IS STEP RATE ASSOCIATED WITH RUNNING INJURY INCIDENCE? AN OBSERVATIONAL STUDY WITH 9- MONTH FOLLOW UP.

Background: Several strategies have been proposed to reduce loading of the lower extremity while running including step rate manipulation. It is unclear however, whether step rate influences the incidence of lower extremity injuries. Purpose: To examine the association between step rate and risk of injury in an adult recreational runner population. Study Design: Prospective Cohort. Methods: A total of 381 runners were prospectively followed for an average of nine months. Two-dimensional video was used to assess preferred step rate during a timed two-mile run or a 5K race. Injury surveillance to record sub-clinical injuries (those for which medical treatment was not sought) was performed via semi-monthly email surveys over the course of one year. Injury surveillance for clinical injuries (those for which medical treatment was sought) was performed via a full medical record review using the Armed Forces Health Longitudinal Technology Application. Clinical, sub-clinical and combined clinical and sub-clinical injury incidence were assessed in separate analyses. Injury was operationally defined as seven or more days of reduced activity due to pain. To assess the predictive validity of running step rate, the step rate of participants who did not develop a musculoskeletal injury during the observation period were compared with the running step rate of participants who did develop an injury during the observation period. Results: Out of 381 runners, 16 sustained a clinical overuse injury for which medical treatment was sought. Mean step rate for clinically un-injured runners was 172 steps/min and mean step rate for clinically injured runners was 173 steps/min which was not statistically significantly different (p = 0.77.) Out of 381 runners, 95 completed all four sub-clinical injury surveys (95/381 = 25%). Out of those 95 runners, 19 sustained a clinical (n=4) or sub-clinical injury (n=15). The step rate of sub-clinically injured and non-injured runners in this sub-sample was also not statistically significantly different (p = 0.08), with a mean of 174 steps/min for the uninjured group and a mean step rate of 170 steps/min for those in the sub-clinical injured group. Conclusion: Preferred step rate was not associated with lower extremity injury rates in this sample of DoD runners. Additional research is needed to justify preferred step rate manipulation as a means to reduce lower extremity injury risk. Level of Evidence: Level 3.