Mandibular movements and jaw muscles' activity while voluntarily chewing at different rates

As a way of learning about the motor control of chewing, we studied how well a subject could voluntarily chew in time with a metronome and defined the changes in the spatial and temporal aspects of the chewing pattern with changes in chewing rate. Timing and extent of mandibular movements were assessed in nine adults from Kinesiograph recordings; timing and level of activity in digastric and both masseter muscles were determined from surface EMGs. Each subject chewed gum in time with a metronome set randomly at 46, 100, 160 beats per minute or at a frequency close to his automatic chewing rate. Cycle-by-cycle analysis showed that subjects varied in their ability to keep pace with the metronome. When chewing at high frequencies, six subjects reduced gape, three did not. Contralateral deflection in opening, when present, was significantly reduced when chewing at high frequencies and this decrease was independent of vertical gape. Durations of opening, closing, and occlusal phases decreased in proportion to the decrease in total cycle duration. Burst duration of digastric activity decreased about 29% compared with a 77% decrease in cycle duration over all chewing rates. At low frequencies onset of digastric activity occurred after onset of opening. At high frequencies digastric onset preceded opening. Burst durations of both masseter muscles decreased in concert with the decrease in cycle duration. Termination of activity in both masseters was synchronous and always occurred 100 ± 20 ms prior to the next opening. Therefore, we conclude that (i) individuals vary in ability and strategy for controlling chewing rate voluntarily and (ii) with increases in chewing rates the shortening of burst duration for the digastrics is significantly less than for the masseters.