Baseline Fitness And Body Composition Influence Spine Areal Bone Mineral Density Following Army Ranger School

U.S. Army Ranger School students must endure 61-days of caloric deficit, sleep restriction, and prolonged movements carrying heavy loads to graduate from the one of the most challenging courses in the U.S. military. This extended period of stress leads to a decline in spine areal bone mineral density (aBMD) from before (PRE) to two-weeks post (2WK) Ranger School. However, baseline factors that contribute to changes in spine BMD during Ranger School are unknown. PURPOSE: This study aimed to identify PRE measures of fitness and body composition associated with spine aBMD at 2WK. METHODS: Twenty-eight male Soldiers (age: 23.3 ± 4.1 yrs, height: 1.8 ± 0.1 m, body mass: 83.3 ± 7.0 kg) attended Ranger School. Total fat-free mass (FFM), fat-mass (FM), and spine aBMD were measured using dual energy x-ray absorptiometry. Fitness was assessed using the Ranger Athlete Warrior assessment, which included the Illinois agility test (IAT), 300-yd shuttle run (SR), 20-m multistage beep test (BT), and maximum repetitions of metronome push-ups (METPU), pull-ups (PU), heel clap (HC), 185-lbs bench press (BP), and 225-lbs deadlift (DL). A backward stepwise linear regression was used to assess the relationship between PRE fitness and body composition on 2WK spine aBMD with and without adjusting for baseline spine aBMD. RESULTS: The unadjusted model, including baseline total FFM, METPU, PU, HC, BP, and DL, was statistically significant (F(6, 21) = 3.542, p = 0.014) and explained 36.1% of the variance in 2wk spine aBMD with a large effect size (adj. R2 = 0.361, Cohen’s f2 = 0.565). Total FFM (β = 0.006, SE = 0.003, p = 0.011) and HC (β = 0.016, SE = 0.006, p = 0.019) were the only statistically significant contributors to the model. The model remained statistically significant after adjusting for baseline spine aBMD (F(4, 23) = 35.330, p < 0.001, adj. R2 = 0.836). However, only METPU significantly contributed to the adjusted model (β = 0.003, SE = 0.001, p = 0.020). CONCLUSION: Baseline total FFM, HC, and METPU may be protective factors for spine aBMD during Ranger School. Improvements in physical fitness, particularly core strength, may preserve spine aBMD during periods of caloric stress and unaccustomed increases in physical activity and prolonged load bearing.