Volumetric Bone Mineral Density Increases Following a Period of Heightened Physical Activity in Female U.S. Army Soldiers

Abstract

Background/Aim: Basic combat training (BCT) for military recruits is a time of heightened physical activity when stress fracture risk is increased. Animal studies demonstrate that bones can mount an anabolic response to physical activity, which confers mechanical benefits that may offset risk of injury. During BCT, this anabolic response may be more robust in those with lower bone density at the start of training, where a greater loading stimulus is perceived by the skeleton. We hypothesized that the BCT setting would result in increases in volumetric bone mineral density (vBMD; suggesting bone anabolism) and that individuals with lowest baseline vBMD would see the greatest change. Methods: We quantified the tibia vBMD of 90 female recruits before and after BCT, using a high-resolution peripheral quantitative computed tomography (XtremeCTII, Scanco Medical). Using a linear mixed model, we estimated the mean change in vBMD following BCT. To evaluate the effect of baseline vBMD on the change in vBMD we used a generalized additive model, applying a naturalized spline to baseline vBMD. We adjusted for baseline 25-hydroxyvitamin D and serum calcium, exercise prior to BCT, and race/ethnicity. Results: Tibial vBMD increased following BCT (Effect Estimate: 1.79%, 95% CI: 1.32, 2.25). The relationship between the change in vBMD and baseline vBMD was significantly non-linear (p<0.001). Individuals with the lowest baseline values for vBMD saw the greatest increases in vBMD where the slope of this effect diminished as baseline vBMD increased. Conclusions: BCT results in new bone formation, as suggested by increased vBMD. This anabolic response to training was most evident in Soldiers with lowest baseline bone density. These findings suggest that Soldiers with less dense bones at BCT entry are capable of bone anabolism—a biological process that can be targeted for stress fracture mitigation.