The aim of the study is to evaluate the influence of 6 week physical training and respiratory rehabilitation performed in outpatients rehabilitation clinic on quality of life, symptoms, physical endurance, mental state, force of respiratory and skeletal muscles and myokines profile in patients after COVID-19 infection.

The study of interventional, prospective character will be conducted in Rehabilitation Department of Medical University of Bialystok on total group of 60 participants less than 12 months after COVID-19 (coronavirus disease) infection. In all participants medical anamnesis, physical examination (including chest wall mobility), anthropometric measures, The Short Physical Performance Battery (SPPB), 6-minute walking test (6MWT six minute walking test) and treadmill stress test will be performed.

Medical history will consist of questions regarding course of COVID-19 and its consequences with special emphasis on dyspnea during daily activities in Borg scale and dyspnea in daily living in mMRC scale, coexisting diseases, treatment, lifestyle, etc. Quality of life will be assessed with use of The Short Form (36) Health Survey - SF-36), Post-COVID-19 Functional Status (PCFS] scale and Modified Fatigue Impact Scale (MFIS). Mental status will be evaluated with the use of Beck Depression Inventory (BDI) and Hamilton Depression Scale (HAMD)) Biochemical measures will be performed including, among others, myokines such as BDNF (brain-derived neurotrophic factor), adiponectin, fibroblast growth factor 21 (FGF21), resistin, myonectin and inflammatory markers, such as interleukin 6 ((Il-6), C-reactive protein (CRP).

Fasting venous blood will be collected in 2 tubes each a 10 ml, one dedicated to obtain serum ("on clot"), second with heparin) to perform biochemical analyses, In clinical evaluation measurement of respiratory (inspiratory and expiratory) muscles force with use of device MicroRPM (respiratory pressure meter), hand grip strength with use of hand dynamometer (Jamar) and strength of quadriceps muscles with use of digital dynamometer (AXIS) will be performed. During first visit also ECG (electrocardiogram), six minute walking test and treadmill stress test will be done. All patients will have diaphragm ultrasound with measurement of diaphragm thickness.

Patient will be randomized to the control and interventional groups. Control group will be subjected to the safe protocol of 6 week physical training (aerobic training, resistance training, general improvement training, stretching training and respiratory rehabilitation) adjusted to the clinical state of patients. Additionally patients will be recommended to perform a set of exercises at home under a supervision of physiotherapist.

Patients from interventional group in addition to the training protocol will be subjected to additional resistance respiratory training with the use of respiratory muscle trainer (Philips Respironics Threshold IMT). Control group will have placebo with no resistance set on respiratory muscle trainer (Philips Respironics Threshold IMT). The study will be double blind, patients will be unaware of being assigned to a group and investigators assessing functional state in the next stages of the study will not conduct rehabilitation and will not be aware of the allocation of patients to groups. Physiotherapists conducting rehabilitation will receive detailed, individual guidelines regarding the training parameters of each patient. In addition, both groups will be educated on rules of safe physical activity and how to deal with troublesome symptoms.

Physical endurance (6 MWT, treadmill stress test), respiratory function, respiratory and peripheral muscle strength, other functional tests (SPPB), biochemical determinations, dyspnea (mMRC, Borg scale), quality of life (SF-36, PCFS, MFIS) mental state (BDI, HAMD), chest wall mobility and diaphragm ultrasound will be assessed at the beginning and end of the intervention. In addition, an analysis of the potential change in lifestyle of all patients will be carried out using accelerometry counting daily number of steps and assessing physical activity. Each patient will be equipped with basic exercise equipment and the necessary monitors of physical activity and vital signs to carry out the recommended activities at home (except for training units in the outpatient clinic). A reassessment of the long-term effects will be performed six months after the end of the exercises and will include 6 MWT, treadmill stress test, SPPB, spirometry, measurement of respiratory muscles force, handgrip strength, strength of quadriceps muscles, SF-36, PCFS, MFIS, BDI, HAMD, assessment of dyspnea in Borg scale during daily activities, mMRC, blood samples for biochemical analyses, ), chest wall mobility and diaphragm ultrasound . Patients in both groups will be monitored with the use of accelerometers in order to asses daily activity, daily number of steps within 7 days prior to intervention, after intervention and after observation period.

Inclusion Criteria:

• COVID 19 infection confirmed with a positive PCR (polymerase chain reaction) SARS-CoV-2 (Severe Scute Respiratory Syndrome coronavirus type 2) test less than 12 moths prior to intervention
• more than 14 days from the day of obtaining a positive PCR test result for SARS-CoV-2 or discharge from hospital.
• mMRC (modified Medical Research Council) score ≥1
• age >18 years
• informed consent signed by patient to conduct the study

Exclusion Criteria:

• severe pulmonary disease (e.g. COPD)
• the functional state that makes it impossible to carry out the pre-examination and improvement program
• severe chest pain
• worsening dyspnea
• hemoptysis
• worsening dry cough
• syncope
• worsening oedema of extremities
• myocarditis (less than 6 months from acute phase)

The study will be double blind, patients will be unaware of being assigned to a group and investigators assessing functional state in the next stages of the study will not conduct rehabilitation and will not be aware of the allocation of patients to groups.