Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The purpose of this study was three-fold: (1) does agonist activity (Ag1) duration remain constant over a wide range of movement amplitudes? (2) can Ag1 be modified by peripheral feedback? and (3) what is the precise relationship between Ag1 and the acceleratory phase of the movement?;In the first set of experiments, subjects performed 5 to 50 degree step-tracking movements about the elbow. In all subjects, small movements (5-20 deg) were initiated by short duration Ag1 bursts (70-80 ms) while large movements (40-50 deg) were initiated by long duration bursts (140-150 ms). The increased Ag1 duration resulted from a second peak (component) of activity which was approximately 70-80 ms in duration. Doubling of Ag1 duration also occurred in movements made by a deafferented patient, suggesting independence of Ag1 duration modulation from afferent feedback.;In a second set of experiments, brief perturbations were randomly applied prior to the onset of elbow flexion movements. In 30 deg movements, perturbations which opposed the movement produced graded increases in the magnitude of both components of Ag1. Perturbations which assisted the movement increased the magnitude of the first component but decreased the magnitude of the second.;The effects of changes in initial joint angle on Ag1 were also examined. Subjects performed flexion movements in which starting joint angle ranged from 65 to 125 deg. The magnitude of Ag1 increased as starting position became more extended. Movement (phase-plane) trajectories associated with different starting positions were not altered despite large differences in Ag1 magnitude. It was hypothesized that these changes in Ag1 activity compensate for angle-dependent changes in limb mechanical properties so as to maintain a prelearned movement trajectory.;Finally, experiments were conducted to examine the relationship between phasic muscle activity and movement trajectory. As movement profiles shifted from short to long acceleration, both Ag1 duration and time of onset of phasic antagonist activity increased. These findings suggest that the timing and magnitude of movement-related muscle activity are not related to a single movement parameter but rather to the profile of the intended movement. (Abstract shortened with permission of author.)



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