Gender-specific differences in electromyographic changes and perceived pain induced by experimental muscle pain during sustained contractions of the upper trapezius muscle.

The aim of the study was to investigate gender-specific differences in the pain-induced alterations of muscle activity during sustained contractions of the upper trapezius muscle. Surface electromyographic (EMG) signals were recorded bilaterally from the upper trapezius muscles of healthy volunteers (10 men and 9 women). The subjects held the arms at 90 degrees abduction for 5 s (short contraction) and 3 min (sustained contraction). These contractions were performed before, during, and after injection of 0.5 ml of hypertonic saline (5.8%) bilaterally in the upper trapezius muscles. Injection of isotonic saline (0.9%) served as control. The root mean square (RMS) and mean power frequency (MPF) were computed from the EMG signals and normalized to the values obtained during maximal voluntary contractions. In the short contractions, injection of hypertonic saline, but not isotonic saline, induced a less pronounced decrease in RMS in women than men, and a significant decrease in MPF in men but not women. In the sustained contractions, RMS slope increased and MPF slope decreased (less negative) with muscle pain in men but not women. It was concluded that the pain-induced changes in motor control strategies during sustained contractions differ in men and women, which may partly explain gender differences in the incidence of musculoskeletal pain.