The biomechanics of knuckle-walking II: muscle activity of chimpanzee extrinsic hand and wrist muscles

Knuckle-walking is a terrestrial hand posture used by chimpanzees, bonobos, and gorillas alone among primates. Given the relevance of these African apes to human origins, considerable research has sought to understand how forces across the hand and wrist have shaped the morphology underlying this unique hand posture. Yet, data on extrinsic hand and wrist muscle activity during knuckle-walking are incomplete; only three extrinsic hand muscles are documented and no new data have been published in the last 45 years. This incompleteness has led to confusing, and sometimes contradictory assumptions being used in comparative functional analyses. Here, we used electromyography (EMG) to investigate eight extrinsic hand muscles (five wrist muscles and three digital muscles) during knuckle-walking in chimpanzees across four commonly used hand postures. We documented a complex pattern of support and swing phase activity in all wrist flexor and extensor musculature, combined with a near silence of digital flexor muscle activity. As in other mammals, wrist flexor activity typically exceeded extensor activity. Hand posture had some effect on muscle activity, notably a general increase in wrist muscle activity when the hand was orientated posteriorly as opposed to medially. Chimpanzee muscle activity showed general similarities with baboons, though chimpanzees showed slightly more activity in radial side muscles while baboons showed more reliance on ulnar muscles. These data help rule out two evolutionary hypotheses concerning the origins of knuckle-walking and whether the earliest known hominin knuckle-walked. More importantly, they form a foundation upon which future modeling and form-function studies can draw.