VO2 max appears both on a laboratory report and on a wrist device, and the two numbers are produced in entirely different ways. The gap between the measurement and the estimate explains why the figures often disagree.
What the measurement represents
VO2 max is the maximum rate at which the body can take in oxygen, transport it and use it in working muscle, expressed relative to body mass.
It is limited mainly by how much oxygenated blood the heart can deliver per minute, with muscle capacity and blood oxygen carrying capacity contributing.
Because it depends on the whole chain, it summarises the cardiovascular system's capability in a single figure better than any other single test.
The laboratory test forces the ceiling
A direct test involves exercise of progressively increasing intensity on a treadmill or bicycle while every breath is analysed for oxygen and carbon dioxide content.
As intensity rises, oxygen uptake rises with it until it stops climbing despite further increases in workload, and that plateau is the maximum.
The test requires a genuine maximal effort, which makes it uncomfortable and makes the result partly dependent on the participant's willingness to continue.
Estimates work backwards from submaximal data
A wearable device has no gas analyser, so it infers the value from the relationship between heart rate and pace or power during ordinary activity.
A fitter person sustains a faster pace at a given heart rate, and models trained on people who underwent real testing convert that relationship into a number.
The estimate therefore inherits the assumptions of the model, including a typical maximum heart rate for age, which varies widely between individuals.
Why the estimate drifts
Anything that changes heart rate independently of fitness moves the estimate: heat, illness, caffeine, poor sleep, dehydration or a stressful day.
Terrain and wind affect pace without affecting effort, so estimates from outdoor running are noisier than those from controlled conditions.
The consequence is that a single reading is weak evidence while a trend over several weeks under similar conditions carries real information.
What changes the underlying value
The value responds to training that spends time near the upper end of aerobic capacity, along with a base of longer easy work that builds the supporting circulation.
It declines with age and with inactivity, and the decline attributable to inactivity is generally larger than the decline attributable to age alone.
It also sits alongside other measures of fitness rather than replacing them, since strength, movement economy and fatigue resistance determine much of what a person can actually do.