The Effect of Dietary Loading on Structural Determinants of Force Production in the Rat Masseter

Purpose: Biomechanical loading associated with feeding is known to direct cranial bone growth, however less is known about its effects on masticatory muscle growth and performance. Peak muscle contractile forces are determined by a combination of factors including mass, fiber length, and fiber type. Here, we test the hypothesis that mechanically challenging diets increase the physiological cross-sectional area (PCSA), an estimate of maximum contractile force at tetanus, in the rat superficial masseter. Methods: Sprague-Dawley rats were raised on either a hard/tough (overuse) diet or a soft (underuse) diet (n=5/cohort). The superficial masseters were dissected and photographed using a trifocal stereo microscope, and muscle fiber length (6/individual) were measured using ImageJ. Muscle volumes were calculated from in-situ diffusible iodine-based contrast-enhanced µCT scans. PCSA was calculated using the formula Vm/Lf [muscle volume (cm3) / fiber length (cm)]. Results: Mean PCSA is greater in the overuse cohort than the underuse cohort (38.41 vs 35.09) however the results are not statistically significant (p=0.55) likely due to small sample size. The increase in PCSA for the overuse cohort is driven largely by muscle volume (432.24 vs 410.30 cm3, p=0.0.69) rather than fiber length (11.33 vs 12.35 cm, p=0.031). Conclusions: Mechanically challenging diets tend to be associated with greater masticatory muscle volumes and thus increased PCSA, however future studies with increased sample sizes are needed to verify these results. Previous work from our lab suggests postnatal changes in muscle fiber phenotype also contribute to increased muscle contractile forces during feeding.