VO2 max is the single best lab measure of aerobic fitness, and it happens to be one of the fitness numbers most tightly linked to how long people live. The number on your watch is a rough estimate, not a verdict, but the direction it moves over months tells you something real. The good news: it is largely trainable, and the recipe is unglamorous.
What VO2 max actually is
VO2 max is the maximum rate at which your body can take in, transport and use oxygen during all-out exercise. It is measured in millilitres of oxygen per kilogram of bodyweight per minute (ml/kg/min), and it is widely considered the single best measure of cardiorespiratory fitness.
Read the units literally and they make sense. A higher number means your lungs, heart, blood and muscles can shuttle and burn more oxygen per kilo of you, every minute, at full effort. Because it is divided by bodyweight, both a stronger engine and a lighter frame can move the figure.
A true reading comes from a lab test: you exercise to exhaustion wearing a mask that measures the oxygen you consume and the carbon dioxide you produce. Everything your watch reports is an estimate of that.
Is your number good?
Reference values shift with age and sex. For men aged 20 to 29, a "good" VO2 max sits roughly around 37 to 41 ml/kg/min; for men over 60, roughly 26 to 30. For women, the equivalent bands are roughly 32 to 36 at 20 to 29 and 21 to 25 at 60-plus.
Women's values run about 10 to 15% lower than men's on average, owing to differences in haemoglobin, body composition and heart size. That is biology, not a smaller effort, so compare yourself within your own sex band.
At the far end, elite endurance athletes typically score 70-plus (men) and 60-plus (women), and the highest cross-country skiers and cyclists exceed 80 to 85. Most people reading this are nowhere near that ceiling, which is the point: there is room to move.
Why fitness tracks so strongly with longevity
This is where VO2 max stops being a gym-bro stat. In the largest study of its kind, 122,007 patients undergoing treadmill testing, cardiorespiratory fitness was inversely associated with all-cause mortality, with no observed upper limit of benefit. Fitter kept meaning lower risk of dying, even at elite levels.
The size of the gap is striking. Compared with the lowest-fitness group, elite-fitness individuals had an adjusted hazard ratio for death of 0.20, a roughly 80% lower risk; even elite versus merely high fitness was lower still (HR 0.77). In the same cohort, the mortality risk linked to low fitness was comparable to or greater than smoking, diabetes and coronary artery disease.
Put plainly: being unfit predicted dying about as strongly as smoking did. As a rough rule of thumb from cohort data, each one-MET gain (about 3.5 ml/kg/min) is associated with roughly a 10 to 12% lower mortality risk.
Cardiorespiratory fitness was inversely associated with mortality, with no observed upper limit of benefit.Mandsager et al., JAMA Network Open, 2018
Here is the honest caveat, and it matters. These are observational studies. They show a very strong association, not proof that a higher number directly buys you years. Fitter people also tend to be leaner, less likely to smoke and more metabolically healthy, and those things travel together. Causation is plausible and biologically sensible, but it is not nailed down by this kind of evidence. Treat VO2 max as one of the best available markers of how robust your cardiovascular system is, not as a guarantee.
How your watch guesses your number
Your watch does not measure oxygen. It infers VO2 max from the relationship between your heart rate and your pace or power during regular activity, then fits that to a model. That inference carries real error.
In a controlled validation against laboratory indirect calorimetry, the Apple Watch underestimated VO2 max by a mean of 6.07 ml/kg/min, with a mean absolute percentage error of 13.31% and very wide limits of agreement (roughly minus 6 to plus 18 ml/kg/min). A separate study of the Series 7 found a mean absolute percentage error of about 15.8%. Garmin's estimate carries an average error around 5% and tends to slightly overestimate, comparable to or better than other consumer wearables.
So the absolute figure can be off by double-digit percentages. Watches also tend to overestimate in less-fit people and underestimate in very fit people, a regression-to-the-mean effect that flatters the unfit and shortchanges the strong. The researchers' own conclusion: these estimates may suit general fitness monitoring and trend-tracking, but they are not accurate enough to inform clinical decisions.
The practical reading: ignore the exact number, trust the trend. If your watch creeps up over eight to twelve weeks of consistent training, that direction is far more trustworthy than any single reading. A lab test only earns its cost when you genuinely need an accurate absolute figure.
What genuinely raises it
Two ingredients, in this order. First, a base of easy aerobic work, the Zone 2 pace where you can still hold a conversation. That volume builds the stroke-volume and mitochondrial foundation that everything else sits on. Second, a small dose of high-intensity intervals to drive the peak.
The most-studied protocol for the peak is the Norwegian 4x4: four 4-minute intervals at 90 to 95% of max heart rate, with 3-minute easy recoveries, done two to three times a week. The long near-maximal efforts hold cardiac output high long enough to push stroke-volume adaptation. In head-to-head trials, this kind of near-maximal interval work has produced substantially larger VO2 max gains than the same volume of steady-state running, which is why it is the protocol most often cited for moving the peak.
Two myths to bury. You do not need only brutal intervals: the base-plus-intervals combination beats doing either alone. And more intervals is not better. Past two or three hard sessions a week, fatigue accumulates and progress stalls. Most of your week should be easy.
Bottom line
Realistic expectations
Can a normal person move this, or is it genetics? Mostly trainable. For untrained-to-recreational people, realistic improvement is on the order of 5 to 15% with consistent training, and larger gains (around 25%) are possible over longer stretches. Trained athletes often plateau within four to six months because they are already near their genetic ceiling.
Response varies between people. The same programme produces high responders and low responders; some of that is heritable, which is why two people on identical training can see very different gains. Your genes shape your ceiling and your speed of response, not whether you can improve at all.
Age does erode the number, but training blunts the slide. VO2 max declines about 1% per year in sedentary adults versus roughly 0.5% per year in regularly active ones. Training meaningfully slows the age-related drop; it does not stop it.
So aim for a few percent over a couple of months, keep going, and judge yourself by the trend line, not by any single watch reading. For most people, a wearable estimate is enough to track progress and place yourself in a fitness band; reserve the lab and the mask for when you truly need the exact number.
This article is informational coverage, not medical advice. Hard interval training is demanding; if you have a heart condition or have been inactive, check with a qualified clinician before starting.
Sources
- Understanding VO2 Max (Fitnescity)
- Association of Cardiorespiratory Fitness With Long-term Mortality (Mandsager et al., JAMA Network Open, 2018)
- Compelling study on fitness and mortality (Exercise is Medicine)
- What's a good VO2 max? Charts by age and sex (Marathon Handbook)
- VO2 max by age and sport norms 2026 (runbikecalc)
- VO2 Max by Age (Cora Health)
- Investigating the accuracy of Apple Watch VO2 max measurements (PLOS One, 2025)
- Validation of Apple Watch Series 7 VO2 max estimation (2024)
- Is VO2 max on smartwatches like Garmin and Apple correct? (4Endurance)
- The 4x4 interval protocol (Myworkout)
- Genes to predict VO2max trainability: a systematic review (2017)
- How much can I grow my VO2 max? (BRL Sports)
