The impact of gape on biting force of the masseter muscle in male and female macaque monkeys


Meeting Abstract

P3-63  Sunday, Jan. 6 15:30 – 17:30  The impact of gape on biting force of the masseter muscle in male and female macaque monkeys. DRAMM, CL*; ORSBON, CP; VARGEESE, JJ; ROSS, CF; GIDMARK, NJ; Knox College; University of Chicago; Knox College; University of Chicago; Knox College cdramm@knox.edu

The force-length relationship of skeletal muscle constrains the relationship between biting force and gape in jaw systems. If muscle forces and bite force are highest at minimum gape (occlusion), then forces should decrease at larger gape angles, as the mouth is opened. Male macaques have wider maximum gapes than female macaques to provide clearance to their longer canines, but how that impacts jaw muscle force production is unknown. Here, we present data on the impact of masseter muscle force-length relationships on biting force in male (N = 3) and female (N = 2) Rhesus Macaque (Macaca mulatta) monkeys. We electrically stimulated the masseter muscle bilaterally at the full range of possible gapes while measuring bite force at the incisors with a force transducer. Surgically implanted markers in the cranium, mandible, and masseter muscle, combined with X-ray videos allowed the use of XROMM to reconstruct 3D jaw/skull position. Both males and females in our study utilized the ascending, plateau, and descending limbs of the force-length curve, and did not differ significantly in the gape angle that correlates with optimal muscle force. The males’ jaws could be depressed to a wider gape angle and distance than the females’ and their masseters utilized a greater portion of the descending (= long) end of the force-length curve. Male macaques have larger canines than females, creating a difference in the relationship of muscle length with gape, depending on how gape is measured – i.e., from incisors versus canines. The correlation of canine gape with biting force is nearly identical across both sexes (which is not true of incisor gape), suggesting that macaque jaw musculoskeletal biomechanics are more aligned to canine gape than molar gape, incisor gape, or gape angle.

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