Objective This study investigated a possible interaction between postural risk factors and job strain on the incidence proportion of self-reported musculoskeletal symptoms in the shoulder-neck, lower back and upper limbs regions.

Methods A cohort of white-collar workers (N=2431) was assessed with a self-administered questionnaire regarding postural risk factors and job strain at work. After a three-year follow-up, the six-month incidence proportion of musculoskeletal symptoms in the three body regions was measured with a modified version of the Nordic questionnaire. The analyses were stratified for gender. Interaction was defined as a departure from the addition of effects of individual risk factors, and its importance was estimated from the attributable proportion due to interaction and its 95% confidence interval (95% CI).

Results A significant attributable proportion of 0.80 (95% CI 0.23–1.37) due to interaction between postural risk factors and job strain was observed for men in the lower back region. An indication of interaction was found for women with attributable proportions due to interaction of 0.44 (95% CI -0.06–0.94), 0.27 (95% CI -0.34–0.88) and 0.36 (95% CI -0.33–1.05) for the shoulder-neck, lower back, and upper limbs regions, respectively.

Conclusions The simultaneous presence of postural risk factors and job strain seems to increase the pathogenic effect of each exposure on the incidence proportion of musculoskeletal symptoms. This interaction effect is important for work intervention practices as success in decreasing any of these two risk factors could have the additional benefit of reducing up to 80% of new cases of musculoskeletal symptoms among participants exposed to both risk factors.

Key terms combined effect; computer work; etiology; job strain; Karasek’s model; musculoskeletal symptom; occupational disease; postural risk factor; prospective study; psychosocial factor; risk factor; self-assessment; self-report; video display unit; workload