The association between changes in weekly running distance and running-related injury: Preparing for a half marathon

BACKGROUND: Sudden changes in training load may play a key role in the development of running-related injury (RRI). Because the injury mechanism depends on the runner's musculoskeletal load capacity, the running schedule followed prior to sudden change in training load may influence the amount of change that a runner can tolerate before the runner is at a higher risk of RRI. OBJECTIVES: To investigate the association between change in weekly running distance and RRI, and to examine whether the association may be modified by the running schedule the runner follows. METHODS: Two hundred sixty-one healthy (noninjured) runners were included in this prospective cohort study over a period of 14 weeks. Data on running activity were collected daily and objectively, using a global positioning system watch or smartphone. Instances of RRIs were collected using weekly e-mailed questionnaires. Primary exposure was defined as changes in weekly running distance. Data were analyzed with time-to-event models that produced cumulative risk difference as the measure of association. RESULTS: A total of 56 participants (21.5%) sustained an RRI during the 14-week study period. Twenty-one days into the study period, significantly more runners were injured when they increased their weekly running distance by 20% to 60% compared with those who increased their distance by less than 20% (risk difference, 22.6%; 95% confidence interval: 0.9%, 44.3%; P = .041). No significant difference was found after 56 and 98 days. No significant effect-measure modification by running schedule was found. CONCLUSION: Significantly more runners were injured 21 days into the study period when they increased their weekly running distances by 20% to 60% compared with those who increased their distances by less than 20%.