Inclusion Criteria:  Diagnosis of DMD confirmed by 1) clinical history with features before the age of five, 2) physical examination, 3) elevated serum creatine kinase level and 4) absence of dystrophin expression, as determined by immunostaining or Western blot (<2%) and/or DNA confirmation of dystrophin mutation. Minimum entry age of 7.0 years Ambulatory  Exclusion Criteria:  Older than 12.0 years of age Contraindication to an MR examination (e.g. aneurysm clip, severe claustrophobia, magnetic implants) Presence of unstable medical problems, significant concomitant illness including cardiomyopathy, uncontrolled seizure disorder, uncontrolled hypo or hypertension Presence of a secondary condition that impacts muscle function or muscle metabolism (e.g. myasthenia gravis, endocrine disorder, mitochondrial disease) Contraindications to Tadalafil (use of nitrates, alpha-adrenergic blockers, other PDE5A inhibitors) anti-hypertensive drugs or other medications known to modulate blood flow or muscle metabolism or inhibit or induce CYP3A4)