Continuous Peak Oxygen Uptake Monitoring in High-Intensity Interval Training Minimizes False Responder Rates Compared with Pre- to Posttesting

PURPOSE: This study investigates the intra- and interindividual time courses of physiological adaptation to high-intensity interval training, comparing single and duplicate pre- to posttesting with session-by-session analysis to more accurately identify "genuine" adaptations. METHODS: Seventeen participants (nine men) engaged in repeated 4 × 4 min high-intensity interval training sessions (2 times per week) until a meaningful change in the primary outcome, i.e., relative peak oxygen uptake (V̇O 2peak ), was observed. RESULTS: Mixed-effects model analysis revealed a significant improvement for V̇O 2peak for both session-by-session (estimate: 0.18, P < 0.01, d = 0.11) analysis and duplicate pre- to postanalysis (estimate: 3.97, P < 0.01, ηp2 = 0.36). Session-by-session analysis revealed significant variability in physiological responses, with a low coefficient of variation (CV) for V̇O 2peak (3.49% + 1.96%) and estimated maximum stroke volume (SV max ) (3.07% ± 1.92%), and indicating their reliability for detecting small changes. With a CV of 22.14% ± 13.80%, submaximal blood lactate ([BLa] submax ) was the least reliable parameter. With session-by-session analysis, V̇O 2peak was the only parameter displaying 100% positive responders after 9.5 ± 3.8 sessions. Additionally, session-by-session analysis revealed lower proportions of participants with positive adaptations for submaximal V̇O 2 and SV max but higher proportions for submaximal respiratory exchange ratio and rating of perceived exertion compared with pre- to postanalysis. CONCLUSIONS: This study highlights the value of longitudinal assessments for understanding the variability and dynamics of training adaptations. By addressing the limitations of pre- to postevaluations, the findings emphasize the importance of frequent monitoring to accurately capture individual responses, thereby advancing strategies for optimizing exercise interventions across diverse populations.