In this context, the human hand is traditionally considered unique in the animal kingdom for its ability to use a variety of forceful precision grips, opposing the thumb to the pads of the fingers, and power squeeze grips (Napier, 1960, Marzke, 1997, Susman, 1998, Tocheri et al., 2008). In contrast, humans are distinct from other primates in typically only using their hands for manipulation and they show morphological features consistent with this more specialized function (e.g., Jones, 1916, Napier, 1993, Marzke, 1997). However, non-human primates use their hands for both locomotion and manipulation and show bony and soft tissue morphology of the hands and forelimbs that reflect these diverse functional requirements (e.g., Straus, 1940, Schultz, 1956, Schultz, 1969, Jouffroy et al., 1993, Zihlman et al., 2011, Myatt et al., 2012, Kivell et al., 2016). In primates, and particularly humans, manipulating objects is a crucial skill for essential behaviors such as feeding, social interactions, tool-making and use (Boesch, 1993, Fragaszy, 1998, Byrne et al., 2001, Hopkins et al., 2007). These musculoskeletal simulation results shed light on the functional consequences of the different hand proportions among extant and extinct hominids and the different manipulative abilities found in humans and great apes. Assuming a human-like range of motion at each hand joint, simulation results show that H. naledi and A. sediba had the biomechanical potential to use the grip techniques considered important for stone tool-related behaviors in humans. Results show that certain grips are more challenging for some species, particularly orangutans, than others, such that they require stronger muscle forces for a given range of motion. This study aims to understand the importance of hand proportions on the use of different grip strategies used by humans, extant great apes (bonobos, gorillas and orangutans) and, potentially, fossil hominins ( Homo naledi and Australopithecus sediba) using a musculoskeletal model of the hand. However, this cause-effect relationship is not fully understood because little is known about the biomechanical functioning and mechanical loads (e.g., muscle or joint forces) of the non-human primate hand compared to that of humans during object manipulation. Differences in grip techniques used across primates are usually attributed to variation in thumb-finger proportions and muscular anatomy of the hand.
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