OBJECTIVE: To investigate the influence of superficial heat on the fatigue cascade of the upper trapezius muscle in subjects with myofascial pain and matched normal controls. DESIGN: In a prospective randomized block crossover pilot study, eight female subjects, ages 20 to 35 yr, with upper trapezius muscle trigger points and eight matched female control subjects without pain were studied. Subjects performed six 100-sec shoulder shrug contractions to fatigue, with subjects randomly chosen to receive heat during the first three or last three trials. The initial median frequency and the slope of the median frequency decline were calculated from the data. RESULTS: In the subjects with pain, the slope of the median frequency was elevated in ambient room temperature as compared with controls. There was no difference in slope of the median frequency under heated conditions. Heat application in controls before fatiguing exercise caused an increase in initial median frequency, whereas exercise before heat treatment resulted in a significantly lower initial median frequency. Subjects with myofascial pain had no significant change in initial median frequency. CONCLUSIONS: Heat seems to have a positive effect on muscle function in normal individuals when applied before exercise. Subjects with myofascial pain respond differently to exercise and heat challenge, which suggests a difference in their muscle physiology.

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