Time-Varying Associations Between Physical Activity and Injury Risk Among Children

Physical activity has time-varying associations with injury risk among children. While previous activity may predispose to injury through tissue damage, fatigue and insufficient recovery, it may protect against injury by strengthening tissues and improving fitness and skills. It is unclear what the relevant time window and relative importance of past activity are with regard to current injury risk in children.

The objectives of this study were to assess how previous activity patterns are associated with injury risk among children.

Our data source was the Childhood Health, Activity, and Motor Performance School Study Denmark (CHAMPS-DK), a prospective cohort study of Danish school children conducted between 2008 and 2014. We applied flexible weighted cumulative exposure methods within a Cox proportional hazards model to estimate the time-varying association between the number of weekly activity sessions and time-to-first injury in each school year. We estimated several models with varying time windows and compared goodness-of-fit.

Out of 1667 study participants, 986 (59.1%) were injured at least once, with a total of 1752 first injuries across school years. The best-fitting model included 20 weeks of past physical activity. Higher levels of activity performed 10-20 weeks ago were associated with decreased injury risk, while higher levels of activity performed 2-9 weeks ago were associated with higher injury risks. Compared to those who remained minimally active for the entire past 20-week period, children who were highly active in the past 10 weeks after being minimally active 11-20 weeks ago had an injury hazard ratio of 1.63 (95% confidence interval 1.18, 2.23).

Flexible weighted cumulative exposure methods suggest a complex temporal relationship between past physical activity history and injury in children.