Electromyography of brachial muscles in Pan gorilla and hominoid evolution

Electromyographic studies on brachial muscles in a gorilla indicate that its elbow joint may be especially adapted for knuckle‐walking and suspensory behavior. A close‐packed positioning mechanism that minimizes muscular effort during full extension of the elbow joint is indicated by remarkably low levels of EMG in the brachial muscles, particularly during knuckle‐walking and suspensory behavior on a trapeze. Extension of the elbow joint is facilitated by reduction of the olecranon process of the ulna, a feature that is attributable initially to aspects of an arboreal heritage in protogorilla and secondarily to selection for efficient knuckle‐walking. Although notable differences exist between gorilla and man in known activity of the brachial muscles, the two species are strikingly similar in many basic features. Available evidence suggests that they share a common heritage of arboreal adaptation, including vertical climbing, hauling, hoisting, and suspensory behavior, perhaps more recently than some authors would care to admit. Knuckle‐walking probably played an inconsequential role in the protohominid career. Selection for tool use, expecially involving powerful and rapid extension of the elbow joint, is the most reasonable explanation for the relatively more protuberant olecranon process in man by comparison with apes.