What’s In a Number? Here’s a 101 on VO2 Max and What it Means for You


Just like you learned in science class, when you exercise, you start to breathe harder to get much needed oxygen into your bloodstream. And of course, if you keep boosting your intensity—whether you’re walking or running faster, for example—eventually you’ll reach a point where you feel “out of breath.” When you’ve reached a point where you can’t breathe hard enough to keep up with your pace, you’ll have to stop or slow down to “catch your breath.” It’s this point in your experience that illustrates V02 Max, one of the measurements scientists use to evaluate fitness, and the measurement Fitbit estimates to provide your Cardio Fitness Score.

The traditional VO2 Max test takes place in a lab setting and results in a measurement that generally falls between 20 and 90*. The volume (V) represents how much oxygen (02) you can process (measured through inhaling and exhaling) when you are working at your maximum aerobic level (Max)—a level you can build up to and sustain for about eight minutes, going all out for the last minute, says coach and exercise scientist Steve Magness, author of The Science of Running. In other words, your VO2 Max is a measurement of how efficiently your body can process oxygen. And when you’re efficient, and can more easily get needed oxygen into your body, you’ll be fitter and better able to complete everyday tasks easily.

How does it all work?

If we took a Magic School Bus into your lungs, and followed the air you inhale, we could chase oxygen through the lung wall where it gets grabbed by red blood cells. The heart then pumps the oxygen-carrying blood cells to the muscles. The oxygen is transferred to muscle tissues, which use it to release energy from stored sugars to power muscular contraction. Carbon dioxide, a waste product of this reaction, has to then be transferred back through the system and exhaled by the lungs.

“Every step along the way is important and affects your V02 Max,” says Magness. While you experience it as shortness of breath, any factor that limits your measurement could be unique to you, whether it may be your heart function, or blood cells. Respected exercise scientist Tim Noakes believes the limit actually comes from your brain, subconsciously shutting down other muscles before they actually reach their physical limits to ensure your heart doesn’t lack the oxygen it needs. Regardless of what limits your VO2Max, the number provides a good measurement of how much work you can do at your maximum oxygen-processing capacity.

How exercise affects V02 Max

Exercise of all kinds can improve the cardiorespiratory system from top to bottom, because your body will want to adapt to any form of stress by becoming stronger and more efficient. As you get fitter, you gain ability to take in more air with each breath, and the strength to breathe in and out faster. You build more alveoli, the sacs in the lungs at the transfer point, and increase the number of capillaries, the tiny blood vessels where oxygen transfer takes place. You gain capillaries at the muscle end of the system as well, so you can transfer more oxygen to the tissues, and, at the cellular level, you increase the number and the effectiveness of your mitochondria, the cellular energy factories that process oxygen.

When you exercise and your body adapts to become more efficient, you increase your ability to take in and use more oxygen, and thus produce more work.

Losing extra weight can also help your number go up, as weight loss can help your body operate more efficiently.

What’s in a number, anyway?

“Knowing your VO2 Max number gives you an indicator of your strengths and weaknesses,” Magness says. “If it is a really high number, you know you have a really big engine.” Translation: You have a great cardiovascular (aerobic energy) system and are able to push hard for a long time.

If you’ve got a lower VO2 Max number or Cardio Fitness Score, you might have a smaller “engine,” and can focus your work on building a higher capacity to burn energy and put it to use. If your number isn’t as high as you might like, there’s good news: While V02 Max or Cardio Fitness Score gains (and measurements) can plateau for highly trained athletes, most people—especially those just starting fitness programs—can see fast improvement by doing aerobic training at all levels of intensity, from long steady workouts in your fat-burning and cardio zones, to high-powered sessions that get your heart-rate near max. You can also elevate your heart rate during high-intensity strength training sessions.

You can then monitor how your number changes as you gain in aerobic capacity—and build that bigger engine—to power you to new personal records.

Cardio Fitness Score is exclusively available on Fitbit Charge 2. Learn more about how an athlete uses Cardio Fitness Score and what you can do to improve yours.


* VO2 Max as measured using standard testing methodology in a lab setting and varies by age and gender. Source: Shvartz, E, Reibold, RC. Aerobic fitness norms for males and females aged 6 to 75 years: a review. Aviat. Space Environ. Med. 1990; 61:3-11.

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  • Without commenting on most of the article, I need to inform you: the correct term is, as you may know, VO2. Several times this article uses “V02” by accident, but the “O” is a letter standing for oxygen, rather than the number zero. Please fix this for the sake of nerds everywhere.

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