Can Your Watch Really Measure Your VO2 Max?

12 Oct

Written by Rich Saxton

You finish a hard run, glance at your wrist, and there it is: VO2 max 52. Maybe it has crept up by one point and you feel a small glow of pride. Maybe it has dropped by two and you start wondering whether the last month of training was wasted.

Either way, that number feels like a lab result. It isn't. It is an estimate built from a few clues your watch can see, and the research on how good those estimates are is more mixed than the marketing suggests. Here is what is happening behind the number, how far you can trust it, and what to do with it.

What is VO2 max, and why does everyone care?

VO2 max is the highest rate at which your body can take in and use oxygen during hard exercise. It is usually quoted in millilitres of oxygen per kilogram of body weight per minute (mL/kg/min). A bigger engine means a higher ceiling for endurance performance.

The gold-standard way to measure it is a lab test: you run or cycle with increasing intensity until you can't continue, breathing through a mask that analyses the air you inhale and exhale. That needs equipment, a trained operator and a genuinely maximal effort, which is why most of us have never done one, and why watches are so appealing.

How your watch makes its guess

Your watch never measures oxygen. It watches your heart rate and your speed, then infers the rest.

• Heart rate against pace or power: The oxygen cost of running or cycling rises predictably with speed or power. If your heart rate is low for a given pace, the watch assumes you are fit and nudges the estimate up. A higher heart rate at the same pace pulls it down.

• Your maximum heart rate: The watch needs this to place your effort on the scale. It may use an age-based formula, a number you typed in, or one it has learned from your hard efforts. If it is wrong, the whole estimate shifts.

• Your profile: Age, sex and weight, entered at setup.

• Extras: Some watches also factor in heart rate variability, resting heart rate, heat, altitude and terrain.

Most need a steady outdoor run or ride of roughly 10 to 15 minutes at a reasonably hard effort before they will update the number. Brands keep their exact formulas private, so this is the general method, not any one company's recipe.

So how accurate is it?

A 2025 review of 13 studies that tested wearables against lab gas analysis found errors ranging from under 5% to almost 16%. Seven of those studies judged the watch's estimate valid or acceptable.

There is a twist. A separate meta-analysis of 14 studies found the average bias across devices was close to zero. That sounds reassuring until you read the fine print: some watches read high, some read low, and they cancel out. The spread around any one person's result stays wide, which means the group average can look fine while your own reading is well off.

Device, group and what was found

Garmin Fenix 6, athletic sample Average error 6.85%, judged accurate and valid

Garmin Forerunner 245, athletes above 60 mL/kg/min, Underestimated by more than 6 mL/kg/min, roughly 9 to 10% error

Apple Watch, healthy adults, Underestimated by about 6 mL/kg/min, average error 13.3%

Apple Watch Series 10, 40 healthy adults, Read 45.3 against a lab average of 51.5, with wide individual variation

The fair summary: a watch is a reasonable guide to where a group of people sits, and an unreliable judge of any one person on any one day.

Why fitter runners get short-changed

The pattern in the research is consistent: the fitter you are, the less your watch tends to be right. In the Forerunner 245 study of 35 endurance athletes, the estimates held up in moderately trained runners but were less valid in highly trained ones. The authors suggest validity may fall as fitness rises.

The likely reason is that the watch assumes your heart rate and running economy follow the typical pattern. Well-trained runners often don't, so the formula under-reads them. One commentary on that study notes it used an older version of Garmin's algorithm, so newer watches may do better, but there is no head-to-head evidence yet.

Several everyday things can also move the number without any change in your fitness:

• Heat and humidity, which raise heart rate at a given pace

• Hills and uneven terrain, which muddy the pace-to-effort link

• Tiredness, caffeine and poor sleep

• A weak optical heart rate signal on a loose or sweaty strap

• A maximum heart rate setting that doesn't match yours

Lab testing, and the lactate route

If you want a number you can lean on, a lab ramp test with gas analysis is still the benchmark. There is a middle option too: performance software such as INSCYD calculates VO2 max from several maximal efforts, finger-prick blood lactate samples and power or speed data, instead of measuring oxygen directly. We use this software in the RSH Performance lab!

The independent evidence for it is encouraging but thin. One study of 11 trained cyclists found its relative VO2 max within 0.13 mL/kg/min of the lab value, which is well inside normal day-to-day variation. INSCYD's own validation work makes similar claims, though company-run data deserves a little more scepticism.

Two caveats matter. Eleven cyclists is a small sample, and we found no study that put a watch, a lactate-based model and a lab test side by side in the same athletes. And it is a structured, hard testing session, not something you get from a Sunday jog.

How to use your watch's number sensibly

1. Watch the direction, not the digit: With errors of several points either way, a one- or two-point wobble tells you almost nothing. A steady climb or slide over a few weeks is more meaningful.

2. Set your maximum heart rate properly: If you know it from a hard race or test, enter it. A wrong value skews everything downstream.

3. Compare like with like: Similar routes, similar weather, well rested. A hot, hilly, tired run will drag the number down.

4. Don't rewrite your training around one reading: Use it alongside how the effort feels, your race times and how you are recovering.

5. Test properly when it matters: If you are prescribing training zones, tracking adaptation closely or chasing a precise figure, book a lab test.

6. Treat it as motivation, not a verdict: A watch is good at keeping you curious about your fitness. It is not a diagnosis.

The part your watch can't see

VO2 max describes your engine. Injuries tend to happen in the chassis: tendons, bones, muscles and joints. Your heart and lungs usually adapt to training faster than those tissues do, so a runner can feel and measure as fitter than ever while their body is not yet ready for the load. That gap is where many overuse injuries start.

No watch metric tells you whether your calf, Achilles, shin or knee can cope with this week's mileage. If your watch says you are in the best shape of your life but something is niggling, trust the niggle. Get in touch with us at Run Serve Physiotherapy and we can look at how your body is handling the load, not just how your engine is performing.