Skilled throwers use physics to time ball release to the nearest millisecond.

Skilled throwers achieve accuracy in overarm throwing by releasing the ball on the handpath with a timing precision as low as 1 ms. It is generally believed that this remarkable ability results from a precisely timed command from the brain that opens the fingers. Alternatively, precise timing of ball release could result from a backforce from the ball that pushes the fingers open. The objective was to test these hypotheses in skilled throwers. Angular positions of the hand and phalanges of the middle finger were recorded with the search-coil technique. In support of the backforce hypothesis, we found that when subjects made a throwing motion without a ball in the hand (i.e., without a backforce), they could not open the fingers rapidly, and they had lost the ability to time finger opening in the 1- to 2-ms range. In addition, relationships were found between the magnitude and timing of hand angular acceleration and finger (joint) extension acceleration. The results indicate that although a central command produced initial hand opening, precise timing of ball release came from a mechanism involving Newtonian mechanics, i.e., hand acceleration produced a backforce from the ball on the fingers that pushed the fingers open. In this mechanism, given the appropriate finger force/stiffness, correction for errors in hand acceleration occurs automatically because hand motion causes finger motion. We propose that skilled throwers achieve ball accuracy by computing finger force/stiffness based on state estimation of hand acceleration and that ball inaccuracy occurs when this computation is imprecise.