Utilization and compensation of interaction torques during ball-throwing movements.

The manner in which the CNS deals with interaction torques at each joint in ball throwing was investigated by instructing subjects to throw a ball at three different speeds, using two (elbow and wrist) or three joints (shoulder, elbow, and wrist). The results indicated that the role of the muscle torque at the most proximal joint was to accelerate the most proximal joint and to produce the effect of interjoint interaction on the distal joints. In the three-joint throwing, shoulder muscle torque produced the assistive interaction torque for the elbow, which was effectively utilized to generate large elbow angular velocity when throwing fast. However, at the wrist, the muscle torque always counteracted the interaction torque. By this kinetic mechanism, the wrist angular velocity at the ball-release time was kept relatively constant irrespective of ball speed, which would lead to an accurate ball release. Thus it was concluded that humans can adjust the speed and accuracy of ball-throwing by utilizing interaction torque or compensating for it.