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.