Timing of Stress Fractures in Soldiers During the First 6 Career Months: A Retrospective Cohort Study

Context: Stress fractures (SFs) are injuries that can result from beginning new or higher-volume physical training regimens. The pattern of clinical presentation of SFs over time after individuals start a new or more demanding physical training regimen is not well defined in the medical literature. Objective: To report trends in the clinical presentation of SFs over the first 6 months of soldiers' time in the service. Design: Retrospective cohort study. Setting: This study was conducted using medical encounter and personnel data from US Army soldiers during the first 6 months of their career. Patients or Other Participants: United States Army soldiers beginning their careers from 2005 to 2014 (N = 701 027). Main Outcome Measures: Weekly SF numbers and incidence were calculated overall, as well as by sex, over the first 6 months of military service. Results: Stress fracture diagnoses (n = 14 155) increased steeply in weeks 3 and 4, with a peak in the overall incidence during weeks 5 to 8. Although the clinical incidence of SFs generally decreased after 8 weeks, incident lower extremity SFs continued to present for more than 20 weeks. The hazard ratio for SFs among women compared with men was 4.14 (95% CI = 4.01, 4.27). Conclusions: Across the 6-month study period, women showed a more than 4 times greater hazard for SFs than men. The results also suggest that health care providers should be particularly vigilant for SFs within 3 weeks of beginning of a new or higher-intensity exercise regimen. The incidence of SFs may continue to climb for several weeks. Even as the SF incidence declines, these injuries may continue to appear clinically several months after a change in activity or training.