Meeting Abstract
Bite force is an important performance variable for animals and is comprised of muscular force input (by the jaw-closing muscle) and jaw mechanics (skeletal morphology). Muscle force input is dynamic across jaw positions (gapes) due to the physiological constraints of skeletal muscle. We examined how the force-length relationship of a jaw-closing muscle (superficial masseter) relates to bite force across various gapes in two Rhesus Macaque (Macaca mulatta) monkeys during supra-maximal stimulation. The female we tested varied in biting force by 37% across gapes, whereas the male varied by 25% across gapes. This variation in muscular input is surprisingly small, given 50% muscle-tendon-unit (MTU) length change in the female and 55% MTU length change in the male. We found that intermediate gape widths produced the highest bite force, and extreme gapes (large and small) produced lower bite forces. We implanted radiopaque markers in the masseter muscle, skull, and mandible of each individual and filmed with x-ray video during the experimental stimulation events for XROMM analysis. We found that markers within the muscle bulged laterally as force increased. At higher gapes, the muscle fibers were more in line with the line of action of the muscle-tendon unit (which we attribute to a decrease in pennation angles) and produced less lateral bulging. We propose that rotation of jaw muscle fibers is a product of both active (fiber shortening and contractile force) and passive (elastic behavior of connective tissue and aponeuroses) mechanisms.This fiber rotation allows Macaca mulatta to bite at near optimal force levels across a wider range of gapes than would otherwise be possible.
