Which physiological change occurs to heart rate with increased fitness levels during rest?

As fitness levels increase, particularly with regular aerobic training, the heart becomes more efficient at pumping blood. This improved efficiency is due in part to physiological adaptations such as an increase in the size and strength of the heart muscle and an improved ability of the cardiovascular system to deliver oxygen to the muscles. As a result of these adaptations, the resting heart rate tends to decrease.

A lower resting heart rate indicates that the heart can pump a greater volume of blood with each beat, requiring fewer beats per minute to maintain adequate circulation at rest. This is a hallmark of cardiovascular fitness and is often used as an indicator of overall heart health. Therefore, as an individual becomes more fit, the heart rate reduces during periods of rest, demonstrating the heart's ability to work less hard when the body's demand for oxygen is lower.