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

Cardiac filling is constrained by a reduced ejection time and diastolic period as heart rate increases during exercise. We compared cardiac filling dynamics in endurance athletes and controls during exercise to determine how athletic status influences hemodynamic constraints during exercise. Thirty-two participants (21 endurance athletes and 11 controls) underwent exercise echocardiography at 20, 40, and 60% of peak power output. We measured cardiac timing, stroke volume (SV), and flow rates (SV indexed to body surface area and phase duration). Data were analyzed using linear mixed models. Athletes demonstrated resting bradycardia (44 ± 8 vs. 68 ± 14 beats/min, P < 0.001), through an extended ejection time (296 ± 38 vs. 247 ± 30 ms, P < 0.001) and diastolic filling (811 ± 229 vs. 560 ± 161 ms, P = 0.005). Preexercise cardiac outputs were matched (P = 0.969). During exercise, there was a significant group × exercise interaction for left ventricular (LV) ejection time (P < 0.001) and diastolic period (P < 0.001), indicating distinct athletic adaptations. Although the systolic-to-diastolic (S/D) ratio was similar at rest, a significant interaction occurred during exercise (P = 0.013). Athletes also achieved a greater cardiac output response (group × exercise interaction: P < 0.001). Indexed stroke volume increased similarly in both groups (interaction: P = 0.271) yet remained significantly higher in athletes (group effect: P < 0.001). Consequently, athletes maintained superior absolute volumes throughout the protocol, resulting in greater high-intensity stroke volumes. Athletes are able to produce greater reductions in both LV ejection and diastolic filling time and generate higher stroke volumes during exercise, permitting greater cardiac outputs and a greater exercise response.NEW & NOTEWORTHY This study reveals that endurance athletes' slow resting heart rates provide additional time for ventricular filling. During intense exercise, athletes compensate for dramatically shortened filling times by achieving remarkably higher flow rates, particularly during systole. These differences allow athletes to maintain larger stroke volumes at both rest and at high heart rates, producing superior cardiac outputs and highlighting a key physiological mechanism underlying endurance performance.