O5 Contact Optimization

Topic

This contact between the athlete and the ground—or, alternatively, with their equipment—represents the final moment when the athlete can alter their momentum via an external force; this reflects the relationship between impulse and momentum established in another model within this set. During the athlete's run, optimal contact minimizes the collision-related energy loss (as defined in another model in this set) while maximizing the athlete's final horizontal propulsion.

The position of the athlete's foot contact point relative to their center of mass—specifically the horizontal distance between the two—determines whether significant braking torque is generated (when the heel strikes first) or minimal braking torque occurs (when the midfoot or forefoot strikes first). If the foot contact point is positioned further ahead of the center of mass, the resulting braking torque acting on the athlete increases.

The angle of the ground reaction force relative to the vertical—a factor addressed in another model in this set—during the propulsive phase of contact determines the orientation of the athlete's propulsion. If this reaction force angle tilts further forward during the propulsive phase, the horizontal component of the propulsion increases, thereby benefiting the athlete.

The athlete's optimal contact time depends on their velocity: the higher the athlete's velocity, the shorter the available contact time—a relationship between contact time and velocity that is also established in another model in this set regarding the transition between modes. A further reduction in available contact time requires the athlete to exhibit significantly greater leg stiffness—a parameter defined elsewhere in this framework—as well as an enhanced capacity to generate force much more rapidly.

During a jump, the propulsive contact phase must generate the maximum possible net vertical impulse within the minimal time available; this net impulse corresponds to the change in the center of mass's momentum, as defined in another model within this framework. If the athlete manages to further compress the propulsion time without sacrificing the peak vertical force applied, the resulting jump height increases.

Consequently, regarding the athlete's striking of a ball, the specific point of impact on the ball's surface determines the ball's final trajectory, spin, and exit velocity—the latter being a parameter defined elsewhere in this framework via the coefficient of restitution. As the point of impact moves further away from the ball's geometric center, the resulting spin increases at the expense of the final exit velocity.

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