Throwing accuracy in the vertical direction during prism adaptation: Not simply timing of ball release

In a previous study, others have hypothesized that the variance in vertical errors that occurs while throwing at visual targets is caused by changes i n any of three throw parameters: hand location in space, hand translational velocity, and hand orientation. From an analysis of skilled throwers, thos e authors concluded that vertical error is best correlated with variance in hand orientation, which in turn is related to the timing of ball release. We used a vertical prism adaptation paradigm to investigate which of these throwing parameters subjects use when adapting to external perturbation. Ou r subjects showed no correlation between hand position or hand translationa l velocity and ball impact height in normal, over-practiced throwing. Howev er, video-based motion analysis showed that modifications both of position and speed of the hand play an important role when subjects are forced to co mpensate for a vertically shifting prism perturbation during a dart-like th row (these factors contribute similar to 30% of the adaptation). We conclud ed that, during adaptation, more degrees of freedom and more sources of pot ential error are modified to achieve the gaze-throw recalibration required to hit the target than are employed in this type of throw during normal con ditions.