Training Modality and Aerobic Fitness in Active Duty U.S. Marines

INTRODUCTION: Aerobic fitness is a key determinant of military readiness. Although resistance training is increasingly emphasized in military physical training programs, concerns persist that concurrent resistance training may compromise aerobic fitness. Evidence from real-world military populations remains limited. MATERIALS AND METHODS: This cross-sectional study examined associations between self-reported training modality and estimated aerobic fitness in active duty U.S. Marines. Training modality was classified as no structured training, aerobic-only, resistance-only, or combined aerobic and resistance training. Aerobic fitness was estimated using VO2max derived from 3-mile run performance (Daniels equation). Multivariable linear regression models adjusted for age and sex were used to estimate adjusted marginal means and pre-specified pairwise contrasts. RESULTS: Estimated VO2max differed significantly by training modality (overall P < .001). Aerobic-only and combined training were associated with the highest adjusted VO2max (41.18 mL·kg-1·min-1 [95% CI: 38.64-43.72] and 41.11 [38.58-43.65], respectively), whereas resistance-only training (39.38 [36.81-41.96]) and no structured training (39.41 [36.80-42.02]) demonstrated lower values. Compared with resistance-only training, aerobic-only (Δ = 1.80 mL·kg-1·min-1; P < .001) and combined training (Δ = 1.73; P < .001) were associated with higher VO2max. Combined training did not differ from aerobic-only training (Δ = -0.07; P = .84). CONCLUSIONS: In this active duty cohort, aerobic-only and combined aerobic-resistance training were associated with similarly higher aerobic fitness compared with resistance-only or no structured training. These findings suggest multiple training pathways to comparable aerobic performance and support modality-specific considerations in fitness programming and readiness assessment.