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Summary for Patients: This study, funded by the Muscular Dystrophy Association, is intended to build on recent findings published in the journal Nature showing beneficial effects of tadalafil (also known as Cialis) in mice with an animal version of Duchenne and Becker muscular dystrophies. Only two doses of tadalafil improved muscle blood flow, allowing the dystrophic mice to perform more exercise with less muscle injury. This new short-term clinical trial will move the testing from animals to human patients with Becker muscular dystrophy and examine the effects of acute tadalafil dosing on muscle blood flow during a bout of exercise. Patients will take two doses of tadalafil prior to exercising. Then doctors will measure whether muscles receive increased blood flow and therefore are better protected during exercise. Scientific Hypothesis: In patients with Becker muscular dystrophy (particularly those with dystrophin gene mutations between exons 41-46), loss of sarcolemmal nitric oxide synthase engenders functional muscle ischemia and thus muscle edema after an acute bout of exercise. The investigators further hypothesize that PDE5A inhibition, which boosts nitric oxide-cGMP signaling, constitutes an effective new countermeasure for these patients.