Abstract
The position of the jaw and teeth contacts affect the evoked potential elicitable from the sternocleidomastoid muscle (SCM), which is activated during bite force production. The purpose of this study was to determine the changes of the activity of the SCM during clenching in the retruded mandibular position (RP). Six subjects were asked to clench at maximal and submaximal voluntary bite force levels when their mandible was fixed in the habitually closed position (HP) and in the retruded contact position (RP). Simultaneously, surface electromyography (EMG) of the masseter, temporal and SCM muscles was conducted. The root mean square (RMS) of the EMG in each clench effort was analyzed, and all data was normalized to the highest RMS found in the HP for each subject. EMG activity in all muscles depended on jaw position: the incidence of RMS values in RP higher than HP averaged 10.6%. The retrusion value of the condylar point was also significant: the subjects who retruded their jaw less than 0.6 mm showed higher SCM activity levels than those whose retrusion values were higher. It was concluded that SCM-EMG activity increases due to RP clenching. This is significant because muscle hyperactivity relates to muscle fatigue and pain.
