VO2 max (maximal oxygen uptake)

VO2 max is the maximum rate at which a person's body can take up and use oxygen during intense, sustained exercise, expressed in milliliters of oxygen per kilogram of body weight per minute (ml/kg/min). It is widely regarded as one of the best laboratory measures of aerobic, or cardiovascular, fitness, since it captures the combined capacity of the heart, lungs, and muscles to deliver and use oxygen under maximal effort.

The reference measurement is a graded exercise test in a lab, where a person exercises at increasing intensity on a treadmill or bike while breathing into a mask that measures oxygen consumption directly until they reach exhaustion. Consumer wearables instead estimate VO2 max from resting heart rate, heart rate variability, pace, and age using proprietary regression models, and these estimates are consistently less precise than a lab test, so day-to-day changes reported by a device are better read as directional trends than exact values.

VO2 max is used by athletes and coaches to track aerobic fitness over a training block and to set training zones, alongside training load and resting heart rate as inputs to load management decisions. It naturally declines with age and inactivity. This is informational context, not a fitness or medical assessment of any individual: interpreting a specific value for health purposes should involve a qualified professional, and VO2 max is one input among several tracked in quantified self practice.