Timing of finger opening and ball release in fast and accurate overarm throws.

How precisely does the CNS control the timing of finger muscle contractions in skilled movements? For overarm throwing, it has been calculated that a ball release window of less than 1 ms is needed for accuracy in long throws. The objective was to investigate the timing precision of ball release and finge opening for 100 overarm throws made using only the arm. Subjects sat with a fixed trunk and threw balls fast and accurately at a 6-cm-square target when it was 1.5, 3.0 and 4.5 m away. Three-dimensional angular positions in space of the clavicle, upper arm, forearm, hand and distal phalanx of the middle finger were simultaneously recorded at 1000 Hz using the magnetic-field search-coil technique. Ball release was determined by pressure-sensitive microswitches on the proximal and distal phalanges of the middle finger (proximal and distal triggers). Variability of ball release, defined in terms of the standard deviation (SD) of the means of release times, was different when synchronized to different hand kinematic parameters. It was highest to the start of movement (when the hand started rotating vertically forward and up around a space-fixed horizontal axis) and was lowest when synchronized to the moment near ball release when the hand was vertical. These values did not depend on target distance. When throws were synchronized to vertical hand position, and SDs were averaged across the 10 subjects, the average interval for 95% of the throws (4xSD) was 9.6 ms for ball release and 10.0 ms for onset of finger opening. Thus, two independent measures of timing precision gave similar results. It is concluded that for 100 fast and accurate throws made by male recreational ball players, timing of finger opening and ball release was controlled precisely but not to fractions of a millisecond.