The purposesof this study were(1) to investigate the relationship between posture and ground reaction force (GRF) by the joint torques, and (2) to clarify the postureused to orient the GRF forward during first stance from the crouch start. Thirty-six male track and field athletes volunteered to participate in the present study. The participants sprinted 10_m from starting blocks. The GRFs of the leg in the first and second stance phasesweredetermined by using a force platform (1000 Hz). Simultaneously, three-dimensionalcoordinates were recorded with a motion analysis system (250Hz) using 20 cameras (MX-T20).The support leg joint torque was calculated using inverse dynamics. On the basis of the collected data, sprint motions that changed the joint angle were created with kinematic simulation. In the simulated motion, the contribution of the GRF by the joint torques of the support leg was calculated by solving equations of motion for the whole body system. We investigated the relationship between posture and GRF by the joint torques. The main results were as follows:(1) The ankle joint planter flexion torque of the support leg was the main generator of GRF. (2) When the shank of the support leg was leaned forward, the propulsive force by the plantar flexion torque was bigger and the upward force was smaller.A previous study suggested that the GRF of a good sprinter is characterized by a forward-oriented GRF. The present study clarified that a forward-oriented GRF is attained by forward leaning of the shank segment.
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