Research in the neural control of movement is only beginning to address the issue that bear on the control of multijoint movements. These issues arise, for the most part, from considerations of dynamics: the rotation about a joint does not depend exclusively on the muscular and external torques acting about that joint, but also depends upon the rotations about other joints. These interactions are due to inertial properties. It is important to study the strategies employed by the healthy nervous system for coping with the complexity of the dynamics of multijoint movement, for two reasons. First, the most apparent consequences of pathology in the neural motor-control system are disruptions in multijoint performance. Second, an understanding of normal multijoint control can be of immediate practical use in designing the strategies for artificial control of muscles by functional electrical stimulation in patients. The first specific aim of the present application addresses the rules whereby muscular torques chosen to initiate the movement. It is proposed that the nervous system may employ simple rules for this purpose, as an alternative to solving the complicated dynamics problem. Such simple rules, however, would not always result in the hand being launched in the target direction.
The second aim i s to test the hypothesis that reflex effects within and across joints are so organized that the direction of motion is corrected automatically. If true, this would provide a new perspective for understanding the role of reflexes in voluntary movements, a role which is currently controversial, perhaps because it has not been studied in the context of multijoint movements. These hypotheses will be tested by impeding or perturbing the movement by mechanical means, for a variety of arm configurations and target directions.
The third aim of the proposal represents an attempt at investigation of the process of braking of multijoint movements, an issue that has not been addressed before.
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