Electromyography of pongid shoulder muscles. II. Deltoid, rhomboid and “rotator cuff”

Electromyographic (EMG) recordings were taken of action potentials in the deltoid, rhomboid, supraspinatus, infraspinatus, subscapularis, and teres minor muscles of pongid apes as they spontaneously elevated their arms and engaged in suspensory activity on a trapeze. During brachial elevation, activities of pongid deltoid, rhomboid, supra‐spinatus, infraspinatus and subscapularis muscles basically resemble those of human subjects. The intermediate deltoid exhibited high potentials particularly near and at the zenith of brachial elevation. The anterior and, to a lesser extent, posterior deltoid were more variable than the intermediate deltoid during arm‐raising. The three heads often acted discretely. Rhomboid potentials also increased steadily with the highest ones at 130–150°. The supraspinatus sometimes acted as an initiator and accelerator of arm‐raising. Supraspinatus potentials commonly decreased markedly near the zenith of brachial elevation. When subjects reached overhead, it was impossible to discern whether the subscapularis acted fundamentally as an arm‐raising muscle or merely as a medial rotator to position the hand. In the orangutan, the teres minor was inconsequentially active during arm‐raising. This is strikingly different from the human activity pattern in which it is supposed to function in the inferior component of a glenohumeral force couple (Inman et al., '44). Remarkably little consequential EMG activity was recorded in the shoulder muscles of gorilla during hoisting activity. Per contra, in the orangutan some muscles were quite active as she hoisted herself on the trapeze. The teres minor seemed to be a prime retractor of the humerus in that context. In the chimpanzee, the rhomboid and especially the supraspinatus were significantly active during hoisting. All muscles tested in the three great apes were silent or inconsequentially active during pendent suspension from the trapeze. We conclude that at least during short‐term pendent suspension, osseoligamentous mechanisms are probably chiefly responsible for maintaining integrity of pongid glenohumeral joints. The important role of the supraspinatus muscle as a brachial elevator should be emphasized in future studies of pongid scapular shape. In view of the variable, contextually dependent, actions of the pongid “rotator cuff” muscles we suggest that they be termed the deep scapulohumeral muscles.