Methods: Electromyographic recordings of masseter activity were made in 10 volunteer subjects. Reflex inhibitions of this activity (mean latency: 41ms) were evoked by electrical stimulation of the hair-bearing skin of the upper lip. Blocks of these stimuli were delivered whilst the subjects maintained masseter activity at levels between 5 and 40% of the voluntary maximum, with the aid of visual feedback. The electromyograms were full-wave rectified, smoothed and averaged to reveal the inhibitory reflexes which were quantified by integration with respect to the mean pre-stimulus level of activity.
Results: When the reflexes were measured in absolute units (yielding an integral in µVms), their magnitudes varied with, and showed a strong positive correlation with, the clenching level (ANOVA, P < 0.0005; Pearson's r = 0.776, P < 0.0005). However, when the electromyographic voltage levels were normalised as percentages of the pre-stimulus level of activity (yielding an integral in %ms), the reflex magnitude did not vary significantly with the clenching level (ANOVA, P = 0.8).
Conclusions: These results indicate that the proportion of activity in the masseter muscle which is reflexly suppressed by a given stimulus, is fairly constant at markedly different levels of muscle activity. This suggests that different types of motor unit are equally susceptible to reflex inhibition, including those units thought to be responsible for static activities (e.g. posture) and those with more dynamic roles (e.g. in chewing).