## 2 Materials and Methods
### Participants and Tools  
Before starting, a comprehensive assessment of patients was performed, including a Basic Assessment (BA) and the UZ Leuven Start To Move protocol (STMP). BA included:  
* (BA1) basic vital signs: blood oxygen saturation, respiratory rate, heart rate, blood pressure, and temperature;
* (BA2) range of motion (passive and active);
* (BA3) the degree of dyspnea. STMP was created to be used in the evaluation of patients hospitalized in Intensive Care Units (this type of patients is exposed to prolonged time spend at bedrest and other factors that can have a direct or indirect impact on the muscle and the body, causing weakness and a decrease in functional performance). It consists of six levels of assessment, each of them being determined through objective measurements [31]. This type of protocol was used in this study because the vast majority of hospitalized patients with COVID-19 became physically deconditioned fairly quickly, and this imposed bed rest for them. According to the STMP guidelines, an STMP level of 0 corresponds to a patient who does not cooperate, so no physiotherapy procedures are applied; at level 1, the patient may cooperate by performing passive mobilizations of the lower and upper limbs and the passive bicycle; at level 2, the patient is able to perform active mobilizations, resistance exercises, and active hand and foot cycling; at level 3, imposed ADL movements (ambulating, feeding, dressing, personal hygiene, continence, and toileting) are possible; at level 4, the patient can walk using a frame in the hospital room; and at level 5, the patient can walk assisted by a medical professional. Inclusion criteria comprised patients aged over 18, with respiratory symptoms (cough and dyspnea) and a confirmed RT-PCR test of COVID-19, with an STMP level between 3 and 5. Patients who did not corresponded to the above mentioned criteria, who were transferred to the ICU ward and whose health had deteriorated or who had various comorbidities such as hepatic, kidney, and circulatory diseases, stroke, or other serious neurological diseases, as well uncooperative patients and those who did not want to perform physiotherapy were excluded. Electronic medical records of all patients admitted in the study were retrospectively screened, and relevant data were independently extracted. The relevant data extracted was included in a de-identified form in an excel spreadsheet. All data was stored in password-protected electronic documents, with access only for the authors of the study. Patient demographic data and baseline clinical characteristics, including age, gender, and comorbidities were recorded. Clinical data consisted of the length of hospital stay, medical treatment, symptoms, and types of physiotherapy. Lab tests included blood levels of C-reactive protein (CRP), IL-6 (interleukin 6), and ferritin, and were also recorded. _Sample preparation_  
The blood samples were collected on admission for all study participants. Taking all aseptic precautions, about 6 mL of blood was drawn by venipuncture from a peripheral vein with a disposable syringe, then collected in a clean dry glass tube (clot activator tube) that allowed it to stand for 10 min at room temperature for the retraction of the clot. This was centrifuged at 4000 rpm for 10 min to separate the serum. All tests were performed in the biochemical laboratory, following standard procedures for clinical biochemistry purposes. The biological parameters measured were: CRP, ferritin, and IL-6. _Biochemical assay_  
Serum parameters (CRP and ferritin) were measured using a Cobas Integra 400 plus (Roche) biochemical autoanalyzer, while IL-6 was assayed by Cobas E411, fully automated analyzer that uses a patented electrochemiluminescence (ECL) technology for immunoassay analysis. Result quality is validated with internal quality control and the laboratory participates in an external quality control scheme. They were selected considering that:  
* CRP was selected for being measured, as it represents a valuable indicator of inflammation.