VO2 max is quoted as a single figure and treated as a summary of fitness. It measures one specific physiological quantity, and knowing which one clarifies what it does and does not describe.
The measurement is of gas, not effort
In a laboratory test, the participant exercises at progressively increasing intensity while breathing through a mask connected to gas analysers.
The equipment measures how much air is moved and the concentrations of oxygen and carbon dioxide in it. The difference between inhaled and exhaled oxygen gives the rate of consumption.
As intensity rises, oxygen consumption rises with it until it stops increasing despite further increases in workload. That plateau is the maximum.
The number is normalised to body mass
Results are usually reported per kilogram of body weight per minute, because a larger body consumes more oxygen in absolute terms.
Normalising allows comparison between people of different sizes, and it is why the figure can change without any change in cardiovascular function if body mass changes.
Absolute values are also used, and matter more for activities where the body is supported rather than carried.
The heart, not the lungs, sets the ceiling
In healthy people the lungs are not the limiting step. Blood leaving the lungs is close to fully saturated with oxygen even during maximal effort.
The constraint is how much oxygenated blood the heart can deliver per minute, which is stroke volume multiplied by heart rate.
Maximum heart rate is largely fixed by age and changes little with training, so improvement comes mainly from a larger stroke volume.
Trained hearts move more blood per beat
Endurance training increases the volume of the left ventricle and improves its filling, so each contraction ejects more blood.
Blood volume itself expands, which supports that filling, and this is among the earliest adaptations to appear.
Muscle changes matter too: more capillaries and more mitochondria mean a greater share of delivered oxygen is extracted and used.
It predicts less about performance than expected
Two athletes with the same maximum can perform very differently, because sustained performance depends on what fraction of that maximum can be held for a long period.
Efficiency of movement is a further variable, since less oxygen used at a given pace produces the same result from a smaller number.
Maximal testing is physically demanding, and anyone with a heart condition or symptoms during exertion should be assessed medically before undertaking it.