Background Physically demanding tasks within blue-collar jobs can impose high aerobic workloads on the work-ers, particularly among workers with low cardiorespiratory fitness. Accordingly, blue-collar workers could benefit from leisure time physical activities (LTPA) that enhance cardiorespiratory fitness to withstand the aerobic strain of their work tasks. Nevertheless, blue-collar workers are predomi-nantly inactive during leisure time. One reason for this could be that the high aerobic workload ex-hausts the workers, thereby increasing the perceived need to rest during leisure time, consequently lowering levels of LTPA. Accordingly, this study aimed to investigate the association between du-ration of work hours spentd at a high relative aerobic workload and LTPA among blue-collar work-ers. Methods This was a cross-sectional study based on heart rate and accelerometer data from 803 blue-collar workers (447 men and 356 women). Relative aerobic workload was measured as percentage of heart rate reserve during work (%HRR). LTPA was expressed in terms of leisure time spent in sedentary and active behaviours in uninterrupted bouts (i.e. <10 min, ≥10-30 min and >30 min). Compositional regression and isotemporal substitution models were used to assess the association between the rela-tive importance of work time spent at ≥40%HRR and leisure time spent in sedentary and active bouts. All analyses were stratified by sex. Results For men, there was no statistically significant association between the relative importance of work time spent at ≥40%HRR and LTPA. Among women, the relative importance of ≥40%HRR at work was negatively associated with relative leisure time spent in ≥10 min bouts of active behaviour (β ̂=-0.21, p=0.02). For average female workers, a theoretical 15 min reallocation of time to work activities at ≥40%HRR was estimated to decrease active behavior by 6 min during leisure time.