The systolic-to-diastolic time ratio in the cardiac cycle differs between endurance athletes and non-athletes during exercise.

Study: A race against time: timing constraints of cardiac filling and athletic performance.

During exercise, elite athletes' hearts change how they spend time pumping versus filling in a different way than non-athletes' hearts — even though both groups work harder, the athletes' hearts adapt more efficiently.

Study: Dynamic right ventricular and atrial volume responses to exercise in endurance-trained and untrained healthy individuals

Athletes' hearts pump more efficiently during exercise by squeezing harder and relaxing faster, which means the time spent pumping blood (systole) increases relative to the time spent filling up (diastole) — something that doesn’t happen as much in non-athletes.

Study: Female athlete's heart: Systolic and diastolic function related to circulatory dimensions

Athletes' hearts work differently during exercise — they spend more time relaxing and filling with blood and contract more strongly, which means the balance between squeezing and relaxing changes compared to non-athletes.

Study: Assessing the relationship between left ventricular diastolic and systolic function during exercise in healthy volunteers using a novel individualised stress echocardiography protocol

During exercise, the hearts of very fit people pump harder without increasing pressure in the filling phase, meaning the timing of squeezing and relaxing changes differently than in less fit people. This matches the claim that athletes’ hearts behave differently during exercise.