Context: Stress fractures (SF) are injuries that can result from beginning new or higher volume physical training regimens. The pattern of clinical presentation of SF over time after individuals start a new or more demanding physical training regimen is not well defined in medical literature.

Objective: Report trends in the clinical presentation of stress fractures over the first six month of soldiers' time in the service.

Design: Retrospective Cohort study

Setting: This study was conducted using medical encounter and personnel data from U.S. Army soldiers during the first 6 months of their career.

Participants: U.S. Army soldiers beginning their careers from 2005–2014 (N=701,027).

Data Collection and Analysis: Weekly SF numbers and incidence were calculated overall, as well as by sex, over the first 6 months of military service.

Results: SF diagnoses (N=14,155) increased steeply in weeks 3 and 4, with a peak in the overall incidence of SF diagnoses occurring during weeks 5–8. Although clinical incidence of stress fracture generally decreased beyond 8 weeks, incident lower extremity stress fractures continued to present for over 20 weeks. The hazard ratio (HR) for SF among women compared to men was 4.14 (95% CI = [4.01, 4.27]).

Conclusions: Across the 6-month study period, women showed over 4 times greater hazard for stress fracture. The results also suggest that health care providers should be particularly vigilant for stress fractures within 3 weeks after the beginning of a new or higher intensity exercise regimen. The incidence of SF may continue to climb for several weeks. Even as stress fracture incidence declines, it should be noted that these injuries may also continue to appear clinically even several months after a change in activity or training.

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Author notes

This study was approved by the institutional review board of the U.S. Army Research Institute of Environmental Medicine.

The opinions or assertions contained herein are the private views of the author(s) and are not to be construed as official or reflecting the views of the Department of the Army or the Department of Defense. Any citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organizations.

The investigators have adhered to the policies for protection of human subjects as prescribed in Department of Defense Instruction 3216.02 and the research was conducted in adherence with the provisions of 32 CFR Part 219.

This research was supported in part by an appointment to the Postgraduate Research Participation Program at the U.S. Army Research Institute of Environmental Medicine administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USARIEM.