Timing finger opening in overarm throwing based on a spatial representation of hand path.

Previous studies on overarm throwing have suggested that throwing accuracy depends on a precise central timing mechanism. In the present study, we investigated an alternative hypothesis: that central control of finger opening is based on an internal positional representation of handpath. Angular positions of each segment of the middle finger, thumb, and arm were recorded with the search-coil technique as subjects made slow, medium, and fast throws at a target 3.1 m away. Onset of ball release from the hand was strongly correlated with extension at the proximal interphalangeal joint (PIJ). The velocity of this finger joint opening varied with the speed of the throw. In agreement with the hypothesis, at a fixed hand angular position in space, there was no difference across subjects in the amplitude of extension at the PIJ for throws of different speeds. That is, for these two parameters, a fast throw was the same as a slow throw that was sped-up. This occurred irrespective of whether the trunk was constrained (sitting throws) or unconstrained (standing throws). No equivalent relation was found between extension at the PIJ and elbow extension. These findings support the idea that precisely timed finger opening in overarm throwing depends, not on a central timing controller that triggers a step-like (ballistic) finger opening at the right moment in throws of different speeds, but on a central spatial controller that matches angular positions of finger opening to the intended handpath.