Post-COVID-19 syndrome (PCS) is a condition that occurs following an acute infection with the SARS-CoV-2 virus (COVID-19). PCS is characterized by symptom persistence for at least 12 weeks after the infection or the emergence of new symptoms during this period. Current guidelines propose several criteria for diagnosing PCS, including persistent symptoms from the acute COVID-19 phase, new symptoms leading to health limitations, symptoms assumed to be related to COVID-19 after the acute phase, and worsening of pre-existing underlying conditions. PCS is a complex and multisystemic disorder, with symptoms ranging from chronic fatigue, decreased physical performance, muscular weakness, and pain to cognitive impairment (often referred to as brain fog) and mental and psychological distress resembling a post-traumatic stress reaction. The exact causes of PCS are not fully understood but may involve processes such as endothelial dysfunction, cytokine storm, and increased oxidative stress affecting various organs and structures in the body. The incidence of PCS varies depending on the population studied and the severity of symptoms considered, with estimates ranging from 7.5% to 41% among non-hospitalized patients with acute infection.

Medical rehabilitation is essential for patients with persistent PCS, but there are limited studies on its efficacy, especially in the context of aerobic endurance training. Physical exercise, including aerobic training, has shown positive effects in various diseases, and it is suggested that it may be beneficial for PCS as well. However, there is limited evidence regarding the effectiveness of aerobic endurance training in reducing the burden of decreased physical performance in PCS. Moreover, there is a lack of guidelines for PCS patients related to exercise-based rehabilitation. Aerobic endurance training has been proven to have positive effects on multiple physiological functions and is recommended for chronic heart and pulmonary diseases. Studies have also suggested that aerobic interval training may be advantageous for PCS patients, as it reduces ventilatory demand and may alleviate dyspnea and breathing effort. This study aims to investigate the efficacy of moderate-intensity aerobic endurance training performed as either continuous training and interval training for the medical rehabilitation of PCS patients.

The study will include PCS patients referred for inpatient medical rehabilitation in Germany with a history of (at least one) Covid-19 infection and ongoing or newly expressed performance deficits lasting for at least 3 months prior to recruitment. Performance deficits will be documented according to the recent consensus statement, with the cluster of lead symptoms including fatigue/exercise intolerance, shortness of breath, and cognitive dysfunction impairing activity of daily living and everyday functioning. A detailed clinical workup will be performed, and the history of comorbidities and current medication will be documented. After admission, patients will be randomized to a continuous training (CT) or interval training (IT) group. Patients will receive individual medical rehabilitation including a combination of strength, respiratory and cognitive training, as well as physio-, psycho- and nutrition therapy, unaltered for both groups. Data on medical rehabilitation including prescriptions of therapeutic actions and participation will be recorded for all patients. Symptom-limited spiroergometry at admission and after 4-6 weeks of inpatient rehabilitation (before discharge) will be performed to analyze improvements in exercise capacity. Validated questionnaires will be used to document patients' disease perception.