Effects of local and remote muscle pain on human jaw reflexes evoked by fast stretches at different clenching levels

Muscle pain imposes significant changes on natural motor tasks, but the con sequences for stretch reflexes are still disputed. The present study examin ed the jaw reflexes to fast (10 ms) stretches of the mandible in an experim ental model with local pain in the masseter muscle and remote pain in the t ibialis anterior muscle. The stretch reflexes were elicited in healthy volu nteers (n = 13) before, during, and after periods with constant levels of e xperimental pain and while the subjects clenched at 0%, 15%, 30%, and 45% o f the maximal voluntary contraction (MVC) levels. Surface electromyography (EMG) was used to record the reflex responses. Pain in the masseter muscle (mean SEM, 3.8 +/- 0.4 on a 10-cm visual analogue scale), but not in the ti bialis anterior muscle (3.4 +/- 0.3; paired t-test, P = 0.318) was associat ed with significant changes in both prestimulus EMG activity (ANOVA, P = 0. 002) and in peak-to-peak amplitudes of the stretch reflex (ANOVA, P = 0.022 ). However, when the changes in prestimulus EMG activity were taken into co nsideration a significant increase in the stretch reflex persisted in the p ainful muscle at 15% and 30% MVC. Local circuits at the trigeminal level in volving the fusimotor system are proposed to mediate a significant part of this modulatory effect.