Masseter muscle hyperactivity and myofascial pain dysfunction syndrome: a relationship under stress.

Myofascial pain dysfunction syndrome (MPD) of the temporomandibular joint (TMJ) is a psychophysiological disorder that develops because of hyperactive muscles of mastication. Ten women meeting criteria for MPD and 12 symptom-free women participated in the study. The rationale for this study was to observe cardiovascular and masseter muscle changes during four contiguous experimental periods: baseline/adaptation, reaction time, recovery, and relaxation. MPD patients showed less masseter muscle activity and higher heart rates at baseline than controls. Controls had significantly higher masseter EMG activity during reaction time. Both groups showed significant elevation in masseter muscle activity and heart rate over the 14-min reaction period. MPD patients' recovery from stress was equivalent to controls' for both heart rate and masseter muscle activity. MPD patients exhibited significantly slower reaction times than controls. The results suggest that masseter muscle hyperactivity may not account for the development and maintenance of MPD.