It's safe to say VO2 Max is all the rage in wearable tech. It's been a focus of high-end sports watches for a while, but now it's going mainstream on the latest fitness trackers.
Fitbit has chosen it as the basis for its "Cardio Fitness" score on the Fitbit Charge 2, and what was once a metric for elite athletes is fast becoming a figure that weekend park runners are being conditioned to obsess over.
But what exactly is VO2 Max? How useful is it as a measure of overall fitness and, most importantly, can you trust a fitness tracker or smartwatch to give you an accurate reading?
We delve into the murky world of VO2 Max and enlist the help of Garmin's Fitness and Wellness product manager, Shane Harman.
What is VO2 Max?
VO2 Max is an abbreviation for the maximum volume of O2 (oxygen) which is transferred to the blood. As this level changes and increases as you start exercising, VO2 Max is the level at which the amount of oxygen you processes hits its maximum and plateaus.
VO2 Max is measured in millilitres of oxygen processed per kilogram of body weight every minute.
The better your aerobic fitness, the more oxygen you can pass into your body – so this is the best metric out there to check your performance.
Which devices track VO2 Max?
There are plenty of wearables to choose from if you're looking to get up close and personal with your VO2 Max.
Why is it essential for fitness fans?
So what are the benefits of knowing your VO2 Max? We're throwing this one over to Garmin, which makes VO2 Max a big part of the data measured by its top-end devices:
"VO2 Max is displayed as a number, anywhere from 40-80 (the total millilitres of oxygen that you can process per minute). You can therefore understand in black and white your current level of fitness," said Harman.
"By implementing training you should hopefully see that number increase. If it is not increasing, your training is not having the desired effect and you can look to make some changes," he continued.
How can I benefit from knowing VO2 Max?
We've written thousands of words here at Wareable about heart rate, and how it's a constant figure that can provide a baseline for your training. While that's true, it's only a constant measure of effort e.g. how hard you're working. In reality, it can be hugely affected by heat, terrain, fatigue to name but a few outside issues.
Harman says that VO2 Max isn't affected by outside factors, and while it does a different job to training by heart rate, it's an unbeatable tool for gauging your fitness.
"It's not like heart rate which is a very personal, subjective matter and open to influence by many external factors. You can compare a VO2 Max figure to a friend, or fellow athlete to understand how you stack up, it may therefore be useful in understanding the level of fitness you need to reach in order be competitive at a certain level of competition," he said.
How is VO2 Max calculated?
Garmin uses a company called Firstbeat for its physiological data – and the company's algorithms are prevalent on a host of wearable devices. Jabra, Suunto, Garmin, Samsung, Microsoft all use Firstbeat's data and there's plenty of detail about how it's worked out.
A proper VO2 Max test done in a lab requires you to wear a mask while completing an increasingly strenuous treadmill run, and the volume of oxygen entering and leaving your lungs is measured. Clearly, that's not within the capability of a wristband or watch, so measurements are always going to be an estimate.
The key is getting your age, sex and weight inputted into the device correctly, as this data underpins many of the assumptions your device will make about you. But the next crucial elements are heart rate and speed, which can both be accurately measured from wearable devices. We're going to ignore the issue of optical heart rate accuracy here, but clearly, bad data is going to cause anomalous VO2 Max scores.
1) The personal background info (at least age) is logged.
2) The person starts to exercise with a device that measures heart rate and speed.
3) The collected data is segmented to different heart rate ranges and the reliability of different data segments is calculated.
4) The most reliable data segments are used for estimating the person's aerobic fitness level (VO2max) by utilising either linear or nonlinear dependency between the person's heart rate and speed data.
How to change your VO2 Max score
Working on your aerobic activity is still the best way to affect your VO2 Max score, and Firstbeat says that you should be able to enact a physiological change in 4-12 weeks.
"A beginner may increase his/her fitness level during 4-10 weeks of successive training by up to 10-20%," its white paper says.
But it's also key to make sure that your exercise intensity matches your VO2 Max. If you're not training hard enough you could see levels decreasing.
How accurate are VO2 Max estimates?
Wearable tech is based on a lot of estimations, and we often compromise on accuracy for convenience. That's certainly the case here, where a proper VO2 Max test is expensive, gruelling and uncomfortable.
There isn't data for all VO2 Max devices, but in its white paper, Firstbeat claimed a 95% accuracy, based on 2,690 runs from 79 individuals. We'd judge that to be a pretty decent compromise, given the simplicity of getting the data.
"In a vast majority of the measurements, the error was below 3.5 ml/kg/min and the error was evenly distributed around the mean value. For perspective, the error in a typical indirect sub-maximal test is 10-15% and in a direct laboratory test about 5%," the report said.
Can your VO2 Max score be wrong?
You do have to be careful about VO2 Max. When running uphill, your heart rate is typically higher and your speed is slower, which could be evaluated as a lower fitness level. The opposite can be true for downhill runs.
Devices such as Garmin and Fitbit keep tabs on your score over time, so if you're mixing up your routes, the average should settle down. Make sure you get out plenty of times before you start obsessing over your scores.