An Approach to Quantitative Electromyography of the Masseter Muscle

¹ Departments of Physiology and Orthodontics, School of Dentistry, Washington University, St. Louis, Missouri

By means of a single-beam oscilloscope, surface electrodes, a bite device to fix the bite opening (6 mm.) and record pressures, and a 16-mm. camera, electromyographic records were taken from the left masseter muscle. A procedure is described for quantitatively expressing the muscle activity in millivolts. Measurements were reproducible, once the electrodes were placed in position. However, variations in the level of activity occurred after the electrodes were removed and replaced. Regardless of the level of activity, the nature of the response in all cases was similar; i.e., increasing bite forces (0-23 lb.) were related directly to muscle activity (0-279 mV x l0^-3). A difference in muscle activity was evident in subjects manifesting a slight temporomandibular joint disturbance or anterior displacement (0.2-1.5 mm.) of the mandible when compared with subjects with no joint disturbance or no anterior displacement, respectively.