Characteristics of electrical activity in trapezius muscles with myofascial pain

Objective: To study the reliability and validity of resting electrical activity from tender areas in muscle. Methods: In seven subjects with tender areas in their trapezius muscles, electromyographic (EMG) signals were obtained from monopolar needles at various depths over the tender area and at the same depths over the control area, and from surface electrodes. At selected depths, multiple epochs were recorded. Results: Increased electrical activity occurred from the electrode inserted toward the tender area in all seven subjects (to a mean of 47.6 μV), but not from control areas. The depths of the maximal EMG amplitudes advancing and withdrawing the needle were usually within 3 mm. The EMG amplitude from the first of multiple epochs (38.4 μV) did not differ from the average (40.0 μV) of subsequent epochs (paired-t test, 6 df, 0.2 < p < 0.3). Conclusions: We suggest that the electrical activity arises from a localized zone and is stable over time. Significance: Our data provide evidence for the reliability and validity of the electrical activity from myofascial tender areas.