Fontan operation is a palliative surgical procedure performed in patients with a functional or anatomical single ventricle. After the Fontan procedure, patients face significant risk of morbidity and mortality, and patients with complicated congenital heart disease need to be followed for life by a cardiologist experienced in the care of patients. Annual follow-up is recommended in uncomplicated patients, but more frequent follow-up is required in patients with postoperative complications. Survival rates of 15-20 years after the operation vary between 60-85%. Since there is no ventricular pump to push blood into the pulmonary artery circulation in patients undergoing the Fontan procedure, there is increased systemic venous pressure compared to normal biventricular circulation. Most of the common complications after Fontan are directly or indirectly related to elevation of central venous pressure. These complications include arrhythmia, cyanosis, decreased exercise capacity, ventricular dysfunction, heart failure, atrioventricular valve insufficiency, protein-losing enteropathy, thrombosis, bleeding, venous insufficiency. Patients undergoing the Fontan procedure generally do not experience a normal increase in cardiac output during exercise; therefore exercise capacity is limited. It has been emphasized that hemodynamic, vascular and muscular factors may be effective in decreasing exercise capacity. In previous studies, it has been reported that cardiac output is the main factor affecting exercise capacity in patients with Fontan, and this is due to insufficient increase in stroke volume. However, in previous studies, it has been reported that factors such as insufficient precision of measurements (resting echocardiography in supine position, resting cardiac MRI) and failure to see optimal functions with maximum effort are insufficient to interpret the effects of hemodynamic profile on exercise capacity. Therefore, it is important to evaluate the global and regional myocardial functions in more detail and to determine the hemodynamic profile with speckle tracking echocardiography. In a study, it was reported that atrial tension parameters measured by speckle tracking echocardiography in the Fontan circulation were affected and this was related to functional exercise capacity.. In addition to the hemodynamic profile, the effects of muscle oxygenation, arterial stiffness and peripheral muscle strength on exercise capacity were also mentioned in some separate studies. Changes in muscle oxygenation in children undergoing the Fontan procedure indicate that the balance between the oxygen demand of the tissues and the amount of oxygen supplied is impaired. It is thought that this may be a mechanism that causes a decrease in exercise tolerance in this patient population. Also, the function of peripheral muscles by acting as a pump is particularly important for venous return in the Fontan circulation, and skeletal muscle mass is reduced in patients with Fontan. Studies have shown that leg lean mass is closely related to an increase in blood flow during exercise, and skeletal muscle contractions can generate pulsatile pulmonary blood flow in some patients with Fontan circulation. A relationship was also found between arterial stiffness and cardiorespiratory fitness in pediatric and adult patients with fontan circulation, and it has been reported that practices to increase exercise capacity may be important for the preservation of vascular structures. Exercise capacity in children and adults undergoing the Fontan procedure is evaluated with field tests such as the cardiopulmonary exercise test (CPET) and the 6-minute walk test (6MWT). CPET is very important in terms of providing objective data in terms of cardiopulmonary fitness in the evaluation of maximal exercise capacity and being the gold standard. However, in terms of determining the functional levels of individuals and maintaining their daily living activities, submaximal exercise capacity, which is evaluated with both cheap and practical 6MWT, also gains importance. When the literature was searched, there was no study examining the effects of hemodynamic, vascular and muscular profile on submaximal and maximal exercise capacity. Therefore, in this study, it is aimed to determine and compare the effects of hemodynamics, arterial stiffness and muscle oxygenation on maximal and submaximal exercise capacity in patients with single ventricle who underwent Fontan procedure.