Intensity Trajectories During High-Intensity Interval Training and Their Impact on Health Outcomes in Adolescents:...

High-intensity interval training (HIIT) is increasingly implemented in school-based physical education, yet little is known about how exercise intensity changes across repeated sessions and whether such trajectories are associated with physiological adaptations in adolescents.

This study, conducted within the PEER-HEART project (ClinicalTrials.gov: NCT06431230), included 145 adolescents from the experimental arms of a broader school-based trial, stratified by sex and training modality: male HIPT (n = 24), male HIIT (n = 45), female HIPT (n = 46), and female HIIT (n = 30). The 8-week intervention was delivered twice weekly, and exercise intensity was monitored during 16 sessions using heart rate sensors. Intensity trajectories were examined using visual trajectory plots and slope-based statistical analyses, and pre-to-post changes in body fat percentage, systolic and diastolic blood pressure, and predicted VO₂max were analyzed.

Visual analyses indicated sex- and modality-specific intensity patterns, with the female HIIT group showing the most dynamic trajectory. Mean changes ranged from -0.63 ± 2.37% to -1.54 ± 2.66% for body fat, from -2.73 ± 5.28 to -5.37 ± 4.32 mmHg for systolic blood pressure, from -0.22 ± 5.00 to -2.62 ± 6.25 mmHg for diastolic blood pressure, and from 0.41 ± 3.25 to 3.81 ± 4.67 mL/kg/min for predicted VO₂max across subgroups. Although most delta values showed no significant main effects, a sex × modality interaction was observed for body fat percentage, and a main effect of sex was observed for predicted VO₂max. Greater intensity progression was associated with larger reductions in diastolic blood pressure in the female HIIT (β = -708.0, p < 0.001, R² = 0.39) and male HIIT groups (β = -377.0, p = 0.014, R² = 0.13) and with greater improvements in predicted VO₂max in the female HIPT (β = 154.0, p = 0.029, R² = 0.104) and male HIIT groups (β = 315.0, p = 0.029, R² = 0.106).

Individual intensity trajectories may help explain variability in physiological responses to school-based interval training and may provide additional insight beyond group-level comparisons alone.