Function of the long head of the biceps at the shoulder: electromyographic analysis.

The purpose of this study was to delineate, through electromyographic analysis, the function of the long head of the biceps at the shoulder. Ten shoulders were examined with dynamic electromyography. The long head of the biceps was instrumented with thin wire electrodes. The supraspinatus, infraspinatus, deltoid, brachialis, and brachioradialis were instrumented as controls. Because the biceps functions primarily as a forearm supinator and elbow flexor, a long arm brace was used to lock the elbow in extension with the forearm in neutral pronation/supination. Each motion was tested in a full arc at fast (170 degrees per second) and slow (36 degrees per second) speeds and repeated with and without a 5-pound weight attached to the distal end of the brace. No electrical activity was identified in the long head of the biceps muscle in response to isolated shoulder motion with the elbow and forearm position controlled. The data demonstrate that the long head of the biceps is not active in isolated shoulder motion when the elbow and forearm are controlled. Thus, any hypothesis on bicipital function at the shoulder must be based on either a passive role of the tendon or tension in association with elbow and forearm activity.