Macaque pterygoid muscles: 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. I dissected 36 medial pterygoid and 36 lateral pterygoid muscles from 30 adult females of 3 maca que species (Macaca fascicularis, M. mulatta, M, nemestrina) using gross an d chemical techniques and calculated PCS. These macaques have mechanically similar dietary niches and exhibit no significant difference in muscle arch itecture or fiber length. Fiber length does not scale with body size (mass) for either total pterygoid muscle or-for menial pterygoid muscle mass. How ever fiber length scales weakly with lateral pterygoid muscle mass. In each case, differences in PCS among species result from differences in muscle m ass not fiber length. Medial pterygoid PCS scales isometrically with body s ize; larger animals have greater force production capabilities. Medial and lateral pterygoid PCS scale positively allometrically with facial size; ind ividuals with more prognathic faces and taller mandibular corpora have grea ter PCS, and hence force, values. This positive allometry counters the less efficient positioning of masticatory muscles in longer-faced macaques. PCS is only M weakly correlated with bone proxies previously used to estimate muscle force. Thus, predictions of muscle force from bone parameters will e ntail large margins of error and should be used with caution.