Prior Physical Activity Influences Changes In Tibial Bone Microarchitecture During U.S. Army Basic Combat Training

Abstract

Bone stress injuries (BSIs) are common among military personnel due to heightened physical activity. Whereas we have previously shown significant tibial bone adaptation following 8 weeks of Basic Combat Training (BCT), it is not known whether prior exercise habits influence the skeletal response to BCT. PURPOSE: Determine the relationship between exercise history and changes in tibial bone microarchitecture induced by BCT. METHODS: We collected high-resolution peripheral quantitative computed tomography images of the distal tibia, pre- and post-BCT, on U.S. Army trainees. We created low (LOW; 0-2 days/wk, n = 312), moderate (MOD; 3-4 days/wk, n = 597), and high (HIGH; 5+ days/wk, n = 696) exercise groups based on self-reported average days/wk of exercise during the 2 months prior to BCT. Linear mixed models were used to estimate within and between group differences from pre- to post-BCT controlling for sex, race, age, height, and weight. RESULTS: We enrolled 552 female and 1053 male trainees (20.7 ± 3.7 yrs) with normal BMI (24.7 ± 3.5 kg/m2). Trabecular thickness (Tb.Th), cortical thickness (Ct.Th), Ct area (Ct.Ar), total volumetric bone mineral density (Tt.vBMD), Tb.vBMD and Ct.vBMD increased significantly among all exercise groups over the BCT period (0.46-1.9%; all p < 0.0001, Fig 1). However, there was a greater increase in Ct.Th (p < 0.0001), Ct.Ar (p = 0.02), Tb.vBMD (p < 0.0001) and Tt.vBMD (p = 0.001) in LOW vs. HIGH. Similarly, MOD had greater increases in Tb.Th (p = 0.003), Ct.Th (p = 0.004), Ct.Ar (p = 0.001), Tb.vBMD (p = 0.0001), and Tt.vBMD (p < 0.0001) compared to HIGH. There were no differences between LOW and MOD CONCLUSION: All groups saw improved bone microarchitecture in response to BCT. LOW compared to MOD had greater increases than HIGH, perhaps due to a greater relative increase in novel physical activity during BCT experienced by LOW and MOD. Ongoing work will determine whether a more robust skeletal response to BCT reduces BSI incidence.