Modern fitness trackers offer an array of stats that can tell you about almost any aspect of your exercise. They can tell you how many calories you’ve burned, how many steps you’ve taken, how fast you ran the last kilometre of your run and even if you’re working hard enough during your workout, if you pay attention to which heart rate zone you’re in. However, perhaps the stat you should pay the most attention to is your VO2 max, because it can tell you how fit you actually are and whether you are getting any fitter over time.

All the major manufacturers of fitness trackers now have at least one device in their line-up that will give you an estimation of your VO2 max, although some call it by a different name – Fitbit calls it a Cardio Fitness Score, for example. For most of them you will have to wear the tracker for a week or so and use it to record some outdoor activities like running and cycling before it’ll give you a score. The accuracy of these scores might not hold up against a full lab-tested VO2 max score, but for the average person it’s a handy way to get a picture of your fitness without submitting to a brutal lung-busting test.

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For more details on what your VO2 max score means, how it’s measured by fitness trackers and what you should be doing if you want to improve it, we spoke to James Phillips, a strength and conditioning coach at Pure Sports Medicine.

What is VO2 max?

Fitness trackers will generally give your two-digit VO2 max score and an indication of whether it’s good or not compared to the rest of your age group, which is handy, but gives little indication of what the number actually means. Here’s the science behind the score.

“VO2 max, or maximal oxygen uptake, is the greatest amount of oxygen that can be used by the entire body,” says Phillips. “The capacity to use oxygen is related primarily to the ability of the heart and lungs to transport oxygen and the ability of the body tissues to use it. VO2 max is measured in millilitres of oxygen per kilogram of mass per minute.”

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Memorise that and impress everyone at your next party, and then when they ask for a simpler explanation, say sometime like this.

“It is a measure of aerobic fitness related to a person’s ability to sustain long periods of moderate to high intensity physical activity with large muscle groups,” says Phillips.

“It is one of the most fundamental human physiology measures, with an impact on health, wellbeing and performance.”

How is VO2 max measured at an elite level? And how do everyday fitness trackers estimate it?

“At elite level, VO2 max is measured in a laboratory setting through a maximal running or cycling test,” says Phillips. “It is measured directly via breathing through a mask while completing an incremental test to voluntary failure.” Essentially, strap on a mask and go hard until you drop.

But if you’re not an elite athlete, fear not – that isn’t the only way to measure it. “Everyday fitness trackers estimate VO2 max through heart rate and speed of exercise,” says Phillips.

Most of the popular consumer fitness trackers on the market use Firstbeat Technologies’ analytical algorithms to calculate VO2 max.

“The Firstbeat method for estimating VO2 max during running is through the linear relationship between VO2 and running speed, which is measured through GPS,” says Phillips.

“The oxygen cost of running increases when running speed increases. As heart rate is measured alongside running speed, Firstbeat has been able to develop an automatic VO2 max estimation method applicable for any uncontrolled workout.”

Firstbeat claims its method is 95% accurate compared with lab measurements when calculated during running, but there are potential problems with relying on a wrist-based tracker to provide GPS and heart rate measurements.

“Outside influences such as running under a bridge or the heart rate strap slipping can alter the data recorded, reducing reliability and skewing the estimation,” says Phillips (although Firstbeat say their tech identifies and removes these events from the final calculation).

If you don’t pair a chest strap with the tracker and rely on the watch to record heart rate, accuracy will probably be reduced further, because wrist devices can’t match a chest strap at monitoring heart rate.

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In short, if you use a chest strap to measure heart rate and run in an open area, you should get a decent estimation of your VO2 max from your tracker.

How is VO2 max a good indicator of someone’s fitness? What benefits are there to improving your VO2 max?

Basically a good VO2 max score for your age means you’re in good shape and it’s a key indicator of how you might perform in endurance events.

“It [VO2 max] has been found to correlate well with the degree of physical conditioning and is recognised as the most widely accepted measure of cardiorespiratory fitness,” says Phillips.

“As the duration of endurance events increases, so does the proportion of the total energy demand that must be met by the aerobic metabolism. The higher an individual’s VO2 max, the greater their ability to use oxygen they inhale and therefore the more energy they are able to produce.”

How do you improve your VO2 max?

If you’re a fitness beginner, improvements to your VO2 max will come relatively easily with sustained aerobic exercise like running, cycling, rowing or swimming. As you get fitter, however, you might need to start doing more intense training sessions.

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“In order to improve something specifically you need to train at or around that variable – so training slightly above, below or at VO max pace/intensity will improve it up to a point,” says Phillips.

For example, high-intensity interval training (HIIT) where you work at close to flat-out for short periods of time will help improve your VO2 max. You can run, cycle, row or do whatever you fancy – the key is to push hard during the work intervals.

Runners working on their VO2 max can also try Fartlek workouts, which is where you change your pace throughout a run. This can be structured – like a track session where you do 400m sprints with a 400m jog to recover in between efforts – or unstructured, where you head to a local park and sprint every time you overtake, say, a dog walker. The latter can be a great way to make HIIT slightly more enjoyable because, if nothing else, every hard effort starts with seeing a dog, which is always nice.