INDEPENDENT COACTIVATION OF SHOULDER AND ELBOW MUSCLES

The aim of this study was to examine the possibility of independent mu scle coactivation at the shoulder and elbow. Subjects performed rapid point-to-point movements in a horizontal plane from different initial limb configurations to a single target. EMG activity was measured from flexor and extensor muscles acting at the shoulder (pectoralis clavic ular head and posterior deltoid) and elbow (biceps long head and trice ps lateral head) and flexor and extensor muscles acting at both joints (biceps short head and triceps long head). Muscle coactivation was as sessed by measuring tonic levels of electromyographic (EMG) activity a fter limb position stabilized following the end of the movements. It w as observed that tonic EMG levels following movements to the same targ et varied as a function of the amplitude of shoulder and elbow motion. Moreover, for the movements tested here, the coactivation of shoulder and elbow muscles was found to be independent - tonic EMG activity of shoulder muscles increased in proportion to shoulder movement, but wa s unrelated to elbow motion, whereas elbow and double-joint muscle coa ctivation varied with the amplitude of elbow movement and were not cor related with shoulder motion. In addition, tonic EMG levels were highe r for movements in which the shoulder and elbow rotated in the same di rection than for those in which the joints rotated in opposite directi ons. In this respect, muscle coactivation may reflect a simple strateg y to compensate for forces introduced by multijoint limb dynamics.