VO2 Max Testing and Wearable Estimates

What VO2 max is

VO2 max is the highest rate at which the body can use oxygen during hard exercise. It is a common measure of cardiorespiratory fitness. The reference method is a graded exercise test in a laboratory with a gas analyzer that measures oxygen use directly. Many watches and apps instead show an estimate calculated from heart rate, pace and personal details. The biology and training evidence are in Cardiorespiratory Fitness and VO2 Max in Aging Research.

Evidence labels: established, promising but limited, mixed, insufficient evidence, marketing claim. Fitness as a predictor of outcomes is established as an association. Watch accuracy for individuals is mixed.

Fitness and survival: what the studies show

• A meta-analysis of 33 cohort studies (about 103,000 people for all-cause mortality) found each 1-MET higher fitness level went with a risk ratio of 0.87 for all-cause death and 0.85 for coronary or cardiovascular events. Low fitness versus high fitness had a risk ratio of 1.70 for death. [1] A MET is a unit of exercise capacity, and 1 MET corresponds to about 1 km/h faster running speed.
• In 122,007 patients at one US medical center who were referred for treadmill testing, lower fitness went with higher long-term mortality. Low versus elite fitness had an adjusted hazard ratio of 5.04. This was a retrospective study of patients sent for testing, and fitness was estimated from treadmill performance, not measured with a gas analyzer. [2]
• An American Heart Association scientific statement argues that cardiorespiratory fitness is a potentially stronger predictor of mortality than smoking, high blood pressure, high cholesterol or type 2 diabetes, and that adding it improves risk reclassification. [3]

These are associations. Observational studies cannot rule out that people who are already healthier can exercise harder, so the direction of cause is uncertain. The studies do not tell any individual what number to aim for.

How accurate are watch estimates?

A systematic review and meta-analysis of 14 validation studies found that wearables using resting-condition information significantly overestimated VO2 max on average (bias 2.17 ml/kg/min), with wide limits of agreement (about minus 13 to plus 17). Devices using exercise-based information had little average bias (minus 0.09) but still limits of about minus 10 to plus 10. The authors concluded that exercise-based estimation may be reasonable at the population level but individual-level error is large. [4]

One validation study of an Apple Watch Series 7 in 19 people aged 18 to 63 compared the watch with a laboratory test. The watch averaged 41.4 versus 45.9 ml/kg/min measured, with a mean absolute percentage error of 15.8% and poor reliability. The watch tended to overestimate in people with poor fitness and underestimate in people with excellent fitness. [5] It was a small study of a single model, and other devices and software versions were not tested here.

Label: mixed. Use a watch value to watch your own trend over time, if at all, but do not read it as a lab result. See Wearables and Sleep Tracking: Accuracy and Limits and Wearables and Health Trackers.

How to evaluate a VO2 max claim

• Is the figure measured or estimated?
• What is the margin of error, and was the device checked against a lab test in people like you?
• Does the product claim to predict lifespan or "biological age"? That would be a marketing claim unless backed by a study.
• Compare with Biological Age vs Chronological Age.