VO2 max on your watch: how it is estimated
The lab test needs a mask and a treadmill. Your watch works it out from pace and heart rate. Here is what the estimate is good for.
Kivorent News desk, , 2 min read
What to know
- VO2 max is the most oxygen your body can use per minute during all-out exercise, adjusted for body weight.
- A watch estimates it from how hard your heart works to hold a given pace.
- The watch number can be off by a meaningful margin, but its direction over months is useful.
VO2 max is one of the most studied measures of aerobic fitness. Many watches now put a version of it on your wrist, sometimes labeled cardio fitness.
What it means
VO2 max is the maximum volume of oxygen your body can take in and use during intense exercise. It is expressed in milliliters of oxygen per kilogram of body weight per minute. A higher number means your heart, lungs and muscles can deliver and use more oxygen, which lets you sustain harder efforts.
Typical values range from the 20s and 30s for less active adults to the 50s and above for well-trained endurance athletes. It differs by sex and falls with age.
The lab version
In a lab, you run or cycle while wearing a mask that measures the air you breathe in and out. The effort increases in stages until you cannot continue. The peak oxygen use recorded is your VO2 max. It is a direct measurement, and it is hard work.
The watch version
A watch measures no oxygen at all. It relies on a relationship: for a given person, heart rate rises with effort in a fairly predictable way, and a fitter person holds a faster pace at the same heart rate.
During an outdoor walk or run, the watch records your speed from GPS and your heart rate from its optical sensor. It combines them with your age, sex and weight, and compares the result with data from people who have had lab tests. Out comes an estimate.
Why it can be wrong
- Heart rate errors. A loose band or cold weather gives the formula bad input.
- Hills, heat and wind. Your heart works harder for the same pace, and the watch may read that as lower fitness. Some devices correct for elevation and temperature and some do not.
- Trail running. Slow pace with a high heart rate looks like poor fitness.
- An unusual maximum heart rate. Formulas that assume a maximum from your age will be off if yours is far from average.
- Easy efforts only. Estimates are better when a workout includes sustained harder running.
- Medications that affect heart rate, such as beta blockers.
Studies comparing watches with lab tests generally find the estimates land within several points for many people and further off for some.
Getting a better estimate
- Wear the watch snug and above the wrist bone.
- Record steady outdoor runs or brisk walks on flat ground.
- Keep your weight current in the app, since the figure is per kilogram.
- If your device lets you set a measured maximum heart rate, use it.
What to do with the number
Use it to see whether your fitness is moving. A rise of a few points over three months of consistent training reflects a real change. A one-point dip after a hot, hilly week does not.
To improve it, the well-supported approach is regular aerobic exercise that includes some harder intervals, built up gradually. If you are new to exercise or have a heart condition, talk to a clinician before starting intense training.
For another fitness signal your watch tracks, see heart rate variability.
This article is general information about how technology works. It is not medical advice. Talk to a clinician about your own health.
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