OBJECTIVES: To examine the effects of different protocols of high-intensity interval training (HIIT) on VO2max improvements in healthy, overweight/obese and athletic adults, based on the classifications of work intervals, session volumes and training periods. DESIGN: Systematic review and meta-analysis. METHODS: PubMed, Scopus, Medline, and Web of Science databases were searched up to April 2018. Inclusion criteria were randomised controlled trials; healthy, overweight/obese or athletic adults; examined pre- and post-training VO2max/peak; HIIT in comparison to control or moderate intensity continuous training (MICT) groups. RESULTS: Fifty-three studies met the eligibility criteria. Overall, the degree of change in VO2max induced by HIIT varied by populations (SMD 0.41 to 1.81, p < 0.05). When compared to control groups, even short-intervals (<= 30s), low-volume (<= 5 min) and short-term HIIT (<= 4 weeks) elicited clear beneficial effects (SMD 0.79 to 1.65, p < 0.05) on VO2max/peak. However, long-interval (>= 2 min), high-volume (>= 15 min) and moderate to long-term (>= 4 to 12 weeks) HIIT displayed significantly larger effects on VO2max (SMD 0.50 to 2.48, p < 0.05). When compared to MICT, only long-interval (>= 2 min), high-volume (>= 15 min) and moderate to long-term (>= 4 to 12 weeks) HIIT showed beneficial effects (SMD 0.65 to 1.07, p < 0.05). CONCLUSIONS: Short-intervals (<= 30s), low-volume (<= 5 min) and short-term (<= 4 weeks) HIIT represent effective and time-efficient strategies for developing VO2max, especially for the general population. To maximize the training effects on VO2max, long-interval (>= 2 min), high-volume (>= 15 min) and moderate to long-term (>= 4 to 12 weeks) HIIT are recommended.

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