Optimising High-Intensity Interval Training to Enhance V̇O₂max in Endurance Trained Adults: A Systematic Review and Bayesian Network Meta-Analysis of Randomised Trials

Abstract

Introduction: Maximal oxygen uptake (V̇O₂max) is a key determinant of endurance performance. Numerous training strategies have been explored to enhance V̇O₂max, with high-intensity interval training (HIIT) demonstrating greater effectiveness than moderate-intensity continuous training (MICT). Despite its advantages, HIIT encompasses a wide range of work-rest combinations, intensities and durations, and the specific protocol(s) that yields the greatest improvement in V̇O₂max remains unclear. Aims: To compare and probabilistically rank distinct HIIT protocols by how effective they were at enhancing V̇O₂max in endurance-trained adults. Methods: A systematic review and Bayesian random-effects network meta-analysis (NMA) of randomised controlled trials was conducted. Eligible studies included endurance-trained adults and prescribed 2–3 training sessions per week for 4–12 weeks. Interventions were grouped into network nodes by intensity domain/work-to-rest ratio, work-interval duration, and total accumulated work-interval duration. The primary outcome was mean difference in ΔV̇O₂max (mL/kg/min) from pre to post-intervention versus each study’s pre-specified baseline comparator (typically MICT). Surface Under the Cumulative Ranking (SUCRA) values were used to rank protocols. Results: Nineteen randomised trials comprising 42 study arms and 410 participants met the eligibility criteria and were synthesised in the NMA. Across the network, the majority of HIIT protocols elicited larger gains in ΔV̇O₂max than MICT. The highest ranking intervention consisted of very-short work intervals (<60 s) performed at the severe–extreme boundary with work:rest ≥1:1 and a moderate total accumulated work-interval duration (10–20 min). This ranking was robust to meta-regressions on baseline V̇O₂max, training frequency, and study duration. However, three protocols formed a high-ranking cluster with clear effects versus MICT: very-short severe-extreme intervals, long severe intervals, and medium severe intervals — all with work:rest ≥1:1. Conclusion: In endurance-trained adults, HIIT resulted in greater improvements in V̇O₂max than MICT. While very-short intervals at the severe–extreme boundary ranked highest, three protocols formed a high-ranking cluster — suggesting multiple HIIT configurations can effectively enhance V̇O₂max when work:rest ratios are ≥1:1 and sufficient work is accumulated within the severe domain.