Inclusion Criteria:  1. Having definite diagnosis of COPD, namely the value of the FEV1 divided by forced vital capacity (FVC) 10 to 15 minutes after beta-2 agonist inhalation being less than 0.7. 2. The score of Mini-Mental State Examination (MMSE) being between 23 and 27  Exclusion Criteria:  1. Being not able to follow instructions on respiratory muscle training or complete the questionnaires of our study due to cognitive impairment. 2. Having difficulty to complete the cardiopulmonary exercise testing or the six minute walking test (6MWT) due to high risk cardiopulmonary diseases or other orthopedic conditions. 3. Having the diagnosis of lung cancer or history of receiving thoracoabdominal surgery 4. Having Body Mass Index more than 30