 Children suffering from chronic disease and receiving long-term glucocorticoid therapy suffer over years from severe growth retardation and profoundly altered body composition. They consist in a marked increase in fat mass and a decrease in lean body mass. Published studies have shown that Growth Hormone GH treatment in children with Juvenile Idiopathic Arthritis can improve body composition by increasing lean mass and by preventing increase in fat mass. The aim of the present protocol is to evaluate whether the increase in lean body mass observed during GH treatment is associated with changes in muscle strength and mass. 