 The overall objective of this pilot study is to assess whether a mild to moderate-intensity strengthening exercise program can be safely implemented in boys with DMD. In Aim 1, the investigators will determine the dose response and safety of mild to moderate-intensity isometric resistance exercise in children with DMD. Twelve ambulatory boys with DMD will participate in an isometric exercise dosing protocol, in which the load is progressively increased. In this early proof of concept pilot study, two large muscle groups will be studied: the knee extensors and the knee flexors. T2 weighted magnetic resonance imaging MRI of the thigh muscles will be used to monitor evidence of muscle damage at each intensity level and determine a safe exercise range. Other safety measures will include a verbal pain rating scale, clinical examination, and serum creatine kinase CK levels. In Aim 2, the investigators will implement a pilot intervention study to examine the feasibility and safety of a 12 week in-home isometric strengthening program in children with DMD. In this study, 20 boys with DMD will be randomized to either an exercise group or a control group. Ten boys with DMD randomized to the exercise group will complete a progressive exercise program using the parameters and dose identified in Aim 1. Assessment of strength and safety will be performed at regular time intervals throughout the study. Ten boys with DMD randomized to not participate in an exercise intervention will be tested at similar time intervals and serve as controls. The data from this pilot study will serve to perform the power analysis needed to design an appropriately powered clinical intervention study. 