How to Improve VO2 Max: A Training Block Is Worth About a Decade. Your Watch Cannot See It Happen.

Here is the whole thing in two numbers.

The average person’s fitness falls by about 4.0 every decade after their twenties. A few months of interval training adds about 5.5. [4][2]

Same units. So a decent training block is worth roughly a decade — and that is the honest, boring, extremely good news at the centre of a topic that has otherwise been turned into a longevity sales pitch.

The catch is that you cannot watch it happen. The device telling you your number misses by more than the amount you are trying to gain.

First, what the number actually is

VO2 max is the most oxygen your body can take in and use in one minute, per kilogram you weigh, when you are working as hard as you can possibly go. That is it. It is measured in millilitres of oxygen per kilogram of bodyweight per minute — written mL/kg/min — and there is no everyday American unit for it, so it stays as the researchers write it.

A proper measurement involves a mask, a treadmill or a bike, and somebody increasing the difficulty until you cannot continue. Everything else is an estimate.

So what is a good one?

That question has its own page, because the answer is a table rather than a sentence: VO2 max by age, and the three things that move your percentile before you train. It carries the American reference standard — 22,379 measured tests — for men and women by decade. [4]

The one number worth carrying into the rest of this page is the slope. Across six decades the 50th percentile — the ordinary middle — falls by an average of 13.5%, or 4.0 mL/kg/min, per decade on a treadmill. [4] That is the figure everything below is measured against.

Why anybody cares: the mortality number, and how it gets abused

This is the part that built the industry, and it is genuinely one of the largest associations in the field.

A 2024 overview in the British Journal of Sports Medicine pooled 26 systematic reviews covering 199 unique cohort studies and over 20.9 million observations. Comparing people with high fitness to people with low fitness, the hazard ratio for dying of any cause was 0.47 — which in plain terms means the high-fitness group had roughly half the risk over the follow-up period. The range of plausible values ran from 0.39 to 0.56, so “no effect” is nowhere near it. [1]

They also report a dose-response: each 1-MET step up in fitness went with an 11% to 17% lower risk of dying. A MET is just a repackaging of the same unit — one MET is 3.5 mL/kg/min. [1]

Now the sentence the sales pitches leave out. The authors’ own certainty rating for this evidence ranges from very low to moderate. [1] Not high. Twenty million observations and the confidence is still capped, because you cannot randomise people into being fit for thirty years. Every one of those cohorts compares people who are fitter with people who are not.

The multiplication that does not work

Here is the move, and once you have seen it you will see it everywhere.

Take the 11-to-17%-per-MET figure. Take your own training gain of 5 mL/kg/min, call it about 1.4 METs. Multiply. Announce that you have bought yourself some specific percentage of extra life.

That arithmetic is not licensed by the study it borrows from. The cohorts show that fitter people die later. They do not show that moving yourself from one group to the other transfers the whole association with you — because the people who are fit at 50 have usually been fit for decades, and being the kind of person whose fitness is high is bundled with a great many other things.

This is the same trap as the sauna research, where a striking Finnish number came out of 201 men and ten deaths, and the randomised trials that followed found much less. A huge observational finding tells you the association is real. It does not tell you it will move when you push on it.

Does training actually raise it? Yes, and this part is settled

This is where the evidence gets much better, because you can randomise somebody into a training programme, and people have, repeatedly.

A 2015 meta-analysis pooled 28 controlled trials, 723 people, healthy adults aged 18 to 45. Against no-exercise control groups: [2]

Endurance training added about 4.9 mL/kg/min. Interval training added about 5.5. Head to head, intervals beat steady work by about 1.2 — real, replicated, and considerably smaller than the internet’s enthusiasm for intervals would suggest. [2]

And the people who gained most were the people who started least fit. [2] Which is the opposite of how almost everything in fitness is sold, and it is the single most encouraging finding on this page.

What the training actually looked like

A 2019 meta-analysis of 53 randomised trials sorted the interval protocols by length, volume and duration to see which shape of session did most. [3]

Two findings, and they point in different directions. Even the minimal version works against doing nothing — intervals of 30 seconds or less, under 5 minutes of total work, over 4 weeks or fewer, still produced clear gains. [3]

But to actually maximise the number, the trials that did best used intervals of 2 minutes or longer, 15 minutes or more of total work in a session, sustained over 4 to 12 weeks. And that longer version was the only one that beat ordinary moderate continuous training. [3]

These are reported as standardised mean differences, which need a scale to mean anything: roughly, 0.2 is a small effect, 0.5 moderate, 0.8 and above is what researchers call large. The maximising protocols ran from 0.50 to 2.48. [3]

So the four-minute miracle workout is not a lie. It is just the floor being sold as the ceiling.

Does raising it actually help you, though?

The best available bridge between the two halves of this page is a 2019 study of 833 adults who each had two proper laboratory tests, an average of 8.6 years apart, then followed for up to about 18 years. [5]

Each 1 mL/kg/min of improvement between the two tests went with roughly 11% lower all-cause mortality. And the later test predicted death better than the earlier one — meaning where you are now matters more than where you started. [5]

That is the most useful finding here and it still is not proof. It is 833 people and 172 deaths, and it is observational: nobody was assigned to improve. People whose fitness went up over eight years were also doing other things. We wrote down how we grade this kind of evidence.

The caveat nobody selling you a programme will mention

In the HERITAGE Family Study, 481 sedentary adults from 98 families did the same supervised 20-week training programme. Same sessions. Same supervision. [6]

The average gain was about 400 mL/min. But in the researchers’ own words there was “considerable heterogeneity in responsiveness, with some individuals experiencing little or no gain, whereas others gained >1.0 l/min”. [6]

Some people did the whole programme properly and barely moved. Others gained more than twice the average. And the trait clustered in families: the study put the heritability of how much you respond at around 47%. [6]

So roughly half of your trainability was decided before you were born. If you have trained honestly and your number has not moved much, that is a documented outcome, not a character flaw. It is also the reason to judge a programme by whether you can keep doing it rather than by somebody else’s results.

And now the watch

Almost nobody reading this has had a mask on their face. The number you have is an estimate produced by a wrist device from your heart rate and your pace.

The Apple Watch Series 7 was checked against a laboratory gas analyser. The watch read 41.37 where the laboratory measured 45.88. The agreement statistic — how reliably the two tracked each other — came out at 0.47, which the authors classify as poor. Typical error was 15.79%, or 8.85 mL/kg/min. [7]

Look at that last number against the top of this page. A training block gains you about 5. The device’s typical miss is about 8.85. The measurement error is larger than the thing being measured.

That study ran nineteen people, and that matters. [7] Nineteen is small, it was one watch model, and we are not going to quote it as though it settled the category. It is the best direct validation we found, and it is thin.

There is also a fair defence of your watch that is worth stating, because leaving it out would be the same overclaiming this page is about. A device can be wrong in a consistent direction and still be useful for tracking change, because a steady offset cancels out when you compare yourself to yourself. That study measured accuracy against the laboratory. It did not test whether the watch tracks improvement over time. Nobody has shown us that it does, and nobody has shown us that it does not.

What you should not do is treat the two-decimal number on the screen as a measurement of your body, or compare it with somebody else’s watch.

What this all adds up to

Fitness is one of the few markers that is also a lever. That is what makes it unusual and it is worth being precise about, because most of the things measured about you are not. Grip strength predicts a great deal and squeezing a gripper does not fix it. VO2 max predicts a great deal and moves when you train. Those are different situations and they get discussed as though they were the same.

The practical version, with nothing added that the studies did not say:

— The gain from training is roughly 5 mL/kg/min, or about a decade’s worth of the average age-related gap. [2][4]
— Intervals beat steady work, by about 1.2. Both work. The gap is smaller than the argument about it. [2]
— Longer intervals and more of them do more — 2 minutes and up, 15 minutes of work and up, across 4 to 12 weeks. [3]
— The less fit you are, the more you gain. [2]
— Roughly half your response is inherited, and some people gain very little on an identical programme. [6]

None of that requires a subscription, a test, or a wearable. It requires being out of breath on purpose, regularly. Running, walking enough of them and lifting all have their own pages here.

And the thing being sold is mostly the measurement, not the fitness. The testing, the wearable, the dashboard, the score you can post. The part that actually moves the number has been free and well-evidenced for forty years, and it is the part with no margin in it.