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From observation of knuckle-walking chimpanzees, it seems that a distinction can be made between ‘high’ and ‘low’ knuckle-walking positions of the hand. The MCPJ and PIPJ in the longest finger D3 will be more hyperextended and hyperflexed, respectively, than in the shorter fingers, at least with vertical metacarpals ( Fig. 2C). To allow the shorter fingers to reach the ground, the MCPJ must hyperextend and the PIPJ flex ( Fig. 2C). Even more trivial, the longest finger would support all load, as the shorter fingers would not reach the ground ( Fig. 2B). Maintaining the neutral MCPJ position would require constant muscle effort.
#CHIMPANZEE HAND XRAY SKIN#
The small ground contact area would lead to high tissue pressure at impact even with a thick skin pad, the more so because a straight joint chain does not buffer impact energy. A straight articular chain aligning ground contact at the proximal phalanx (PP) head with the MCPJ, wrist and (fore)arm is unsuited for dynamic locomotion ( Fig. 2A). In wild gorillas, a wider variability in hand postures has recently been documented, including fist walking ( Thompson et al., 2018).Īs an introduction to the biomechanical models, some basic aspects of knuckle walking are here considered. However, gorillas more often use a palm-back position with loading of digits 2 to 5 ( Inouye, 1994 Matarazzo, 2013 Samuel et al., 2018).
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In captive chimpanzee observations, weight is mainly borne by digits 2, 3 and 4, and knuckle-walking positions vary over a spectrum between palms facing backwards and palms facing inwards ( Wunderlich and Jungers, 2009 Thompson, 2020). Between gorillas and chimpanzees, the hand position in knuckle walking differs. The ground reaction forces generally do not apply at the proximal phalanx head but are distributed over the middle phalanx dorsum, which is covered by a thick skin pad ( Matarazzo, 2008 Wunderlich and Jungers, 2009 Matarazzo, 2013). To enable this, the proximal interphalangeal joints (PIPJs) are hyperflexed and the metacarpophalangeal joints (MCPJs) hyperextended ( Inouye, 1994 Thompson et al., 2018 Thompson, 2020) ( Fig. 1). In this distinctive locomotor mode, the hands contact the substrate with the dorsum of the middle phalanges (MP). The African great apes – Gorilla spp., Pan troglodytes (chimpanzee) and Pan paniscus (bonobo) – are emblematic knuckle walkers. In conclusion, the current biomechanical analysis in accumulation of previous EMG findings suggests that finger flexors play no role in impact buffering in knuckle walking. Impact buffering by large flexion moments at the MCPJs from active finger flexors would result in impacts at the knuckles themselves, which is dysfunctional for various biomechanical reasons and does not occur in real knuckle walking. In knuckle walking, the finger flexors are not elongated to lengths where passive strain forces would become important. The biomechanics do not support the finger flexor impact buffering hypothesis. Therefore, various aspects of knuckle walking were modeled and the finger flexor tendon displacements in the load-bearing fingers were measured in a chimpanzee cadaver hand, of which also an MRI was taken in knuckle stance. Although these data by themselves question the finger flexor impact buffering hypothesis, the present study aimed to critically investigate the hypothesis from a biomechanical point of view. However, EMG studies did not report significant finger flexor activity in knuckle walking. This stretching of the finger flexor muscle–tendon units would absorb impact energy. In the literature, it was hypothesized that the finger flexors help buffer impacts because in knuckle stance the metacarpophalangeal joints (MCPJs) are strongly hyperextended, which would elongate the finger flexors. As these muscular apes are given to high-velocity motions, the question arises of how the ground reaction forces are buffered so that no damage ensues in the load-bearing fingers. Chimpanzees are knuckle walkers, with forelimbs contacting the ground by the dorsum of the finger's middle phalanges.