Cardiac performance is a major determinant of aerobic fitness. It depends on cardiac output that can be varied by changes in stroke volume (SV) and heart rate (HR). SV is driven by numerous factors such as loading conditions (i.e. preload and afterload) and intrinsic myocardial properties (i.e. contractility, relaxation). During exercise, cardiac sympathetic activity increases HR and both ventricular contractility and relaxation. Venous return increases, due to both sympathetic vasomotor nerves that induces venoconstriction in the splanchnic circulation and skelatal muscle contractions that compress veins in the limbs. Moreover, afterload decreases consequently to the vasodilation in the exercise skeletal muscles that induces a fall in systemic vascular resistances. Human studies observed that, as a result of these adaptations within the heart and outside it, SV increases from rest to submaximal exercise (about 50 of maximal aerobic power), and then remains constant from submaximal to maximal exercise, that can be explained by a lower LV filling time compensated by a higher myocardial contractility. HR increases in proportion to oxygen consumption and reaches a maximum of about 180–200 beat per minute during maximal exercise in young adults. The changes in SV and HR result in a drop of cardiac output from 4–7 L. min⁻¹ at rest to 15–20 L. min⁻¹. Cardiac performance can be improved by aerobic training. At rest, trained subjects exhibited a cardiac enlagement named as “athlete's heart” and including increases in LV chamber size and wall thickness. LV systolic function appears to be normal in athletes, both when measured at rest and during exercise. LV diastolic function is on average normal at rest, but enhanced during exercise wich favours adequate filling of the LV at high heart rate. These adaptations result in a higher cardiac reserve during effort in athletes. On overall, cardiac performance during exercise has been evaluated mainly using hemodynamics parameters such as mitral filling patterns or ejection fraction. New non-invasive tools based on Doppler echocardiography (e.g. Tissue Doppler or speckle tracking imaging) and cardiac magnetic resonnance imaging have the potential to evaluate intrinsic myocardial performance in the future.