For the NIV intervention, two levels of positive airway pressure (bi-level) were used: the positive inspiratory airway pressure (IPAP) was fixed at 6 cmH2O (centimeters of water), which could be gradually increased up to 14 cmH2O (2 cmH2O every minute), depending on the patient's comfort. and gradually increased to 14 cmH2O (2 cmH2O every minute), While the expiratory airway pressure (EPAP) was set at 3 cmH2O, it could be gradually increased up to 7 cmH2O (1 cmH2O every minute), depending on the patient's comfort.while expiratory airway pressure (EPAP) was fixed at 3 cmH2O and gradually increased to 7 cmH2O (1 cmH2O every minute).

Lung function was assessed using a spirometer (Koko Digidoser, Longmont, USA), according to the American Thoracic Society/European Throracic Society recommendations14. At least three tests were performed, with a variation of less than 5%, and the highest value obtained was compared with predicted values for the Brazilian population.

Respiratory muscle strength was assessed using a digital manometer (MVD 300 - Brazil). Maximum inspiratory and expiratory pressures were performed from residual volume and total lung capacity, respectively, and values were compared with previously reference values for the Brazilian population

The 6-minute walk test (6MWT) was performed in a flat 30-m corridor, according to the ATS recommendations17. Subjects were instructed to keep walking for six minutes, and standardized verbal incentives were given each minute. The perception of dyspnea (using the 10-point Borg scale) were assessed at rest, at the end of the 6MWT, and two minutes after the test. In this test, the walked distance (in meters) was considered for analysis.

The Portuguese version of the COPD Assessment Test (CAT) questionnaire was used to assess the quality of life. Subjects were instructed to choose only one option in each item of the questionnaire (cough, phlegm, chest tightness, breathlessness, limited activities, confidence leaving home, sleeplessness, and energy). Item scores range from 0 to 5 points resulting in a total score ranging from 0 to 40 points. The clinical impact of COPD was assessed according to the following stratification: mild (6-10), moderate (11-20), severe (21-30), and very severe (31-40)20.

QF (Quadriceps femoris) strength was evaluated with subjects performing 20 repetitions at the same speed. The protocol established was designed to reach a fatigue threshold in the QF muscle. Dyspnea and perceived level of exertion (Borg scale) were also assessed before and after isokinetic evaluation.