The purpose of this study was to investigate the change in kinetics of the lower limb joints in 100 m sprint running. Nine male sprinters, running 100 m with the maximum effort, were videotaped at every 10 m mark from the start to 90 m mark with five high-speed cameras (250 Hz) and five normal VTR cameras (60 Hz). Two dimensional coordinates of the body landmarks were obtained by digitizing VTR images over at least, two steps at every 10 m. Performance descriptors, kinematics and kinetics such as stride length, stride frequency, joint torque and joint torque power of the lower limb joints were calculated by an inverse dynamics approach, with estimating ground reaction forces from the acceleration of the whole body center of gravity. The relationships between changes in running velocity and the joint torque and joint torque power of the leg were analyzed. (1) 100 m sprint could be divided into four phases: the first and second acceleration phases, the maximum speed phase, and the deceleration phase. (2) In the first acceleration phase, 0-30 m, where the stride length and stride frequency increased. There were significant relationships between increases in running speed, stride length, and stride frequency and increases in negative power exerted by knee extensors in the early recovery phase, positive power by hip extensors and negative power by knee flexors in the late recovery phase. (3) In the second acceleration phase, 30-50 m, where the stride length increased and the increase in the running speed was smaller than that of the previous phase. Positive power exerted by hip flexors in the early recovery phase and negative power by knee flexors decreased. (4) In the maximum speed phase, 50-70 m, significant increase in powers to the previous phase were found in positive power exerted by hip flexors in the early recovery phase, negative power by knee flexors in the late recovery phase, and power by plantar flexors during the support phase so as to allow the sprinters to acquire higher speed. (5) In the deceleration phase, 70-100 m, where the stride frequency as well as the running speed decreased. The decreases were found in positive power exerted by hip flexors in the early recovery phase and negative power by knee flexors in the late recovery phase.
These results revealed that the critical factors in the joint kinetics of 100 m sprint running would be power exertions of hip extensors, knee extensors and knee flexors in the late recovery phase for the acceleration, of hip flexors in the early recovery phase, and of knee flexors in the late recovery for high speed and speed endurances.
