The relationship between jaw-muscle mechanical advantage and activity levels during isometric bites in humans

Mathematical analyses of mandibular biomechanics often assume a close relation between the relative activity levels of the jaw-adductor muscles and their mechanical advantage. However, this relation has not been clearly demonstrated. This study examined the relation between muscle activity levels, force generation, and mechanical advantage in 48 (23 male, 25 female) normal subjects. Mechanical advantage in the sagittal plane was calculated from standard lateral cephalograms. Activity for the anterior and posterior temporalis and masseter muscles was recorded from surface electrodes during incisor and molar bites. Muscle activity was standardized by calculating a least-squares regression for electromyographic versus occlusal force. At each bite position the more efficient muscles were recruited at higher rates but muscle size was more important in determining efficiency than was mechanical advantage. Differences in jaw-muscle recruitment rates between males and females were also best explained by differences in muscle size. Decreased muscle recruitment rates at molar bite positions matched increased mechanical advantage for the anterior temporalis and masseter. However, posterior temporalis recruitment patterns seemed to reflect jaw position more closely than mechanical advantage.