  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.
