This project will bring together a multidisciplinary team of pediatric rheumatologists, neurologists, metabolic geneticists and exercise physiologists to determine the effect of creatine (CR) supplementation on muscle function and muscle metabolism in children with Dermatomyositis (DM). The investigators propose using well-established exercise testing techniques as well as new, powerful exercise imaging protocols in order to better delineate the effects of CR on muscle pathophysiology in a non-invasive way. Evidence from this study will provide information regarding the effect of creatine supplementation on muscle function in DM. Improvements in muscle function and fatigue through CR use may also contribute to an improvement in quality of life and have significant clinical implications for the treatment of children with DM.

Inclusion Criteria:

• Ages 7 to 18 years
• Diagnosis of probable JDM (onset <16 years) according to Bohan and Peter criteria
• Subjects on a stable course of medication (unlikely to change over study treatment period as determined by the treating physician)
• Minimum height of 132.5cm

Exclusion Criteria:

• Subjects newly diagnosed with JDM within the previous 6 months
• Subjects unable to cooperate with study procedures, or too weak to participate in the exercise testing
• Subjects with impaired kidney function as determined from baseline visit screening lab values
• Subjects who are currently pregnant or planning to become pregnant within the study period
• Subjects who are shorter than 132.5cm