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In 2018 JAMA Network Open published a Cleveland Clinic study that followed 122,007 patients who had undergone treadmill testing and compared their survival (Mandsager et al.). People with low cardiorespiratory fitness died more often than the rest, and the best, “elite” group lived longest. The authors found no upper limit beyond which the benefit disappears. The study is observational, so it cannot prove causation, but the association is so strong that VO₂max came to be seen as a key health measure.

What VO₂max is and how it is measured

VO₂max is the maximum amount of oxygen the body can use per minute per kilogram of body weight. Lab measurement requires a mask, a treadmill or cycle ergometer, and effort to exhaustion. It is accurate but expensive, unpleasant and rarely repeated.

How watches estimate it

Watches and bands do not measure gas exchange. They estimate VO₂max from the relationship between heart rate and speed (or power) during running or brisk walking, plus age, weight and sex. Garmin, Apple Watch, Polar and others display the number automatically. For people who train regularly and give the device enough outdoor data, estimates usually correlate with lab results, but notable scatter remains; watches often underestimate very fit athletes and err for people who rarely run.

So the rule is: compare yourself with yourself. A rise in the estimate over several months using the same method is an informative signal. An argument about “I have 48, my friend has 52” is not.

What matters for interpretation

  • Results depend on conditions: heat, fatigue, hills and poor GPS can shift the number.
  • Watches work better if heart rate is measured accurately; a chest strap is more reliable than optical sensors.
  • Test in similar conditions: same route and time of day.
  • If the number drops, do not panic. Compare with sleep, illness and training load over recent weeks.

How to improve it

The foundation is plenty of easy aerobic work (you can hold a conversation), plus one or two interval sessions a week, for example 4 × 4 minutes at a hard but controlled pace. These schemes are the best studied, but they do not suit everyone: people with heart disease and those who have not trained for a long time should talk to a doctor first.

When you need a real test

A lab test makes sense if you train endurance seriously, want precise training zones, or have questions about heart and lungs. For everyone else a watch gives enough information to see the trend.

How VO₂max fits into healthy aging overall is in our piece on healthspan .