The pain-induced change in relative activation of upper trapezius muscle regions is independent of the site of noxious stimulation

OBJECTIVE: To assess the effect of local excitation of nociceptors at different locations of the upper trapezius muscle on the spatial distribution of upper trapezius electromyographic (EMG) amplitude during sustained contraction.

METHODS: Surface (EMG) signals were recorded from the upper trapezius muscle with a grid of 10x5 electrodes from nine healthy men during 90 degrees shoulder abduction sustained for 60s. In one experimental session, the subjects received separate injections of 0.4 ml of hypertonic saline (experimental muscle pain) into the cranial and caudal region of the upper trapezius. In a separate experimental session the same subjects received two injections of 0.2 ml each of hypertonic saline simultaneously in the cranial and caudal region. The EMG root mean square (RMS) values were computed for each electrode location to provide a topographical map of EMG amplitude.

RESULTS: The RMS value averaged across all electrode locations decreased following injection of hypertonic saline (P<0.05) by a similar amount for the two experimental sessions. The pain-induced decrease was larger in the cranial than in the caudal region for both experimental sessions, as evidenced by a shift of the EMG amplitude distribution towards the caudal region of the muscle (P<0.0001).

CONCLUSION: Muscle pain induces a consistent change in the spatial activation of the upper trapezius muscle which is independent of the site of noxious stimulation.

SIGNIFICANCE: Pain-induced changes in the spatial distribution of muscle activity may induce overload of specific muscle regions in the long term.