Eight healty participants performed horizontal isometric jaw movements with maximum voluntary contractions of the jaw muscles until they could not tolerate the task. Electromyographic activities bilaterally recorded from the superior and inferior regions of the sternocleidomastoid muscle and the nuchal and shoulder regions of the trapezius muscle were observed.
The results obtained were as follows:
1. In more than 94% of 32 trials, the amplitude of EMGs during the task were higher than those at rest, and in more than 78%, temporal changes in EMG burst were synchronized with the onset-offset of the exercise.
2. In more than 72% of all trials, small amplitude EMG lasted after cessation of the task.
3. Muscle activities of the lateral cervical region during the exercise were superior to the activity of the nuchal and shoulder regions, and amplitude of EMGs in the neck and shoulder regions tended to be the highest during isometric retrusion.
4. These results don't contradict with our hypotheses that occlusal habits might be one of cause of the unpleasant sensation in the neck and shoulder regions and that sustained isometric retrusion might have heavier effects on it.
