This hypothetical postural difference has since been relatively well adopted within the literature, and has subsequently been upheld as kinematic evidence of fundamentally different forms of knuckle-walking ( Le Maître et al., 2017 Macho et al., 2010 Saunders et al., 2016 Schilling et al., 2014 Simpson et al., 2018 Thorpe et al., 2014). Indeed, such is the dearth of data that a recent study utilized bony morphology to hypothesize kinematic differences in wrist and metacarpophalangeal joint angles between chimpanzees and gorillas ( Kivell and Schmitt, 2009). Without kinematic data, they remain assumptions. Such studies often make predictions that require knowledge of bone and force orientations ( Barak et al., 2017 Chirchir et al., 2017 Dunmore et al., 2019 Lazenby et al., 2011 Macho et al., 2010 Matarazzo, 2015 Patel and Carlson, 2007 Tsegai et al., 2013 Zeininger et al., 2011). The lack of accurate, 3-D data on knuckle-walking has become particularly apparent in the light of an increasing numbers of studies correlating knuckle-walking with internal hand and wrist bone morphology. Without such data, the evaluation of musculoskeletal adaptations must necessarily rely on assumptions of knuckle-walking biomechanics. Yet remarkably little detailed quantitative biomechanical data on knuckle-walking exist from which such biomechanical inferences can be drawn. Knowledge of bony motion, combined with force magnitudes, orientations and muscle activations are all crucial to drawing conclusions relating form to function.
Establishing and evaluating such form–function correlations requires not only robust extant morphological comparisons, but also specific locomotor data.
Richmond et al., 2001 and citations therein, and more recently Begun and Kivell, 2011 Kivell and Schmitt, 2009 Kivell et al., 2013 Lovejoy et al., 2009 Orr, 2005 Orr, 2016 Sayers et al., 2012 Simpson et al., 2018 Williams, 2010 among many others). Correspondingly, identification of skeletal correlates in the hands of apes reflecting knuckle-walking or other hand postures, has been the focus of an enormous amount of research within paleoanthropology (e.g. Why, when and how many times this peculiar locomotor hand posture evolved has been the subject of intense scrutiny over the last 100 years in paleoanthropology (see the reviews of Begun, 2010 Crompton et al., 2008 Richmond et al., 2001 among others). 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.Ĭhimpanzees, bonobos and gorillas are unique among primates in their predominant use of knuckle-walking hand postures when terrestrial. 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. These data also reveal that maximum metacarpophalangeal extension angles during knuckle-walking (26–59 deg) were generally higher than previously considered. Metacarpophalangeal joint motion displayed distinct differences between digits in both species, likely related to the timing of force application. Maximum extension of the chimpanzee wrist was slight (5–20 deg) and generally overlapped with macaque digitigrady.
Notably, chimpanzees utilized more wrist ulnar deviation than any macaque hand posture. These results clarify the unique characteristics of, and commonalities between, knuckle-walking and digitigrady/palmigrady in multiple planes of motion. 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. This lack of data has limited the testability of proposed adaptations for knuckle-walking in African apes. Nor do any comparative 3-D data exist for hand postures used during quadrupedalism in monkeys. 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. The origin and evolution of knuckle-walking has long been a key focus in understanding African ape, including human, origins.
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