What physiological mechanism most directly contributes to the increase in stroke volume observed in athletes?

The increase in stroke volume in athletes is primarily attributed to increased preload. Preload refers to the volume of blood in the ventricles at the end of diastole, which stretches the heart muscle fibers just before contraction. In athletes, several adaptations contribute to increased preload, such as enhanced venous return and greater blood volume due to improved cardiovascular fitness.

During exercise, the muscles create a demand for increased blood flow, leading to enhanced venous return to the heart. This higher volume of blood returns to the heart and fills the ventricles, increasing the preload, which stimulates the heart to pump more forcefully. This is in accordance with the Frank-Starling mechanism, where a greater preload results in a more powerful contraction and ultimately an increase in stroke volume.

While heart rate does play a role in cardiac output, it is the combination of heart rate and stroke volume that determines overall output, and in this case, the primary contributor to the increase in stroke volume in athletes is the increased preload. Decreased venous return and increased afterload are factors that do not typically lead to improved stroke volume in this context.