Creatine supplementation may improve strength, muscle mass, bone mass and muscle function in healthy and elderly people. The investigators speculate that creatine supplementation could have therapeutic effects in pediatric rheumatic (i.e., juvenile systemic lupus erythematosus and juvenile dermatomyositis) patients who usually present muscle weakness, muscle wasting and bone mass loss.

Inclusion Criteria:

• patients diagnosed with lupus systemic erythematosus and juvenile dermatomyositis
• physically inactive
• stable pharmacological treatment

Exclusion Criteria:

• macroalbuminuria
• GFR < 30 mL/min/1.73m2
• use of oral anticontraceptive agents
• pregnancy
• diabetes mellitus
• hypothyroidism