The role of predictive visual temporal information in the coordination of muscle activity in catching.

This study addresses the question as to the nature of the information on which the preactivation of the appropriate muscles in the grasping of the ball in a one-handed catching task is initiated and coordinated. High speed film and electromyograms were recorded while experiences subjects (N = 4) caught balls--projected towards them by a ball-machine at different speeds (11.9, 13.9 and 16.2 m/s--resulting in significantly different flight times of 508, 443 and 355 ms, respectively). Tau-margins (times to contact) values were calculated at the time of the initiation of the grasp movement for each subject at each speed. No significant differences were found between tau-margins at different speeds. Further, the onset of the muscle activity for the initiation of the grasp movement was shown to be independent of ball speed. These findings lend support to the contention that the initiation of the grasp movement in catching is controlled and coordinated by the optical variable tau which specifies (directly) this time-to-contact. Given that the muscle group selected includes both flexors and extensors, co-activation on the basis of tau information is evidenced.