Body composition and cardiopulmonary exercise testing (CPET) are vital for optimizing sports performance, but the correlations between them are still underexplored. Our study aimed to investigate the relationships between body composition and specific CPET variables describing physical fitness in young athletes, also adjusting for age and height, in a less-studied, female population. Seventy players participated in our study (age: 16.10 ± 1.63 y). After determining body composition using dual-energy X-ray absorptiometry, we conducted treadmill-based maximal-intensity CPET. Data were analyzed in R using multivariate linear regression, accounting for age and height as confounders. Lean body mass (LBM), body fat mass (BFM), and bone mineral content (BMC) showed no effect on resting, maximum, or recovery heart rates and no correlation with resting or maximal lactate values. LBM positively correlated with maximum ventilation (VE-max) (Est: 1.3 × 10⁻³; SE: 6.1 × 10⁻⁴; p < 0.05) and maximum absolute oxygen consumption (VO2_abs-max) (Est: 7.710⁻⁵; SE: 6.9 × 10⁻⁶; p < 0.001)—with age as an influencing factor for VE-max and height as an influencing factor for VO2_abs-max. Conversely, BFM showed a negative correlation with maximum relative oxygen consumption (VO2_rel-max) (Est: −4.8 × 10⁻⁴; SE: 1.2 × 10⁻⁴; p < 0.001). Moreover, BFM and BMC were also negatively correlated with maximal exercise duration (Est: −2.2 × 10⁻⁴; SE: 8.0 × 10⁻⁵; p < 0.01; Est: −3.2 × 10⁻³; SE: 1.4 × 10⁻³; p < 0.05) with height as an influencing factor. Our findings indicate complex correlations between body composition and CPET parameters, providing important information for the analysis of individual ergospirometric data. Our results draw attention to the fact that body composition is more precise than weight and height in the evaluation of athletes’ physical fitness.

Background: Despite numerous data on whole-body responses, we have less information about local muscular changes during physical exercise in athletes. Oxygen saturation (SmO₂) changes in the working muscles follow phases of load and are useful, as local metabolism could influence physical fitness. Methods: A total of 100 asymptomatic elite water polo players (63% male, age: 17.2 (interquartile range: 16.1–18.9) years) were examined using near-infrared spectroscopy to measure SmO₂ in both vastus lateralis and left deltoid muscles during continuous uphill running treadmill exercise. Results: Differences were observed between upper and averaged lower limb resting SmO₂ (82.1% (77.0–89.0%) vs. 68.3% (59.2–73.6%), p < 0.001). During exercise, the relative decrease in averaged lower limb SmO₂ was greater compared to the upper limb at the anaerobic threshold (−0.371 (−0.539–−0.200) vs. –0.224 (−0.340–−0.099), p < 0.001) and at maximal exercise (−0.557 (−0.750–−0.411) vs. –0.420 (−0.556–−0.271), p < 0.001). Higher averaged lower limb relative SmO₂ was recorded compared to the upper limb after 5 min cool-down (+0.081% (−0.046–+0.195%) vs. –0.047% (−0.140–+0.000), p < 0.001). No differences were found between males and females in resting lower limb SmO₂. Both sexes showed a monotonic decrease in SmO₂ during exercise, with differences in the relative values at the anaerobic threshold and at maximal intensity. Females exhibited a rebound in SmO₂ after a 5 min cool-down. Conclusions: We provide insights into SmO₂ alterations during maximal-intensity exercise and recovery through the measurements of elite water polo athletes, also highlighting sex differences in SmO₂. Measuring local SmO₂ changes is a promising additional method in physical fitness follow-up.