VO2 max: why your wearable estimate can be wrong (and how to use it anyway)
VO2 max is one of the most popular fitness numbers on the internet.
If you have a Garmin, Apple Watch, COROS, Polar, or another wearable, you have probably seen an estimated VO2 max score that goes up and down over time.
Sometimes the changes make sense.
Other times you feel fitter, you are training well, and your wearable drops your VO2 max anyway.
Over the last week, this exact frustration has been trending again in wearable discussions. If you want to see the conversation, here is a simple X search:
This guide explains what wearables are really doing, why the number can be wrong, and how to use VO2 related data without getting misled.
TL;DR
- Lab VO2 max is measured by analyzing oxygen consumption. Wearables estimate VO2 max indirectly.
- The estimate is sensitive to heart rate accuracy, heat, hills, fatigue, and pacing strategy.
- Treat wearable VO2 max as a trend and compare it only within similar conditions.
- A better approach is combining multiple signals: performance at a given heart rate, training load, and recovery trends.
- If you want one reliable experiment, repeat the same submax test route every 2 to 4 weeks.
What VO2 max actually is
VO2 max is the maximum amount of oxygen your body can use per minute during very hard exercise.
In a lab test, VO2 max is measured using a mask that tracks your breathing gases while workload increases.
Most people will never need a lab test.
But understanding what is measured helps you interpret the wearable estimate:
- it is a measure of aerobic capacity
- it is influenced by cardiac output, blood oxygen transport, and muscle oxygen use
- it responds to training, but it is not the only thing that predicts performance
How wearables estimate VO2 max
Wearables do not measure oxygen consumption.
Instead, they infer VO2 max using a model that links three things:
- your speed or power (pace, cycling watts)
- your heart rate response
- your personal stats (age, weight, sex, sometimes training history)
In simple terms:
- If you can go faster at a given heart rate, the model assumes higher aerobic fitness.
- If your heart rate is higher than expected for a pace, the model assumes lower aerobic fitness.
This is why the estimate is condition dependent.
If conditions change, the number can change even if your true fitness did not.
The most common reasons the estimate is wrong
1) Heart rate measurement issues
Wrist based optical heart rate can struggle with:
- cold weather and poor blood flow
- vibration (trail running, cycling on rough roads)
- tattoos or darker skin tones for some sensors
- loose watch fit
If heart rate is overestimated, VO2 max is often underestimated.
2) Heat, humidity, and dehydration
In heat, your heart rate climbs for the same pace.
That is normal physiology.
But your wearable can interpret it as reduced fitness.
If your VO2 max drops every summer and rises every winter, that is often the explanation.
3) Hills and terrain changes
VO2 max models work best on steady, flat efforts.
If you run hillier routes, stop for traffic, or do lots of intervals, the data is less clean.
4) Fatigue and training phase
If you are in a high load block, your heart rate and pacing can look worse temporarily.
That does not mean you lost aerobic capacity. It means you are tired.
5) Pacing strategy
If you start too fast and fade, your heart rate stays high while pace drops.
The model usually punishes that.
A practical way to make VO2 max useful
You do not need to ignore the estimate.
You just need to control for noise.
Use it as a trend, not a daily verdict
A good rule:
- look at a 4 to 8 week trend
- ignore single step changes unless they persist
Compare like with like
When you compare efforts, keep these constant when possible:
- similar route and elevation
- similar weather
- similar time of day
- similar freshness
Pair it with one performance marker
Pick one marker that matters for your sport:
- a 30 minute steady run at an easy heart rate
- a 20 minute cycling effort on the trainer
- a standardized aerobic threshold test (if you know your zones)
If performance improves while wearable VO2 max stays flat, you are still getting fitter.
Checklist: how to get cleaner VO2 max estimates
- Tighten the watch one notch for workouts
- Use a chest strap on key sessions if you have one
- Do one steady outdoor run per week on a similar route
- Avoid judging VO2 max changes during heat waves
- Watch your recovery: poor sleep can raise heart rate and distort the estimate
- Use the number to guide patience, not panic
Source: lab VO2 max vs wearable estimate
VO2master published a recent explainer on why wearable VO2 max can diverge from measured VO2 max.
You do not need their device to benefit from the key point: the measurement method matters.
Video: VO2 max explained
Disclaimer: this embedded YouTube video is from a third party creator and is not affiliated with Century. It is for education, not medical advice.
Where Century fits
Century helps you avoid the trap of chasing a single number.
Instead of asking "Did my VO2 max go down?", you can look at the bigger picture:
- your training load and intensity distribution
- your recovery trend (sleep, HRV, resting heart rate)
- your performance at a given heart rate over time
That is how you make aerobic progress measurable, even when the wearable estimate is noisy.