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The adaptation to aerobic exercise that contributes to lower resting and sub-maximal heart rates is increased stroke volume. When an individual engages in regular aerobic training, the heart becomes more efficient at pumping blood. This means that with each contraction, the heart can eject a larger volume of blood—known as stroke volume—compared to when a person is sedentary.

As stroke volume increases, the heart doesn't need to beat as frequently to supply the body with the necessary oxygen and nutrients during both rest and sub-maximal exercise. Thus, over time, this leads to a lower resting heart rate because the heart is able to meet the body's demands with fewer contractions.

In addition to improving cardiovascular efficiency, this adaptation is beneficial for overall athletic performance and health, as a lower heart rate can indicate a well-conditioned heart and improved cardiovascular function. While factors such as increased oxygen uptake and changes in body composition also play roles in fitness, the most direct link to the reduction in heart rate during rest and sub-maximal exercises is the enhancement of stroke volume through cardiovascular adaptations from consistent aerobic exercise.