Effects Of Fat-free And Fat Mass On Injury Risk During U.S. Army Basic Combat Training

Army research has shown a relationship between body mass index (BMI) and musculoskeletal injury (MSKI) risk during Basic Combat Training (BCT), with the greatest risk occurring at low and high ends of BMI. It is generally presumed that low BMI is reflective of low muscle mass while high BMI is reflective of excess body fat; however, data has not previously been available to test whether low muscle mass and high body fat are truly independent predictors of MSKI. PURPOSE: To determine the independent effects of fat-free and fat mass on MSKI risk during BCT. METHODS: Dual-energy X-ray absorptiometry (DXA) scans were obtained in female (n = 901) and male (n = 1,556) trainees (aged 17-41 years) during the first week of BCT to assess fat-free and fat mass (kg). To account for differences in height, fat-free mass index (FFMI) and fat mass index (FMI) were calculated as their respective masses divided by height (m) squared. MSKIs were identified through medical records. Two-way ANOVAs (sex x injured) were used to determine whether FFMI and FMI differed between those with an MSKI and those who didn’t. Binomial regression with sex, FFMI, FMI, and sex interactions were used to establish differences in injury risk and 95% confidence intervals. RESULTS: For female and male trainees respectively, average FFMI was 15.7 and 18.6 kg·m-2 and average FMI was 7.7 and 5.9 kg·m-2. FFMI was significantly lower, and FMI was significantly higher in individuals who were not injured (p < 0.01). There was a significant sex interaction in FFMI (p = 0.01) such that for each unit increase in FFMI, the risk of MSKI was decreased by 7.8% (2.7%-12.5%) in male and 19.5% (11.7%-26.5%) in females (p < 0.01 for both). Each unit increase in FMI increased the risk of MSKI by 9.9% (4.7%-15.4%) in both sexes (p < 0.01), with no sex interaction (p = 0.96). For each unit increase in the FFMI/FMI ratio, risk of MSKI decreased by 10.2% (3.7%-16.3%; p < 0.01). CONCLUSION: These data demonstrate that both low muscle mass and high fat mass at the beginning of BCT are independent predictors of MSKI during BCT. Optimizing body composition prior to the beginning of BCT may decrease the risk and the associated negative consequences of MSKI during BCT. Military Operational Medicine Program, US Army Medical Research and Development Command