E8 Impact and Contact

Topic

An impact generally consists of a very brief interaction during which momentum is exchanged between two distinct bodies via an extraordinarily high contact force. Such impacts occur, for instance, during the contact between an athlete's foot and the ground—where the duration is significantly longer during running than during a landing—or when striking a ball (an even briefer contact) or colliding with another player.

The coefficient of restitution quantifies the elasticity of the impact: a perfectly elastic impact conserves all the kinetic energy of both bodies, whereas a perfectly plastic impact entails the maximum dissipation of that kinetic energy.

In the case of foot-ground contact, a "passive impact peak" occurs shortly after initial contact; this peak is a key feature of ground-contact diagnostic models and is associated with a high loading rate that propagates as a shock wave toward the athlete's skeleton—a stress wave also modeled in analyses of cortical bone behavior.

Natural shock absorbers—such as the athlete's plantar fat, cartilage, and soft tissue—along with their footwear, serve to attenuate this passive impact peak. Greater energy absorption by these natural shock absorbers results in a lower passive impact peak reaching the athlete's skeleton. Consequently, this model—characterized by the momentum-exchange impact with which it begins—allows a single-degree-of-freedom impact model (comprising a specific mass, spring, and damper—the very same components used in another model in this set representing a viscoelastic tendon) to describe the ground reaction force throughout the contact phase. The resulting loading rate of this vertical ground reaction force is the best-known biomechanical predictor of bone stress injury in the runner—specifically the type of stress fracture associated with exceeded bone remodeling capacity in another model within this set. A higher loading rate for this vertical ground reaction force therefore results in an even greater risk of bone stress injury for the runner.

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