The biomechanics of knuckle-walking: 3-D kinematics of the chimpanzee and macaque wrist, hand and fingers.

The origin and evolution of knuckle-walking has long been a key focus in understanding African ape, including human, origins. Yet, despite numerous studies documenting morphological characteristics potentially associated with knuckle-walking, little quantitative three-dimensional (3-D) data exist of forelimb motion during knuckle-walking. Nor do any comparative 3-D data exist for hand postures used during quadrupedalism in monkeys. This lack of data has limited the testability of proposed adaptations for knuckle-walking in African apes. This study presents the first 3-D kinematic data of the wrist, hand and metacarpophalangeal joints during knuckle-walking in chimpanzees and in macaques using digitigrade and palmigrade hand postures. These results clarify the unique characteristics of, and commonalities between, knuckle-walking and digitigrady/palmigrady in multiple planes of motion. Notably, chimpanzees utilized more wrist ulnar deviation than any macaque hand posture. Maximum extension of the chimpanzee wrist was slight (5-20 deg) and generally overlapped with macaque digitigrady. Metacarpophalangeal joint motion displayed distinct differences between digits in both species, likely related to the timing of force application. These data also reveal that maximum metacarpophalangeal extension angles during knuckle-walking (26-59 deg) were generally higher than previously considered. In macaques, maximum metacarpophalangeal extension during digitigrady and palmigrady overlapped for most digits, highlighting additional complexity in the interpretation of skeletal features that may be related to limiting metacarpophalangeal motion. Most importantly, however, these new 3-D data serve as a fundamental dataset with which evaluation of proposed musculoskeletal adaptations for knuckle-walking can be tested.