Time-Course of Physiological Adaptations to High-Intensity Interval Training-Based Cardiac Rehabilitation After Myocardial Infarction

Background: High-intensity interval training (HIIT) is increasingly used in exercise-based cardiac rehabilitation (ebCR) after myocardial infarction (MI), yet the temporal sequence of physiological, cardiac, biochemical, and functional adaptations remains incompletely characterized. Methods: Stable post-STEMI (ST-segment elevation myocardial infarction, MI-group) and previously inactive participants without known cardiovascular, metabolic or systemic disease (CTRL group) completed 12-week supervised outpatient HIIT (4 × 4 min intervals at 85-90% HRpeak (peak heart rate), ~80-90% of VO2peak, 3 sessions/week). Assessments were performed at baseline (T1), 4 (T2), 8 (T3), and 12 weeks (T4), including cardiopulmonary exercise testing (CPET), echocardiography, blood biomarkers, body composition, six-minute walk test (6MWT), and RAND-36. Longitudinal changes were analyzed using Friedman tests with Dunn post hoc comparisons; between-group differences used Mann-Whitney U tests with Holm correction. Results: VO2peak increased significantly in both groups (p < 0.001), increasing by ~22% from T1 to T4 in MI (median 20.1 to 24.5 mL·kg-1·min-1) and ~23% from T1 to T4 in CTRL (median 22.3 to 27.6 mL·kg-1·min-1). LVEF (left ventricular ejection fraction) improved early in MI, increasing from 52.5% (50.0-55.0) at T1 to 57.5% (55.2-58.7) at T2 and up to 60% (55.8-60.0) at T4 (all p < 0.001), while LV dimensions remained stable. NT-proBNP (N-terminal pro-B-type natriuretic peptide) showed no significant longitudinal change (p = 0.510), and CRP (C-reactive protein) decreased from 2.1 to 0.7 mg·L-1 (p = 0.008) in MI. Both groups improved body fat % and 6MWT distance (p < 0.001). Conclusions: In low-risk stable post-STEMI patients, longitudinal changes during supervised HIIT-based ebCR were consistent with improved VO2peak and LVEF, without clinically relevant increases in cardiac stress biomarkers. However, due to the observational design and absence of clinical comparator groups, these findings should be interpreted as descriptive and support further evaluation in larger randomized studies.