Electromyographic Analysis of Jaw Closing Frorces: Influence of an Instantaneous Center of Rotation of the Jaw

Abstract

Biomechanical analyses of jaw movement suggest that considerable differences in mechanical advantage may exist for individual masticatory muscles as a function of the inferior-superior position of the jaw. These variations in mechanical advantage have been hypothesized from the knowledge that the jaw opening and closing muscles apply force around a center of rotation that moves with variations in the magnitude of jaw depression. The influence of these peripheral mechanical properties would appear significant for models of lip-jaw motor control. In the present study, to verify empirlcally the influence of a moving, instantaneous center of rotation for the jaw, changes in mechanical advantage were analyzed by recording EMG from the medial pterygoid, masseter, and anterior and posterior heads of the temporalis during isometric jaw closing manuevers. lntramuscular comparisons of EMG level for each closing muscle were obtained for a range of positions of the jaw while it was acting against a static load. Relative changes in EMG level for each muscle, as a function of jaw position, were interpreted in relation to variations in mechanical advantage predicted from an analysis of the instantaneous center of jaw rotation. These data will be discussed in relation to the motor control of the jaw in speech production.