Objective To examine whether heavy physical workload in young adulthood increases the risk of local and radiating low back pain (LBP) in midlife. Methods Longitudinal nationally representative Young Finns Study data among women (n=414) and men (n=324), aged 18-24 years in 1986 (baseline), were used. Physical heaviness of work was reported at baseline and follow-up (2007), and local and radiating LBP at follow-up. Covariates were age, smoking and body mass index. Logistic regression was used to examine the associations between physical heaviness of work and LBP. Additionally, the mediating effect of back pain at baseline was examined (the Sobel test). Results After adjustment for the covariates, and as compared with sedentary/light physical workload, heavy physical workload was associated with radiating LBP among women (OR 4.09, 95% CI 1.62 to 10.31) and men (OR 2.01, 95% CI 1.06 to 3.82). Among men, early back pain mediated the association ( p value from the Sobel test=0.006). Among women, early exposure to physically heavy work showed the most consistent associations, while early and late exposures were associated with radiating and local LBP among men. Persistently heavy physical work was associated with radiating LBP among women and men. Conclusions Physically heavy work at a young age can have a long-lasting effect on the risk of LBP, radiating LBP in particular. These results highlight the need to consider early and persistent exposures to prevent the adverse consequences of physical workload for the low back.Low back pain (LBP) is a highly prevalent and persistent condition and its determinants have been widely examined for decades.1 2 Work-related factors are of particular interest, as acute and chronic pain among employees increase the risk of sickness absence and disability retirement.3 Globally, LBP attributable to occupation has a notable contribution to the number of disability-adjusted life years lost each year. 4 Several physical workload factors, such as nonneutral trunk postures and various manual handling activities have been associated with LBP.5 A recent individual participant data meta-analysis showed that the risk estimates for posture and force-related variables were small to moderate.6 Also, heavy physical work in general has been associated with LBP.7 Physically heavy work usually involves forceful activities and awkward postures that are loading for the low back. The importance of life course perspective in the study of LBP has been highlighted. [8][9][10] Although there are several studies on the associations between physical exposures and LBP, the focus has largely been on non-specific pain among midlife or older employees. A cross-sectional study showed that work-related physical factors are associated with LBP already in the first job.11 Similarly, in a Finnish study 12 among individuals aged 16 years, physical workload was associated with incident LBP at the age of 18. However, the long-term consequences of early exposure to heavy physical work for the low back are largely u...