Endurance athletes exhibit a lower systolic-to-diastolic time ratio during high-intensity exercise (0.82) compared to sedentary controls (1.00), indicating a preserved diastolic filling advantage under maximal cardiac stress.

The heart of an endurance athlete becomes larger and more elastic from years of training, allowing it to fill with blood faster and more completely between beats. Even when the heart beats very fast during intense exercise, it maintains more time for filling than a non-athlete's heart by using its extra volume and faster relaxation to move blood in quickly. The heart also starts using this extra filling capacity early during exercise, shifting its timing to prioritize speed, which lets it keep pumping more blood without sacrificing the time needed to refill.