Optimising Exercise Prescription: A Meta-Analysis Examining the Dose Response of Exercise Duration on Cardiorespiratory Fitness Following HIIT and MICT

Background High-intensity interval training (HIIT) is often promoted as a time-efficient alternative to moderate-intensity continuous training (MICT) for improving cardiorespiratory fitness, yet the duration of HIIT sessions varies considerably across studies. Objective We aimed to characterise the dose–response relationship between exercise session duration and the improvement in cardiorespiratory fitness for HIIT and MICT. Methods A dose–response meta-analysis of randomised controlled trials comparing exercise duration in HIIT and MICT, following Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines and registered in PROSPERO (CRD42022335590). Effect sizes were calculated using a random-effects meta-analysis. The primary outcome was maximal oxygen uptake (VO 2max ). Secondary outcomes included blood pressure, lipid profiles, glucose metabolism markers and body composition measures. A one-stage random-effects dose–response meta-analysis was performed to examine the relationship between exercise duration and adaptations. We searched PubMed and Google Scholar; eligibility criteria for selecting studies were randomised controlled trials in humans, published in English and exercise interventions lasting at least 4 weeks. Results We identified 69 randomised controlled trials (2387 participants). High-intensity interval training elicited greater improvements in VO 2max than MICT (d = 0.38, 95% confidence interval 0.27–0.49, p < 0.001). High-intensity interval training demonstrated a non-linear dose–response relationship between exercise session duration and VO 2max , with 80% of maximal effect (changes in VO 2max = 3.45 mL/kg/min) achieved with only ~11 min/session (95% confidence interval 9.5–40.2). Moderate-intensity interval training showed a linear dose–response relationship between exercise session duration and VO 2max , requiring ~52 min/session to achieve 80% of the maximal observed effect (95% confidence interval 30.4–55.8). The dose–response relationship was consistent across populations. High-intensity interval training and MICT had comparable effects in improving cardiometabolic risk factors. Conclusions High-intensity interval training demonstrated a non-linear dose response, with 80% of maximal effect on VO 2max in ~11 min/session, whilst MICT required four to five times longer to reach similar responses. The different types of training had comparable effects on cardiometabolic risk factors.