VO2 max
VO2 max is the maximum rate at which your body can take in and use oxygen during intense exercise, measured in millilitres of oxygen per kilogram of body weight per minute (ml/kg/min). It sets the ceiling of your aerobic engine.
How VO2 max is measured
The gold standard is a lab test on a treadmill or bike, breathing through a mask while the intensity rises to exhaustion. Sports watches estimate VO2 max from heart rate and pace or power on your runs and rides; the number is useful for spotting trends even if it isn't exact.
What a good VO2 max is
Values depend heavily on age, sex and genetics. Many untrained adults fall around 30–45 ml/kg/min, trained endurance athletes often reach 50–65, and elite men and women can exceed 70–80. It declines with age, though training slows that decline.
How to improve it
Intervals of three to five minutes at a hard, near-maximal effort — around 106–120% of FTP on the bike, or close to 3K–5K race pace running — with equal recovery are the classic VO2 max workout. A big base of easy training supports it, and VO2 max is only one part of endurance performance alongside threshold and efficiency.