Muscle activity as a releasing factor for pain in the shoulder and neck.
In this review, the evidence for trapezius muscle activity as a releasing factor for shoulder and neck pain is considered, mainly on the basis of studies in our laboratory. Two lines of evidence are produced, (i) vocational studies in an occupational setting, where muscle activity pattern is recorded by surface EMG and a clinical examination of the shoulder region of the subjects performed; and (ii) laboratory studies where muscle activity patterns and pain development are recorded in an experimental situation with mental stress and minimal physical activity. The vocational studies demonstrate pain development in the shoulder and neck despite very low muscle activity recorded, making it very difficult to assume muscular involvement for all cases with such complaints. However, the hypothesis of pain development through overexertion of a subpopulation of low-threshold motor units also makes it difficult to draw a firm negative conclusion. The laboratory experiments, on the other hand, show that trapezius activity patterns in response to stress have many features that would be expected if muscle activation induces pain symptoms. It is further noted that the trapezius is the only muscle with activity patterns that show these features. Possibly, we observe the effects of parallel physiological phenomena, e.g., a systemic autonomic activation that induces pain symptoms and also facilitates the motor response of some muscles. Evidence of autonomic activation of trapezius is presented by the observation of low-level, rhythmic EMG activity during sleep. However, this is not firm evidence for the above hypothesis, which at present best serves as a basis for further experimentation.