A simple rule for controlling overarm throws to different targets.

We investigated the central programming of overarm throws by determining whether throws to spatially separate targets in the vertical direction (sagittal plane) are produced by changes in hand (i.e., finger) path direction or by changes in the timing of ball release. Six skilled throwers made 30 throws at the same speed with a baseball, from a sitting position with the chest fixed, at targets at different heights and distances. Arm segment angular positions in 3D were recorded with the search-coil technique. Videotaping revealed that ball direction was not, as commonly assumed, the tangent to the finger path at ball release. Rather ball direction was the tangent to the finger path at a point about half way between initial uncoupling of the ball from the hand and final ball release. When viewed from the side, finger paths were tilted upwards for the high and the far targets and downwards for the low and near targets. This was associated with changes in angular orientation of the upper arm in space. Throwing at spatially different targets was not associated with changes in the timing of ball release. We propose that there is a simple rule by which throws to targets in different directions and at different distances are controlled: throws of the same speed are produced by different finger path directions, but the same timing of ball release. Such a mechanism would simplify the neural control of throwing to different targets.