Abstract
It has been reported that the tapping movement of the jaw is centrally programmed, as is chewing. However, the functional involvement of the neck muscles in the coordination of jaw and head movements has not been fully clarified. The present study carried out simultaneous recordings of surface electromyography (EMG) of the jaw and neck muscles along with three-dimensional jaw and head movements to ascertain how these muscles were coordinated in their functions.
Eleven healthy male subjects had EMGs recorded from their right masseter and suprahyoid muscles and both sides of their sternocleidomastoid (SCM) and posterior cervical (PC) muscles. First, the subject was instructed to retract his jaw as much as possible from an intercuspal position to the so-called central relation (CR) to compare EMG activities between the positions. Second, the subject was instructed to jaw tap at 75/60 Hz and chew gum on the right side for 12 seconds.
All EMG activities increased in the CR. The amplitudes of jaw and head movements were larger in the tapping motion than in the chewing one while the coefficient variations of these values were less in tapping than in chewing. Although the amplitude of the jaw movement tended to be related to the head movement, this relationship was not linear among the subjects. SCM and PC muscles were active in the jaw opening phase. The SCM muscle showed unique activity in the jaw closing phase of chewing; this was significant on the working side.
The present results strongly suggested that the SCM and PC muscles are differentially involved in the coordination of jaw and head movements.
