VO2 max is the highest rate at which your body can take in and use oxygen during all-out exercise. It sets a ceiling on how much energy you can produce aerobically. In a lab, it is measured by analysing the air you breathe while you exercise to exhaustion.
For decades it was a sports-science number. Now smartwatches estimate it and put it on your wrist, often next to a “fitness age”. Two separate questions sit under the hype. Does cardiorespiratory fitness really predict how long you live? Strongly, although the evidence is observational. And can a watch measure it well enough to matter? Partly: it is useful for trends and loose for any single reading.
Why fitness became a health number
Cardiorespiratory fitness is one of the most consistent predictors of long-term health in medical research. An American Heart Association scientific statement argued it should be assessed routinely as a clinical vital sign, noting that low fitness is associated with a high risk of cardiovascular disease, all-cause mortality and deaths from various cancers.
A widely cited meta-analysis in JAMA found a dose-response pattern. Each additional unit of measured aerobic capacity was associated with a decrease of 13 percent in risk of all-cause mortality.
The big cohort: how strong is the link?
The study that pushed this into the mainstream analysed data from 122,007 patients who had a treadmill exercise test at the Cleveland Clinic. Their fitness was estimated from treadmill performance and ranked against others of the same age and sex.
Risk of death fell as fitness rose, with no clear upper limit. Compared with the least fit group, the most fit (“elite”) group had an adjusted hazard ratio for death of 0.20. Put simply, after adjusting for other risk factors, their risk of dying during follow-up was about a fifth of the low-fitness group’s.
That is a strong association, but it has limits worth stating plainly:
– It is observational. Very fit people may differ in genetics, income, diet and existing illness, and statistical adjustment cannot remove all of that. – Illness lowers fitness. Some people score low because they are already unwell, which can exaggerate the apparent protective effect. – Everyone in the study had been referred for a clinical exercise test, so they are not a random sample of the population. – Fitness was estimated from how long people lasted on the treadmill, not measured directly by gas analysis.
What the data cannot tell you is how much benefit you personally would gain by raising your fitness. Answering that would need long-term trials that deliberately change fitness and track deaths, and those do not exist at this scale.
How trainable is VO2 max, and for whom?
Very trainable on average, with large differences between individuals.
A meta-analysis of interval-training studies in young, healthy, sedentary or recreationally active adults found an average increase in VO2max of 0.51 L per minute after programmes lasting a few weeks to a few months. For an untrained person, that is a meaningful change.
Averages hide the spread. In the HERITAGE Family Study, sedentary adults followed the same standardised endurance programme. Some individuals experienced little or no gain, whereas others gained more than 1.0 l/min. Genetics play a part: the heritability of that training response was estimated at 47%.
That does not mean some people simply cannot improve. In a later study, healthy adults classed as non-responders after an initial programme went on to add extra weekly training, and non-response in cardiorespiratory fitness (measured by maximal power output) was universally abolished. The practical reading is that a standard dose does not suit everyone, and some people need more training to see a change.
Most of these studies involved younger, healthy volunteers over short periods. How well the findings transfer to older adults, people with chronic illness, or gains maintained over many years is less well established.
What your watch is actually measuring
Watches do not measure oxygen. They estimate VO2 max from signals such as heart rate during runs or walks, pace, and details like age, sex and weight, using proprietary algorithms.
A systematic review and meta-analysis by an international research network found that wearables using exercise-based algorithms are more accurate than those relying on resting data, and reasonable for estimating group averages. But the estimation error at the individual level is large, and an individual is exactly what you are.
Recent validation studies of the Apple Watch show the size of that error. One found a mean absolute percentage error of 13.31% against lab testing, and a study of the Series 10 found 13.2%. Both found the watch tended to underestimate VO2 max. These were small studies, and other brands use different algorithms, so the figures do not transfer across devices.
How to use the number sensibly
– Treat your watch’s VO2 max as a trend line, not a lab result. A steady change over months tells you more than any single figure. – Keep conditions comparable, for example similar outdoor routes and consistent wear, so the algorithm has like-for-like data. – Do not read a small drop as a health warning, because normal estimation error is larger than most short-term changes. – If you want to raise it, regular aerobic exercise with some harder interval sessions has good support, and adding more training is a reasonable next step if progress stalls. – If you need an accurate figure for clinical or sporting reasons, a lab test remains the reference.
Common questions
What is a good VO2 max?
It depends heavily on age and sex, so any single “good” number is misleading. The research linking fitness to longevity ranks people against others of the same age and sex. Watch category labels also carry the device’s estimation error, so treat them as rough.
How accurate is Apple Watch VO2 max?
Two recent validation studies found average errors of around 13% compared with lab testing, with the watch tending to underestimate. That is good enough to follow trends but too imprecise for a firm judgement about one person’s fitness.
How can I increase my VO2 max?
Regular aerobic training raises VO2 max for most people, and interval-training studies show meaningful average gains. Responses vary widely, and people who do not improve on a standard programme have been shown to improve when they train more.
Does a higher VO2 max mean I will live longer?
Higher fitness is strongly associated with a lower risk of death in large studies. Those studies are observational, so they cannot prove that raising your own number will add a particular amount of life, although the evidence consistently points the same way.
The bottom line
Cardiorespiratory fitness is one of the best-supported markers of long-term health, and VO2 max is trainable for most people, with wide individual variation. The figure on your wrist is an estimate whose error can swamp small changes. Use it to see the direction of travel, and put the effort into training rather than the decimal places.
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