As the focus of this study is on validating novel measures of cardiac and respiratory function in individuals with DMD, this study will utilize a single-group design with up to 60 males with DMD between 5 and 15 years old upon entry to the study. The participant selection is limited to children since the detrimental effects of DMD begin in early childhood, and the life expectancy of these boys is shortened to the early- to mid-20s. Only males will be eligible to participate in the study because DMD is an X-linked recessive genetic disorder that only leads to the characteristic disease in males. Although females may be carriers, they do not exhibit the same phenotype as males.