Background The shock absorption qualities of running shoes are thought to have an important influence on running-related injuries (RRIs). However, scientific evidence of this relationship is still lacking.

Objective To test if midsole hardness of standard running shoes was related to RRIs.

Design Double-blind randomized controlled trial.

Setting Free-roaming leisure-time distance runners.

Participants Study participants were recruited through newspaper advertisements, posts on specialized internet sites and via mouth-to-ear. They were required to report any running activity, sport practice and injury using a dedicated internet platform. Recorded information was systematically verified, if necessary through direct contacts with the participants.

Risk factor assessment Leisure-time runners were provided with identical running shoes, except for their midsole hardness which was either hard (H; n=113) or soft (S; n=134). They were followed prospectively for 5 months.

Main outcome measurements The primary outcome measure was exposure volume (hours) to running practice until first-time RRI (Cox proportional hazards regression analyses).

Results Overall, 12.1 RRIs were recorded per 1000 hours of running. There was a 15% difference between the shoe types regarding midsole stiffness. The type of shoe used for running was not related to RRI (hazard ratio (HR)=0.92; 95% confidence interval (CI)=0.57–1.48). Furthermore, no difference was detected regarding RRI patterns (location, severity, etc.). Significant risk factors for RRIs were body mass index (HR=1.13; 95% CI 1.03–1.23), injury in the 12 months preceding the study (HR=1.74; 95% CI 1.04–2.90) and average session intensity (HR=1.40; 95% CI 1.04–1.87). Previous regular running practice (HR=0.42; 95% CI 0.23–0.78) and volume of other sport activities (HR=0.70; 95% CI 0.56–0.88) were significant protective factors.

Conclusions RRI risk was not associated with running shoe midsole hardness in this study.