Meeting Abstract
Biting forces exerted by an animal are the product of skeletal morphology (jaw mechanical leverage, joint architecture, muscle insertion angle, etc) and muscular force input. Within an individual, skeletal morphology remains static; by contrast, force production of the muscle can be dynamic because of the physiological constraints of skeletal muscle. We examined how the force-length relationship of muscle relates to biting performance across gapes of Rhesus macaque (Macaca mulatta) monkeys by measuring bite force during supra-maximal stimulation of the nerve to the masseter muscle via nerve cuff. In two individuals (one male, one female), bite force varied only slightly (less than 25% drop) across gapes, despite a nearly 50% change in whole-muscle length from min to maximum gape. Markers within the muscle (filmed with X-ray video and integrated in XROMM analysis of skull and mandible postures) showed that at large gapes, markers were more in line with the line of action of the muscle-tendon-unit, suggesting decreasing pennation angles with increasing gape. Using fatigue testing, we demonstrated that decreases in force had the similar effect on muscle shape of decreasing pennation angles. These results suggest that fibers rotate within a muscle relative to the line of action in macaque masseter muscles, and this rotation is driven through both active (i.e. fiber shortening and force production) and passive (i.e. elastic) mechanisms. These influences could extend the range of gapes at which macaques bite at or near optimal fiber length. Further analyses will incorporate contrast-enhanced CT scans to quantitatively determine fiber angles and lengths.
