Minimal Impact of Age on Adaptive Bone Formation in the Distal Tibia of US Army Basic Combat Trainees: The ARMI Study

ABSTRACT Introduction Although bone formation has been observed in the distal tibia of trainees during Basic Combat Training (BCT), some animal studies have demonstrated greater mechanosensitivity in the juvenile compared to the mature skeleton. How age affects skeletal adaptations to military training is unknown. The purpose of this study was to determine if skeletal adaptations to US Army BCT differ by age. Methods We enrolled 2085 trainees (1303 males, 782 females). High-resolution peripheral quantitative computed tomography was used to assess total, cortical, and trabecular bone mineral density (BMD) and the microarchitecture of the distal tibia before and after 8 wk of BCT. Generalized linear models were used to compare mean percent change (95% confidence interval (CI)) in each bone parameter by age group (<20, 20–25, 26–29, and ≥30 yr) and to evaluate differences in bone parameter changes between age groups (effect estimates (95% CI)). Models were stratified by sex with multivariable adjustment. Results Male and female trainees in all age groups had increases in total BMD, trabecular BMD, and trabecular bone volume fraction from baseline ( P < 0.01 for all). Male and female trainees also had increases in trabecular thickness from baseline in all groups ( P < 0.05) except females ≥30 yr old. Female trainees 20–25 yr old had a significantly larger increase in trabecular thickness than those <20 yr old (effect estimate (95% CI), 0.33% (0.02%–0.64%); P < 0.05). Trabecular microarchitecture was largely not otherwise different between age groups for either sex. Male and female trainees <20 yr old had greater changes in cortical BMD compared to older age groups (males, P < 0.01; females, P < 0.05 for all). Conclusion Trainees <20 yr old experience the greatest improvements in cortical BMD at the distal tibia during US Army BCT, suggesting that adaptive bone formation in cortical bone may be attenuated with skeletal maturity. However, the mechanosensitivity of trabecular bone to exercise training remains intact regardless of age.