8285483 Stress fracture in male royal marines recruits: working toward identification of those at highest risk of injury

<h3>Objective</h3> Stress fractures (SF) occur more frequently in military recruits than in the general population, causing morbidity, training delays, and costly rehabilitation. We aimed to report the incidence of SFs and identify characteristics associated with SF in male Royal Marines recruits <h3>Material and Methods</h3> We studied 1,815 male RM recruits (aged 16-33 years) starting a 32-week training programme at the Commando Training Centre Royal Marines, UK, between November 2017 and March 2020. Baseline exposures included age, weight, body mass index (BMI), waist circumference, waist-to-height ratio, smoking status, estimated maximum oxygen consumption (VO2max), and physical fitness measures (sit-ups, press-ups, pull-ups). Incident SFs were determined through radiography or MRI. Associations between baseline exposures and SF were examined using univariate logistic regression. <h3>Results</h3> The median age at entering training was 21 years. Baseline cardiorespiratory fitness was high (median VO₂max of 52.2 mL/kg/min), and mean BMI was 24.5 kg/m2. Most recruits (85.6%) had healthy central adiposity (waist-to-height ratio: 0.4–0.49), with none classified as high (≥0.6), and 10.4% were smokers. The incidence of SF was 5.6%, with 102 recruits sustaining at least one; the highest rate was among those aged 16–18 years (7.2%). The highest quintiles of BMI (odds ratio 0.16, 95% CI 0.06-0.42, p&lt;0.001), weight (0.50, 0.26-0.95, p=0.036), waist circumference (0.40, 0.19-0.83, p=0.014), and waist-to-height ratio (0.37, 0.18-0.78, p=0.009) demonstrated significantly lower odds of SF compared to respective lowest quintiles. No other baseline exposures were associated with SF risk. <h3>Conclusions</h3> SFs were common in Royal Marines recruits, especially those who were less skeletally mature. Higher BMI appeared protective against SF risk, and we hypothesise that greater BMI is likely related to reduced SF risk via greater muscle mass rather than greater adiposity. Further validation in other cohorts is planned.