Background The IOC recommends using the acute:chronic workload ratio (ACRatio) to quantify changes in relative activity. The ACRatio has been used in adult and youth but not in children.

Objective Determine the relationship between the ACRatio and new onset pain in children.

Design We used the CHAMPS-DK data, which includes prospectively collected weekly activity and pain data using SMS-texting in Danish schoolchildren.

Setting Recreational children’s sports.

Participants Parents of 1152 schoolchildren aged 5–10 in Svendborg, Denmark provided information on their child’s activity and pain during 5.5 years of follow-up.

Interventions/Assessment of Risk Factors We measured the coupled ACRatio using activity in the index week (week of new onset pain) divided by average activity in the index and past 3 weeks (total 4 weeks) and the uncoupled ACRatio using activity in the week before the index week divided by average of the 4 weeks before the index week.

Main Outcome Measurements Development of new onset pain.

Results The total incidence rate of new onset pain was 2 events per person-year. Coupled ACRatios ranged from 0 to 3.20 (mean 1.00) and uncoupled ACRatios ranged from 0 to 9.33 (mean 1.03). The risk ratio for new onset pain using the coupled ACRatio was 1.26 [1.18, 1.36] for each unit increase in ACRatio. The risk ratio using the uncoupled ACRatio was 1.21 [1.16, 1.27]. There was no U-shaped curve, unlike what has previously been observed in injury studies in adults (Figure 1).

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Abstract 118 Figure 1

Probability of new onset pain at different A) coupled and B) uncoupled ACRatios. Logistic regression curves with 95% CIs were fitted. The number of observations are shown for outliers

Conclusion The risk of new onset pain in children increases relatively linearly with increases in the coupled and uncoupled ACRatio.