E8 Impact and Contact

Title

Impact is a short-term interaction in which momentum is exchanged between two bodies through very high contact forces. In sports, impacts occur in foot-ground contact (duration 100200 ms in running, 1050 ms in jumping with landing), in hitting balls (1020 ms in soccer), and in collisions between players. The coefficient of restitution (COR) quantifies the elasticity of the impact: COR = 1 (perfectly elastic) conserves kinetic energy; COR = 0 (perfectly plastic) implies maximum dissipation. At foot-ground contact, there is a passive impact peak (transient force) 2030 ms after initial contact, with loading rates of 50100 BW/s, which propagates as a shock wave to the skeleton. Natural shock absorbers (plantar fat, cartilage, soft tissue) and footwear reduce the peak of impact. The one-degree-of-freedom (mass-spring-damper) impact model describes the GRF during contact. The loading rate of the vertical GRF is the best biomechanical predictor of bone stress injuries in runners.

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