Influence of Postural Position on Cardiac Output During Incremental Exercise in Endurance-Trained Athletes

Background: Recent evidence indicates that the stroke volume of endurance-trained athletes may continue to increase throughout incremental to maximum exercise in the upright position. However, little is known about the effects of changes in postural position on cardiac function during incremental exercise in endurance-trained cyclists. Objective: To evaluate the effects of changes in postural position on cardiac function during incremental exercise in endurance-trained athletes. Methods: The cardiovascular function of 10 endurance-trained cyclists (Age = 26 ± 5 (SD) yr, VO2max = 68 ± 4 mL/kg/min) was evaluated during incremental exercise in the supine and upright positions (counterbalanced) using radionuclide ventriculography. Results: The athletes were able to increase their stroke volume during incremental to maximal exercise in both the upright and supine positions, in the face of a reduced time for diastolic filling. The percent changes in end-diastolic volume and stroke volume were greater in the upright position versus the supine position (P < 0.05). The increase in cardiac output throughout incremental exercise in both postural positions was related to changes in heart rate, myocardial contractility and the use of the Frank-Starling mechanism. Forward stepwise regression revealed that heart rate and end-diastolic volume were the strongest predictors of cardiac output throughout incremental exercise in both postural positions. Postural position had a large influence on the impact heart rate and end-diastolic volume had on cardiac output. In the supine position, heart rate and end-diastolic volume explained 63 and 33%, respectively, of the variance in cardiac output during incremental exercise. In the upright position, heart rate and end-diastolic volume explained 27 and 68%, respectively, of the variance in cardiac output during incremental exercise. Conclusions: Highly trained endurance athletes make use of the Frank-Starling mechanism throughout incremental to maximal exercise in the supine and upright positions. Postural position has a significant effect on the relative contribution of heart rate, myocardial contractility, and the Frank-Starling mechanism to the increase in cardiac output during exercise conditions. The relative changes in end-diastolic volume and stroke volume are limited during supine exercise in comparison to upright exercise perhaps owing to the impact of the pericardium on ventricular filling.