Background Previous studies have linked high physical activity at work to increased risk of heart disease. However, it is not known whether beneficial psychosocial working conditions such as high job influence may buffer the increased risk of heart disease from high physical activity at work.

Objective To investigate whether high job influence has a modifying effect on the association between high physical activity at work and ischaemic heart disease (IHD)

Methods A total of 12,093 nurses aged 45–54 years form the Danish Nurse Cohort Study were followed for 20.6 years by individual linkage to the Danish National Register of Hospital Discharges for occurrence of incident IHD. Information on physical activity at work, job influence, other occupational factors and known risk factors for IHD was collected by self-report in 1993. Physical activity at work was categorised as low (covering sedentary and moderate), demanding and strenuous and job influence was dichotomised as high and low (covering no to some job influence).

Results During follow-up 869 nurses were hospitalised with incident IHD. In an analysis using nurses with high job influence and low physical activity at work as reference group, nurses with low job influence and strenuous physical activity at work had a significant 44% higher risk of IHD (hazard ratio (HR) 1.44 (95% confidence interval (CI) 1.02–2.05)), whereas those with high job influence and strenuous physical activity at work did not have a significantly increased risk of IHD (HR 1.09 (95% CI: 0.59–2.03)). However, no statistically significant multiplicative interaction between physical activity at work and job influence was found.

Conclusion Tendencies for a buffering effect of high job influence for IHD from strenuous physical activity at work were observed, but no statistically significant effect modification was found. Future studies on this potential buffering effect in other job groups are recommended