Integrated Electromyography of the Masseter on Incremental Opening and Closing with Audio Biofeedback: A Study on Mandibular Posture

Purpose: The purpose of this study was to test the hypothesis of a minimum electromyographic (EMG) rest position based on masseter surface EMG recordings of incremental opening and closing of the mandible with simultaneous audio EMG biofeedback. Materials and Methods: Nineteen alert subjects in an upright seated position opened and closed the mandible in 1-mm increments 20 mm interincisally. An electronic recording device allowed each subject to maintain the vertical dimension of each increment while simultaneously reducing right masseteric muscle activity to the minimum possible level using audio EMG biofeedback. Integrated EMG masseteric activity was recorded at each static opening and closing increment. Results: A mean plateau of integrated EMG output for all subjects with no minimum EMG point or circumscribed minimum EMG range for any of the nineteen subjects was shown. Analysis of variance for repeated measures showed no difference in opening and closing EMG levels (P = 0.27) and no interaction between opening, closing, and change in vertical dimension (P 0.0001). Conclusion: These results, with those of other studies, raise questions regarding the validity of the concept of a unique physiologic rest position of the mandible with the masseter or associated muscles at minimum muscle activity. The idea of overlapping postural ranges appears to be more appropriate.