Stroke volume

Stroke volume is the amount of blood the heart ejects with each beat, typically around 70 mL at rest in an average adult. Multiplied by heart rate it gives cardiac output, the total volume pumped per minute.

It matters because it is the trainable half of that equation. Maximum heart rate is set largely by age and barely moves with training, so improvements in the heart's contribution to aerobic capacity come almost entirely from ejecting more per beat.

Endurance training increases it through several routes: a larger, more compliant left ventricle that fills more completely, greater blood volume, and stronger contraction. Those changes take months, which is why cardiac adaptations lag behind the faster mitochondrial changes in muscle.

It is also the explanation for a low resting heart rate in trained people. If each beat moves substantially more blood, fewer beats are needed to meet the same resting demand, so a resting rate in the forties in an endurance athlete is a consequence rather than a curiosity.

During exercise it rises early and then plateaus at moderate intensities in most people, with further increases in cardiac output coming from heart rate. Highly trained athletes are a partial exception, continuing to increase it further into the intensity range.

Falling stroke volume is what heart failure looks like physiologically, producing breathlessness, fatigue and poor exercise tolerance, which is why exercise capacity is a genuinely useful clinical measure rather than a fitness vanity.

In practice you cannot measure it outside a lab, and its consequences are free to watch: resting heart rate, the pace you hold at a given heart rate, and how quickly your heart rate falls after a hard effort.