Why does resting heart rate decrease with regular aerobic activity?

The decrease in resting heart rate with regular aerobic activity is primarily attributed to the increase in stroke volume due to stronger heart contractions. When an individual engages in regular aerobic exercise, such as running, swimming, or cycling, their heart becomes more efficient and stronger over time. This means that the heart can pump a greater volume of blood with each beat, thereby increasing stroke volume.

As stroke volume increases, the heart doesn't need to beat as frequently to supply the body with the same amount of blood—this leads to a lower resting heart rate. When the body is more fit, the efficiency with which it delivers oxygen and nutrients to muscles and removes waste products also improves, contributing to overall cardiovascular health. Thus, a lower resting heart rate is an indicator of improved heart function, reflecting both the benefits of aerobic conditioning and overall fitness levels.