Exteroceptive suppression periods in masseteric EMG: Use of stimulus-response curves

The present study documented quantitative changes in the duration and the depth of the exteroceptive suppression (ES) period in the masseteric electromyogram (EMG) using stimulus-response (SR) curves. From six males and six females (20-41 years), the surface masseteric EMG was recorded bilaterally. Visual feedback from the left masseter muscle (MM) allowed a constant clenching level of 30% or 50% maximal voluntary contraction (MVC). Automatic triggering of the stimulus to the left mental nerve occurred when the background EMG remained constant. Eight to twelve stimulus intensities (range, 0-40 mA) were determined based on the subjects' sensory threshold (T). The late ES (ES2) occurred consistently and could be analyzed using SR curves. ES2 in the right MM appeared at 7.3+/-3.1T at 30% MVC, while it was significantly decreased to 5.2+/-2.2T at 50% MVC (P<0.05). The depth of ES2 suppression in the left MM was 69.7+/-5.7% at 30% MVC, and this increased significantly to 76.6+/-5.5% at 50% MVC (P<0.05). In addition, the depth of ES2 suppression in the left MM was significantly higher than that in the right MM at 50% MVC (70.0+/-7.4%, P<0.05). Consequently, it was confirmed that both the appearance and depth of ES may be affected by the level of background EMG. More importantly, the use of multiple stimulation intensities allowed development of reliable SR curves. These SR curves may be used to determine inter-individual differences in ES.