Urinary Titin Biomarker in DMD

The investigators are recruiting ambulatory individuals with Duchenne Muscular Dystrophy (DMD) or Becker Muscular Dystrophy (BMD), and healthy volunteers. At 3 regular clinical visits, participants will perform a descending stair walk down 2 flights of stairs. Urine will be collected before and after to measure how urinary N-terminal fragment of titin (NTFT) levels are impacted by the activity. Participants will also perform a standard battery of neuromuscular tests, including North Star Ambulatory Assessment and Timed Function Tests. These will be performed as part of the regular clinic visit. Participants will wear an activity monitor during all tests. After both the first and second visits, participants will be sent home with an activity monitor to measure the participants activity over the course of 7 days. The participant's urine will be collected and frozen 3 times a day (morning, afternoon, and evening) during this period. The investigators will correlate physical activity intensity with the urinary NTFT (titin) levels and the severity of the disease (which will be based on the results of clinical neuromuscular tests). If there is a clinically-indicated blood draw, participants will have the option to allow collection of an additional sample for exploratory analysis of additional biomarkers.