One Size Does Not Fit All: Cardiorespiratory Fitness Adaptations to Basic Military Training are Attenuated in Female Recruits and Recruits With High Baseline Fitness

ABSTRACT: Saner, NJ, Kuang, J, Cheng, I-T, Drain, JR, and Bishop, DJ. One size does not fit all: Cardiorespiratory fitness adaptations to basic military training are attenuated in female recruits and recruits with high baseline fitness. J Strength Cond Res 38(10): 1724-1731, 2024- A focus of basic military training (BMT) is to improve the physical fitness of recruits. However, significant individual variation in the response to BMT has been reported, and the prevalence of injury is high. This study investigated the relationship between baseline cardiorespiratory fitness (CRF), sex, and age to changes in CRF and musculoskeletal injury (MSKI) during BMT. Cardiorespiratory fitness and injury prevalence were prospectively assessed in a large mixed-sex cohort of Australian Army recruits (n = 1,581) undergoing 12 weeks of BMT. There was a significant group-level increase in estimated V̇o2max during BMT (6.6 ± 7.9%, p < 0.001); however, there was significant individual variation in responses. Baseline CRF and sex were significant predictors (p < 0.001) of change in estimated V̇o2max, but age was not (p = 0.115). Recruits within the 2 highest quintiles for baseline CRF improved estimated V̇o2max significantly less than recruits in the lowest 3 quintiles (p < 0.001). Male recruits improved estimated V̇o2max to a greater extent than female recruits (mean difference ± SD, 1.9 ± 0.2 mL·kg-1·min-1, p < 0.001), even when baseline fitness was accounted for. There were 153 recruits that reported 1 or more MSKI during BMT, and there was approximately 2.5-fold higher MSKI incidence in female recruits. Overall, we report that CRF improved during BMT and that baseline CRF and sex partially explain these improvements. However, female recruits demonstrated modest gains in CRF and were disproportionately injured when compared with male recruits. This highlights the need for ability-based training strategies to yield consistent improvements and reduce injury prevalence in military personnel.