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

ABSTRACT Purpose This study investigates the intra- and inter-individual time courses of physiological adaptation to high-intensity interval training (HIIT), comparing single and duplicate pre-to-post testing with session-by-session analysis to more accurately identify “genuine” adaptations. Methods Seventeen participants (nine men) engaged in repeated 4x4 min HIIT sessions (2 times/week) until a meaningful change in the primary outcome i.e. relative peak oxygen uptake (VO 2peak ) was observed. Results Mixed-effects model analysis revealed a significant improvement for VO 2peak for both session-by-session (estimate: 0.18, p < 0.01, d = 0.11) analysis and duplicate pre-to-post analysis (estimate: 3.97, p < 0.01, ηp 2 = 0.36). Session-by-session analysis revealed significant variability in physiological responses, with a low coefficient of variation (CV) for VO 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 VO 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 VO 2 and SV max , but higher proportions for submaximal respiratory exchange ratio and rating of perceived exertion compared with pre-to-post analysis. 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-post evaluations, the findings emphasize the importance of frequent monitoring to accurately capture individual responses, thereby advancing strategies for optimizing exercise interventions across diverse populations.