Your fitness wearable will tell you what your VO2 max is with great confidence but is that confidence deserved? We’ve recently published an article about what VO2 max is and whether it even matters. On the theme of VO2max, today we’ll answer the question of whether your wearable is telling you something useful or just giving you a random number.
How can we estimate VO2max without a lab?
Your VO2max is a measure of how much oxygen your body can make use of per kilogram of body mass each minute. It’s highly correlated with running performance, and is a key determinant of performance across all endurance sports. VO2max is a key outcome measure in many studies of training interventions and has gained fame as a result of this. A sports science study will take a cohort of athletes, put them through some kind of training plan and measure their VO2max before and after - if VO2max goes up, the training sessions being tested are deemed effective.
As a result of heavy use in sports science, as well as its heavy featuring in the discourse surrounding endurance sport training, a market for testing, estimating and measuring VO2max grew. Traditionally it’s measured with an incremental exercise test where the test subject will wear a mask that measures how much oxygen is going in and out. These tests are expensive and time consuming, so watch manufacturers went about coming up with an estimate to satisfy the demands of this market.
GPS watches measure a lot: your resting heart rate; your speed during running; your heart rate for every step of every run; your elevation; and some of them measure HRV or other health metrics. Each time you strap your wearable to your wrist, it collects more and more of this data and over time, each observation can create a possible estimation for VO2 max.
This is how GPS watches estimate your VO2max:
When you go for a run, your watch measures your speed and heart rate continuously throughout the run.
Your watch then converts this speed into an estimated oxygen demand using estimated values for each velocity.
It uses your resting heart rate, your maximum heart rate and the heart rate you’re running at to estimate the fraction of your VO2max that you’re using while running.
From this, your watch is able to estimate your VO2max from different fragments of every single run and average them all together over time.
It throws out data that can impact the relationship between your heart rate and pace including dodgy GPS, up hill or down hill running and more.
The estimate is refined using other factors your watch and the software knows about you including weight, HRV and more.
You get a VO2max estimate that changes over time.
At each point in the 6 steps that lead up to your VO2max estimate, there’s many possible sources of uncertainty and error in the measurements that will drag the estimated value from the “true” number. If you are measuring your heart rate with the watch’s optical sensor, it won’t be accurate. If you are fatigued, the oxygen demand for a given pace will be higher than if you’re fresh. If you’ve never reached your true maximum heart rate, your VO2 max will be underestimated. If your watch’s model fails to throw out dodgy GPS data, it could push the estimate in either direction. If your weight in your watch’s app is wrong, it’ll cause your relative VO2 estimate to be wrong and much more. Provided all of these errors remain consistent, the trend in your VO2 max could be correct though many of these errors could push your VO2max estimate in different directions on any given day.
As a runner, should I care about it even if it’s right?
If you went to the lab to test your VO2max, you’d almost certainly get a different number from that which your watch gives you. There have been a number of scientific studies that have looked into this and they all come out with the same (fairly obvious) conclusion - the estimate your watch gives you is usually different from what they measure in the lab. The thing that many sports scientists fail to mention is that the error in the measurement in their equipment can actually mean your VO2max measured at two different labs can be different. Any athlete using VO2max to track changes in fitness should always get tested at the same lab, with the same test protocol using the same equipment.
VO2max has become fashionable in longevity circles recently as it’s a determinant of life expectancy, but improving it isn’t very complicated and doesn’t really require measuring it. If you’re a runner, increasing your VO2max should not be a primary concern - getting faster should be. When you do a race, only one thing matters and that’s the time it takes you to get from one end of the course to the other, whether that’s a big city marathon or a Parkrun.
You can tell you’re getting fitter using just one step of the VO2 max estimation process. If you’re running at a faster pace for the same heart rate or internal sense of effort, you’re probably getting fitter, especially if you have an accurate heart rate monitor or a good sense of effort. These two metrics can be used to calibrate one another too, as heart rate suppression is commonly observed in highly fatigued athletes too - so heart rate may drop for a given pace if you’re very tired.
All of that is to say, even if your watch’s estimate for your VO2max is correct you don’t need to pay much attention to it anyway.
If VO2max doesn’t matter, why do I always see professional athletes in the lab?
Performance doesn’t exist in a vacuum, the relationship between your VO2max and your threshold can also be a key determinant of race day performance. Marathon runners have a threshold very close to their VO2max, whereas 1500m runners often have a threshold that’s lower but a VO2max that’s higher. At very high levels of performance gains can become difficult to find.
Professional athletes use laboratory testing to decide which components of their physiology to work on in order to prepare for whatever goal they have coming up next. The majority of amateur runners do not need to worry about this. One of the key differences between a professional and amateur runner is that the professional is much closer to their genetic potential as they have tens of thousands of hours of training in their legs.
For recreational runners, just getting a balanced training routine is the optimal way to improve your VO2max. The key determinant of performance for amateur runners is total volume and the thing that kills volume - injury! A balanced training program that avoids too much intensity and volume is the way to achieve your goals. Ignore your VO2max estimate on your watch and focus on race day results.
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