What is the primary effect of endurance training on the heart?

The primary effect of endurance training on the heart is an increase in heart chamber size, particularly the left ventricle. This adaptation occurs as a response to the increased volume of blood that needs to be pumped during prolonged physical activity. As an individual engages in consistent endurance training, the heart undergoes structural changes, such as myocardial hypertrophy, which enhances its ability to accommodate a greater volume of blood.

This increased chamber size allows for a greater stroke volume, which is the amount of blood ejected from the heart with each beat. As a result, although the resting heart rate may decrease over time due to improved efficiency, the overall cardiovascular system becomes more effective in delivering oxygenated blood to working muscles, ultimately enhancing endurance performance.

Endurance training promotes a more efficient cardiovascular response to exercise, evidenced by a higher stroke volume leading to improved cardiac output during activity, rather than a decrease in these values. Thus, option B correctly identifies a fundamental adaptation of the heart resulting from endurance training.