COORDINATION OF MULTIPLE MUSCLES IN 2-DEGREE-OF-FREEDOM ELBOW MOVEMENTS

The present study quantifies electromyographic (EMG) magnitude, timing , and duration in one and two degree of freedom elbow movements involv ing combinations of flexion-extension and pronation-supination. The ai m is to understand the organization of commands subserving motion in i ndividual and multiple degrees of freedom. The muscles tested in this study fell into two categories with respect to agonist burst magnitude : those whose burst magnitude varied with motion in a second degree of freedom at the elbow, and those whose burst magnitude depended on mot ion in one degree of freedom only. In multiarticular muscles contribut ing to motion in two degrees of freedom at the elbow, we found that th e magnitude of the agonist burst was greatest for movements in which a muscle acted as agonist in both degrees of freedom. The burst magnitu des for one degree of freedom movements were, in turn, greater than fo r movements in which the muscle was agonist in one degree of freedom a nd antagonist in the other. It was also found that, for movements in w hich a muscle acted as agonist in two degrees of freedom, the burst ma gnitude was, in the majority of cases, not different from the sum of t he burst magnitudes in the component movements. When differences occur red, the burst magnitude for the combined movement was greater than th e sum of the components. Other measures of EMG activity such as burst onset time and duration were not found to vary in a systematic manner with motion in these two degrees of freedom. It was also seen that sev eral muscles which produced motion in one degree of freedom at the elb ow, including triceps brachii (long head), triceps brachii (lateral he ad), and pronator quadratus displayed first agonist bursts whose magni tude did not vary with motion in a second degree of freedom. However, for the monoarticular elbow flexors brachialis and brachioradialis, ag onist burst magnitude was affected by pronation or supination. Lastly, it was observed that during elbow movements in which muscles acted as agonist in one degree of freedom and antagonist in the other, the mus cle activity often displayed both agonist and antagonist components in the same movement. It was found that, for pronator teres and biceps b rachii, the timing of the bursts was such that there was activity in t hese muscles concurrent with activity in both pure agonists and pure a ntagonists. The empirical summation of EMG burst magnitudes and the pr esence in a single muscle of both agonist and antagonist bursts within a movement suggest that central commands associated with motion in in dividual degrees of freedom at the elbow may be superimposed to produc e elbow movements in two degrees of freedom.