VO2 max (maximal oxygen uptake, sometimes written V̇O2max) is the maximum rate at which an individual can consume oxygen during intense exercise. It is measured in millilitres of oxygen per kilogram of body weight per minute (ml/kg/min).
The measurement requires metabolic gas analysis (indirect calorimetry) under a progressive exercise test to exhaustion — in a lab, on a treadmill or cycle ergometer, with a face mask measuring inspired and expired oxygen and carbon dioxide. The point at which oxygen consumption stops increasing despite increasing effort is VO2 max.
VO2 max is one of the strongest predictors of endurance running performance, though not the only one. Running economy (how efficiently oxygen is converted to forward motion) and fractional utilisation of VO2 max (what percentage of VO2 max can be sustained at race pace) also contribute significantly.
Typical values: untrained adults 35–45 ml/kg/min; recreational runners 45–60 ml/kg/min; competitive runners 60–75 ml/kg/min; elite male distance runners often 75–90 ml/kg/min. The highest recorded VO2 max values are around 97 ml/kg/min (cross-country skiers).
VO2 max is trainable but largely genetically determined — you can improve it by 10–20% through endurance training, but the ceiling is set by genetics. Most elite runners have both high VO2 max and excellent running economy.
Do not confuse VO2 max with VDOT. VDOT is a performance-derived estimate, not a laboratory measurement. They are numerically similar but conceptually distinct.