Work-induced pain, trapezius blood flux, and muscle activity in workers with chronic shoulder and neck pain

Pain, trapezius microcirculation, and electromyography (EMG) were recorded during 90 min of simulated office work with time pressure and hand precision demands in 24 full-time working subjects with chronic shoulder and neck pain. The responses were compared with those of a reference group of 28 healthy subjects without pain. Pain intensity was rated on a visual analogue scale. Intramuscular blood flux was measured by laser-Doppler flowmetry (LDF) and muscle activity by surface EMG bilaterally in the upper trapezius. Pain increased during the work task, and the increase was larger in women than in men and in the reference group. Muscle activity was low: <4% of EMG during maximal voluntary contraction. LDF showed elevated intramuscular blood flux above baseline throughout the work task in both groups and during recovery in the pain group. Pain in the active side correlated positively with blood flux in the pain-afflicted subjects and negatively in the reference group. In conclusion, office work induced pain, and trapezius vasodilation that did not return to resting values during recovery. These data show that pain is associated with trapezius vasodilation but not with muscle activity. Interaction between blood vessels and nociceptors may be important in the activation of muscle nociceptors...