Purpose: While the development of cardiorespiratory fitness in normally active children and adolescents is well-documented, longitudinal data on physiological adaptations to training in youth athletes remain limited. This study aimed to investigate the long-term development of cardiorespiratory fitness in young competitive athletes over a period of 2-6 years. Methods: A total of 397 young athletes (48 girls), aged 8-20 years, from a variety of sports underwent up to 6 repeated assessments between 2012 and 2024. Peak exercise performance (Wmax) and peak aerobic power (VO2peak), both were measured via cardiopulmonary exercise testing on an electronically braked cycle ergometer. A linear mixed model analysis was used to evaluate longitudinal changes in VO2peak and Wmax (both normalized to body mass), including body surface area and training intensity (MET-hours/week) as fixed effects, with sex-stratified analyses. Results: A total of 1009 cardiopulmonary exercise testing were analyzed. At baseline, boys showed higher age-specific VO2peak and Wmax, while girls had higher age-specific VO2peak despite similar training intensity. Longitudinally, VO2peak increased significantly with age in mid-adolescent girls and boys, particularly in endurance athletes, and was positively associated with training intensity. Wmax rose with age but was less influenced by training or sport type, showing a stronger relationship with growth-related factors like body surface area. Conclusions: VO2peak development in youth athletes is influenced by age, body size, training intensity, and sport type, making it a sensitive marker of aerobic adaptation. In contrast, Wmax reflects primarily maturational growth and is less responsive to training-specific factors.
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