Abstract
High-intensity interval training (HIIT) has emerged as one of the most widely studied exercise modalities of the past two decades. This systematic review synthesizes findings from peer-reviewed trials published between 2010 and 2024 to evaluate the cardiovascular adaptations associated with HIIT protocols in healthy and at-risk adult populations, comparing outcomes with moderate-intensity continuous training (MICT).
Introduction
Cardiovascular disease remains a leading cause of mortality globally. Exercise prescription has long served as a cornerstone of both prevention and rehabilitation. While MICT has historically dominated clinical guidelines, the growing body of literature on HIIT presents a compelling case for reconsidering optimal training intensity for cardiovascular health outcomes.
HIIT involves repeated bouts of vigorous effort — typically at 80–95% of maximal heart rate — interspersed with active or passive recovery periods. This structure imposes unique physiological demands that appear to elicit distinct adaptive responses compared to steady-state exercise.
Methods
Studies were identified through searches of PubMed, SPORTDiscus, and Web of Science databases. Inclusion criteria required randomized controlled trial design, adult participants (18–65 years), intervention duration of at least 8 weeks, and documented measurement of at least one cardiovascular outcome variable (VO₂max, resting heart rate, stroke volume, or arterial stiffness).
Key Findings
VO₂max Improvements
Across eligible trials, HIIT consistently produced equal or superior improvements in maximal oxygen uptake (VO₂max) compared to volume-matched MICT programs. Several studies reported mean VO₂max gains of 8–15% following 12-week HIIT interventions, with shorter time commitments per session.
Cardiac Structural Changes
Evidence suggests HIIT promotes greater increases in left ventricular stroke volume and cardiac output at peak exercise. These structural adaptations are indicative of enhanced cardiac efficiency and are particularly relevant for clinical populations where exercise tolerance is limited.
Arterial Stiffness and Blood Pressure
Findings on arterial stiffness were more mixed. While some trials demonstrated significant reductions following HIIT, others reported no significant difference versus MICT. Resting blood pressure reductions were broadly comparable between modalities, though HIIT appeared to confer modest additional benefit in hypertensive subgroups.
Discussion
The mechanisms underlying HIIT-driven cardiovascular adaptation likely involve repeated ischemic preconditioning, amplified shear stress responses, and greater sympathetic nervous system engagement. These stimuli appear to upregulate endothelial nitric oxide synthase (eNOS) expression and improve vascular reactivity beyond what is typically achieved through MICT.
It is important to note that HIIT is not uniformly appropriate. Individuals with pre-existing cardiac conditions should undergo medical screening prior to commencing high-intensity programs. Adherence rates in real-world settings may also differ from controlled trial conditions.
Conclusion
The current body of evidence supports HIIT as a time-efficient and effective strategy for improving cardiovascular fitness in adults. Future research should prioritize long-term adherence data, dose-response relationships, and outcomes in older or clinically complex populations. Standardization of HIIT protocols across studies remains a key methodological priority.
Keywords
HIIT, cardiovascular adaptation, VO₂max, exercise training, MICT, cardiac function, arterial stiffness