Children with Juvenile Idiopathic Inflammatory Myopathies (JIIM) are weak and get tired because their muscles aren't able to work like healthy muscles. This can make it hard for them to do normal everyday things and can make them less happy about their lives compared to children without the disease. There are two nutritional supplements that help muscles use energy and recover after exercise: creatine and coenzyme Q10. If the muscle has more energy, it may not be as weak and may not feel as tired or sore after exercise. Because of this we want to see if having children with JIIM take creatine and coenzyme Q10 can make them stronger and less tired. If this works, we hope it will let them be able to do the things that healthy children can do, and make them feel better about their lives.

Inclusion Criteria:

• Ages 7 to 18 years
• Diagnosis of juvenile idiopathic inflammatory myopathy (JIIM) according to the 2017 European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) criteria: ≥90% probability, age of onset <18 years
• Subjects on a stable course of medication (unlikely to change over study treatment period as determined by treating physician)
• Minimum height of 132.5cm (required to fit on the cycle ergometer)

Exclusion Criteria:

• Subjects with newly diagnosed JIIM 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 pre-baseline visit screening lab values (eGFR <90 ml/min/1.73m^2)
• Subjects currently taking anti-hypertensive drugs or blood thinners
• Subjects who are currently pregnant or planning to become pregnant within the study period