Objectives This study focused on determining whether musculoskeletal and psychological complaints reported monthly over a 4-month period predicted muscular activity during and immediately after standardized worktasks and whether muscular activity during and immediately after these tasks predicted changes in complaint severity in the following 12 months.

Methods Surface electromyography (EMG) was recorded bilaterally from the upper trapezius, middle deltoid, and forearm extensor muscles of 45 postal workers (30 women) during sustained submaximal (25% of peak force) isometric contractions (wrist extension and shoulder abduction). Self-reported health complaints were recorded monthly. Musculoskeletal and psychological complaint-severity indices (MSI and PI, respectively) were computed from complaint-severity scores (intensity score x duration score). The history of complaints over the previous 4 months was included in adjusted regression models to predict muscular activity during and immediately after submaximal contractions. Muscular activity was included in adjusted models to predict changes in the complaint severity over the subsequent 12-month period.

Results A higher MSI predicted a lower EMG level in the trapezius muscle during submaximal contractions (P<0.014), whereas the PI did not predict the level of EMG in any of the muscles studied (P>0.194). The EMG activity did not predict changes in the complaint severity over the subsequent 12 months.

Conclusions These findings may support the hypotheses of pain adaptation or the dysfunction of synergistic muscular control in relation to musculoskeletal complaints. However, the findings did not indicate that increased or decreased muscular activity is a risk factor for heightened levels of complaints in the subsequent 12 months.

Key terms electromyographic activity; isometric muscular contraction; muscular activity; musculoskeletal complaint; musculoskeletal pain; prospective study; psychological complaint; subjective complaint; working population