Juvenile dermatomyositis (JDM) and juvenile systemic lupus erythematosus (JSLE) are systemic autoimmune rheumatic diseases (RD) of unknown etiology with diverse clinical manifestations, namely, weakness, muscle dysfunction and atrophy, fatigue, chronic pain, stiffness, joint deformities, and significant loss of muscle strength and function and quality of life. It has been suggested that physical inactivity may account for these manifestations, exacerbating atrophy, dysfunction, muscle weakness and chronic fatigue in these patients, thus constituting a vicious circle, which can only be broken by physical exercise. In fact, it is known that specific a supervised exercise program may greatly benefit adult and juvenile rheumatic patients. Considering the importance of an appropriate physical activity program for these patients, it becomes essential to evaluate their physical activity levels. In this context, accelerometers have been extensively use as an accurate and objective tool to measure physical activity, and have been already validated for children and adolescents. Very few studies have measured the physical activity levels in rheumatic patients, especially in children and adolescents. Thus, it becomes extremely relevant to assess the physical activity level of this population and observe its influence on the disease activity, quality of life and health status of patients. These project will be conducted in the Clinical Hospital of Medicine School of Sao Paulo University (HC-FMUSP) and the methodology will include the assessment of the physical activity level through the use of accelerometers, clinical evaluation of disease, quality of life, fatigue and muscle function using validated disease-specific questionnaires, and analysis of hematological and inflammatory parameters through laboratory exams.