The influence of masseter and temporalis sarcomere-length operating ranges as determined by laser diffraction on architectural estimates of muscle force and excursion in macaques (Macaca fascicularis and Macaca mulatta)

Abstract Objective Determine sarcomere length (L s ) operating ranges of the superficial masseter and temporalis in vitro in a macaque model and examine the impact of position-dependent variation on L s and architectural estimates of muscle function (i.e., fiber length, PCSA) before and after L s -normalization. Design Heads of adult Macaca fascicularis (n = 4) and M. mulatta (n = 3) were bisected postmortem. One side of the jaw was fixed in occlusion, the other in maximum gape. L s was measured bilaterally using laser diffraction and these measurements were used to estimate sarcomere-length operating ranges. Differences in fiber length and PCSA between sides were tested for significance prior to and following L s -normalization. Results Sarcomere-length operating ranges were widest for the anterior superficial masseter and narrowest for the posterior temporalis. Compared with other mammals, macaque operating ranges were wider and shifted to the right of the descending limb of a representative length-tension curve. Fibers were significantly stretched by as much as 100%, and PCSAs reduced by as much as 43%, on the maximally gaped compared with occluded sides. L s -normalization substantially reduced position-dependent variance. Conclusions The superficial masseter ranges between 87-143% and the temporalis between 88-130% of optimal L s from maximum gape to occlusion, indicating maximum relative L s for these macaque muscles exceeds the upper end range previously reported for the jaw muscles of smaller mammals. The wider macaque operating ranges may be functionally linked to the propensity for facially prognathic primates to engage in agonistic canine display behaviors that require jaw-muscle stretch to facilitate production of wide jaw gapes.