Intensity Trajectories During High-Intensity Interval Training and Their Impact on Health Outcomes in Adolescents: Evidence from School-Based PE Settings (Peer-Heart Studies)

Figure 1. Sankey diagram showing transitions in average exercise intensity across the 8 weeks of the intervention. For clarity, weekly mean intensity values were grouped into four categories: ≤60%, 61–80%, 81–90%, and >90% of HRmax. Each vertical bar represents one intervention week, and the width of each flow corresponds to the number of participants transitioning between adjacent intensity categories. Weekly values were calculated as averages of the two sessions performed within each week.

Figure 2. Arc diagram illustrating transitions between exercise intensity categories aggregated across all 16 training sessions. Each node represents a predefined %HRmax category, with node size indicating the frequency of observations within that range. The connecting arcs represent transitions between categories, and line thickness reflects the number of such transitions. The diagram highlights the predominance of moderate-to-high intensity regulation across the intervention period.

Figure 3. Quadratic-smoothed exercise intensity trajectories (%HRmax) across 16 training sessions in four subgroups defined by sex (female, male) and training modality (HIIT, HIPT). Grey lines represent individual participant trajectories, whereas colored curves represent subgroup-level quadratic trends.

Figure 4. Relationship between individual intensity slope across 16 sessions and pre-to-post change in body fat percentage (ΔBF%) in the four experimental groups. Each point represents one participant, and thick lines represent subgroup-level linear regression models.

Figure 5. Relationship between individual intensity slope across 16 sessions and pre-to-post change in systolic blood pressure (ΔSBP) in the four experimental groups. Each point represents one participant, and thick lines represent subgroup-level linear regression models.

Figure 6. Relationship between individual intensity slope across 16 sessions and pre-to-post change in diastolic blood pressure (ΔDBP) in the four experimental groups. Each point represents one participant, and thick lines represent subgroup-level linear regression models.

Figure 7. Relationship between individual intensity slope across 16 sessions and pre-to-post change in predicted VO2max (ΔVO2max) in the four experimental groups. Each point represents one participant, and thick lines represent subgroup-level linear regression models.