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OBJECTIVE: To examine the associations between (i) various types of physical activity and the risk of back pain incidence, and (ii) the influence of substituting sedentary behaviours with physical activities on back pain incidence. DESIGN: A prospective cohort study. METHODS: We analyzed UK Biobank data collected from 365,307 participants who were free of back pain at baseline. The exposures were total, light, moderate and vigorous physical activity, and sedentary behaviours. The outcome was back pain incidence. The main statistical models were the Cox proportional hazard model and the isotemporal substitution model. RESULTS: In the follow-up time (median, 12.97 years; inter-quartile range, 12.10-13.71), 25,189 individuals developed back pain. The associations between all types of physical activity and incident back pain were significantly non-linear (p < 0.001) among the general population and other subgroups. High physical activity was associated with a decreased risk of back pain compared with no physical activity. The lowest risk occurred in the 1801-2400 MET-min/week subgroup of total physical activity (HR 0.64, 95% CI 0.59-0.69), approximately consisting of 1200, 600, and 600 MET-min/week of light, moderate and vigorous physical activity, respectively. Extremely high vigorous physical activity was related to high risk, specifically in males (HR 1.13, 95% CI 1.02-1.25). Replacing 1 hour/day of sedentary behaviours with an equal time of physical activity reduced the risk of incident back pain by 2%-8% (p < 0.05). CONCLUSION: Physical activity was related to a reduced risk of back pain incidence (except over-high vigorous physical activity). Substituting sedentary behaviours with physical activities reduced the risk of future back pain.