Investigation of Tibial Responses to Military Training Reveals Consistency between Adaptation and Markers of Stress and Osteogenic Potential.

INTRODUCTION Arduous military training consists of physically demanding activity that increase tibial bone formation. The intensity of these programs, however, greatly elevates the risk of bone stress injuries, especially in women. This cross-sectional study aimed to profile individuals whose tibia's most and least adapted to the 10-week Marine Officer Candidates School (OCS). METHODS Officer Candidates (n = 168; 49 Female) included were those with the greatest (Adapters; top quintile) and least (Non-Adapters; bottom quintile) change in Bone Strength Index at the distal metaphysis (4% site) of the tibia after OCS. Tibial peripheral quantitative computed tomography (pQCT) scans, blood draws, and demographics were taken before and after OCS. A cell culture model was utilized to measure the impact of circulating factors on osteoblast activity. Circulating markers of stress, bone turnover, and iron status were measured. RESULTS Analysis of tibial parameters revealed Adapters had greater changes in bone mineral density and/or strength index at each measured site (4%, 38%, and 66%) after OCS compared to Non-Adapters. Demographically, women were more likely to have no adaptation response. After adjusting for sex differences between groups, Adapters had no stress hormone decline, unlike non-Adapters, and began training at a lower body mass than non-adapting counterparts. Completion of OCS in Adapters improved the osteogenic capacity of circulating factors, as evident by increased osteoblast mineralization, alkaline phosphatase activity, and expression of key genes. Additionally, iron status was higher in Adapters following OCS, with little differences in bone metabolism biomarkers. CONCLUSIONS Overall, this study provides a novel investigation into those with the greatest and least changes in tibial bone measures following military training and identifies demographic and circulating biomarker differences between groups.