High-Intensity Intervals
Short threshold-or-above work, once a week. Pushes VO₂ max where zone 2 alone can't.
What it does
The high end of the aerobic system adapts to a stimulus zone 2 doesn't reach. Ten to twenty minutes of threshold-or-above work, once a week, captures most of the available top-end gain — as a layer on the aerobic base, not the whole program.
Supra-threshold work drives adaptations in stroke volume, capillary density, and mitochondrial enzymes that submaximal training doesn't fully reach; the cardiac-output ceiling rises with consistent exposure.
What the evidence shows
Overall evidence strength: High certainty
Who it's for, who should skip
Anyone with an aerobic base wanting to raise VO₂ max efficiently. Format matters little — what matters is that heart rate gets uncomfortable and stays there. Once a week is enough; more accumulates stress fast.
Measuring it
Retest VO₂ max — No guideline interval — retest after a significant training block..
Stacking & alternatives
Access & cost
Self-directed — no purchase required.
Last reviewed 27 August 2026.