Aim:  The objective was to assess left ventricular (LV) volumes at rest and during upright submaximal exercise in endurance athletes to see whether changes in heart volume could explain the large predicted increase in cardiac output in endurance athletes.

Method:  Contrast echocardiography was used to assess changes in LV volumes during upright bicycle exercise in 24 healthy male endurance athletes. Maximal oxygen uptake and oxygen pulse were measured by using cardiopulmonary exercise testing.

Results:  From rest to exercise at a heart rate of 160 beats min⁻¹ end‐diastolic volume increased by 18% (P < 0.001) and end‐systolic volume decreased by 21% (P = 0.002). Stroke volume showed an almost linear increase during exercise (45% increase, P < 0.001). The increase in end‐diastolic volume contributed to 73% of the increase in stroke volume. No significant differences were observed between stroke volume calculated from LV volumes with contrast echocardiography and stroke volume calculated from oxygen pulse at heart rates of 130 and 160 beats min⁻¹. Using the linear regression equation between oxygen uptake and cardiac output assessed by echocardiography during exercise (r = 0.87, P = 0.002), cardiac output at maximal exercise was estimated at 33 ± 3 L min⁻¹, with an estimated increase in stroke volume by 69% from rest to maximal exercise.

Conclusion:  By using contrast echocardiography, a large increase in stroke volume in endurance athletes could be explained by an almost linear increase in end‐diastolic volume and an initial small decrease in end‐systolic volume during incremental upright exercise.