Dose–Response Effects of High-Intensity Interval Training on Body Fat, Blood Pressure, and Cardiorespiratory Fitness in Adolescents: A School-Based Randomized Controlled Trial with Responder Analysis

<b>Background</b>: High-intensity interval training (HIIT) is effective for improving body composition and cardiorespiratory fitness, but individual variability in responsiveness remains a challenge. This study examined the dose-response effects of three HIIT session durations (6, 8, and 10 min) and whether previously non-responsive adolescents could benefit from a modified program. <b>Methods</b>: A total of 137 adolescents completed one of three school-based HIIT interventions. Body fat percentage (BF%), systolic and diastolic blood pressure, and maximal oxygen uptake (VO₂max) were assessed before and after the intervention. Responders and non-responders were classified using a Bayesian approach. Statistical analyses included ANOVA, McNemar's test, logistic regression, and generalized estimating equations. <b>Results</b>: All protocols improved outcomes, with the 10 min sessions producing the most consistent VO₂max gains. No significant differences were observed for BF% or blood pressure. Individual analysis showed that more than half of the participants responded positively to training, depending on the outcome. Among prior non-responders, 70-100% showed improvements after the modified intervention. The number of previously non-responsive outcomes strongly predicted improvement (Odds Ratio > 2.4, <i>p</i> < 0.01). <b>Conclusions</b>: School-based HIIT can induce meaningful adaptations even in previously non-responsive adolescents. Individualized monitoring and adjustment of training dose may optimize responsiveness and support health promotion in youth.