Associations Between Physical Fitness Test Scores And Tibial Bone Microarchitecture In Young Adults Entering Military Training

Abstract

Participation in physical activity (PA) leads to more robust bone microarchitecture and decreased risk of bone stress injury (BSI); a common injury during U.S. Army Basic Combat Training (BCT). As self-reported PA may be subject to recall bias, objectively measured physical fitness (PF) may provide new insights into the relationship between PA and skeletal health. PURPOSE: To assess the association between objectively-measured PF and bone microarchitecture in young adults entering BCT. METHODS: We obtained high resolution peripheral quantitative computed tomography (HRpQCT) scans to assess distal tibia microarchitecture and Occupational Physical Assessment Test (OPAT) scores from U.S. Army accession records for 1141 men and 630 women at BCT entry. The OPAT consists of a strength deadlift (SDL), standing long jump (SLJ), seated power throw (SPT), and interval aerobic run (IAR), each scored 1 (best) to 3 (worst). Scores in each event were summed and grouped to create low (LOW), moderate, and high (HIGH) PF groups for overall score, cardiorespiratory (CR; IAR score only) and strength/power (S/P; summed SDL, SLJ, and SPT scores) components. We used linear regression models to determine the relationship between PF and bone parameters, adjusted for age, race/ethnicity, and BMI. RESULTS: Men and women enrolled in the study were healthy, young (20.8 ± 3.7 yrs) and had normal BMI (24.7 ± 3.6 kg/m2). Cortical area (Ct.Ar) and thickness (Ct.Th.) were significantly higher in both men (Ct.Ar = 6.9%; Ct.Th = 6.0%) and women (Ct.Ar. = 7.1%; Ct.Th. = 5.6%), in HIGH vs. LOW (p < 0.01 for all). Total volumetric bone mineral density (vBMD) (4.3%), trabecular (Tb.) vBMD (5.0%), and Tb. thickness (1.9%) were higher in men (p < 0.02 for all) in HIGH vs. LOW, but there were no significant differences in HIGH vs LOW women. When CR and S/P were assessed separately men and women in the HIGH groups had significantly greater measures of Ct. bone parameters vs. LOW; only men had significantly greater Tb. bone parameters. CONCLUSION: Bone microarchitecture was more favorable for individuals with better PF test scores, with the overall OPAT score most strongly associated with favorable bone microarchitecture. Ongoing work will determine whether individuals scoring higher on the OPAT at BCT entry have lower risk for BSIs.