VO2 max has gone from a lab term for elite athletes to a number on millions of wrists. That's a good thing, because few measures say as much about your long-term health. The catch is that the number on your watch may not be your real VO2 max.
What is VO2 max?
VO2 max is the maximum amount of oxygen your body can take in and use during hard exercise. It's usually reported in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min). Think of it as the size of your engine: how well your heart, lungs, blood and muscles work together to deliver and use oxygen.
It matters well beyond sport. Higher cardiorespiratory fitness is associated with lower rates of coronary artery disease, diabetes and cancer, and VO2 max has a strong inverse association with all-cause mortality (PLOS One, 2025).
VO2 max is one of the most important health numbers you can know. It's worth measuring properly.
VO2 max and longevity: why it may be the most important number you know
When researchers look for the single measurement that best predicts how long someone will live, cardiorespiratory fitness keeps rising to the top. Many longevity experts now call VO2 max the strongest predictor of lifespan we have.
The Cleveland Clinic study of 122,007 patients
Researchers followed more than 122,000 adults who had a treadmill fitness test, for a median of 8.4 years. People in the top fitness group were about 80% less likely to die during the study than people in the lowest group. Low fitness raised the risk of death as much as, or more than, smoking, diabetes and coronary artery disease. And there was no upper limit: the fitter people were, the better they did (read the study).
Why does one number say so much?
- It measures the whole system at once. Your VO2 max depends on your heart, lungs, blood vessels, blood and the mitochondria in your muscles all working together. If any one of them is struggling, the number shows it.
- It is your reserve for aging. VO2 max naturally declines as you get older. Climbing stairs, carrying groceries and playing with grandkids all require a minimum level of aerobic capacity. The higher you start, the longer you stay above that line.
- You can change it. Unlike your age or your genes, VO2 max responds to training at almost any age. That makes it one of the most powerful levers you have for a longer, healthier life.
Do you know your real number? A lab-grade VO2 max test takes about 15 to 20 minutes and gives you a true baseline, plus heart rate zones built for your body.
How VO2 max is measured
| Method | How it works | Accuracy |
|---|---|---|
| Lab test (indirect calorimetry) | You wear a mask that measures the oxygen you breathe in and the carbon dioxide you breathe out while exercise gets progressively harder on a treadmill or bike. | The gold standard: it measures VO2 directly |
| Submaximal test | You exercise below your max, and your VO2 max is predicted from heart rate and workload. | An estimate; errors vary by person |
| Field test (e.g., 12-minute run) | Distance covered in a set time is plugged into a formula. | An estimate; can underestimate in athletes |
| Smartwatch | An algorithm estimates VO2 max from heart rate, pace and your profile during outdoor walks and runs. | The least direct; can be off significantly |
Only the lab test actually measures the oxygen you use. Everything else predicts it.
Why you shouldn't trust your Apple Watch with VO2 max
Your watch never sees your oxygen. It estimates VO2 max from heart rate and movement, then runs that data through an algorithm. Independent research shows how far that estimate can drift:
Underestimated by about 6 points
Thirty people wore an Apple Watch for 5 to 10 days, then did a maximal treadmill test with a mask. On average, the watch underestimated VO2 max by 6.07 mL/kg/min, and the authors concluded the estimates need more refinement before clinical use (read the study).
Wrong in both directions
Testing the Apple Watch Series 7 found it tends to overestimate VO2 max in people with poor fitness and underestimate it in people with excellent fitness, the same pattern seen in other smartwatches (read the study).
Six points is a big deal. It can move you an entire fitness category, set your training zones in the wrong place, or hide real progress. Your watch is useful for spotting trends, but it isn't a measurement. For a deeper dive, read our review: How Accurate Is Apple Watch VO2 Max?
How to improve your VO2 max
- Build an aerobic base. Most of your cardio should be steady, conversational effort (often called Zone 2), several hours a week.
- Add high-intensity intervals. One or two sessions a week, such as four 4-minute hard efforts with recovery in between, push the top end of your fitness.
- Strength train. Stronger muscles use oxygen more efficiently and support the training that raises VO2 max.
- Improve your body composition. VO2 max is measured per kilogram of body weight, so losing fat while keeping muscle raises your number. A DEXA scan shows exactly what you're losing.
- Recover well. Sleep, nutrition and rest days are when your body actually adapts to training.
- Train with altitude-style oxygen intervals. Endurance athletes have long used altitude to challenge the body's oxygen systems. Simulated altitude training brings a version of that indoors.
Meet Mito2: simulated altitude training
Mito2 is a professional simulated altitude therapy system, known clinically as Intermittent Hypoxic-Hyperoxic Training (IHHT). Each session alternates short intervals of low-oxygen air with higher-oxygen air, recreating the cellular signal of training at altitude without leaving the facility. Every session includes real-time blood oxygen (SpO2) monitoring for safety.
Unlike hyperbaric therapy, which is passive, IHHT is active cellular training: your body responds to a programmed oxygen challenge and adapts. Mito2 offers five training modes and is built for medical spas, clinics, performance gyms and wellness centers. Research on IHHT is still growing, so pair it with measured testing to see how your own body responds.
Watch how it works:
Can't see the video? Watch it on YouTube.
Measure it, then improve it
Your watch can tell you whether your fitness is trending up or down. It can't tell you where you actually stand. Because VO2 max is so closely tied to how long and how well you live, it deserves a real measurement.
A lab-grade VO2 max test measures the oxygen you actually use while exercise gets gradually harder. You'll get your true VO2 max, how you compare with others your age, and personal heart rate zones to train smarter. Retest every few months to see your progress in real numbers, not estimates. Pair it with a DEXA scan to see how your muscle and fat change as your fitness improves.
Get your VO2 max measured
Find out where you really stand, then build a plan to improve it. Want to add altitude-style training? Ask us about Mito2.
Learn more about Mito2: contact@dexascan.com
Invest in your health. Optimize your future.
Sources
- Investigating the accuracy of Apple Watch VO2 max measurements: a validation study. PLOS One, May 2025. Link
- Assessing the accuracy of smartwatch-based estimation of maximum oxygen uptake using the Apple Watch Series 7: validation study. JMIR Biomedical Engineering, July 2024. Link
- Mandsager K, et al. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Network Open, 2018. Link
- Mito2 simulated altitude therapy system. mito2.com
This article is for educational purposes and is not medical advice. Talk to your healthcare provider before starting a new training program.