Background: Limited research exists on the association between increased physical activity and injury in children. Objective: To assess how well different variations of the acute:chronic workload ratio (ACWR)  predict injury in children. Methods: We conducted a prospective cohort study using data from 1670 Danish schoolchildren measured over 5.5 years  . Coupled 4-week, uncoupled 4-week, and uncoupled 5-week ACWRs were calculated using activity frequency in the past week as the acute load  and average weekly activity frequency in the past 4 or 5 weeks as the chronic load . We used new onset pain as a proxy for injury, and modelled its relationship with different ACWR variations  using generalized linear and generalized additive models, with and without accounting for repeated measures. Results: The  relationship between the ACWR and injury risk was best represented using a generalized additive mixed model for the uncoupled 5-week ACWR. This model predicted an injury risk of ~3\% when activity increased by up to 50\% or decreased by up to 20\% ( 0.8 <= ACWR< = 1.5). Larger decreases in activity were associated with a decreased injury risk to a minimum of 1.5\% . Larger increases in activity were associated with an increased injury risk, from 3\% up to a maximum of 6\% at ACWR = 5. Girls were at significantly higher risk of injury than boys. Conclusion: Increases in physical activity in children are associated with much lower increases in injury risk compared to previous results in adults.
Background: Limited research exists on the association between changes in physical activity levels and injury in children. Objective: To assess how well different variations of the acute:chronic workload ratio (ACWR) , a measure of change in activity, predict injury in children. Methods: We conducted a prospective cohort study using data from 1670 Danish schoolchildren measured over 5.5 years (2008 to 2014) . Coupled 4-week, uncoupled 4-week, and uncoupled 5-week ACWRs were calculated using activity frequency in the past week as the acute load (numerator), and average weekly activity frequency in the past 4 or 5 weeks as the chronic load (denominator). We modelled the relationship between different ACWR variations and injury using generalized linear and generalized additive models, with and without accounting for repeated measures. Results: The prognostic relationship between the ACWR and injury risk was best represented using a generalized additive mixed model for the uncoupled 5-week ACWR. It predicted an injury risk of ~3\% for ACWRs between 0.8 (activity level decreased by 20\%) and 1.5 (activity level increased by 50\% ). When activity decreased by more than 20\% ( ACWR< 0.8), injury risk was lower ( minimum of 1.5\% at ACWR=0). When activity increased by more than 50\% (ACWR > 1.5), injury risk was higher ( maximum of 6\% at ACWR = 5). Girls were at significantly higher risk of injury than boys. Conclusion: Increases in physical activity in children are associated with much lower injury risks compared to previous results in adults.
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