Chronic pain after soft-tissue injury of the cervical spine: trapezius muscle blood flow and electromyography at static loads and fatigue.

Microcirculation in the upper portion of the right and left trapezius muscles was measured percutaneously by laser-doppler flowmetry (LDF) during two 10-min-long series of alternating 1-min periods of static contraction and rest determined electromyographically (EMG). Twenty-five patients with pain persisting after a soft-tissue injury of the neck were studied. Pain assessments by using visual analogue scales and drawings showed 13 patients with predominantly unilateral and 12 with bilateral neck-shoulder pain, in some cases with arm pain and numbness. Mean age was 41 (23-58) and 39 (22-54) years and a female/male ratio 8:5 and 10:2, respectively. Stepwise increased contraction was induced by keeping straight arms at 30 degrees, 60 degrees, 90 degrees and 135 degrees of elevation, and repeated with a 1 kg (women) or 2 kg (men) hand loads. Signal processing was done on-line by using a 386SX computer. LDF and EMG values were normalized. Spectral shift of EMG mean power frequency (MPF) for fatigue was analyzed. Muscle blood flow on the "normal" side in the unilateral pain group showed an ordinary increase at increased angle of arm elevation, shoulder torque and EMG amplitude. On the painful side, during increased muscle tension and fatigue, the ability to increase blood flow appeared to be impaired, and there was no consistent increase in either side of the bilateral pain group. EMG amplitude showed a significant positive correlation to the angle of arm elevation and shoulder torque. The rms-EMG (root mean squared EMG) increase was lower in the painful side at high force contraction (non-normalized data).(ABSTRACT TRUNCATED AT 250 WORDS)