Abstract
Numerous anthropological studies have been aimed at estimating jaw-adductor muscle forces, which, in turn, are used to estimate bite force. While primate jaw adductors show considerable intra- and intermuscular heterogeneity in fibre types, studies generally model jaw-muscle forces by treating the jaw adductors as either homogeneously slow or homogeneously fast muscles. Here, we provide a novel extension of such studies by integrating fibre architecture, fibre types and fibre-specific tensions to estimate maximum muscle forces in the masseter and temporalis of five anthropoid primates: Sapajus apella (N = 3), Cercocebus atys (N = 4), Macaca fascicularis (N = 3), Gorilla gorilla (N = 1) and Pan troglodytes (N = 2). We calculated maximum muscle forces by proportionally adjusting muscle physiological cross-sectional areas by their fibre types and associated specific tensions. Our results show that the jaw adductors of our sample ubiquitously express MHC α-cardiac, which has low specific tension, and hybrid fibres. We find that treating the jaw adductors as either homogeneously slow or fast muscles potentially overestimates average maximum muscle forces by as much as approximately 44%. Including fibre types and their specific tensions is thus likely to improve jaw-muscle and bite force estimates in primates.
Highlights
Considerable effort in anthropology has been directed at modelling feeding mechanics in extant primates with the goal of gaining insights into feeding-system adaptations in extant and extinct taxa
Our results show that the jaw adductors of our sample ubiquitously express myosin heavy chain (MHC) α-cardiac, which has low specific tension, and hybrid fibres
Anthropological studies that have estimated maximum muscle and bite forces in extant human and non-human primates have generally applied either a slow-type (e.g. 22.5 N cm−2) [51] or fast-type (e.g. [16,119,120]) specific tension value uniformly across the chewing muscles; faster tension values have been applied to studies that have modelled bite forces in extinct primates and non-primate mammals (e.g. [21,54,121])
Summary
Considerable effort in anthropology has been directed at modelling feeding mechanics in extant primates with the goal of gaining insights into feeding-system adaptations in extant and extinct taxa. Research linking feeding-system design and function has relied heavily on biomechanical models based fundamentally on engineering principles. Finite-element analysis (FEA) has become widely employed in biological anthropology to characterize the stress and strain patterns of the craniofacial complex under a variety of loading conditions In FEA models of stress and strain patterns associated with feeding, bone tissue and bone material properties must be assigned, as well as applied muscle force vectors and muscle forces [24,25,27,28,29]. Bite force calculations require the input of muscle forces, often estimated from the anatomical (ACSA) or physiological cross-sectional area (PCSA)
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