Asymmetry of masticatory muscle activity during the closing phase of mastication.

The purpose of this study was to investigate the asymmetry of masticatory muscle activity between working and nonworking sides in the closing phase during mastication. Fifty adult subjects displaying normal oral function and occlusion participated in this study. Electromyographic (EMG) activity of the anterior temporalis and the superficial masseter muscle were recorded during mastication, simultaneously with motion data of the mandible. EMG activities of elevator muscles and their Asymmetry Index (AI) were analyzed depending on the vertical deviation of the lower incisal point with a two mm gap from the intercuspal position (ICP). EMG activities of both the anterior temporalis and the masseter on the working side were significantly greater than those on the nonworking side. Masseter muscles tended to show greater AI than the anterior temporalis muscles. Thus, asymmetry of the elevator muscles during mastication was a common finding in normal subjects. The normal range of variability of EMG activity and AI was confirmed in each section.