CHARACTERISTICS OF MUSCLE FATIGUE IN PATIENTS WITH MYOFASCIAL PAIN-DYSFUNCTION SYNDROME

The purpose of this study was to compare the properties of muscle fati gue in the masseter and temporalis muscles of normal individuals and t hose with myofascial pain-dysfunction syndrome (MPD). The MPD muscle i s presumed to have different characteristics of fatigue than its healt hy counterpart; these characteristics can be quantitated using standar d electromyogram (EMG) signal-processing techniques. A total of 18 pat ients diagnosed as having MPD comprised the experimental group and 15 adults with no history or present symptoms of temporomandibular joint pain and dysfunction served as controls. Surface EMG recordings were m ade for both the masseter and anterior temporalis muscles while the su bject held an incisal bite force level of 10N for as long as possible. The EMG data were transferred to a microcomputer where the power-dens ity spectrum of the signals were calculated for 2-s samples at 10-s in tervals. The mean power frequency (MPF) and power (root mean square, r .m.s.) of the signals were calculated from the power-density spectra. Results showed: (1) the endurance times were significantly shorter for the MPD patients; (2) the masseter was not active in three of 17 MPD patients; (3) decreases in MPF over time were significantly greater fo r the MPD patients than normal subjects; (4) increases in r.m.s. power were significantly greater over time for the MPD patients; (5) bandwi dths of the power-density spectra were similar for the two groups. The implication of these results is the MPD muscle is not in a state of c onstant fatigue, as is currently believed, but rather demonstrates acc elerated fatigue.