What is VO2 max, and why does it matter so much?

VO2 max is the ceiling on your aerobic engine. Every step in the chain that gets oxygen to working muscle feeds into it: how much air your lungs move, how much blood your heart pumps with each beat, how much oxygen that blood can carry, and how good your muscles are at pulling the oxygen out and using it.

In most healthy people the heart is the limiting factor. A bigger stroke volume (the amount of blood pumped with each beat) is a large part of what separates a fit person from an unfit one.

Because the number is expressed per kilogram, body weight matters too. Lose some body fat and your VO2 max goes up a little, even if nothing else changes. That's worth knowing, because it means two people with identical hearts can get quite different results.

A VO2 max only makes sense next to people of the same age and sex. The figures below are the median (50th percentile) treadmill results from the FRIEND registry, drawn from more than 16,000 laboratory tests of healthy US adults, in mL/kg/min. Half of people tested scored above these numbers and half below.

Sitting at the median is perfectly respectable. The research is fairly clear on direction, though: higher is better, with no point at which the benefit seems to stop, and the biggest difference in risk tends to be between the least fit people and everyone else. If your result lands in the bottom group for your age, that's the most valuable problem to fix.

The strongest evidence comes from very large groups of people followed for years.

In a Cleveland Clinic study of 122,007 patients who had treadmill tests, the risk of death linked to low fitness was comparable to, or greater than, the risk linked to smoking, diabetes and coronary artery disease. The fittest group had the lowest risk of all, and the researchers found no upper limit to the benefit.

An even larger study followed 750,302 US veterans for about ten years. The least fit had roughly four times the risk of death of the fittest, and being unfit was more strongly linked to death than smoking, diabetes or high blood pressure.

A 2009 analysis of 33 studies put a number on it. Every 1 MET of extra fitness, which is 3.5 mL/kg/min of VO2 max, was associated with a 13% lower risk of death from any cause and a 15% lower risk of heart disease and cardiovascular events. That's roughly what many inactive adults gain from a few months of consistent training.

These are observational studies, so on their own they can't prove that raising your VO2 max will add years to your life. What makes the link persuasive is how consistent it is. It shows up in study after study, it follows a dose-response pattern, and it holds for men and women, younger and older people, and across racial groups.

VO2 max falls as we get older. Across adulthood the drop averages around 10% per decade, and it speeds up later in life. One long-running study found the decline went from about 3 to 6% per decade in people's twenties and thirties to more than 20% per decade in their seventies and beyond.

That sounds bleak until you look at the other half of the picture. Training moves the whole curve up, and plenty of active people in their sixties post numbers above the average for people in their fifties.

The practical reason to care is independence. If your VO2 max falls far enough, ordinary things like a long flight of stairs, a hilly walk or carrying luggage through an airport start to use most of what you have. Building a bigger reserve in your forties and fifties is how you stay well clear of that point in your seventies and eighties.

The gold standard is a cardiopulmonary exercise test, usually on a treadmill or bike. You breathe through a mask while the machine measures the oxygen you take in and the carbon dioxide you breathe out, and the effort increases steadily until you can't continue. The hard part typically lasts around 8 to 12 minutes.

A good lab test gives you more than one number. It also shows your heart rate at the points where your body shifts between energy systems, and those heart rates become your personal training zones. That's far more precise than the "220 minus your age" formula most people use.

Smartwatches estimate VO2 max from your heart rate and pace. A 2022 review of 14 studies found that watches using workout data were close on average, but an individual reading could still be off by around 10 mL/kg/min in either direction. For most age groups, that's the gap between below average and well above it. A small 2025 study of the Apple Watch found it read about 6 mL/kg/min low on average. Watches are handy for following the trend over months, but we wouldn't lean on the absolute number, especially for decisions about your health.

The approach that works for most people has two parts: a lot of easy aerobic exercise, and a small amount of genuinely hard work.

Easy, steady exercise (Zone 2)

This is exercise at a pace where you could hold a conversation, but you'd rather not sing. Brisk walking on an incline, easy cycling, rowing and swimming all count. It should make up most of your weekly training. It builds the foundation: a stronger heart, more blood vessels feeding your muscles, and muscles that are better at using oxygen.

A little high-intensity work

Hard intervals are what push the ceiling up. The best-studied format is the "4x4" from Norway: four minutes at roughly 90 to 95% of your maximum heart rate, three minutes of easy recovery, repeated four times. In the original study, moderately fit young men doing the 4x4 three times a week improved their VO2 max by about 7% in eight weeks, while groups doing steady, moderate running saw no meaningful change. For most people, one interval session a week is plenty to start with.

A realistic week

You don't need to train like an athlete. For a busy adult, something like this is a solid starting point:

• Three easy sessions of 30 to 45 minutes, such as an incline walk, an easy bike ride or a swim
• One interval session, like the 4x4
• Two short strength sessions, because muscle is the other half of aging well

That lines up with official guidance in both the US and the UK, which recommend at least 150 minutes of moderate activity a week (or 75 minutes of vigorous activity), plus muscle strengthening on at least two days.

How quickly will you improve? In the HERITAGE Family Study, previously inactive adults who trained three times a week for 20 weeks raised their VO2 max by about 17% on average. The range was enormous. Some people barely changed or even slipped back slightly, while others improved by nearly 50%. So if a friend on the same plan improves faster than you, that's largely genetics, not a lack of effort.

One important caveat. If you have known heart disease, get chest pain or dizziness when you exert yourself, or haven't exercised in years, speak to a doctor before you start doing intervals. That's exactly when a supervised test is worth doing first.

VO2 max testing is part of our longevity assessments in Chicago and London, usually alongside a DEXA scan so we can see fitness and body composition together. We use the result to set your heart rate zones, build a training plan around Zone 2 and VO2 max, and then re-test so you can see whether the plan is working.

If you're new to this way of thinking about health, our guide to what Medicine 3.0 is explains where fitness testing fits in the bigger picture.