Head posture and masticatory muscle function: An EMG investigation

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Abstract
The investigation aimed to determine to what extent head posture affected the EMG activity of the jaw opening (suprahyoid) and jaw closing (masseter) muscles during chewing. The subjects comprised twenty adult males with normal occlusion. The EMG parameters analyzed were: maximal integrated EMG activity; duration, and coordination of jaw opening and closing muscle activity. Recordings were made with the head in three positions: normal upright, inclined forward, and inclined backward. Maximal integrated EMG activity was reduced for the masseter muscle when the head was flexed forward, and for the suprahyoid muscle group when the head was flexed backward. The duration of the masseter chewing phase was reduced when the head was flexed forward and increased when the head was flexed backward. The duration of the suprahyoid chewing phase was unaffected by variation in head position. No absolute reciprocity was found between the suprahyoid and masseter muscle activity during chewing. Suprahyoid overlap at the end of the masseter chewing phase was reduced when the head was flexed forward. The amount of masseter overlap at the end of the suprahyoid chewing phase was unaffected by variation in head position.

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