Cardiorespiratory Fitness and Body Composition in CrossFit® Participants: Associations with Lean Mass and Adiposity

Background: Body composition may be associated with the expression of cardiorespiratory fitness, particularly when peak oxygen uptake (VO2peak) is normalized to body mass or lean mass. This exploratory, cross-sectional, single-center study primarily examined associations between body composition and absolute, body-mass-normalized, and lean-mass-normalized VO2peak. Secondarily, we compared male and female CrossFit® participants and tested whether the between-sex difference in VO2peak varied by expression method. Methods: Fifteen trained CrossFit® participants recruited from a single affiliate were included, comprising 9 males (30.0 ± 4.2 years; 82.6 ± 4.0 kg; 176.3 ± 4.2 cm; mean ± SD) and 6 females (36.3 ± 6.6 years; 58.7 ± 3.4 kg; 158.3 ± 5.1 cm). Participants underwent dual-energy X-ray absorptiometry and an incremental treadmill test to volitional exhaustion with breath-by-breath gas analysis. Sex differences were assessed using Welch’s independent-samples t-test, and associations using Pearson and sex-adjusted partial correlations, with false discovery rate correction. A log-scale linear mixed-effects model with participant as a random intercept was used to test the sex × VO2peak-expression interaction. Results: Males presented greater total and lower-limb lean mass and lower body fat percentage than females. Absolute VO2peak was ~61% greater in males (4.1 ± 0.5 vs. 2.5 ± 0.4 L·min−1), whereas the difference was reduced to ~15% when normalized to body mass and ~5% when normalized to lean mass. The sex × expression interaction was significant (p < 0.001), indicating that the magnitude of the between-sex difference was not constant across expression methods. Absolute VO2peak was strongly associated with total lean mass (r = 0.908), lower-limb lean mass (r = 0.879) and body mass (r = 0.891; all pFDR < 0.001). Body-mass-normalized VO2peak was inversely associated with body fat percentage (r = −0.555) and positively associated with total lean mass (r = 0.558; both pFDR = 0.044). None of the sex-adjusted associations remained significant after FDR correction. Conclusions: Body composition was associated with aerobic fitness expression in CrossFit® participants, with lean mass related to absolute VO2peak and adiposity to body-mass-normalized VO2peak. The significant sex × expression interaction showed that the between-sex difference depended on the scaling method.