['The evidence on the impact of physical activity on back pain in children and adolescents has been contradicting. It has also been shown that the physical activity cannot accurately be estimated in children using questionnaires.', 'The aim of this study was to establish if physical activity in childhood had any impact on back pain reporting in early adolescence (3 years later), using an objective instrumental measurement of physical activity.', 'Prospective cohort study.', 'Representative random sample of Danish children from the city of Odense sampled at age 9 years and followed-up at age 12 years.', 'The 1-month period prevalence of back pain (neck pain, mid back pain, and low back pain) was established using a structured interview.', 'Physical activity was assessed with the MTI-accelerometer. The accelerometer provides a minute-by-minute measure of the physical activity performed. An overall measure of physical activity and time spent in high activity were studied in relation to back pain using logistic regression. The analyses were performed on the total sample and then stratified on back pain (yes/no) at baseline.', 'High physical activity (HPA) levels seem to protect against future low back pain and appear to actually "treat" and reduce the odds of future mid back pain. When comparing the least active children to the most active children, the least active had a multivariate odds ratio of 3.3 of getting low back pain and 2.7 of getting mid back pain 3 years later. When stratified on back pain at baseline, this effect on mid back pain was especially noticeable in children who had had mid back pain already at baseline, with an odds ratio of 7.2.'] Does high-level physical activity in childhood seem to protect against low back pain in early adolescence?