Braking of elbow extension in fast overarm throws made by skilled and unskilled subjects.

A previous computer simulation study of overarm throws in 2D showed that reversal of elbow torque by antagonist muscle action late in the throw led to increased wrist flexion velocity and to increased ball speeds. We tested the hypothesis that the skill of making fast overarm throws in 3D involves deceleration (braking) of elbow extension before ball release, and that this is an active mechanism. Skilled and unskilled throwers were instructed to throw baseballs at a fast speed. Arm segment angular positions in 3D at 1,000 Hz were recorded with the search-coil technique (which records angular motions). In skilled throws, but not in unskilled throws, there was a period (mean 17 ms) of rapid elbow extension deceleration before ball release. However, there was relatively little biceps EMG activity associated with the very large magnitude of elbow deceleration. This finding and other work suggests that elbow extension deceleration results in part from interaction torques associated with late-occurring shoulder rotations, and only in part from elbow flexor contraction. During the period when elbow extension was decelerating, the forearm in space was undergoing angular acceleration (because of internal rotation at the shoulder) which would be expected to produce a torque at the wrist in the extensor (not flexor) direction. The results show that elbow extension deceleration occurs before ball release in fast (skilled) 3D throws, and that it does not produce forearm angular deceleration. Whether it produces forearm translational deceleration, which could increase wrist flexion velocity, remains to be determined.