Defining Endpoints in Becker Muscular Dystrophy

Inclusion Criteria: For ages 6-12 1. Clinically affected (defined as weakness on bedside evaluation in a pattern consistent with BMD) 2. Genetic confirmation of an in-frame dystrophin mutation 3. Ambulatory 4. Willing and able to give informed consent and follow all procedures and requirements For ages 13 and older 1. Clinically affected (defined as weakness on bedside evaluation in a pattern consistent with BMD) 2. Genetic confirmation of a dystrophin mutation 3. Willing and able to give informed consent and follow all procedures and requirements For participants in the MRI substudy: 1. Ambulatory, defined as able to walk 10 meters without assistive devices (orthotics allowed)

Exclusion Criteria: For ages 6-12 1. Out of frame dystrophin mutation 2. Use of chronic corticosteroids at baseline, defined as greater than 6 months of chronic use, will be limited to 20% of the overall population 3. Non-ambulatory, defined as the inability to walk 10 meters without assistive device (excluding orthotics) 4. \>16 hours of ventilatory support 5. Any other illness that would interfere with the ability to undergo safe testing or would interfere with interpretation of the results in the opinion of the site investigator. 6. Under the age of 6 at time of enrollment 7. For MR Cohort: Have contraindications to MRI or MRS (e.g., non-MR compatible implanted medical devices or severe claustrophobia) For ages 13 and older 1. Loss of ambulation prior to age 16 2. Use of chronic corticosteroids, defined as greater than 6 months of chronic use, will be limited to 20% of the overall population 3. Less than 30% of the overall population will be non-ambulatory, defined as the inability to walk 10 meters without assistive device (excluding orthotics) 4. \>16 hours of ventilatory support 5. Subjects aged 13-16 only: time to rise \>10 seconds 6. For MR Cohort: Have contraindications to MRI or MRS (e.g., non-MR compatible implanted medical devices or severe claustrophobia)