The purpose of this study was to identify functions of the torso and lower limbs in baseball pitching. Pitching motions of ten baseball players were analyzed by three dimensional motion analysis method with two high-speed video cameras (200 Hz) and two force platforms. Three dimensional coordinates of body segment endpoints were obtained by a DLT technique, and ground reaction forces acting on the pivot and stride legs were sampled at 250 Hz and synchronized with the coordinate data. Major variables computed were the joint torques, joint torque powers, and works done by the torso, hip, knee and ankle joints.
Forward twisting torque of the torso was exerted during the twisting phase (from the starting of torso twist, TS, to the stride foot contact with the ground, SFC), and increased up to it's peak during early cocking phase (from SFC to the instant of the minimum ball velocity, Min. BV), in which joint angular velocity of twisting changed from backward to forward direction and as a result the negative joint torque power changed to positive direction. During late cocking phase (from Min. BV to the instant of the maximum external rotation of the shoulder, MER), the torso exerted the torques of extension, backward twisting and left bending and the negative powers.
The hip and knee joints of the pivot leg exerted the extension torque and the ankle joint exerted plantar flexion torque before SFC, but their joint torque powers were not so large as expected. During the second half of the twisting phase, the positive joint torque power of hip extension for the pivot leg rapidly increased.
After SFC, the hip joint of the stride leg exerted the extension torque, the knee exerted flexion torque, and the ankle joint showed the plantar flexion torque. Negative power by the hip extension torque was the greatest in the stride leg joints.
Large moment about the vertical axis of the lower torso was due to the hip extension torque of the pivot leg during twisting phase and the hip adduction torque of the stride leg during early cocking phase.
Works done by the torso and hip joint torques during pitching phase (SFC to REL) were much larger than those of the knee and ankle joints, although there were no significant relationships between the ball velocity and works done by the torso and hip joints.
These results suggest that one of the major functions of the torso and legs is not to directly increase the ball velocity but to exert joint torques and powers for the twisting of the torso and for the generating mechanical energy to transfer to the throwing arm and the ball.
