Sprinting speed is the final result of sprinting movement. Though sprint runners always try to sprint as rapidly as possible, their speed changes with time. Such changes in speed are caused mainly by physical movement in the support phase of sprinting, because sprint runners encounter deceleration force and generate acceleration force in only this phase. Especially, the support leg has an immediate effect on the sprinting speed.
The purpose of this study was twofold : (a) to determine whether functions of the support leg segments change with changes in sprinting speed and the sprinting situation, and (b) to investigate the characteristics of changes in the functions of the support leg segments.
Ten male sprint runners (age 20.7±1.2 yr, height 1.72±0.06m, body mass 63.3±4.7 kg) participated in the study. Subjects performed three sprints from starting blocks, and were instructed to execute exhaustive sprint runs. Their movement patterns were analyzed for five support phases : (S1) at 77.4±3.1% of maximum sprinting speed (MSS) during the acceleration period, (S2) at 95.4±1.9% of MSS during the acceleration period, (S3) at MSS, (S4) at 94.1±1.4% of MSS during the deceleration period, and (S5) at 90.8±2.2% of MSS during the deceleration period. These phases were designed to allow comparison of movement patterns in different situations with equal sprinting speeds. The linear dynamics approach was adopted to determine the direct effects of segmental movement on the sprinting speed. This is based on the theory that the velocity of a segmental center of mass (SCM) is the vector sum of the velocity of the adjacent lower joint and the relative velocity of the SCM with respect to the adjacent lower joint.
The force acting on each segment of the support leg created a stereotypical pattern in spite of differences in sprinting speed and the sprinting situation. However, the support time and the magnitude of the force acting on the shank and the foot changed with the sprinting speed. The sprinting speed was closely related to impulses acting on the shank and the foot. In contrast, the magnitude of the force acting on the thigh remained unchanged. The impulse acting on the thigh was not related to the sprinting speed, but there was a close relationship between impulses acting on the thigh and the shank, and the foot.
In conclusion, (1) The performance of the function of the thigh remains unchanged in spite of differences in sprinting speed and the sprinting situation. The thigh always works to attain its task, which is to sprint as rapidly as possible ; (2) The functions of the shank and the foot have a direct influence on changes in sprinting speed. These changes are caused by changes in the magnitude of the forces acting on the shank and the foot, and the support time ; (3) While the function of the thigh does not affect the sprinting speed directly, it affects the functions of the shank and the foot. The function of the thigh thus has an indirect influence on sprinting speed ; (4) The thigh and the foot work to compensate for decreases in the performance of the shank during the deceleration period.