Hans-Peter Wiesinger ^1,2,3* Thomas L. Stöggl ⁴ Nils Haller ^1,5 Julia Blumkaitis ¹ Tilmann Strepp ¹ Francesca Kilzer ¹ Anna Schmuttermair ⁴ Will Hopkins ⁶

• ¹ Department of Sport and Exercise Science, University of Salzburg, Salzburg, Salzburg, Austria
• ² Institute of Nursing Science and Practice, Center for Public Health and Healthcare Research, Paracelsus Medical University, Salzburg, Austria
• ³ Institute of General Practice, Family Medicine and Preventive Medicine, Center for Public Health and Healthcare Research, Paracelsus Medical University, Salzburg, Austria
• ⁴ Red Bull Athlete Performance Center, Thalgau, Austria
• ⁵ Department of Sports Medicine, Rehabilitation and Disease Prevention, Johannes Gutenberg University Mainz, Mainz, Rhineland-Palatinate, Germany
• ⁶ Internet Society for Sport Science, Auckland, New Zealand

Introduction: Meta-analysts have found that high-intensity interval training (HIIT) improves physical performance, but limited evidence exists for its effects on highly trained athletes, for measures beyond maximum oxygen uptake (V ̇O2max), and for moderating effects of different types of HIIT. Here, we present meta-analyses of the effects of HIIT focused on these deficits.Methods: Effects of six types of HIIT and other moderators from 34 studies of highly trained endurance and elite other athletes were derived in percent units via log-transformation from separate meta-regression mixed models for sprint, time-trial, aerobic/anaerobic threshold and peak speed/power, and for repeated-sprint ability, V ̇O2max, and exercise economy. Level of evidence for effect magnitudes was evaluated via effect uncertainty and a smallest important change of 1%. Results: Compared with control training, HIIT showed good to excellent evidence for substantial enhancement of most measures for some athlete subgroups in practically important study settings defined by effect moderators (maximum of 12.6%, for endurance females after six weeks of aerobic traditional long intervals). Assessment of the moderators indicated good evidence of greater effects, as follows: with more aerobic types of HIIT for V ̇O2max (+2.6%); with HIIT added to conventional training for most measures (+1.1 to 2.3%); during the competition phase for V ̇O2max (+4.3%); and with tests of longer duration for sprint (+5.5%) and time-trial (+4.9%). Sex and type of athlete were unclear moderators. Heterogeneity of HIIT effects within a given type of setting varied from small to moderate (standard deviations of 1.1 to 2.3%) and reduced the evidence of benefit in some settings.Although athletes in some settings can be confident of beneficial effects of HIIT on some measures related to competition performance, further research is needed. There is uncertainty in the mean effects on exercise economy and in the modifying effects of sex, duration of intervention, phase of training, and type of HIIT with most measures.