Skeletal Muscle Biomarkers in People With Fragile Sarcolemmal Muscular Dystrophy

Background: \- Some kinds of muscular dystrophy affect the skeletal muscle membrane. In these conditions, the muscle membrane is more fragile. This affects how the muscles contract and relax, which causes movement problems. Researchers are looking at several muscle enzymes, or chemicals that affect how muscle cells function. By studying changes in these enzymes, they may be able to better understand how muscular dystrophy affects the cells. Researchers want to collect biomarkers (chemicals from blood samples) from people with fragile sarcolemmal muscular dystrophy. This information may provide better treatments for this condition. Objectives: \- To study biomarkers that may affect the muscles of people with fragile sarcolemmal muscular dystrophy. Eligibility: \- Individuals at least 18 years of age with fragile sarcolemmal muscular dystrophy. Design: * Participants will be screened with a medical history and physical exam. * Participants will be asked to come for four visits to the National Institutes of Health Clinical Center. The visits will be at least 2 months apart. Each visit will require participants to stay for 5 days at the clinical center. * During each visit, participants will provide frequent small blood samples. These samples will be collected while at rest and after physical exercise. * Participants will also have a physical therapy assessment. They will perform standard motor function tests and imaging tests (MRI, MRS). These tests may take up to 1 hour each time. * Treatment will not be provided as part of this study.