VO2Max is a powerful metric of longevity: every 1 ml/kg/min increase in VO2 max reduces risk of death by 9 percent. Clinical testing of VO2Max typically uses a respiratory mask during intense exercise, such as a treadmill or cycling. But wearables like Apple Watch estimate cardio fitness (their version of VO2 max) to within an accuracy of 1.2 ml/kg/min without requiring a mask. How do they do that? This article explains the science behind VO2 max, how the Apple Watch’s cardio fitness algorithm w...