Last time I said Zone 3 was "not hard enough to push your ceiling." That framing needs a correction. It's true from an efficiency standpoint. But it's not the full picture.

What vigorous exercise actually does

The epidemiological record here is clear. Arem et al. (2015) followed 661,137 adults and found that any vigorous physical activity — Zone 3 and above — independently reduces all-cause mortality by 20-37%. Gebel et al. (2015) analyzed 204,542 adults and found a dose-response: the higher the proportion of vigorous activity in your total volume, the lower your mortality risk.

These studies don't distinguish between Zone 3, 4, and 5. They measure vigorous activity as a category.

If you've been doing group fitness classes three times a week, you've been doing something real. Your spin class isn't wasted. It's reducing your mortality risk.

Where polarized training comes in

So why does the polarized model matter?

Stoggl and Sperlich (2014) compared four training structures in trained athletes over nine weeks. The polarized group — roughly 80% low intensity, 20% high intensity — improved VO2max by 11.7%. The threshold group, spending more time in Zone 3, improved by 3.9%. Same weekly hours. Very different return.

The mechanism: Zone 2 builds the aerobic foundation (mitochondrial density, fat oxidation, cardiac output). Zone 4-5 forces the cardiovascular ceiling upward. Zone 3 stimulates both pathways, but neither as effectively per hour as dedicated work in each zone.

For someone with 5-6 hours a week, that efficiency gap matters. It's the difference between a protocol that moves your VO2max and one that maintains it.

The professional's framing

Here's what I want to be honest about: the Stoggl research studied well-trained athletes averaging 10-20 hours of training per week. At that volume, the fatigue-versus-adaptation tradeoff becomes real — too much Zone 3 accumulates fatigue without sufficient low-intensity recovery. At 5-6 hours a week, that tradeoff barely exists.

The case for polarized training at our volume isn't about avoiding fatigue. It's simpler: you have limited hours, and structured sessions return more than undifferentiated ones. If Monday is easy, make it genuinely easy. If Thursday is hard, make it hard. Defaulting to moderate for everything is leaving adaptation on the table.

My own training ran at one speed for years: moderate-hard on most days, which felt productive. Structuring sessions to be distinctly easy or distinctly hard improved my results without adding time. The sessions felt different from each other, and the adaptations followed.

What this looks like in practice

Two sessions a week, go genuinely hard. Zone 4-5. Can't-talk hard. Not a spin class at 80% — actual intervals where the last minute is a fight. One to two of those per week is enough. More is not better here.

Everything else: go easy, go long. The app targets 150 minutes a week as a minimum. Structured sessions are the optimal case — but a long walk with the kids, an uphill hike, taking the stairs counts too. Improvise when life doesn't cooperate. Zone 2 responds to volume, not perfection.

That's 14 minutes of work. It won't replace a full 4x4 session, but it counts.

Closing the cardio series

This is where the four cardiovascular foundation newsletters connect:

• ​Zone 2: the aerobic base and why most people undertrain it​
• ​VO2max: the number that predicts how long you'll live​
• ​The 4x4 protocol: the interval format with the strongest evidence​

The same hours, structured deliberately, produce different outcomes. That's the case for the 25 percent.

From the archive

New here? The Beginner's Guide to Longevity Training covers the full framework in one place.

One question

When you look at last week's cardio: how much was genuinely easy, how much was genuinely hard, and how much landed somewhere in the middle?

Alessandro