Locomotor Hand Postures, Carpal Kinematics During Wrist Extension, and Associated Morphology in Anthropoid Primates.

The biomechanics of wrist extension (or dorsiflexion) are important for understanding functional adaptation of the primate hand because extension mobility varies with habitual locomotor hand posture and facilitates certain manipulative tasks. Here, intercarpal kinematics are employed to investigate mechanisms underlying wrist extension in a sample of anthropoids representing various arboreal and terrestrial locomotor modes. Carpal kinematics are studied using computed-tomography of cadaveric forelimbs, and these data are combined with a morphometric analysis of biomechanically-informative anatomical features. The results indicate that stiff-wristed knuckle-walking chimpanzees and digitigrade baboons are characterized by low ranges of motion (ROMs) at the radiocarpal and midcarpal complexes. Palmigrade-capable monkeys have high extension ROMs at both the radiocarpus and midcarpus, while palmigrade-capable orangutans achieve wrist extension through moderate radiocarpal ROMs and high midcarpal ROMs. Morphometrics demonstrate that a more projecting dorsal ridge of the distal radius corresponds with low-to-moderate radioscaphoid mobility in the apes, but that baboons resemble palmigrade-capable monkeys in this metric. Thus, the dorsal ridge of the radius alone is not a good indicator of wrist mobility and hand posture. However, the extent of the lunate's articular arc on the dorsum of the capitate head is correlated with midcarpal mobility across taxa. These findings suggest that although a precise relationship between wrist extension ROM and morphology is difficult to define, the presence of a pronounced dorsal ridge combined with an abbreviated dorsal capitate articular arc reflects limited overall dorsiflexion with attendant constraints on the adoption of palmigrade hand postures. Anat Rec, 300:382-401, 2017.