Twice Weekly Steroids and Exercise as Therapy for DMD

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 immunostain or Western blot (\<2%) and/or DNA confirmation of dystrophin mutation. * Age 5.0 to 9 years: a lower age limit of 5.0 years is selected as children younger than that are likely unable to cooperate and comply with all of the exercise measures as needed. An upper age limit of 9 years has been set as boys with DMD tend to reach a rapid progression into a late ambulatory phase soon after this age. * Ambulatory at the time of the first visit, defined as the ability to walk for at least 100 m without an external assistive device and able to climb four stairs. * Aim 1 only: GC-naïve at baseline (and prior 6 months) * Aim 2 only: on stable daily GC regimen for 6 months prior to baseline Exclusion Criteria: * Contraindication to an MR examination (e.g. aneurysm clip, severe claustrophobia, magnetic implants) * Presence of unstable medical problems, significant concomitant illness including cardiomyopathy or cardiac conduction abnormalities * 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) * Presence of an unstable medical cond…