M7 Address Changes

Topic

Changing direction involves, in general, decelerating the athlete's center of mass along the initial line of movement, subsequently redirecting that center of mass, and finally accelerating it in the new direction. The available friction between the athlete's footwear and the supporting surface limits the lateral force the athlete can exert—and consequently the final speed of the change of direction—reflecting the interaction with the environment established by another model in this set. A friction cone defines the maximum direction of the force the athlete can exert on the surface before slipping occurs. The athlete's energy is partially dissipated during the initial deceleration and must be regenerated during the subsequent re-acceleration toward the new direction; this makes the change of direction metabolically costly—a metabolic cost also addressed by another model in this set regarding efficiency. The inclination of the athlete's center of mass during the turn represents the strategy used to balance the turn's centrifugal force against available friction and gravity—a center of mass dynamic also established by another model in this set. Consequently, given the initial deceleration and re-acceleration of the center of mass, higher entry speeds and tighter turning radii result in a greater required centripetal force; this increases the risk of anterior cruciate ligament (ACL) injury—a risk addressed by another model in this set—precisely due to the resulting lateral force on the athlete's knee. The actual duration of this change of direction depends, consequently, on the angle of the very turn that initiated the maneuver: a wider-angle turn requires, therefore, both a greater initial deceleration and a greater final re-acceleration than a much tighter turn.

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