Inclusion Criteria:  Signed informed consent from legal guardians/patients and patient (where applicable)  Subject must have a confirmed congenital myopathy(CM) diagnosis defined as:  Clinical symptoms consistent with CM with pathohistological findings on muscle biopsy and known genetic mutation consistent with CM OR Clinical symptoms consistent with CM with unspecific pathohistological changes but known genetic mutation consistent with CM Stabile motor function tests over at least 6 months (between baseline and screening) If on other medications- stabile dose for at least 6 months prior to start At least 1 point on Motor function measure 32 test is (MFM32) at screening visit.  >5- <31 years of age (from 6 years to 30 years of age)  Women of fertile age must be on oral contraceptives Underwent cardiac evaluation with ECG and 2D echocardiography in the last 2 years and has no signs or symptoms of cardiac abnormality.  Exclusion Criteria:  Subject with clinical symptoms consistent with CM but has no confirmed genetic mutation and only unspecific changes on muscle biopsy that are not confined to just CM but can be seen in other disorders. Younger than 6 years of age and older than 30 years Subject receives 94 or more points on MFM32 test at screening visit. Subject doesn't not speak Swedish and a translator is needed in order to perform the tests included in the study. Subject smokes more than 10 cigarettes a day or has smoked more than 10 cigarettes in the last year Subject has tracheostomy Subject receives no points on motor function measure test at screening Subject has other concomitant chronic diagnosis that can affect the patients motor function, in the opinion of the investigator Subject is currently or has been on oral corticosteroids in the last 6 months Subject has arrhythmia as seen on electrocardiogram(ECG), confirmed by cardiologist Subject has cardiomyopathy as seen on ultrasound, confirmed by cardiologist Subject has severe behavioural and/ cognitive problems that preclude participation in the study, in the opinion of the investigator Subject is allergic or hypersensitive to study drug or any of its constituents Subject has previous or ongoing medical condition, medical history, physical findings or laboratory abnormalities that could affect safety, make it unlikely that treatment and follow- up will be correctly completed or impair the assessment of study results, in the opinion of the Investigator; Subject is currently taking any other investigational drug or has taken any other investigational drug within 3 months prior to the first dose of study medication Subject is planning on participating in any other study during the duration of this study. Female subjects of fertile age that are or are planning to become pregnant during the study. Female subjects that have given birth up to 1 year prior to baseline visit and/or are nursing up to 1 month prior of baseline visit Subject has a fracture in the last 6 months before the study start or has acquired a fracture during the study.