Studying Skeletal Muscle, Heart, and Diaphragm Imaging in Boys With Duchenne Muscular Dystrophy

Background: \- Duchenne muscular dystrophy (DMD) is a disease in which the muscles are unable to make the protein dystrophin. Without this protein, the muscles become gradually weaker. A new medicine called GSK2402968 is being tested to see if it can help prevent or slow down this loss of muscle strength. In this study, boys with DMD and healthy volunteers will have different types of imaging studies to see which ones provide the best images of the muscles. This information will help researchers use these imaging techniques to test the safety and effectiveness of GSK2402968 and other agents. Objectives: \- To test magnetic resonance imaging and ultrasound techniques that can detect changes in muscles of boys with DMD. Eligibility: * Boys who have DMD and are in the GSK2402968 drug test study. * Healthy boys of the same age as the above study participants. Design: * Participants will be screened with a medical history and physical exam. * Healthy volunteers will have one 2-hour visit with three tests. Magnetic resonance imaging (MRI) scans of the skeletal muscles and heart and diaphragm muscles will be carried out. Muscle ultrasound imaging of leg and arm muscles will also be done. Participants should not perform heavy physical activity like school sports or long walks during the week before the visit. * Participants in the GSK2402968 study will have the same series of tests as the healthy volunteers. The tests will be given during the study screening phase. They will be repeated after 3 months and 6 months of receiving the study agent (GSK2402968 or placebo) and at 6 months after stopping the GSK study.