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)