(Received 14 May 1996; accepted 24 May 1996)
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Little controversy exists regarding the costs to society, both financial and in terms of human pain and suffering, associated with slip and fall injuries. Yet, for all the very valuable research published to date in the United States and abroad, controversy still exists regarding the biomechanics of initial foot-to-ground contact (that is, footstrike). Studies of foot kinetics have been limited by the use of force plate sensitivities chosen to capture the entire stance phase, rather than footstrike alone, necessitating the use of calculation thresholds. The present study of young adults (2 male, 2 female; 8 foot contacts each) was conducted to address these limitations. Two strain gage force plates, with sensitivities set to optimize amplitude resolution at footstrike (+100 ms), were sampled at 1000 Hz. A net shear force Fs was calculated as the vector sum of medio-lateral Fy and anteroposterior Fx shear forces. Since Fx predominantly affected the direction of impending slip, a novel shear force Fs′ was given the amplitude of Fs and the sign of Fx. In this way, impending slips could be identified as predominantly forward or backward. Utilized slip resistance μsr was calculated for each sampled instant as the ratio of Fs′ to the normal force Fz. For comparison purposes, a geometric approximation of μsr was calculated using leg length and half step length (from force plate center-of-pressure data) to derive the tangent of an “apex angle.” Results showed two distinct patterns for μsr over the first 100 ms of foot contact. Pattern 1 began with an initial backward slip tendency, with peak μsr values near −0.4 occurring at approximately 10 ms. This was followed by a forward slip tendency peaking between 0.2 and 0.3, at approximately 45 ms. Pattern 1 did not seem to put the subjects at risk for slipping. Pattern 2 began with a profound forward slip tendency, with μsr values above 1.0 occurring during the first 20 ms. This was followed by a brief period of backward slip tendency (
Director, Biomechanics Laboratory, Santa Rosa Outpatient Rehabilitation Center for Children and Adults, San Antonio, TX
Technical Director, Orthopaedic Research Laboratory, Good Samaritan Medical Center, West Palm Beach, FL
Stock #: JTE11457J
ISSN: < 0.3), and a second forward slip tendency (0.2 < μsr < 0.3) for the remainder of the 100 ms sampling interval. Pattern 2 did seem to put the subjects at risk for an initial forward slip, though none were reported. The geometric approximations for μsr, typically had magnitudes near 0.4 and always predicted forward slips. We concluded that variations can be expected in foot contact patterns among normal adults. Although Patterns 1 and 2 were divided according to sex here, this may have been a coincidental finding confounded by small sample sizes, more related to shoe construction or preferred gait kinematics than to sex. Finally, geometric approximations for μsr grossly oversimplified initial foot contact, and should be discontinued in favor of more accurate methodology.