One Size Does Not Fit All: A Meta-Analysis of 115 Trials Comparing High-Intensity Interval and...

This study compares the effects of high-intensity interval training (HIIT) and moderate-to-vigorous-intensity continuous training (MVICT) on physiological adaptations and physical performance across a broad population, from the general public to athletes. Additionally, it also explores how participant characteristics (e.g., sex, age, and training status) and training protocol parameters (e.g., mode, interval type, and intensity) influence the comparison. Following PRISMA guidelines, a systematic search was conducted in PubMed, Web of Science, and CNKI databases, completed on September 21, 2024. Eligible studies were randomized controlled trials comparing the chronic effects of HIIT and MVICT. A three-level meta-analysis was employed to calculate standardized mean differences (SMD, Hedge's g), with subgroup analyses and meta-regression used to examine potential moderators of any observed effects. A total of 115 studies involving 3196 participants were included, with a mean age range from 8 to 68 years, spanning populations from untrained sedentary individuals to elite/world-class athletes. Compared to MVICT, HIIT demonstrated significantly superior improvements in relative maximal oxygen uptake (SD = 1.30 mL·kg^-1·min^-1, g = 0.39, 95% CI [0.27, 0.51]), absolute maximal oxygen uptake (SD = 0.09 L·min^-1, g = 0.29, 95% CI [0.15, 0.43]), maximal aerobic power/speed (g = 0.31, 95% CI [0.17, 0.47]), and mean anaerobic power (g = 0.47, 95% CI [0.08, 0.86]). No significant differences were observed between HIIT and MVICT in peak anaerobic power (g = 0.31, 95% CI [-0.06, 0.68]), first intensity thresholds (g = 0.43, 95% CI [-0.38, 1.25]), second intensity threshold (g = 0.06, 95% CI [-0.25, 0.36]), exercise economy (g = 0.26, 95% CI [-0.03, 0.54]), and on indices of physical performance (g = 0.04, 95% CI [-0.46, 0.54]). Subgroup analyses revealed that training status (6-tiered participant classification framework), age, sex, interval type, and exercise mode significantly moderated the effect. Specifically, compared to MVICT, HIIT demonstrated greater improvements in maximal oxygen uptake among individuals at Tier 0 (inactive; g = 0.34), Tier 1 (recreationally active; g = 0.57), and Tier 3 (elite/national; g = 0.83), in males (g = 0.43) and mixed-sex populations (g = 0.42), using short-interval (g = 0.55) or long-interval HIIT (g = 0.57), and with rowing (g = 0.71), running (g = 0.53), or cycling (g = 0.29) as the training modes. Compared to MVICT, HIIT offers superior benefits for improving maximal oxygen uptake and anaerobic capacity, whereas both modalities show comparable outcomes for intensity thresholds, exercise economy, and physical performance. The relative superiority of HIIT compared to MVICT is influenced by participant characteristics (e.g., training background, age, and sex) and by the characteristics of the HIIT protocol.