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Limitation of mouth opening, one of the symptoms of TMD, is commonly estimated by measuring the range of motion of the mandible (ROM). The values of ROM in previous studies show that ROM becomes smaller by fixation of the head while being recorded. Recently it has become obvious that the head postures, such as head extension, have a great effect on the stomatognathic function.In this study, we investigated the influence of head extension and body posture on ROM to find out the reliability of measured ROM as a diagnostic aid in TMD and the mechanism of mouth opening; and simultaneously, we observed the EMG activity of the anterior belly of digastric muscle (Da).The results were as follows:1. As the head was extended, the ROM increased significantly. It is considered that the space under the mandible increased by head extension enables wider mandibular movement. This suggests that more consideration should be given to the head posture to obtain accurate ROM records of individuals.2. The muscle activity of Da also increased significantly with head extended mouth opening. This confirms that Da plays an important role in mandibular rotation of maximum mouth opening.

Increase in both Range of Motion of the Mandible and Activity of Anterior Belly of Digastric Muscle with Head Extension.