Step-tracking movements of the wrist. IV. Muscle activity associated with movements in different directions

We examined the patterns of muscle activity associated with multiple direct ions of step-tracking movements of the wrist in humans and monkeys. Human s ubjects made wrist movements to 12 different targets that required varying amounts of flexion-extension and radial-ulnar deviation. Wrist muscles disp layed two patterns of electromyographic (EMG) modulation as movement direct ion changed: amplitude graded and temporally shifted. The amplitude-graded pattern was characterized by modulation of the quantity of muscle activity that occurred during two distinct time periods, an agonist burst interval t hat began before movement onset and an antagonist burst interval that began just after movement onset. The timing of muscle activity over the two inte rvals showed little variation with changes in movement direction. For some directions of movement, EMG activity was present over both time intervals, resulting in "double bursts." Modulation of activity during the agonist bur st interval was particularly systematic and was well fit by a cosine functi on. In contrast, the temporally shifted pattern was characterized by a grad ual change in the timing of a single burst of muscle activity. The burst oc curred at a time intermediate between the agonist and antagonist burst inte rvals. The temporally shifted pattern was seen less frequently than the amp litude-graded pattern and was present only in selected wrist muscles for sp ecific directions of movement. Monkeys made wrist movements to 8-16 differe nt targets that required varying amounts of flexion-extension and radial-ul nar deviation. These movements were performed more slowly than those of hum an subjects. The wrist muscles of the monkeys we examined displayed the amp litude-graded pattern of activity but not the temporally shifted pattern. S timulation of individual wrist muscles in monkeys resulted in wrist movemen ts that were markedly curved, particularly for the wrist extensors. These r esults indicate that step-tracking movements of the wrist are generated mai nly by using the amplitude-graded pattern to modulate muscle activity. We p ropose that this pattern reflects a central process that decomposes an inte nded movement into an agonist, "propulsive" component and an antagonist, "b raking" component. Separate bursts of muscle activity then are generated to control each component. On the other hand, we argue that the temporally sh ifted pattern may function to reduce the amount of movement curvature assoc iated with the activation of wrist muscles.