A standardized exercise echocardiography protocol using fixed heart rate (HR140) and absolute workload detects functional differences in cardiac performance between endurance-trained athletes and untrained individuals that are not observable using relative exercise intensities or resting measurements.

Endurance training makes the heart's main pumping chamber larger and more stretchy, so it can hold more blood. At rest, the heart beats slower, giving more time to fill. When the heart rate is forced to 140 beats per minute during exercise, the trained heart still fills faster and more completely because its chamber stretches more easily and pulls in more blood, even though the filling speed measured by standard ultrasound markers doesn't change. This lets it pump out more blood than the untrained heart under the same heart rate, something you can't see at rest or when comparing how hard people feel they're working.