Effects of expected perturbations on the velocity control of fast arm abduction movements

A triphasic electromyographic pattern of sequential activation of agonist, antagonist, and again agonist muscles underlies rapid or ballistic limb movements in humans. The first agonist burst reflects muscular force accelerating the limb, the antagonist burst is mainly related to the braking process of movement, while the second agonist burst is considered a reactive adjustment to the deceleration. The duration of the first agonist burst has been reported to be constant for movements of different amplitudes, thus suggesting that only changes in its amplitude contribute to the velocity control of movement. The present research has been undertaken to investigate the strategy whereby the nervous system increases agonist impulsive force for ballistic performance in response to experimental conditions requiring perceptual and/or previsional processes related to expected changes in load or accuracy constraints. The effects of expected perturbations of different strengths on some kinematic and electromyographic variables of fast arm abduction movements performed in a step-tracking task and in an outer-stop terminated task were analyzed in normal subjects. All motor performances were characterized by triphasic electromyographic patterns. In the absence of expected perturbations, the mean velocity of the movements was markedly higher in the outer-stop terminated task. Correspondingly, greater amplitudes and durations of the first agonist burst were observed. In both types of motor tasks, when expected perturbations were inserted, the velocity of the movement increased as well as the amplitude and the duration of the first agonist burst. These results, in aggrement with previous observations, indicate that the normal mechanism, whereby the nervous system increases agonist impulsive force in rapid movements, comprises changes not only in the amplitude but also in the duration of the first agonist burst.