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.