A cadaveric study of wrist-joint moments in chimpanzees and orangutans with implications for the evolution of knuckle-walking.

Understanding the mechanism underlying the evolution of knuckle-walking in African great apes but not in humans may provide important implications about the origin and evolution of human bipedal locomotion. In this study, aiming to reveal possible structural adaptations of the chimpanzee's forearm and hand musculature related to knuckle-walking, we measure the passive elastic moment of the chimpanzee's and orangutan's wrist as it was rotated into extension, immobilizing the metacarpophalangeal joint at three different positions: extended (as in knuckle-walking), flexed (as in fist-walking), and an intermediate position. Our findings demonstrate that when the metacarpophalangeal joints are extended, the rigidity of the wrist joint in the extended direction increases.