Mental stress of long duration: EMG activity, perceived tension, fatigue, and pain development in pain-free subjects.

The study examined the relationship between pain development in the shoulder, neck, and facial regions and the EMG activity of underlying muscles, during prolonged exposure to a mental stressor. The subjective perception of tension and fatigue was recorded. Thirty-six subjects were exposed to a two-choice reaction-time test for 1 hour. Electromyographic (EMG) recordings were performed bilaterally over the frontalis, temporalis, splenius, and trapezius muscles. Pain and perceived tension were scored on a visual analog scale, and fatigue on a Borg scale. Pain development was most pronounced in the shoulder and neck region. There was a weak tendency of those reporting pain in the shoulder region to generate higher EMG activity in the trapezius relative to those with no shoulder pain at the end of the test. No such relationship was observed for the other muscles. Perceived tension during the test was weakly related to pain and strongly related to fatigue at the end of the test, but not to EMG level. It is concluded that the mean level of the EMG response is of little consequence for pain development during stressful conditions. It is argued that other physiological responses such as prolonged activity in low-threshold motor units, whereby the surface EMG response can serve as a marker, can be important for shoulder pain originating in the trapezius muscle.