Effect of stimulus parameters and contraction level on inhibitory responses in human jaw-closing muscles: Implications for contingent stimulation

Objective: Examine the effect of stimulus duration, intensity and level of muscle contraction on the inhibitory responses evoked by electrical stimuli in human jaw-closing muscles applied to the right mental nerve. Design: The inhibitory jaw-reflexes, short-latency (ES1) and long-latency (ES2), were recorded in the surface electromyogram (EMG) of masseter and temporalis muscles in 16 healthy subjects. Three stimulus durations (1 ms single square-wave pulse, 10 and 450 ms square-wave pulse train), two stimulus intensities adjusted to perceived intensity of 3 (non-painful) and 7 (distinct painful) on a 0-10 verbal rating scale were applied to the right mental nerve while the subject was biting at 25% and 50% of the maximal voluntary contraction (MVC). Results: The magnitude of suppression in the ES2 evoked by 1 and 10 ms stimuli was dependent on stimulus intensity (P < 0.002 and P < 0.001, respectively) but not contraction level. However, ES1 could not be observed in most of the recordings. There were significant decreases evoked by the 450 ms stimuli in RMS-EMG values in the 400-500 ms compared with the pre-stimulus interval (P < 0.001) which was dependent on contraction level (P < 0.01) but not on stimulus intensity (P = 0.486). Conclusions: The present results suggest that the ES2 reflex response is associated with the duration of the electrical stimuli, the intensity level but not the contraction level. In contrast, the inhibitory effects of ultra-long stimuli (450 ms) are not specifically related to the intensity level suggesting that this is a non-nociceptive response. (C) 2009 Elsevier Ltd. All rights reserved.