Control Of Human Limb Movement Trajectories

It is common experience that movements we make do not end at the same position during repeated attempts. Variability must, however, develop during movements and not just at their endpoint. Recent studies of the effects of perturbations on limb movements have suggested that the entire trajectory of movements, rather than their endpoint, may be controlled. Experiments were carried out to further examine this hypothesis by studying the variability in human arm movement trajectories and related muscle electromyographic (EMG) activities as a function of movement speed and practice.;Variability in both movements and the EMGs of involved muscles depended on both movement speed and practice. Increases in movement speed produced greater variability in movement trajectories and muscle EMG activities. With practice, variability in the trajectories was reduced. The effects of practice on variability in the EMGs of related muscles depended on changes in movement speed during practice. If movement speed was kept constant during practice, EMG variability decreased in association with decreased trajectory variability. In contrast, increases in movement speed during practice resulted in greater EMG variability in spite of decreased movement variability.;The variations in movement related phasic EMGs were not random. Deviations from intended movement trajectories produced by variations in the first agonist burst which initiates movement were accompanied by variations in antagonist muscle activity which returned the limb toward the intended trajectory. Reduced variability in movements in spite of greater variability in the agonist-antagonist EMG patterns was produced by this mechanism.;It was concluded that the entire movement trajectory, rather than final position only, is controlled by the nervous system. The control is complex, however, because of the inherent variability in the motor system. During practice, refinement of the linkage of agonist and antagonist muscle activation patterns is therefore critical in reducing variability in movements as speed increases