This study seeks to develop and validate non-invasive assessments of cardiac and respiratory muscles with magnetic resonance imaging (MRI) to better predict the natural disease progression of Duchenne muscular dystrophy (DMD) in affected individuals over time, as well as determine whether peripheral skeletal muscle dysfunction can predict cardiopulmonary dysfunction. The central hypothesis is that non-invasive MRI measures of the heart, muscle, and peripheral skeletal muscles can sensitively predict future cardiopulmonary decline.

Magnetic Resonance Imaging (MRI) of the heart and breathing muscles, special breathing tests (called pulmonary function testing), special exercise tests (using a stationary bike), and possibly an echocardiogram (ultrasound of the heart, commonly known as an "echo") will be completed up to 4 times per year for up to 4 years. Most participants will complete testing once or twice each year; however, some participants will be asked to repeat some of the tests twice during each visit.

Inclusion Criteria:

• Male
• 5-15 years old at the time of enrollment
• Diagnosed with DMD (as defined by parent project)
• Written parental informed consent (and assent where appropriate) before any study procedures take place

Exclusion Criteria:

• Contraindication to an MRI examination
• Presence of a secondary condition that impacts muscle function or metabolism, that leads to developmental delay or impaired motor control, or that is not stable
• Participant is unable to comply with study requirements
• Congenital structural abnormality of the heart, repaired or unrepaired
• Clinically contraindicated participation

As the focus of this study is on validating novel measures of cardiac and respiratory function in individuals with DMD, this study will utilize a single-group design with up to 60 males with DMD between 5 and 15 years old upon entry to the study. The participant selection is limited to children since the detrimental effects of DMD begin in early childhood, and the life expectancy of these boys is shortened to the early- to mid-20s. Only males will be eligible to participate in the study because DMD is an X-linked recessive genetic disorder that only leads to the characteristic disease in males. Although females may be carriers, they do not exhibit the same phenotype as males.