Aim: Leisure-time physical activity (LTPA) has a beneficial effect on cardiovascular health and longevity, while occupational physical activity (OPA) is associated with an increased risk of cardiovascular diseases and mortality. The physiological mechanism of this apparently contrasting relationship between LTPA and OPA on cardiovascular health remains unknown. A possible explanation is that OPA and LTPA exert different effects on the autonomic nervous system. Thus, we aimed to investigate whether objectively measured OPA and LTPA are differentially associated with cardiac autonomic regulation in an occupational sample.

Methods: Cross-sectional data from the Danish cohort DPHACTO were analysed. The study comprised 514 bluecollar workers who took part in ambulatory monitoring of physical activity and heart rate variability (HRV). Physical activity (i.e. time spent in walking, climbing stairs, running and cycling) was assessed objectively using accelerometers (Actigraph) worn on the thigh, hip and trunk over 4–5 working days. Simultaneously, a heart rate monitor (Actiheart) was used to collect inter-beat intervals from the ECG signal. Heart rate and HRV indices were analysed during nocturnal sleep as markers of cardiac autonomic activity. Multiple regression analysis was used to determinethe main effects of OPA and LTPA and their interaction on heart rate and HRV indices, adjusting for multiple confounders (age, gender, body mass index, smoking and cardiovascular ailments).

Results: Time spent in physical activity was on average (SD) 1.4 (0.6) hours/day for OPA and 0.9 (0.4) hours/day for LTPA. OPA showed generally negative associations with nocturnal HRV indices, while positive estimates were observed for LTPA. There was a statistically significant interaction effect between OPA and LTPA on heartrate (P < 0.0001) and HRV indices in time (root mean square of standard deviation, P = 0.004: standard deviation of normal to normal sinus beat, P = 0.019) and frequency domains (high frequency power, P = 0.022; low frequency power, P = 0.033). The favourable effect of LTPA on nocturnal HRV clearly diminished with higher levels of OPA, and high levels of both OPA and LTPA had a detrimental effect. The observed associations persisted after adjustment for possible confounders.

Conclusion: Our findings suggest that time spent in OPA and LTPA have interactive effects on nocturnal autonomic regulation. Future longitudinal studies should examine whether autonomic regulation is a mediator for the effect of OPA and LTPA on cardiovascular disease and mortality.