The Norwegian 4×4: the interval session everyone is suddenly doing
Four intervals of four minutes hard, three minutes easy between them, twice a week. The Norwegian 4×4 has spent two decades in exercise-physiology labs and about two years in everyone's training plan, and the reason is simple: it is the most reliable way anyone has found to move VO₂ max in a hurry.
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What the session actually is
Warm up for ten minutes. Then four bouts of four minutes at a hard but sustainable effort — roughly 85–95% of maximum heart rate, the pace where talking is down to a word or two and you are counting down the clock. Three minutes of easy work between each. Cool down. The whole thing takes about forty minutes and can be run, cycled, rowed or done uphill; the protocol cares about the effort, not the mode.
Why it took over
Two things collided. The Norwegian group's trials kept showing the 4×4 producing larger VO₂ max gains than steady moderate work of the same duration — the four-minute bout is long enough to drive the heart to near-maximal output and repeated enough to hold it there. Then VO₂ max stopped being an athlete's statistic and became a longevity one: it is among the strongest single predictors of all-cause mortality in the epidemiology, which moved the session from the track to the general-interest podcast.
What it costs
This is the part the enthusiasm skips. A true 4×4 is a hard session, and hard sessions are withdrawals. Expect the night after to show it — resting heart rate a little higher, heart-rate variability a little lower than your own normal, and sometimes a shallower overnight fall. That is the training stimulus, not damage, and it should resolve in a day or two. What matters is whether it does resolve. Two 4×4s a week on top of everything else is where people quietly dig a hole.
How to know it is working
VO₂ max moves slowly — think six to twelve weeks, not a fortnight — so the session-to-session evidence has to come from somewhere else. The useful signal is your recovery pattern: an adaptation shows up as the same session costing you less over time, not as a heroic number on the day. If your resting heart rate is trending up across weeks rather than bouncing back, the dose is too high for the rest of your life right now.
Watching it on your own baseline
Vitra reads your ring on your own machine and holds a cardio-capacity estimate alongside your own thirty-day baselines for HRV and resting heart rate, so the question "is this session still costing me what it used to?" has an answer rather than a feeling. Tag your 4×4 days and the tag correlations will tell you what those days actually do to the night that follows — measured against your normal, not against a population average.
Frequently asked questions
Pedro Thomaz builds Vitra, a desktop app that reads Oura data against your own baseline instead of a population average. He has worn a ring daily for years and reads these same numbers every morning — which is where most of what is written here comes from. Vitra is not a medical device and nothing on this blog is medical advice.
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