Shoulder motion during quadrupedal walking in Cercopithecus aethiops: Integration of cineradiographic and electromyographic data

Abstract

This study describes the motion of the scapula and proximal humerus during walking in a vervet monkey (Cercopithecus aethiops), and relates it to the activity patterns of shoulder muscles as revealed by electromyography. Cineradiography shows that the protraction and retraction of the forelimb during a stride is composed of a complex series of scapular and humeral movements, some unified, some disjoint. At the beginning of support phase, the greater tuberosity of the humerus is moved against the cranial lip of the glenoid, a configuration that we suggest constitutes a close-packed position. Throughout most of support phase, the glenohumeral joint remains in this configuration and the scapula and humerus move as a unit, confirming Rose's (1989) deduction. Major activity in the supraspinatus and infraspinatus during this period contributes additional stabilization to the joint. The first half of swing phase involves cranial scapular rotation and translation while the humerus is still being retracted. Humeral retraction is brought about mainly by the teres major and is part of the mechanism for lifting the hand off of the ground. Protraction of the arm to swing the distal end of the humerus forward does not begin until about the middle of swing phase. The trapezius and atlantoscapularis anterior are probably responsible for the motion of the scapula during swing phase, but the mechanism for swinging the humerus forward remains unclear.