EMG Analysis of Masseter and Temporal Muscles in Patients with skeletal Class III

The aim of this study was to elucidate the relationship between the craniofacialmorphology and the activity of the masticatory muscles.The samples for this study consisted of 5 patients with mandibular protrusion (skeletal Class III group) and 5 control subjects with normal occlusion (normal group).Electromyographic activities in the masseter and anterior part of temporal muscles were recorded using bi-polar surface electrodes when subjects were instructed to bite in the intercuspal position (ICP) at several levels of bite force.We evaluated the relationship between total. amount of activity on bilateral masseter and temporal muscles and the activity of each muscle, using normalized value relative to maximum voluntary clenching (MVC).Simple linear regression analysis was performed in each subject. The results were as follows:In normal group, slopesof regression line of the masseter muscle were gentler than those of the temporal muscle, while in skeletal Class III group, slopes of regression line of the masseter muscle were steeper than those of the temporal muscle.Therefore, innormal occlusion, the masseter muscle might predominantly be working as a majorpower generator at any clenching level.In contrast, in skeletal Class III group, the temporal muscle appeared to be predominant compared with the masseter muscle in its activity at any clenching level. Further, at high clenching level, the temporal muscle is possibly working as a major power generator, and working as apower generator as well as a jaw positioner at low clenching level.