How Much Should You Trust
 Your VO₂max Score

What expensive sports watches can tell us about fitness and training

Your watch can give you useful information, but its VO₂max score is still an estimate. Before I would use that number to change your training, I want to understand how it was calculated and whether your performance supports it.

Imagine finishing a hard run and seeing your score drop. You put in the work, so why does your watch seem unimpressed? Before you add another interval session, let’s look at what the number actually means.

VO₂max describes the maximum rate at which your body can take in and use oxygen during exercise. The familiar mL/kg/min value expresses that rate relative to body mass. A watch predicts it from available sensor and profile data. Apple, for example, uses heart-rate and motion information from eligible outdoor walks, runs and hikes, together with personal information. It does not measure the oxygen in your breathing. [1, 3, 4]

I use a Garmin myself. For VO₂max, I want evidence for the particular model and software, in people and activities relevant to me. A higher price alone cannot establish that accuracy.

Two published validation studies illustrate the range of results.

These were different groups and protocols, so the figures do not establish a brand ranking. They describe the tested devices and software, not every current model. An average error of 7% also does not guarantee that your reading is within 7% of your reference result.

The Apple study found an average underestimate of 6.25 mL/kg/min, with wide variation between people. Its analysed sample was mostly young and highly fit, which limits generalisation. We cannot simply add 6.25 to everyone’s watch score. [2]

Consistency and accuracy are different. A device might repeatedly give a similar score and still disagree with a reference test. Detecting a real training improvement is another question. The Series 10 researchers specifically called for further validation over time, so I would not assume that a smooth trend proves an accurate change in fitness. [2]

The conditions around the workout matter too. Apple documents several influences that can affect its estimate. Heat, dehydration, caffeine and recent altitude exposure can raise heart rate and contribute to a lower estimate. Sand, carrying a heavy backpack or pushing a stroller can change the workload in ways the watch may not fully recognise. Incorrect profile information and medications that affect heart rate can also alter the result. [3]

That gives us a practical coaching question. Was the lower number recorded during a hotter run, on a different surface or with extra load? A change in those conditions is a reason to investigate before deciding you have lost fitness. Some changes reflect real acute physiological strain; others reflect how the model interprets it.

The exercise mode matters. Cycling and running VO₂max results can differ, and the size of that difference depends partly on training background. They share underlying physiology, but a cycling result cannot automatically replace a running result. There is no universal conversion factor I would apply to every athlete. [5]

For HYROX, I would use VO₂max as one part of the assessment. I also want to see your running pace after a station, how you manage the sled, and whether you can maintain technique as fatigue builds. A watch score alone cannot answer those coaching questions.

The same standard applies to testing services. Power-based INSCYD PPD derives physiological estimates from performance inputs; it does not directly measure respiratory gases. I want an athlete to know the method, exercise mode and any concern about test execution. A carefully conducted maximal gas-exchange test in the relevant mode offers a stronger reference, while still depending on calibration, protocol and effort. [2, 6]

For a recreational exerciser, I see value in an accessible fitness indicator, especially if it encourages curiosity and consistent activity. For an athlete or coach, it can add context when considered with pace, power, heart rate, perceived effort and repeatable performance tests. These are practical uses I would support, with appropriate caution about small fluctuations.

Researchers can also use wearable estimates when the device and method are validated for the population and outcome. The estimate should be identified clearly. [1, 2]

When your score changes, here is how I would work through it with you.

1. Check the setup. Confirm your profile information, watch fit and eligible workout type. Note any device or software change.
2. Compare similar sessions. Record the route, conditions and any extra load so the comparison has context.
3. Look at what you could actually do. Consider pace or power, heart rate, perceived effort and recovery alongside the score.
4. Investigate a persistent mismatch. Review the data and testing method with your coach before making a major training change. Use a suitable assessment when greater precision would change the decision.

If your watch number is confusing you, bring me the workout and a little context about the day. We can make much better use of the data when we understand what was happening around it. That is where I want your attention: making training decisions that help you perform and recover well.

Research and source links

[1] Carrier B, Marten Chaves S, Navalta JW. 2025. Validation of Aerobic Capacity (VO2max) and Pulse Oximetry in Wearable Technology. Sensors, 25(1), 275. VO₂max findings cited here.

[2] Lambe R et al. 2026. Accuracy of VO₂ max Estimates From Apple Watch Series 10. Mayo Clinic Proceedings: Digital Health, 4(2), 100357.

[3] Apple. 2021. Using Apple Watch to Estimate Cardio Fitness with VO₂ max. Manufacturer technical report; cited for method and limitations.

[4] Apple Support. Track your cardio fitness levels. Official explanation of eligible activities and profile inputs.

[5] Basset FA, Boulay MR. 2000. Specificity of treadmill and cycle ergometer tests in triathletes, runners and cyclists. European Journal of Applied Physiology, 81, 214–221.

[6] INSCYD. Validation and verification. Manufacturer documentation; see the power-only PPD example.