Gender differences in the associations of body mass index, physical fitness and tobacco use with lower extremity musculoskeletal injuries among new US Army soldiers.

Background: Musculoskeletal (MSK) injuries are a leading cause of morbidity among US Army soldiers, especially among women and the newly enlisted. While extremes of body mass index (BMI), low physical fitness and tobacco use have been identified as risk factors, no prior studies have determined the combined effects of these risk factors or tested for gender differences in the associations.

Methods: We conducted a retrospective cohort study utilising the Stanford Military Data Repository to study all enlisted soldier accessions to the US Army between January 2011 and January 2014, with follow-up through December 2014 (n=238 772). We used discrete-time logistic regression to test for associations between demographic, socioeconomic and health behaviour-related predictors and incident lower extremity injuries. We included interaction terms to test for gender differences.

Results: Among men, there were greater adjusted odds of injury among underweight (OR 1.27), overweight (OR 1.14) and obese soldiers (OR 1.54) (all p<0.001) relative to normal-weight soldiers. No statistically significant differences in the odds of injury on the basis of BMI were observed for female soldiers. Physical fitness was a significantly stronger predictor of injury for female as compared with male soldiers, while the association between tobacco use and injury was stronger for men (OR 1.47) than for women (OR 1.30) (p<0.001 for both).

Conclusion: This is the largest known study to simultaneously assess important, modifiable musculoskeletal injury risk factors and to test for gender differences in these associations. These findings provide critical gender-related nuances for clinicians and leaders seeking to reduce the risk of these problems.