Methods: To date, six asymptomatic volunteers have participated. Bite forces between the anterior teeth, and electromyograms (EMGs) from both masseters, were recorded. The participants were provided with visual feedback of the bite force and were asked to maintain this around 50N. Reflexes were evoked by electrical stimulation of the skin of the upper lip. A range of stimulus intensities below and above a nociceptive threshold was used. The EMGs were processed by rectification, averaging and smoothing. The bite force recordings were simply averaged. Deviations following the stimuli, in any of the recordings (EMGs or bite force), were analysed with respect to a 300ms pre-stimuli period, to determine whether they could be designated as reflexes.
Results: Inhibitory reflexes (relaxations) were seen in the force records of all 6 subjects. The minimum latency and maximum duration of this response were 47.9±4.1 and 122.1±14.7ms respectively (means±S.E.M.). In all respects, this mechanical response was slower and longer lasting than the equivalent EMG inhibitions. In 4 subjects, the relaxation was immediately followed by a period of increased force generation (contraction) which again was more prolonged than the equivalent excitation in the EMGs (latency: 135.0±13.2ms; duration: 326.4±92.6ms). Interestingly, the ratio of the durations of the mechanical responses to EMG responses appeared to be substantially greater for the contractions (approx. 4:1) than for the relaxations (approx. 2:1).
Conclusion: These preliminary results show as one would expect, that the mechanical parameters of this jaw reflex are more prolonged than the electromyographic ones. More surprisingly, this discrepancy was much greater for the excitatory than for the inhibitory responses.