Sex-differences In Bone Density, Geometry, And Estimated Strength Adaptations To 10-weeks Of Military Officer Training

Mechanical loading (e.g. physical activity) is associated with changes in bone density and structure; however, few investigations have examined the adaptive bone response to arduous military training in men and women. PURPOSE Investigate the effects of military training on volumetric bone density (vBMD), geometry, and strength in men and women who complete Marine Corps Officer Candidates School (OCS). METHODS Male and female candidates (n=267) completed a tibial peripheral quantitative computed tomography (pQCT) scan before and after a 10-week physically intensive military training course. Three-dimensional vBMD, geometry, and estimated bone strength were assessed at the 4%, 38%, and 66% sites. Wilcoxon signed-rank tests assessed changes across training. Data are mean±SEM, α=0.05. RESULTS Subjects were aged 19-35 (25.3±0.2 yrs) with a BMI 25.5±0.1 kg/m2. At the distal (4%) tibia, increases in total vBMD (pre: 354.5±2.7, post: 356.3±2.7 mg/cm3), trabecular vBMD (294.3±2.2, 295.6±2.2 mg/cm3), and estimated compression strength (BSI; 154.7±2.2, 156.2±2.1 mg2/mm4) were observed in men (n=222, p<0.001). In women (n=39), total vBMD (324.2±5.1, 326.5±5.2 mg/cm3 p=0.03), trabecular vBMD (262.7±4.8, 264.4±4.9 mg/cm3 p=0.01), and BSI (105.9±3.3, 107.4±3.4 mm3 p<0.01) also increased. At the midshaft (38%) tibia, total vBMD (938.1±3.7, 938.9±3.7 mg/cm3 p=0.03), cortical thickness (6.8±0.1, 6.8±0.1 mm, p<0.01), periosteal circumference (77.0±0.3, 77.1±0.3 mm p<0.01) and estimated bending strength (SSI; 2182.7±25.9, 2193.8±25.1 mm3 p=0.02) increased in men (n=208). In women (n=40), only periosteal circumference increased (70.0±0.6, 70.1±0.6 mm p=0.05). At the proximal (66%) tibia, no significant changes (p>0.05) were observed in men (n=200). In women (n=38), total vBMD decreased (735.9±9.0, 732.7±8.8 mg/cm3 p=0.04) and periosteal circumference increased (82.5±0.9, 82.8±0.9 mm p<0.01) following training. CONCLUSION Bone adaptations in response to 10 weeks of military training are slight (≤1.5%), but statistically significant and may be sufficient to improve estimates of bone strength. Changes are further dependent on biological sex and anatomical location.