Inclusion Criteria:

• * Diagnosis of DMD confirmed by genetic report
• * Minimum entry age of 6.0 years old
• * Ambulatory
• * On stable glucocorticoid regimen (for \> 3 months)

Exclusion Criteria:

• * Contraindication to a Magnetic resonance Imaging examination (e.g. severe claustrophobia, magnetic implants, unable/unwilling to perform test)
• * Presence of unstable medical problems, including severe cardiomyopathy, left ventricular ejection fraction \<45%, cardiac conduction abnormalities as evidenced on ECG, 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)
• * Presence of a secondary condition leading to developmental delay or impaired motor control (e.g., cerebral palsy) or previous history of unprovoked rhabdomyolysis
• * Contraindications to phosphodiesterase 5 inhibitors (use of nitrates, alpha-adrenergic blockers, other phosphodiesterase 5 inhibitors) or other medications known to modulate blood flow or muscle metabolism
• * Participation in currently approved FDA trials or other investigational clinical trials during the period of the study