Macaque masseter muscle: Internal architecture, fiber length and cross-sectional area

Models of mastication require knowledge of fiber lengths and physiological cross-sectional area (PCS): a proxy for muscle force. Yet only a small numb er of macaques of various species, ages, and sexes inform the previous stan dards for masseter muscle architecture. I dissected 36 masseters from 30 ad ult females of 3 macaque species-Macaca fascicularis, M. mulatta, M. nemest rina-using gross and chemical techniques and calculated PCS. These macaques have mechanically similar dietary niches and exhibit no significant differ ence in masseter architecture or fiber length. Intramuscular tendons effect ively compartmentalize macaque masseters from medial to lateral. Fiber leng ths vary by muscle subsection but are relatively conservative among species . Fiber length does not scale with body size (mass) or masseter muscle mass . However PCS scales isometrically with body size; larger animals have grea ter force production capabilities. PCS scales positively allometrically wit h facial size; animals with more prognathic faces and taller mandibular cor pora have greater PCS, and hence force, values. This positive allometry cou nters the less efficient positioning of masseter muscles in longer-faced an imals. In each case, differences in PCS among species result from differenc es in muscle mass not fiber length. Masseter PCS is only weakly correlated with bone proxies previously used to estimate muscle force, Thus prediction s of muscle force front bone parameters will entail large margins of errors and should be used with caution.