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Introduction: Activity of the upper trapezius muscle during work has been associated with neck pain. However, studies on the possible influence of bilateral asymmetric activation of the upper trapezius muscle on neck pain are scarce. The aim of this study was to investigate the relationship between bilateral asymmetric activation of the upper trapezius muscle and neck pain.

Methods: Seven Scandinavian research institutes provided surface electromyographic (EMG) data on upper trapezius muscle activity during work and related questionnaire data on neck pain intensity. After harmonization and pooling, EMG and questionnaire data were available for 530 participants (men: N = 284, women: N = 246) working in 26 occupations. 256 of the participants responded to longitudinal neck pain measures. Asymmetry of muscle activity during work was calculated for the whole working day as a whole and in relation to maximum voluntary effort (MVE).

Results: In cross-sectional analyses, asymmetry in upper trapezius muscles activation in the ranges 0–0.05% and 0.05–2% MVE was significantly positively associated with neck pain intensity in both unadjusted and adjusted analyses (adjusted for sex, adjusted for sex and height). Asymmetry in the 6–8% MVE and 8–10% MVE ranges was significantly negatively associated with neck pain, but only in unadjusted analyses. In longitudinal analyses, a significant negative association was found between upper trapezius muscles activity asymmetry in the 20–50% MVE range and neck pain intensity in unadjusted analyses.

Discussion: An asymmetry in the activation of the upper trapezius muscles during the working day has both positive and negative effects on neck pain intensity. While asymmetry in the higher ranges of muscle activity related to MVE appears to be protective against higher neck pain intensity in both the short and long term, asymmetry in the very low ranges of muscle activity related to MVE (muscular rest and static activities) may increase neck pain intensity in the short term.

Conclusion: During work, conditions should be created to achieve simultaneous relaxation during breaks and simultaneous activation of both muscles during static activities to prevent neck pain.