Last time we covered Zone 2: the base. The long, boring sessions that build your aerobic engine.

Now: the ceiling.

Why VO2max matters more than any other fitness metric

In 2018, Mandsager and colleagues published a study tracking 122,007 patients over two decades. They measured cardiorespiratory fitness and followed who lived and who didn’t.

The findings were stark. People in the bottom 25% of fitness had a mortality risk comparable to smoking. Those in the top 2.3%? 80% lower all-cause mortality. And here’s what makes this study different from most: they found no upper limit to benefit. The fitter you are, the longer you live. They found no plateau. No diminishing returns.

This study is why the app is called the25percent. The single largest drop in mortality risk comes from moving out of that bottom quartile.

You’ve been building the base with Zone 2. Now you need to know where your ceiling is.

I tested mine

Ten weeks into my training program, I did a Wattbike submax test. You ride at progressively harder stages while the bike estimates your oxygen uptake. No lab required, no mask on your face.

My result: 33.84 ml/kg/min.

For men aged 40-49, the 50th percentile among healthy adults is around 38 ml/kg/min (FRIEND registry - the gold standard for directly measured fitness data). I’m below that. My Garmin confirms it: Poor.

Knowing the number changed how I train. Before the test, I had a vague sense that I “should improve my cardio.” I’ve had asthma since birth. I hate cardio. But it is what it is.

After the test, I had a concrete target: get above 38, then push toward 45 – the 75th percentile for my age group. Peter Attia’s longevity framework goes further: aim for elite fitness of a person one decade younger. That’s a long road from 33.84. But you can’t close a gap you haven’t measured.

The base-building is working. My power output on the bike improved 6.9% in those ten weeks. The foundation is getting stronger. But 33.84 tells me I have a long way to go.

How to test yours

Before you can push the ceiling, you need to know where it is. Three options, from most accurate to most convenient:

Bike submax test (what I use)

A Wattbike or similar smart trainer runs you through staged efforts and estimates VO2max from your power output and heart rate response. No maximal effort required. Repeatable. Takes about 15 minutes.

If you have access to a Wattbike at your gym, this is the one I’d recommend.

Cooper 12-Minute Run Test

Run as far as you can in 12 minutes. That’s it. Use a track or a flat, measured route. Then convert your distance:

VO2max = (distance in meters - 504.9) / 44.73

Well-validated against lab testing. Free, no equipment, repeatable. The catch: it requires genuine maximal effort. If you held back, your number will be too low.

Wearables (Apple Watch, Garmin)

Your watch probably already shows a VO2max estimate. Use it for tracking trends over time, not for absolute values. Studies show Apple Watch underestimates by about 6 ml/kg/min on average, with a mean error of 10-13%. Garmin is better for consistency but still off on the actual number.

Direction matters more than precision here. If your watch shows your VO2max climbing month over month, that’s real signal.

Where do you land?

Find your age group. These categories come from the Cooper Institute - the same classification your Garmin uses. Apple Watch uses similar ranges from the FRIEND registry.

Men (ml/kg/min)

Women (ml/kg/min)

Source: The Cooper Institute (via Garmin). Values show lower threshold for each category.

Don’t like where you are? Good. That’s the starting point.

What’s next

Now you know the number. Next time: the protocol to raise it.

I’ll break down the Norwegian 4x4 interval method, one of the most validated VO2max protocols in the research. Four minutes of near-maximal effort, three minutes of recovery. Repeat four times. It’s not pleasant. It works.

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From the archive

New here? Start with The Beginner’s Guide to Longevity Training for the full framework behind everything we cover in this newsletter.

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One ask

Do you know your VO2max? Have you ever tested it? Reply and tell me. I read every response.

Alessandro