How it works
You repeat efforts in Zone 5, at or near the intensity that elicits VO2max, holding that high oxygen demand long enough to stress the system, then recover and repeat. Spreading the work into intervals lets you accumulate far more time near VO2max than a single all-out effort would allow. The adaptation raises your aerobic ceiling: a bigger maximal oxygen uptake and stronger high-end fitness.
How much you actually need
HIIT is the small hard portion of a polarized or pyramidal week, typically one to two quality sessions, sitting on top of a large base of easy aerobic volume. It is a high-dose stimulus, not a daily diet. Because the fatigue cost is steep, piling on more hard sessions is the fastest route to stagnation, burnout, or injury rather than to faster progress.
How to apply it
Two reliable formats are the Norwegian 4x4 (four efforts of about four minutes near VO2max, each followed by roughly three minutes of easy recovery) and short 30/15s (30 seconds hard, 15 seconds easy, repeated in blocks). Slot HIIT after a thorough warm-up, when you are fresh rather than already fatigued, and surround it with easy or rest days. Treat the easy days as part of the workout: the adaptation happens during recovery, not during the intervals.
FAQ
Is HIIT better than long, slow distance for endurance?
Neither replaces the other. Easy aerobic volume builds the durable base and lets you train often, while HIIT sharpens the top end and VO2max. The best endurance plans combine a large easy base with a small, well-placed dose of high intensity.
How often should I do HIIT?
For most endurance athletes, one to two HIIT sessions per week is plenty, with the rest of the week kept easy. Doing more rarely adds fitness and usually just adds fatigue. Let your recovery, sleep, and motivation guide whether to push or hold back.
What is the Norwegian 4x4 session?
It is four intervals of about four minutes each at a hard, near-VO2max effort, separated by roughly three minutes of easy recovery. The four-minute duration is long enough to drive oxygen uptake toward its maximum, and the recovery lets you repeat that quality across all four reps.