Telemetered electromyography of the supinators and pronators of the forearm in gibbons and chimpanzees: Implications for the fundamental positional adaptation of hominoids

Extant apes are similar to one another, and different from monkeys, in feat ures granting them greater range of forearm rotation and greater size of th e muscles that produce this motion. Although these traits may have been ind ependently acquired by the various apes, the possibility arises that such f eatures reflect adaptation to the stem behavior of the hominoid lineage. An ticipating that knowledge of forearm rotatory muscle recruitment during bra chiation, vertical climbing, arm-hanging during feeding, and voluntary reac hing might point to this stem behavior, we undertook telemetered electromyo graphic experiments on the supinator, pronator quadratus, ulnar head of pro nator teres, and a variety of other upper limb muscles in two gibbons and f our chimpanzees. The primary rotator muscles of the hominoid forearm were r ecruited at high levels in a variety of behaviors. As had been suspected by previous researchers, the supinator is usually active during the support p hase of arm-swinging, but we observed numerous instances of this behavior d uring which the muscle was inactive. No other muscle took over its role. Ki netic analyses are required to determine how apes can execute body rotation of arm-swinging without active muscular effort. The one behavior that is c ommon to most extant apes, is rare in monkeys, and which places a consisten tly great demand on the primary forearm rotatory muscles, is hang-feeding. The muscles of the supporting limb are essential to properly position the b ody; those of the free limb are essential for grasping food. Since the grea ter range of forearm rotation characterizing apes is also best explained by adaptation to this behavior, we join previous authors who assert that it l ies at the very origin of the Hominoidea. (C) 2001 Wiley-Liss, Inc.