Cardiovascular fitness has a specific technical definition: the maximum rate at which the body can take in, transport and use oxygen during sustained effort. Everything else described as fitness is either a proxy for that or a different quality altogether.
The measure is a rate of oxygen use
Sustained effort is powered by processes requiring oxygen, so the ceiling on sustained output is set by how fast oxygen can be delivered to and consumed by working muscle.
Laboratory testing measures this directly by analysing inhaled and exhaled air while workload increases until the rate stops rising despite further effort.
The result is expressed relative to body mass, which allows comparison between people of different sizes.
Three separate systems set the ceiling
Oxygen must be taken into the lungs, carried by blood the heart pumps, and extracted and used by the muscle. Any of the three can be the limiting factor.
In healthy people, the constraint is usually cardiac output — how much blood the heart moves per minute — rather than the lungs.
Training raises capacity partly by increasing the volume the heart ejects per beat and partly by increasing the density of capillaries and mitochondria in trained muscle.
Field and wearable estimates work indirectly
Direct testing requires equipment and maximal effort, so most estimates infer capacity from the relationship between heart rate and workload at submaximal intensities.
A fitter person sustains a given pace at a lower heart rate, and that relationship can be extrapolated towards the maximum.
The extrapolation depends on assumptions about maximum heart rate that vary substantially between individuals, so wearable figures track change over time better than they establish an absolute value.
Threshold matters more than maximum for most people
Maximum capacity is only part of the picture. What determines endurance performance is the proportion of that maximum a person can hold for a long period.
That sustainable fraction responds strongly to training and can improve considerably even when maximum capacity has plateaued.
It is also the quality most relevant to daily life, since ordinary activity is sustained rather than maximal.
Why the measure gets clinical attention
Aerobic capacity is one of the stronger predictors of long-term health outcomes across large populations, and it responds to training at any starting point.
Improvements are typically largest, proportionally, among those starting from the lowest capacity, which is the opposite of how fitness advice is usually pitched.
Anyone with cardiac symptoms, a diagnosed heart condition or a long inactive period should have starting intensity set with a clinician rather than by a formula.