Effect of experimental pain on EMG-activity in human jaw-closing muscles in different jaw positions

We examined the effect of a standardized painful stimulus on the surface EMG-activity of the human jaw-closing muscles at rest and during two levels of jaw opening. Sixteen healthy women participated in two experimental sessions. In randomized order, hypertonic saline (HS: 5.8%) was infused into the left masseter muscle on one occasion, and isotonic saline (IS: 0.9%) on the other. The subjects scored the pain intensity continuously on a 0-10 visual analogue scale (VAS). The subjects were asked to hold the jaw in three different positions (rest, half-maximal, and maximal opening). Before, during, and after infusion, the EMG-activity was recorded from the masseter and temporalis muscles with the jaw in each of the three positions. HS induced significantly higher pain-levels than IS (mean VAS: HS: 5.2+/-1.3, IS: 0.7+/-0.2, P<0.05). At rest, the EMG-activity in most of the jaw muscles increased significantly during both infusions (P<0.05). At half-maximal opening, the EMG-activity in the infused muscle increased significantly with both HS and IS (P<0.05). At maximal opening, the EMG-activity during infusion of HS decreased significantly in the right masseter and temporalis (P<0.05). There was no significant difference in the position of the jaw at rest during infusions. However, the vertical opening distance was significantly decreased during infusion of HS at half-maximal and maximal opening (P<0.05).

It is concluded that experimental pain affects EMG-activity differentially in jaw-closing muscles in different opening positions of the jaw.