Thanks to more advanced features on the Apple Watch, Garmin, Polar, and other running watches, VO2max is the latest buzzy metric getting runners all worked up. People track this measurement because, in theory, the higher that number is, the more efficient you are while burning aerobic energy. Translation: You’ll be able to run faster while expending less effort.
But 'VO2 is a measurement of cardiorespiratory fitness, not of performance,' says Melissa M. Markofski, Ph.D., an assistant professor in the Department of Health and Human Performance at the University of Houston and an ASCM-certified exercise physiologist. It refers to the maximum rate at which your body can deliver oxygen to your exercising muscles; it affects how fast or far you can run but isn’t the only factor at play.
And a higher VO2max isn’t always better when it comes to performance, a 2019 case study published in the Journal of Applied Physiology found.
In the case study, an 18-year-old world junior champion time trial cyclist recorded the highest-ever value in a VO2max test; three years later, he ended up retiring after a short, underwhelming pro career. The reason he didn’t perform better? There could be a number of different reasons, but one is that he had the capacity for aerobic power, but his body didn’t use it efficiently.
'Let’s use a car as a metaphor,' says Filip Larsen, a researcher from the Swedish School of Sport and Health Sciences. 'If you try to boost your car’s performance by adding a turbo-charger, you will indeed increase top speed and horsepower, but the price to pay is worse fuel economy. The same thing seems to happen in human metabolism.'
Larsen was one of the authors of a response to that case study, which was also published in the Journal of Applied Physiology in 2019. 'Metabolic processes follow a bell-shaped trade-off between efficiency and power output,' he and his co-author, Mikael Flockhart, wrote. 'If the initial level of efficiency is low, an increase in efficiency indeed has a positive effect on power output (performance level). Conversely, if efficiency increases too much, the trade-off is a slower rate of the metabolic processes.'
Improving maximum power output and efficiency at the same time, they argued, isn’t possible. The reason why has to do with your mitochondria, the 'powerhouse' of each cell, says Larsen. 'They have to choose between being powerful (being able to produce lots of adenosine triphosphate, or ATP) or being efficient (producing ATP with as little cost of oxygen as possible).'
Here’s another example: Paula Radcliffe showed no changes in her VO2max between being tested as an 18-year-old and at 29 years old, a year before she set the women’s marathon record (a record that would stand for 16 years). If there were no changes in her VO2, her record-breaking improvement would have been largely due to an increase in running economy—which accounts not just for aerobic fitness, but also biomechanics and muscular endurance—an article published in the International Journal of Sports Science & Coaching found.
Following that logic, if you have two runners with the same VO2max, but one is more biomechanically efficient (i.e. they have better running posture, a better stride, or fewer imbalances or asymmetries), that runner is going to outperform the other if all other variables can be controlled.
When it comes to recreational runners, most of us are now measuring VO2max via wrist-based heart rate tracking. Most running watches use heart rate data (which can be affected by factors such as heat and altitude) and exercise data (like pace) to estimate VO2max. But using just those two data points can only provide an estimated VO2max. 'Differences in efficiency (i.e. running economy), maximum heart rate, running surface, and environmental factors can all affect your VO2max estimate,' says Larsen.
Compare that to the methods used in a lab setting: During a VO2max test, you run on a treadmill and progressively increase the speed while wearing a mask that analyzes your exhalation to determine the highest rate of oxygen your body can utilize. This provides a far more accurate VO2max estimate than a wrist-based sensor can.
Scientists have indeed determined that wrist-based heart rate tracking isn’t always accurate. Research published in the journal Cardiovascular Diagnosis & Therapy 2019 found that inaccuracies in runners’ data increased as they ran faster. Another issue: The darker your skin, the less accurate the reading, according to a 2017 study published in the Journal of Personalized Medicine.
Sure, putting a VO2max tracker on your wrist is a great way to make that data more accessible to runners who can’t afford the fancy lab test, which usually costs around $100 and $250 depending on location—especially because you may want to get tested regularly as your VO2max changes as you train. But a watch can’t account for every variable, and letting one metric dictate how you feel about your performance is a recipe for mentally bonking.
Instead, look at your heart rate at a given speed on a run. 'As your performance increases, your heart rate should be lower at that pace,' says Markofski. And if you’re running outside, just make sure to take the environment into account: If you’re comparing your performance on a flat run to a hilly run and you’re seeing a slower pace and higher heart rate, that’s not necessarily a sign of decreased fitness, but a sign that the terrain is a different kind of workload.
You can also look at how long it takes you to finish a specific distance rather than VO2max. 'If we’re concerned about performance, let’s say in a 5K, the best way to look at that is to run 5Ks consistently and see if your performance is increasing, decreasing, or staying the same,' says Markofski. If your performance is increasing, keep doing what you’re doing; if it’s not, then you may need to reevaluate your training to better serve your goals.
And for those runners who aren’t logging super high weekly mileage, 'it has clearly been shown that if you are running less than 75 miles per week, just trying to build up a higher mileage (at a good pace) will have tremendous effects on performance,' says Larsen.
Having all this technology at the touch of a fingertip is great—as long as you remember that you (not the computer on your wrist) are the best judge of your performance and what may or may not be affecting it.
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