Kinetic sources of lower-limb angular displacement in the recovery phase of sprinting.

One female sprinter was filmed at the 100-m mark (speed 6.5 m X S-1) of a 400-m run. Four moments occurring at each end of the thigh and shank segments during lower-limb recovery were calculated. These were: proximal and distal net muscle moments, a moment due to proximal joint accelerative force and, a moment due to distal joint-force resulting from motion and inertia of the distal connected segment. Individual contributions of each moment to segmental angular displacement were calculated by double integration, and angular velocity at toe-off was multiplied by time to yield its contribution. Contributions of the proximal muscle moments throughout recovery were 21 rad and 7.5 rad for the thigh and shank segments, respectively. Such large angular displacements did not occur because the three remaining moments opposed the proximal muscle moment. These large moments are mutually offsetting and interactive, acceleration of proximal joints provides a substantial moment during contralateral stance, and segmental angular velocity at toe-off is a significant contributor. Consequently, a phenomenon producing a change in any moment (e.g. muscular fatigue, slippery surface) will require modification of other moments for angular displacement to be maintained within reasonable limits.