HIIT: what intervals do, the 4×4 protocol, and how to use it without burning out

What it means

High-intensity interval training alternates efforts at 85–95% of maximum heart rate (or above) with easier recovery periods. The best-studied format is the Norwegian 4×4: four minutes hard, three minutes easy, four times, after a warm-up. Shorter formats (30 seconds on, 30 off; Tabata; sprint intervals) exist and work, with more muscular and less cardiac emphasis as the efforts get shorter.

Where zone 2 trains the muscle's ability to use oxygen, intervals train the heart's ability to pump it — stroke volume — and recruit the fast-twitch fibres that steady work never reaches.

Why it matters for longevity

VO2max is the strongest single predictor of lifespan, and intervals raise it faster than any other training. In older adults they also reverse part of the age-related decline in mitochondrial function. The cost is recovery: hard sessions raise cortisol and inflammation for 24–48 hours, which limits how many a week are useful.

What the evidence says

Helgerud 2007 compared 4×4 intervals with steady training at matched work and found about twice the VO2max gain (7.2% versus 3–4%) in eight weeks. Robinson 2017 (Cell Metabolism) gave older adults twelve weeks of intervals and found a 69% rise in mitochondrial respiratory capacity and a reversal of age-related changes in muscle gene expression — larger effects than in the young.

The Generation 100 trial (Stensvold 2020, 1,567 adults aged 70–77, five years) compared intervals, moderate training and advice: all-cause mortality was low in every group and about 37% lower in the interval group than the moderate group, though the difference did not reach statistical significance. Meta-analyses show HIIT is safe in cardiac rehabilitation when supervised.

How to measure or use it

• The 4×4: warm up 10 minutes, then four rounds of four minutes at an effort you could hold for about five, three minutes easy between, cool down. Heart rate should reach 85–95% of max by the end of each effort.
• Dose: one session a week on a zone-2 base; two for the well-recovered. More than two usually shows up as a falling HRV and stalled progress.
• Track VO2max every six months and resting heart rate weekly; a rising resting rate means too much.

Related

• VO2max and longevity — the weekly structure
• Mitochondria and energy — the Robinson trial in detail
• The recomposition protocol — intervals inside a fat-loss plan