Last time, I shared my VO2max number: 33.84. Below the 50th percentile for my age. Poor on my Garmin. Now the question that matters: how do I raise it?

The answer has been studied for decades. And one protocol has the strongest evidence behind it.

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The Norwegian 4x4

Four intervals. Four minutes each. At 90-95% of your max heart rate. Three minutes of active recovery between intervals at around 70% HRmax. Ten minutes of warm-up, a few minutes of cool-down. Total session: about 40 minutes.

That’s the protocol.

In 2007, Helgerud and colleagues at the Norwegian University of Science and Technology tested four different training methods head-to-head. Same total work output across all groups. Eight weeks. Three sessions per week.

The results:

• 4x4 intervals: +7.2% VO2max
• 15/15 intervals (15 seconds on, 15 seconds off): +5.5%
• Lactate threshold training (85% HRmax): +2.0%
• Long slow distance (70% HRmax, 45 minutes): -0.6%

Read that last one again. The long slow distance group went backwards. Same total work as the 4x4 group. Radically different results.

Then there’s the Generation 100 study: 1,567 Norwegian adults aged 70-77, followed for five years. The group doing 4x4 intervals had a 3.0% mortality rate. The moderate-intensity group: 5.9%. The difference didn’t reach statistical significance, but the trend is hard to ignore.

The 4x4 works. It’s one of the most studied, most repeatable interval formats in the literature.

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The principle matters more than the format

Here’s what I want to be clear about: the 4x4 is one of the most studied protocols. But it’s not the only one that works.

The principle behind it is what drives the adaptation: hit 90-95% of your max heart rate, sustain it long enough, recover, repeat. Whether you do that on a Wattbike, a treadmill at incline, a rowing machine, or running hills, your cardiovascular system doesn’t care about the label on the workout.

Helgerud’s own 15/15 group improved VO2max by 5.5%. A different format, and statistically not significantly different from 4x4. Both high-intensity protocols crushed the moderate and low-intensity groups.

Your Zwift workout, your gym’s stairmaster, a hill sprint session: if you’re hitting the right intensity and recovering between efforts, you’re getting the adaptation.

I’ve been doing structured intervals on the bike — different plans, different formats. Not the 4x4 specifically. But the same principle applies: push the intensity, recover, repeat.

The 4x4 is what I’m building toward as I shift to more interval-focused training. But I’m not going to pretend I started there.

Fun matters too. Variance keeps you on the bike. And consistency is what actually produces results over months and years.

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How it fits into your week

The polarized training model: roughly 80% of your training at low intensity (Zone 2), 20% at high intensity.

In practice, that looks like:

• 3–4 Zone 2 sessions (45-60+ minutes each, at least 150+ minutes)
• 1–2 interval sessions per week

Two interval sessions per week means about 32 minutes of work at high intensity. That’s it. The rest is Zone 2.

Allow 48-72 hours between interval sessions. Your cardiovascular system needs recovery time to adapt.

And a critical point: don’t replace Zone 2 with intervals. They serve different purposes. Zone 2 builds the base. Intervals push the ceiling. You need both.

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Start here

If you’re currently only doing Zone 2, add one interval session per week.

• Week 1-2: One session. Start shorter if needed: 3x4 instead of 4x4, or a 30-minute structured workout from whatever app you use.
• Week 3-4: Full session length, still once per week.
• Week 5+: Add a second session if your recovery allows it.

The intervals should feel hard. If they don’t, you’re not in the right zone. At 90-95% HRmax, holding a conversation is not possible. That’s the point.

One common misconception: a hard spin class or boutique cycling session that keeps you at 80-85% HRmax for 45 minutes is neither Zone 2 nor VO2max training. It’s the grey zone between them — too hard for aerobic base building, not hard enough to push your ceiling. More on this next time.

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What’s next

Zone 2 builds the base. VO2max training pushes the ceiling.

Next time: why intensity has a multiplier effect that most people underestimate, and what the research says about vigorous exercise and longevity.

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From the archive

New here? Start with The Beginner’s Guide to Longevity Training.

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One ask

What interval protocol are you using? Or what’s stopping you from starting? Reply and tell me.

Alessandro