Study aim: The purpose of the present study is to examine the influence of 8 weeks of respiratory muscle training on pulmonary function and respiratory muscle strength in MS patients. Design: A controlled clinical trial with parallel, double-blind, randomized groups on 36 patients with MS. computer-generated randomization was used for randomization. Settings and conduct: It will be conducted on 36 patients with MS who will be referred by a neurologist at Sina Hospital. Patients are divided into two groups by computer randomization. A researcher who is not involved in any other part of the study assigns them to two groups. Participants/Inclusion and exclusion criteria: Inclusion criteria are patients over 18 years of age, with a definitive diagnosis of relapsing-remitting MS that at least one year has passed since the diagnosis, patients are able to walk, have no relapse in a recent month, and have not received corticosteroid pulses in the last month. Exclusion criteria include patients with a history of any respiratory disease, or have an active pulmonary infection and will also be excluded from the study if they have contraindications to do aerobic activities (such as orthopedic and muscular problems), smoker, pregnancy, and participating in another research program. Intervention groups: The intervention group will be educated exercises with a pressure threshold device (Power-breath), then patients will do exercises for 8 weeks. Patients will be followed up by telephone and the intensity of the device will be increased on a weekly basis. After 4 weeks, patients will be re-visited and side effects will be recorded. The control group will be explained only the training and importance of regular physical activity in MS patients and lifestyle modification, and both groups will be given a booklet in this regard. Main outcome variables: PImax; PEmax; 6MWT; TUG test; quality of life; and spirometric parameters