Step-tracking movements of the wrist in humans. II. EMG analysis.

We asked human subjects to make accurate step-tracking movements of the wrist to targets that required 5 degrees-30 degrees of radial or ulnar deviation. Speed instructions were given prior to each trial. Muscle activity was recorded from extensor carpi radialis longus (ECRL) and extensor carpi ulnaris (ECU) using surface electrodes. The agonist muscle initiated each movement with a brief burst of activity which began approximately 45 msec before movement onset. Then, the antagonist muscle displayed a brief burst of activity which began approximately 10 msec after movement onset. The magnitude, but not the timing, of these bursts was modulated by changes in the task requirements. The area of the initial agonist burst varied with changes in both displacement and intended speed. This burst was most highly correlated with the initial peaks of acceleration and jerk. In contrast, the area of the initial antagonist burst varied with changes in intended speed and was less well modulated by changes in displacement. This burst was highly correlated with the reciprocal of movement duration. Some small, fast movements had the same agonist bursts as some large, slow movements. However, the antagonist bursts for these movements differed greatly. This observation provides clear evidence that the magnitudes of the agonist and antagonist bursts are independently controlled. In a prior paper (Hoffman and Strick, 1986b), we proposed that step-tracking movements of different amplitudes and intended speeds are centrally generated by adjusting 2 kinematic variables: (1) the peak value and (2) the duration of a derivative of displacement. The present results suggest that these 2 kinematic parameters are separately generated by independently modulating the magnitudes of the agonist and antagonist bursts. Thus, the peak displacement of a step-tracking movement must be determined by the appropriate adjustment of both bursts of muscle activity.