Errors in the control of joint rotations associated with inaccuracies in overarm throws

Abstract

1. Accurate overarm throwing requires precise control of joint rotations so that the ball is released at the appropriate time on the appropriate hand trajectory. Inaccuracy in throws, in turn, must result from errors in the control of joint rotations. But do high and low throws result from disorders in the joint rotations that produce the hand trajectory or in those that cause ball release? Are they due to error at a particular joint or to accumulation of errors across a number of joints? The objective was to answer these questions and thereby to gain insight into the CNS control of joint rotations in a skilled arm movement task. 2. Ten subjects--male, right-handed recreational ball players, all accurate throwers--sat with a fixed trunk and threw tennis balls at a 9 x 9 grid of 6-cm target squares 1.5 or 3 m away. Rotations of five arm segments in three dimensions were measured at 1,000 Hz with the magnetic-field search-coil technique. Hand trajectory (translation) was computed from these rotations. 3. The cause of ball high-low inaccuracy was investigated by determining its relation with hand kinematic parameters that could potentially affect it. No statistically significant relation was found between height of ball impact on the target and height of the hand trajectory. In contrast, statistically significant relations appeared between height of ball impact on the target and both hand trajectory length at ball release (for 8 of 10 subjects) and finger and hand orientation in space at ball release (for all 10 subjects). 4. Three hypotheses were proposed to explain the variable finger and hand orientations in space at ball release, i.e., that they resulted from errors in velocity of rotation at one or more proximal joints (wrist, elbow, shoulder), timing of onset of rotation at one or more proximal joints, or timing of ball release (due to incorrect velocity or timing of onset of finger opening). All three mechanisms could result in inappropriate finger and hand orientations in space at ball release, but the pattern of joint space trajectories would be different in each case. 5. High and low throws did not follow the joint space paths predicted by the first two hypotheses. Instead, as predicted by the third hypothesis, a separation of traces occurred when finger extension was plotted against wrist flexion or against elbow extension, e.g., for a given amplitude of wrist flexion, finger extension was large for the high throws and small for the low throws. 6. In agreement, when all throws were considered, a statistically significant (P < 0.005) relation was found between ball impact height on the target and the amplitude of finger extension, for a fixed amplitude of wrist flexion (10 subjects), and for a fixed amplitude of elbow extension (8 subjects). Only two subjects showed a statistically significant relation between ball impact height and the amplitude of wrist flexion, for a fixed amplitude of elbow extension. 7. The separation of finger extension-wrist flexion traces in joint space for high and low throws was ...

Timing Rules for Synaptic Plasticity Matched to Behavioral Function

“…In throwing a ball, for example, there is a delay of a few hundred milliseconds for the ball to reach the target and visual feedback about the accuracy of the throw to reach the nervous system. Nevertheless, the timing of the finger movements controlling the release of the ball can be learned with remarkable precision -a difference of just 10 ms can cause the throw to be too high or too low (Hore et al, 1996).…”

Disruption of kinematic coordination in throwing under stress

“…The variability in the horizontal position decreased significantly in the stress session, despite a decline in throwing performance. This result is somewhat surprising, because in many studies the consistent release position has been highly correlated with skilled throwing (e.g., Hore et al, 1994Hore et al, , 1995Hore, Watts, & Tweed, 1996;McDonald et al, 1989).…”

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

“…the handpath, which, in turn, was determined by the timing of finger opening. Given that finger opening was not driven by proprioceptive feedback (Hore et al 1999a), we assumed that PIP joint extension was caused by a central command to the finger muscles (Hore et al 1996;Timmann et al 1999). The present results show that this view needs to be modified.…”

“…In keeping with this, it might be expected that throwing inaccuracy would be caused by lack of precision in the timing of the central command to the fingers. Such evidence has been found in overarm ball throwing, where modeling and experimental studies have shown that ball high/ low inaccuracy could be caused by variability in the timing of finger opening on the handpath (Calvin 1983a(Calvin , 1983bChowdhary and Challis 1999;Hore et al 1996). However, in dart throwing, inaccuracy has been associated with errors in hand trajectory speed (Smeets et al 2002).…”

“…Angular positions of finger and arm segments were measured using the search-coil technique as described previously (e.g., Hore et al 1992Hore et al , 1996. Although the search-coil technique records angular motions of arm segments robustly and with great precision, it does not record translational motion.…”