Analysis Lab), Klinik und Poliklinik für Allgemeine Orthopädie, University Hospital Münster, Germany, and Institute of Sports Medicine, University Hospital Münster, Germany Purpose: To characterize in-shoe pressure measurements during different soccer-specific maneuvers on two playing surfaces to identify the main loading areas of the foot. Methods: Twenty-one experienced male soccer players participated in the study (25.5 ± 1.8 years, 78.7 ± 5.4 kg, and 182.9 ± 5.7 cm). The Pedar Mobile system was used to collect plantar pressure information inside the soccer shoe. Four soccer-specific movements were performed (normal run, cutting maneuver, sprint, and goal shot) on both a grass and a red cinder surface. Results: Results showed characteristic pressure distribution patterns with specific loading areas of the foot that correspond to the evaluated movements. In addition, loading patterns with higher pressure values than those observed during normal run were found. In cutting, the medial part of the foot; in sprinting, the first and second ray; and in kicking, the lateral part of the foot are predominantly loaded. No global effect of the two surfaces on pressure parameters was found. Conclusion: The results of the present investigation suggest that the high load in soccer in combination with a high repetition may have an important influence in the development of overuse injuries. Keywords: soccer; pressure distribution; biomechanics; overuse injuries; playing surface Soccer is the most popular sport worldwide. The Fédération Internationale de Football Association represents 203 countries including more than 200 million licensed participants. 9 The Union des Associations Européennes de Football represents 49 European nations with approximately 20 million participants, and the German Soccer Association listed 6.25 million participants in 2000. 6,28 These statistics do not include players who play soccer on an unorganized basis. Stress fractures and overload injuries are a common problem in amateur as well as professional athletes. 17 For example, 38% of the members of the 1994 U.S. National World Cup Soccer team were diagnosed with stress fractures. 17 In a 1-year prospective study in 264 soccer players, 23% of overuse injuries concerning the knee and the lower This work was presented in the Foot Biomechanics Award session of the 18th Congress of the International Society of Biomechanics, ETH Zürich, July 8 13, 2001. Address correspondence and reprint requests to Dr. Eric Eils, Funktionsbereich Bewegungsanalytik (Movement Analysis Lab), Klinik und Poliklinik für Allgemeine Orthopädie, University Hospital Münster, Domagkstr. 3, 48129 Münster, Germany (e-mail: eils@uni-muenster.de). The American Journal of Sports Medicine, Vol. 32, No. 1 DOI: 10.1177/0363546503258932 2004 American Orthopaedic Society for Sports Medicine leg were found. 25 It was also reported that 97% of the players from the U.S. men s senior national and Olympic soccer team had extra bony growths such as ostheophytes as a result of repeated micro and macro traumata. 22 Different extrinsic and intrinsic risk factors might be involved in the etiology of these injuries. 7,13,17 Intrinsic risk factors are related to individual biological or psychosocial characteristics (for example, age, previous injuries, and inadequate rehabilitation), and extrinsic risk factors are related to variables of the environment (for example, exercise load, equipment, and playing field conditions). Although the soccer shoe as one extrinsic factor is thought to contribute to the risk of overuse injuries, 13,17 there is no quantitative information available concerning the footloading characteristics during soccer-specific movements. Knowledge about the location and the amount of load acting on the sole of the foot is important for the development of specific shoe/insole designs and may also help to prevent overuse injuries. Therefore, the purpose of the present investigation was to characterize and to compare in-shoe pressure measurements during soccer-specific movements on two different surfaces to identify the predominantly loaded anatomical structures and to quantify the specific load distribution under the foot. Two hypotheses were investigated: 1) different soccer-specific movements have a significant influ- 140

Vol. 32, No. 1, 2004 Plantar Pressure Distribution Patterns 141 ence on pressure distribution patterns, that is, specific high-loading areas of the plantar foot inside the soccer shoe are clearly identifiable for different movements; and 2) surface conditions have a significant influence on pressure distribution patterns inside the shoe. 7 m 12 m Stop Start MATERIALS AND METHODS Twenty-one experienced male soccer players participated in the study. Their mean age, mass, and height were 25.5 ± 1.8 years, 78.7 ± 5.4 kg, and 183 ± 6 cm, respectively. At the time of the study, all players were free of injuries and had no history of injuries to the lower legs within the last 3 months before the start of the study. Prior to participation, all subjects signed an informed consent form. All subjects had a shoe size between 9.5 and 11.5 and were fitted with new soccer shoes with a 12-stud traditional (elliptic) plate (Air Zoom Brasilia F.G., Nike, Inc.). All players played soccer on a regular basis, and the performance level of the players varied between the sixth (Landesliga) and the ninth (Kreisliga B) highest league in Germany. The Pedar Mobile system (Novel GmbH, Munich) was used to collect plantar pressure distribution. The system consisted of two flexible insoles (each containing 99 sensors in a matrix design) and a portable datalogger for data storage. The sampling frequency was 50 Hz, and the system had a resolution of 1.7 cm 2 per sensor. The sensor measurement range was 30 kpa to 1.2 MPa with a sensitivity of 10 kpa. The accuracy was better than 5% over a temperature range of 10 C to 40 C. The system is reported to have good to excellent reproducibility during walking and running. 15,16,20 In pretests, peak pressures under some areas exceeded the maximum calibration of the standard system (600 kpa). Therefore, the sensors were calibrated up to 1 MPa with an air pressure device (True Blue, Novel GmbH, Munich). The insoles were fitted into the shoe, and four soccer-specific movements were performed on both a grass and a red cinder surface on an athletic field in the following order: a normal run at 4.2 m/s, a cutting maneuver at approximately 70% of maximum speed, a sprint at maximum speed, and a goal shot. For the normal run, subjects ran a distance of 150 m and measurements were made within the last 25 m. The velocity was controlled by a combination of acoustic signals and cones that were positioned in discrete time/distance intervals. For the cutting maneuver, a 7-m wide and 40-m long area was prepared. Three cones with a distance of 12 m were set outside of each borderline and shifted to each other that a slalom course was developed (Fig. 1). Subjects ran with approximately 70% of their maximum speed from cone to cone and performed a sidecut with the outer leg at each cone. The sprint was performed over a distance of 30 m, and the velocities for both cut and sprint were controlled by timing lights. For the goal shot, subjects performed five individual steps in an approach before kicking the ball. Pressure distribution was collected under the supporting foot. To obtain enough measurements of each movement, cutting = cones = running direction Figure 1. Schematic representation of the slalom parcours for the cutting maneuver. Three cones with a distance of 12 m were set outside the prepared area on each side. Subjects ran from the start to each cone along the dashed line and performed a sidecut with the outer leg at each cone. and sprinting were repeated three times and goal shooting five times. Finally, nine valid steps for the normal run, the cutting maneuver, and the sprint and five steps of the supporting leg in the goal shot were extracted for each subject for further analysis. Each footprint was subdivided into 10 different areas using a standardized mask that was available in different sizes and corresponded to the sizes of the insoles. The different areas were the medial and lateral heel; the medial and lateral midfoot; the medial, central, and lateral forefoot; the hallux; the second toe; and the lateral toes. The same mask for each insole was applied to all subjects footprints, so it was ensured that the same areas of the insole were always compared to each other in this intraindividual comparison. Relative loads and peak pressures for all areas were extracted for each step. Loads are defined by taking the product of force and time and are represented by the force-time integral, that is, the impulse. Relative loads are calculated by dividing the local load of an area by the total load of the foot. Relative loads take into account both amplitude and duration of loading (force over time). They are suitable to obtain a better understanding of the load-bearing role of each anatomical area and may therefore be used for the characterization of foot loading. Peak pressures are related to the maximum loading of anatomical structures at different areas of the foot. Finally, the mean of all steps was calculated for each area. For statistical analysis, a two-way repeated measures analysis of variance with the treatment factors movement (run, cut, sprint, kick) and surface (grass, red cinder) was used. The alpha level was set to 5%, and the Scheffé test was used for post hoc comparisons. In case of significant interactions between factors, the type of the interaction was identified using the descriptive approach from Bortz and Döring (2001). 1 The knowledge of the type of interaction (ordinal, hybrid, disordinal) is essential for the interpretation of significant main effects. With no significant interaction or a significant ordinal interaction, the significant main effects can be globally interpreted, that is, without regarding the second factor. A hybrid interaction means that only one of the two factors (movement, surface) can be globally interpreted. 2 With disordinal interactions, none of the main effects can be globally interpreted.

142 Eils et al. The American Journal of Sports Medicine TABLE 1 Means and Standard Deviations for Parameters of Pressure Distribution Significant Movement Surface main effects and/or Run Cut Sprint Kick Grass Cinder interactions a Relative loads (%) Medial heel 7.8 ± 2.4 15.9 ± 3.6 b 1.5 ± 0.9 b 9.2 ± 2.4 8.9 ± 6.0 8.3 ± 5.3 c M, S, I α Lateral heel 8.8 ± 2.4 11.3 ± 3.0 b 1.8 ± 1.1 b 13.7 ± 3.5 b 8.8 ± 5.1 9.0 ± 5.3 M Medial midfoot 2.4 ± 1.1 5.3 ± 1.6 b 0.5 ± 0.4 b 2.7 ± 1.1 2.9 ± 2.2 2.6 ± 1.9 c M, S, I α Lateral midfoot 9.8 ± 1.8 3.5 ± 1.3 b 3.5 ± 1.1 b 13.0 ± 3.3 b 7.4 ± 4.8 7.4 ± 4.4 M, I β Medial forefoot 18.7 ± 3.3 25.8 ± 3.6 b 27.7 ± 5.0 b 11.1 ± 4.6 b 21.0 ± 8.1 20.7 ± 7.5 M, I β Central forefoot 15.6 ± 2.4 10.5 ± 2.1 b 19.2 ± 2.3 b 11.5 ± 3.0 b 13.9 ± 4.5 14.5 ± 4.0 c M, S, I β Lateral forefoot 18.2 ± 2.1 5.7 ± 1.5 b 17.6 ± 4.4 19.7 ± 2.5 15.0 ± 6.4 15.6 ± 6.2 c M, S, I β Hallux 9.6 ± 2.2 13.3 ± 3.4 b 14.7 ± 2.9 b 8.8 ± 2.0 11.6 ± 3.6 11.6 ± 3.6 M Second toe 4.9 ± 1.6 5.3 ± 1.5 8.1 ± 2.6 b 5.6 ± 1.6 5.9 ± 2.2 6.0 ± 2.3 M Lateral toes 4.1 ± 1.7 3.3 ± 1.6 5.4 ± 1.5 b 4.7 ± 1.8 4.4 ± 1.8 4.4 ± 1.8 M, I β Peak pressure (kpa) Medial heel 298 ± 59 655 ± 145 b 59 ± 24 b 680 ± 120 b 438 ± 296 407 ± 258 c M, S, I β Lateral heel 294 ± 58 489 ± 96 b 56 ± 23 b 728 ± 150 b 398 ± 274 386 ± 257 M, I β Medial midfoot 140 ± 30 233 ± 73 b 57 ± 28 b 271 ± 108 b 179 ± 111 171 ± 104 c M, S, I β Lateral midfoot 191 ± 33 175 ± 57 95 ± 30 b 374 ± 98 b 208 ± 126 210 ± 112 M, I β Medial forefoot 414 ± 92 653 ± 131 b 595 ± 171 b 295 ± 106 b 491 ± 200 488 ± 184 M, I β Central forefoot 336 ± 70 313 ± 72 406 ± 75 b 310 ± 86 335 ± 81 347 ± 88 c M, S, I β Lateral forefoot 293 ± 46 133 ± 33 b 295 ± 65 403 ± 93 b 274 ± 117 288 ± 113 c M, S, I β Hallux 348 ± 72 487 ± 98 b 486 ± 130 b 380 ± 92 428 ± 114 423 ± 121 M, I β Second toe 187 ± 48 230 ± 65 b 253 ± 77 b 264 ± 71 b 237 ± 68 230 ± 76 c M, S, I β Lateral toes 199 ± 67 180 ± 57 227 ± 51 273 ± 75 b 221 ± 72 218 ± 72 M, I β a M and S indicate significant main effects of movement and surface; I indicates a significant interaction between movement and surface. The type of interaction is marked by α (= ordinal) or β (= hybrid). b Indicates significant differences in movement. Only differences compared to the normal run are presented. c Indicates significant differences between surfaces. RESULTS The analysis of variance revealed significant interactions between independent factors for almost all parameters and areas. Thus, a simple presentation and discussion of their main effects on dependent variables was not directly possible without the analysis of the type of interaction. Analysis of the type of interactions showed ordinal and hybrid interactions. Further hybrid interaction analysis revealed that the main effect of movement can be discussed for all parameters and areas independently of the surface but not vice versa (that is, movement parameters are inconsistent under the influence of the different surfaces). The main effect of surface can be globally interpreted only for the relative loads under the medial heel and midfoot because of ordinal interactions. Movement had a significant effect on relative loads and peak pressures for all areas. Post hoc analysis showed distinct differences in cutting, sprinting, and kicking as compared to running. Analysis of relative loads revealed a significant shift of load to the heel, medial midfoot, medial forefoot, and hallux in cutting as compared to running. Significantly less loaded areas were the central and lateral forefoot and the lateral midfoot. In sprinting, a load shift to the first and second ray and to the lateral toes could be identified. The midfoot and the heel were significantly less loaded areas. In kicking, the load under the supporting foot was shifted to the lateral part of the foot (heel and midfoot) as compared to running. Significantly less loaded areas were the medial and central forefoot (Table 1, Fig. 2). Peak pressure analysis revealed characteristic patterns for the four different soccer-specific movements (Table 1, Fig. 3). In running, the main loading areas were found under the heel, the metatarsal heads, and the hallux. In cutting, the medial heel, medial forefoot, and hallux were loaded with average peak pressures of 655 ± 145 kpa, 653 ± 131 kpa, and 487 ± 98 kpa, respectively. In sprinting, the predominant loading areas were found in the forefoot (medial forefoot and hallux, central forefoot, and second toe). Especially the medial forefoot was exposed to pressures of 595 ± 171 kpa and 486 ± 130 kpa. In kicking, the highest pressures were found under the lateral part of the foot, especially under the heel (728 ± 150 kpa). Compared to running, peak pressures were higher for some areas during cutting and kicking. An increase of 220% and 160% for the medial heel and forefoot was found in cutting, respectively. In kicking, an increase of 250% was found for the lateral heel. The increase of peak pressures on the medial aspect of the foot in cutting, the first and second ray in sprinting, and the lateral part of the foot in kicking were significant (Table 1).

Vol. 32, No. 1, 2004 Plantar Pressure Distribution Patterns 143 Relative loads significant increase significant decrease Figure 2. Shift of load in soccer-specific movements. The arrows indicate a significant increase (gray) or decrease (white) of load compared to running. Surface had no global effect on relative loads and peak pressures for almost all areas except for the relative loads under the medial heel and midfoot (ordinal interaction, indicated by an alpha in Table 1). Differences between grass and red cinder in these areas were small (for example, 2.9% versus 2.6%). Significant hybrid interactions indicated that the order of the mean values for movement differed under the influence of the main effect of surface; that is, some values of movement were higher for grass and others for cinder for the same area. Significant interactions between movement and surface were mainly due to cutting and kicking. Run Cut DISCUSSION Sprint Kick In the present investigation, a pressure distribution analysis inside the soccer shoe was successfully performed to obtain information about the magnitude and distribution of load acting on the plantar surface of the foot during soccer-specific movements on two different playing surfaces. The results showed characteristic pressure distribution patterns with specific loading areas of the foot that corresponded to the evaluated movements. To our knowledge, there is no pressure data for running in soccer available in the literature. Therefore, the pressure distribution analysis for the running movement in soccer should be compared to pressure data published about running activities. Several factors have to be considered when relating pressure distribution measurements to results presented in the literature, especially when they were not obtained with the same system. The measuring system has an influence on the pressure distribution values due to the underlying measurement principle as well as the type and the size of the transducers. 3,4,12 It was shown that walking or running speed, shoe design, and the type of movement influenced pressure distribution. 5,11,21,26 The most common Figure 3. An example of the characteristic peak pressure pattern of the four tested movements in soccer. parameters for the evaluation of pressure distribution measurements are relative loads and peak pressures. Relative loads are dependent on the defined areas of the foot (masks) thus complicating comparisons to other investigations unless the same masks are used. Since peak pressures are related to single sensors within a mask, they are comparable to results in the literature. Therefore, the

144 Eils et al. The American Journal of Sports Medicine above-mentioned factors should be kept in mind when comparing results from different investigations, especially from different measuring systems. In the present investigation, a typical pressure distribution pattern for running with the main loading areas under the heel, the metatarsal heads, and the hallux was found. The reported peak pressures are comparable to results reported for running using the same measurement system and a similar running speed, 5,29 although different types of footwear were used (jogging shoe versus soccer shoe). Chen et al. measured peak pressures under the heel, the medial forefoot, and the hallux of 290 kpa, 390 kpa, and 380 kpa. 5 Weist and Rosenbaum reported values of 250 kpa, 430 kpa, and 380 kpa under the heel, the first metatarsal head, and the hallux during running. 29 However, peak pressures in walking were reported to be higher in soccer shoes than in running shoes when testing on the same surface. 27 In addition, soccer shoes provide less protection, support, and cushioning compared to running shoes. 22 Therefore, similar results in peak pressures for running with a soccer and a running shoe indicate that the grass and red cinder surfaces under the present conditions of the study are counterbalancing the stiffness of the soccer shoe. A harder surface would probably have resulted in higher loading of the foot. 24 The comparison of cutting, sprinting, and kicking to normal running shows characteristic pressure distribution patterns for all parameters that correspond well to the tested movements. In cutting, the load is shifted from the lateral parts of the midfoot and forefoot to the medial parts of the foot. In sprinting, the load is shifted to the first and second ray as well as to the lateral toes. In kicking, the load is shifted to the lateral part of the supporting foot (heel and midfoot). This load transfer leads to a significant increase of peak pressures in most of the presented areas. As a consequence of these results, the insole design of soccer shoes could gain from modifications aimed at reducing or transferring the load from characteristic areas to less loaded areas of the foot. In this context, it has to be considered that a simple increase of the thickness of the insole on the medial side for cushioning purposes is not an acceptable solution to the problem because it may force the foot into a more inverted position that may lead to an increased potential of lateral ankle sprains. Although there is no absolute threshold for the development of overuse injuries, the results of the present investigation indicate a potential danger for overloading specific areas of the foot that may help to understand the incidence of stress fractures in soccer players. Focusing on peak pressures, twofold values as compared to the normal run were found in some areas of the foot. In one subject, peak pressures exceeding the maximum calibration of 1 MPa were found for the medial part of the foot. However, other important factors besides maximum values, like duration of exposure or frequency of loading, also have to be considered in the development of stress fractures. 13,23 Furthermore, shear stresses at the sole of the foot may play an important role because they influence direction and amount of resultant forces acting on anatomical structures of the foot. 8 The actual stress that acts on the foot may therefore be underestimated. However, the vertical component has the major influence on the resultant force. To our knowledge, such high-pressure values have not been reported previously in the literature for the present in-shoe pressure measurement system. Therefore, high load in soccer in combination with a high repetition or inadequate rehabilitation may have a potential for the development of stress fractures. A direct relation of the observed pressure patterns to specific overuse injuries or stress fractures is difficult. It is imaginable that the pressure distribution patterns of the measured running and sprinting movements may lead to typical stress fractures that are reported for running or jogging (a high repetition and inadequate rehabilitation assumed). 18,19 However, there are some hints that specific stress fractures might be related to characteristic footloading patterns. Kavanaugh et al. stated that fractures of the proximal part of the diaphysis of the fifth metatarsal (Jones fracture) mainly occurred in basketball and soccer players. 14 Maximal loading of the lateral aspect of the foot was reported by the patients at the time of the injury. Wright et al. reported that patients with Jones fractures had a twofold increase in peak pressures at the base of the fifth metatarsal head compared to a control group and that athletes involved in running and cutting may be predisposed to these fractures. 30 They also stated that three of the four observed professional soccer players developed a refracture of the fifth metatarsal within 1 day of return to full activity after surgery and rehabilitation. The observed excessive loading patterns of kicking and cutting might, therefore, be contributing to these injuries. Although the kicking movement leads to extreme loading of the lateral aspect of the foot in the stance leg, it should not be considered as the main factor developing stress fractures because of the relatively low frequency of occurrence during a soccer game compared to sprinting and cutting. Attention should therefore be paid to more frequent movements. In this context, further game analyses in soccer should be performed to identify specific movement and activity patterns of single players or positions. 10 Significant interactions between movement and surface are mainly due to cutting and kicking. Focusing on mean values separated for movement and surface, it was revealed that most mean values of the characteristic loading zones in cutting and kicking on grass are increased compared to red cinder, whereas the values for the other movements remain similar. The question arises about whether different shoes should be recommended for different surfaces. Although there are significant interactions between movement and surface due to cutting and kicking, the absolute values for these movements differ only slightly between grass and red cinder. In addition, the interactions are not relevant for all areas. Therefore, a recommendation for shoes for different surfaces cannot be supported. Focusing on the two surfaces, the analysis of variance revealed that the surface seldom had global effects on dependent variables for the actual surface conditions at the time of the study. However, caution should be taken when trying to generalize these results, for example, stat-

Vol. 32, No. 1, 2004 Plantar Pressure Distribution Patterns 145 ing that these surfaces have in general no influence on pressure distribution values. It is imaginable that weather conditions may have a different influence on the grass or red cinder surface. It has been reported that the hardness of a surface is negatively correlated with the soil moisture content. 24 Therefore, the weather conditions and the properties of the surface should be taken into account as well. In conclusion, the results of the present study showed characteristic loading patterns of the foot during soccerspecific movements. The different surfaces had a minor influence on the results under the predominant conditions. Excessive loading values of specific areas suggest that there is an increased potential for the development of overuse injuries. As a consequence, the design of insoles could gain from modifications aimed at reducing the load in these areas, and in combination with further game analyses in soccer, insole designs that are customized to movement patterns of specific playing positions might be considered. ACKNOWLEDGMENT The authors would like to thank Nike, Inc. for supporting the project and Dr. M. Tietjens for her assistance with the statistics. REFERENCES 1. Bortz J, Döring N: Research methods and evaluation for human- and social scientists [Forschungsmethoden und Evaluation für Humanund Sozialwissenschaftler]. Third edition. Berlin, Springer Verlag, 2001, pp 532 536 2. Bredenkamp J: Verfahren zur Ermittlung des Typs einer statistischen Wechselwirkung [Procedures for the identification of the type of statistical interaction]. Psychologische Beiträge 24: 56 75, 1982 3. Cavanagh PR, Hewitt FG, Perry JE: In-shoe plantar pressure measurement: A review. The Foot 2: 185 194, 1992 4. Cavanagh PR, Ulbrecht JS: Biomechanics of the diabetic foot: A quantitative approach to the assessment of neuropathy, deformity, and plantar pressure, in Jahss MH (ed): Disorders of the Foot and Ankle. Second edition. Philadelphia, Saunders, 1991, pp 1864 1907 5. Chen H, Nigg BM, de Koning J: Relationship between plantar pressure distribution under the foot and insole comfort. Clin Biomech 9: 335 341, 1994 6. German Soccer Association [Deutscher Fußball-Bund]: http://www. dfb.de. Accessed 2001 7. Dvorak J, Junge A, Chomiak J, et al: Risk factor analysis for injuries in football players: Possibilities for a prevention program. Am J Sports Med 28: S69 74, 2000 8. Ekstrand J, Nigg BM: Surface-related injuries in soccer. Sports Med 8: 56 62, 1989 9. Fédération Internationale de Football Association: http://www.fifa. com. Accessed 2001 10. Hennig EM, Briehle R: Game analysis by GPS satellite tracking of soccer players. XXV Conference of the Canadian Society for Biomechanics, Département de kinésiologie, Université de Montréal, Montréal, Canada, 2000, p 44 11. Hennig EM, Milani TL: In-shoe pressure distribution for running in various types of footwear. J Appl Biomech 11: 299 310, 1995 12. Hughes J, Clark P, Linge K, Klenerman L: A comparison of two studies of the pressure distribution under the feet of normal subjects using different equipment. Foot-Ankle 14: 514 519, 1993 13. Inklaar H: Soccer injuries. II: Aetiology and prevention. Sports Med 18: 81 93, 1994 14. Kavanaugh JH, Brower TD, Mann RV: The Jones fracture revisited. J Bone Joint Surg Am 60: 776 782, 1978 15. Kernozek TW, LaMott EE, Dancisak MJ: Reliability of an in-shoe pressure measurement system during treadmill walking. Foot Ankle Int 17: 204 209, 1996 16. Kernozek TW, Zimmer KA: Reliability and running speed effects of inshoe loading measurements during slow treadmill running. Foot Ankle Int 21: 749 752, 2000 17. Knapp TP, Mandelbaum BR, Garrett WE: Why are stress injuries so common in the soccer player? Clin Sports Med 17: 835 853, 1998 18. Korpelainen R, Orava S, Karpakka J, Siira P, Hulkko A: Risk factors for recurrent stress fractures in athletes. Am J Sports Med 29: 304 310, 2001 19. Matheson GO, Clement DB, McKenzie DC, Taunton JE, Lloyd-Smith DR, MacIntyre JG: Stress fractures in athletes: A study of 320 cases. Am J Sports Med 15: 46 58, 1987 20. McPoil TGC, Cornwall MW, Yamada, W: A comparison of two in-shoe plantar pressure measurement systems. Lower Extremity 2: 95 103, 1995 21. Milani T: Biomechanische Belastungsanalysen der Fußstruktur bei Absprung- und Landebewegungen im Sport. Dissertation. Universität Konstanz, 1992 22. Monto RR: Time to redesign the trusty football boot? New Scientist April: 1993 23. Mueller MJ: Application of plantar pressure assessment in footwear and insert design. J Orthop Sports Phys Ther 29: 747 755, 1999 24. Orchard J: Is there a relationship between ground and climatic conditions and injuries in football? Sports Med 32: 419 432, 2002 25. Peterson L, Junge A, Chomiak J, Graf-Baumann T, Dvorak J: Incidence of football injuries and complaints in different age groups and skill-level groups. Am J Sports Med 28: S51 s57, 2000 26. Rosenbaum D, Hautmann S, Gold M, Claes L: Effects of walking speed on pressure distribution patterns and hindfoot angular motion. Gait & Posture 2: 191 197, 1994 27. Santos D, Carline T, Flynn L, Feeney D, Patterson C, Westland E: Distribution of in-shoe dynamic plantar foot pressures in professional football players. The Foot 11: 10 14, 2001 28. Union des Associations Européennes de Football: http://www.uefa. com. Accessed 2001 29. Weist R, Rosenbaum D: Changes of plantar pressure and muscle activity patterns under the influence of fatigue under exhausting treadmill running. Emed Scientific Meeting, Kananaskis, Canada, 2002 30. Wright RW, Fischer DA, Shively RA, Heidt RS Jr, Nuber GW: Refracture of proximal fifth metatarsal (Jones) fracture after intramedullary screw fixation in athletes. Am J Sports Med 28: 732 736, 2000