Bite through pain – an experimental pain force-controlled study on EMG-activity of the masseter muscle

Aim of this high-density surface electromyography (EMG) study was to evaluate the effect of experimentally-induced pain in the masseter on the performance of a constant vertical bite-force. Twenty healthy participants performed two ramp-and-hold contractions at baseline and after an injection of either hypertonic saline (HS) or, as control, isotonic saline (IS) based on random assignment. The measurement was repeated after 15 min. with IS or HS, respectively. The target level of vertical bite-force was 15% of maximum voluntary contraction registered using an intra-oral device. The average root-mean-square (RMS) values showed significantly lower (p<0.001) EMG activity for all selected monopolar (14.9%) and bipolar (17.1%) derivations for the HS condition despite unchanged mean force. Bipolar EMG RMS values for the contralateral masseter did not change significantly. The centre of mass obtained from the interpolated monopolar RMS amplitude map showed a mean shift away from the injection site of 0.85mm (p<0.001). Experimental pain induced a general reduction of RMS EMG amplitude of the ipsilateral masseter and a small but significant shift away from the site of injection. The masticatory motor system seems to possess a unique adaptability to produce a constant force despite the presence of experimentally induced pain. This could represent a feature of the high resilience and adaptability of the stomatognathic system in acute painful conditions.