How it works
You repeat efforts in Zone 5, at or near the intensity that elicits VO2max, then recover and repeat. Intervals accumulate far more time near VO2max than one all-out effort. The result is a bigger maximal oxygen uptake.
How much you actually need
HIIT is the small hard portion of a polarized or pyramidal week, typically one to two sessions on top of a large easy base. The fatigue cost is steep. More hard sessions lead to stagnation, burnout or injury.
How to apply it
Two reliable formats: the Norwegian 4x4 (four efforts of about four minutes near VO2max, three minutes easy between) and 30/15s (30 seconds hard, 15 seconds easy, in blocks). Do HIIT fresh, after a thorough warm-up, with easy days around it.
FAQ
Is HIIT better than long, slow distance for endurance?
Neither replaces the other. Easy aerobic volume builds the base and lets you train often. HIIT sharpens the top end and VO2max. The best plans combine a large easy base with a small dose of high intensity.
How often should I do HIIT?
One to two HIIT sessions per week is plenty for most endurance athletes, with the rest of the week easy. More rarely adds fitness and usually adds fatigue. Let recovery and sleep decide.
What is the Norwegian 4x4 session?
The Norwegian 4x4 is four intervals of about four minutes at a hard, near-VO2max effort, separated by roughly three minutes of easy recovery. Four minutes drives oxygen uptake toward its maximum, and the recovery lets you repeat that quality.