My watch says my VO2max is 44. It has never measured a molecule of my oxygen. Aerobic capacity is one of the strongest mortality predictors we have: in a 122,007-patient Cleveland Clinic cohort, the least-fit carried five times the risk of dying, a bigger jump than smoking (1.41), diabetes (1.40), or heart disease (1.29). That part is solid. The watch is not. Against the lab, the Apple Watch was off by about 6 units on average, with an error band roughly 24 wide, the difference between "unfit" and "athlete." No watch measures oxygen; it extrapolates from pace and heart rate against a population template. So read the trend, not the digits. If your estimate climbs over a season, that direction is probably real. Aerobic capacity is trainable, and that part is free. The watch is the one piece that costs four hundred dollars and knows the least.