VO2 max is an intimidating word for an easy-to-understand biometric: It’s how well your body uses oxygen when you push yourself. Short for “maximal oxygen uptake,” it’s been the gold standard for assessing cardiorespiratory fitness since the 1950s. Until recently, it’s mostly lived in research labs and elite training centers, helping coaches squeeze every last drop of performance out of elite skiers, runners, and cyclists. Today, VO2 max has escaped the lab and gone mainstream, landing on fitness trackers, longevity podcasts, and wellness clinics that promise to put a number on how fit you are.

I tested mine last year at Canyon Ranch’s Performance Lab Center, where they fitted me with a heart rate monitor and a tight, Darth Vader-esque mask at 8 am. After a night of little sleep and zero food, the lab technician put me on a treadmill and ramped up the speed and incline until I couldn’t keep going. No music. No hyper-caffeinated instructor yelling positive affirmations. Every few moments, the technician would ask me to rate my suffering how close I was to maxing out on a scale of 1 to 10.

The test is simple, if mildly sadistic: As the workout intensifies, your oxygen consumption rises until it doesn’t. That plateau marks your VO2 max, the upper limit of your aerobic capacity. When my test ended (right around the moment my patience did), I was told my results would arrive the next day. Still gasping, my first thought was that I could’ve gone longer. That lingering doubt is part of what makes VO2 max such a maddening and compelling metric.

VO2 Max, Explained

“VO2 max is an objective measure of how a human energy system can take in oxygen and utilize it during exercise,” says Elizabeth Gardner, an associate professor of orthopedic surgery at Yale University School of Medicine.

Your body is always consuming oxygen to produce energy, but VO2 max captures the maximum amount of oxygen your system can use at peak effort. It’s essentially what happens when you’re working as hard as you possibly can. A high VO2 max means your heart and lungs are more efficient at delivering blood to your muscles, and your muscles are better at extracting oxygen from your blood to generate adenosine triphosphate (ATP).

“It’s the best predictor we currently have for premature mortality,” says Malene Lindholm, an exercise physiologist and senior research engineer at Stanford Cardiovascular Medicine. Higher VO2 max levels correlate with a lower risk of cardiovascular disease, type 2 diabetes, stroke, and certain cancers. Beyond disease prevention, stronger cardiovascular fitness is also linked to better sleep, improved mood, and a higher overall quality of life.

“It’s especially useful for anyone serious about peak performance and endurance sports,” says Tyler McQuality, associate director of the Center for Sports Innovation at Illinois Tech University. “There are certain intervals you can run based on VO2 max pacing and effort.”