E7 Interaction with the Environment

Topic

The athlete's interaction with their environment encompasses the ground reaction force—a key element of this model—as well as aerodynamic drag, buoyancy, hydrodynamic drag, and the friction between the athlete and the surface.

The ground reaction force represents the Newtonian response (based on the third law of motion) to the force the athlete exerts on the ground; its vertical, anteroposterior, and mediolateral components are directly linked to the athlete's propulsion, deceleration, and lateral control, with the force's propulsive orientation serving as a key indicator of horizontal force production. In elite runners, for instance, the vertical component of the ground reaction force can reach several times the runner's total body weight.

Aerodynamic drag is proportional to the square of the athlete's speed; consequently, it dominates other forms of resistance during high-speed cycling, though it also plays a significant role in sprinting and swimming.

The friction between the athlete's footwear and the supporting surface determines both their ability to change direction and their capacity to generate horizontal propulsion against that surface.

Thus, within the framework of this model—which began by considering ground reaction force—the lift generated by the swimmer's arm and hand contributes significantly to their total propulsion, complementing the friction and aerodynamic drag factors initially identified. The athlete's final impulse—the integral of this ground reaction force over time—is consequently exactly equal to the final change in the momentum of their center of mass (the very center of mass that defines this model). If this ground reaction force acts on the athlete for a longer time interval, the resulting final impulse is greater, as is the final change in the momentum of the center of mass that completes the model.

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