How strong is the evidence for HIIT?

High-intensity interval training (HIIT) is supported by several meta-analyses and randomized controlled trials showing improvements in cardiorespiratory fitness and body composition, including in clinical populations such as people with rheumatoid arthritis, stroke survivors, and adolescents with obesity. That evidence is currently rated strong (75/100), built on 6 published studies, including 4 randomized controlled trials and 2 meta-analyses -- enough for real confidence in the effect.