Effect of cardiovascular and muscular endurance is not associated with stress fracture incidence in female military recruits: A 12-month follow up study

Stress fracture (SF) is a common injury among military recruits, especially among women, during the army basic training (ABT). The purpose of this study was to evaluate the effects of health habits and physical activity before recruitment on the fitness level and the incidence of SF during the 4-month ABT. We screened 226 female recruit volunteers (weight: 60.5±10 kg; height: 163±6 cm) from an integrated combat unit and 124 aged-matched female controls (weight: 57.0±8.3 kg, height 162±7 cm) from a non-combat unit. A self-report questionnaire on their habits pertaining to smoking, physical activity, and orthopedic injuries prior to recruitment were analyzed in relation to the incidences of SF during ABT. Aerobic fitness was similar between the two groups. The overall incidence of SFs was 10.2%. Physical training prior to recruitment had no significant effect on the incidence of SF during ABT (11.7% vs. 9.6% in those who trained and did not train before recruitment, respectively) (Odds ratio, OR)=1.24, p=0.236). Nearly 42% of the female recruits smoked regularly, and the incidence of SFs among smokers was 10.5% compared with 9.9% among the non-smokers (OR=1.07, p=0.188). The overall incidence of SFs 12 months after recruitment was 1.78%. The use of contraceptive medication did not affect the incidence of SF: 10.0% among prior-trained vs. 6.4% in non-prior trained (p>0.05) recruits. SFs were not correlated to these variables at the end of the ABT program and 16 months after recruitment. In the present female cohort, physical activity prior to recruitment had no protective effect against SF during or after ABT. The incidence of SFs during the 12-month period after ABT was negligible.