Skeletal macro- and microstructure adaptations in men undergoing arduous military training

Purpose: Short periods of basic military training increase the density and size of the tibia, but the adaptive response of bone microarchitecture, a key component of bone strength, is not fully understood. Methods: Tibial volumetric bone mineral density (vBMD), geometry, microarchitecture and mechanical properties were measured using high-resolution peripheral quantitative computed tomography in 43 male British Army infantry recruits (mean +/- SD, age 21 +/- 3 years, height 1.76 +/- 0.06 m, body mass 76.5 +/- 9.4 kg). Bilateral scans were performed at the distal tibia at the start (week 1) and end (week 13) of basic military training. Concurrent measures were obtained for whole-body areal bone mineral density (aBMD) using DXA, and markers of bone metabolism (beta CTX, P1NP, PTH, total 25(OH)D and ACa) from venous blood. Results: Training increased areal BMD for total body (1.4%) and arms (5.2%) (P <= 0.031), but not legs and trunk (P >= 0.094). Training increased trabecular (1.3 to 1.9%) and cortical vBMD (0.6 to 0.9%), trabecular volume (1.3 to 1.9%), cortical thickness (3.2 to 5.2%) and cortical area (2.6 to 2.8%), and reduced trabecular area (- 0.4 to -0.5%) in both legs (P < 0.001). No changes in trabecular number, thickness and separation, cortical porosity, stiffness or failure load were observed (P >= 0.188). beta CTX decreased (-0.11 mu g.l(-1), P < 0.001) and total 25(OH)D increased (9.4 nmol.l(-1), P = 0.029), but no differences in P1NP, PTH or ACa were observed between timepoints (P >= 0.233). Conclusion: A short period of basic military training increased density and altered geometry of the distal tibia in male military recruits. The osteogenic effects of basic military training are likely due to an increase in unaccustomed, dynamic and high-impact loading.