Throwers Use Physics to Time Ball Release to the Nearest

15 Skilled throwers achieve accuracy in overarm throwing by releasing the ball on the handpath 16 with a timing precision as low as one millisecond. It is generally believed that this remarkable 17 ability results from a precisely timed command from the brain that opens the fingers. 18 Alternatively, precise timing of ball release could result from a backforce from the ball that 19 pushes the fingers open. The objective was to test these hypotheses in skilled throwers. Angular 20 positions of the hand and phalanges of the middle finger were recorded with the search-coil 21 technique. In support of the backforce hypothesis, we found that when subjects made a throwing 22 motion without a ball in the hand (i.e., without a backforce) they could not open the fingers 23 rapidly, and they had lost the ability to time finger opening in the 1-2 millisecond range. In 24 addition, relationships were found between the magnitude and timing of hand angular 25 acceleration and finger (joint) extension acceleration. The results indicate that although a central 26 command produced initial hand opening, precise timing of ball release came from a mechanism 27 involving Newtonian mechanics, i.e., hand acceleration produced a backforce from the ball on 28 the fingers that pushed the fingers open. In this mechanism, given the appropriate finger 29 force/stiffness, correction for errors in hand acceleration occurs automatically because hand 30 motion causes finger motion. We propose that skilled throwers achieve ball accuracy by 31 computing finger force/stiffness based on state estimation of hand acceleration, and that ball 32 inaccuracy occurs when this computation is imprecise. 33