Regional properties of the superior head of human lateral pterygoid muscle.

The aim of this study was to investigate functional heterogeneity within the superior head of the human lateral pterygoid (SHLP) muscle by comparing the functional properties (e.g. firing rates) of single motor units (SMUs) between different arbitrarily defined regions of the SHLP, namely, medial, middle or lateral; origin or insertion; and superior or inferior regions. Jaw movement and electromyographic (EMG) activity was recorded from computed tomography-verified locations within the SHLP of 27 asymptomatic human subjects during goal-directed contralateral, ipsilateral, and protrusive jaw movements. The SMU firing rates for protrusion in the medial, origin, and inferior regions were significantly lower than, respectively, the firing rates in the middle, insertion, and superior regions. For contralateral movement, the firing rates were significantly greater in the medial and middle regions than those in the lateral region. The data provide additional evidence that the SHLP is functionally heterogeneous and, together with previous evidence for functional heterogeneity within the inferior head of the lateral pterygoid, support the proposition that both heads should be regarded as functionally heterogeneous.