HIIT vs. Steady-State: What the Latest Meta-Analysis Says

Dr. Martin Gibala's Exercise Metabolism Research Group at McMaster University has spent two decades studying interval training. A 2026 meta-analysis published in the British Journal of Sports Medicine — co-authored by Gibala and researchers from 12 institutions — pooled data from 54 randomized controlled trials comparing high-intensity interval training to moderate-intensity continuous training. The conclusion is nuanced: HIIT is more time-efficient but not categorically superior.

For VO2 max improvement, HIIT outperformed steady-state by a small but significant margin. The pooled effect size was 0.28, meaning HIIT produced roughly 3-5% greater VO2 max gains when training volume was matched. When total time was matched — not volume — the gap widened.

For fat loss, the difference was negligible. Both modalities produced nearly identical reductions in body fat percentage when caloric expenditure was equivalent. The old claim that HIIT burns more fat through EPOC is technically true but practically irrelevant — the excess amounts to about 50 calories.

For muscle preservation, steady-state had a slight edge. High-intensity intervals above 90% of max heart rate produced measurably higher cortisol responses, which over time can impair recovery and muscle protein synthesis. Recreational athletes doing more than three HIIT sessions per week showed signs of overreaching.

The practical recommendation: do both. Two HIIT sessions per week produce the biggest VO2 max gains. Fill the remaining cardiovascular work with steady-state at 60-70% of max heart rate — zone 2 — which builds mitochondrial density without significant recovery cost. This polarized approach is what most elite endurance coaches already prescribe.

If you only have three hours per week for cardio, two sessions of 20-minute HIIT and one session of 45-minute zone 2 is a near-optimal split. The HIIT drives adaptation. The zone 2 builds the aerobic base that lets you recover between intervals.