Background Running distance has previously been identified as a risk factor for running-related injury. It is likely that the runner's fitness level may also influence injury risk.

Objective To investigate if running level modifies the association between cumulated running distance over the previous week and injury risk.

Design Prospective cohort study with a 6-month follow-up.

Setting Recreational runners.

Participants Injury free recreational runners (n=372) were required to report all running activity as well as any injury using a dedicated Internet platform.

Assessment of Risk Factors The main exposure was cumulated running distance over the week prior to a given session (time-dependent exposure) and was dichotomized into low<20 km (reference group), and high≥20 km. Running level was defined as the average running speed during follow-up and was dichotomized into fast runners≥10 km/h and slow runners<10 km/h.

Main Outcome Measurements First running-related injury based on a 1-day time loss definition.

Results In the crude analysis, a lower injury rate was observed when the runners cumulated a high running distance prior to a given session (hazard ratio (HR)=0.43; 95% confidence interval (95% CI)=0.28–0.66). After stratification according to running level, a significant lower injury rate was found for slow runners (HR=0.32; 95% CI=0.18–0.60) when they cumulated a high running distance prior to a given session, while the injury rate did not differ between exposure states for fast runners (HR=0.78; 95% CI=0.37–1.64). Based on the hazard ratios from the stratified analysis, it appears that low running level modifies the association between cumulated running distance over the previous week and injury risk with a factor of 2.43.

Conclusions Running level should be taken into account when analysing the association between training-related risk factor and injury risk.