High-Intensity Interval Training (HIIT) vs. Moderate-Intensity Continuous Training (MICT) in the Optimization of Cardiorespiratory Fitness (VO2max)

Abstract

Introduction: Physical inactivity is a primary risk factor for mortality and chronic diseases, with cardiorespiratory fitness (VO2max) being a robust health predictor. Given the low adherence to traditional guidelines, High-Intensity Interval Training (HIIT) emerges as a time-efficient method. Objective: The objective of this review is to compare the efficacy of HIIT versus Moderate-Intensity Continuous Training (MICT) in optimizing $\text{VO}_2\text{max}$ across different age groups and levels of evidence. Results: Physiological evidence indicates that HIIT generates a more potent metabolic signal, resulting in superior mitochondrial biogenesis when volume is matched. Systematic reviews and umbrella reviews in adults consistently demonstrate that HIIT is superior to MICT in increasing VO2max, particularly in older adults. However, randomized controlled trials (RCTs) in young, healthy adults have sometimes found an equivalence inVO2max, though MICT may be superior for secondary outcomes, such as systolic blood pressure reduction. Conclusion: HIIT is a robust and, generally, superior stimulus to MICT for enhancing VO2max, supporting its inclusion in exercise guidelines. Nevertheless, it is crucial to note that primary RCTs often exhibit methodological limitations related to small sample sizes and analyses of highly specific groups. Furthermore, the literature is marked by significant heterogeneity regarding the ideal HIIT protocols, the precise definition of which falls outside the scope of this review.