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VO2 max is<\/span> an intimidating word for an easy-to-understand biometric: It\u2019s how well your body uses oxygen when you push yourself. Short for \u201cmaximal oxygen uptake,\u201d it\u2019s 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.<\/p>\n I tested mine last year at Canyon Ranch\u2019s 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 The test is simple, if mildly sadistic: As the workout intensifies, your oxygen consumption rises until it doesn\u2019t. 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\u2019ve gone longer. That lingering doubt is part of what makes VO2 max such a maddening and compelling metric.<\/p>\n Jump To<\/strong><\/p>\nmy suffering<\/del> how close I was to maxing out on a scale of 1 to 10.<\/p>\n