Purpose: The purpose of this prospective study was to investigate the epidemiology of overuse injuries and to identify common risk factors for stress fractures among female and male recruits in a new light infantry basic training designed to minimize the incidence of overuse injuries.
Methods: Study subjects were male and female recruits in the 16-wk light infantry basic training. A control group of noncombat female medics whose military service did not include demanding physical activities was recruited to assess the female recruits' preinduction physical preparedness. Pretraining survey of all participants' medical and sports participation histories was conducted. Anthropometric measurements were performed. Subjects were followed every 3 wk for overuse injuries. Stress fractures were diagnosed by radiography or scintigraphy.
Results: Ninety-nine female recruits, 36 male recruits, and 55 controls participated. Although 31% of the controls reported regular preinduction sports participation, less than 25% of both male and female recruits did. Stress fractures incidence was 0% among males and controls but 12% among female recruits (P = 0.03). The mean body mass index of female recruits with stress fractures was 19.2 +/- 2.6 versus 22.5 +/- 3.3 kg x m of female recruits without stress fractures (P = 0.02, odds ratio = 1.397, 95% confidence interval = 1.065-1.833). No statistically significant difference was found between female and male military trainees in the incidence of other overuse injuries, but there was a statistical trend (P = 0.07) for more back pain among females.
Conclusions: Lower body mass index was the only variable identified as a risk factor for stress fractures among female recruits in the present study. It does not explain the markedly different response of female and male recruits' bones to the low demand training. There may be an intrinsic difference between male and female bone resistance to fatigue.