Activity of wrist muscles during step-tracking movements in different directions

Human subjects made step-tracking movements of the wrist in 12 different directions while electromyographic activity was recorded from the long and short heads of the extensor carpi radialis. Three basic patterns of muscle activity were observed from a single muscle. The agonist pattern consisted of a large burst of activity beginning about 40 ms before movement onset, followed by relative inactivity during the movement. The antagonist pattern consisted of a large burst of activity beginning at or just after movement onset. The third pattern appeared to be a combination of agonist and antagonist patterns in a single muscle. These observations lead us to hypothesize that the nervous system has simplified the task of generating wrist movements in different directions by limiting the patterns of muscle activity to combinations of two basic patterns: agonist and antagonist.