We plan to randomly assign 80 children with arthritis to one of two groups. The experimental group undergoes a vigorous exercise training program consisting of twelve weekly supervised sessions as well as twice weekly at home sessions using an exercise video. The control group also has twelve supervised sessions and two home sessions. However, their exercises are non-strenuous, based on Qi gong. All children have comprehensive fitness testing before and after the training at the exercise lab at the Hospital for Sick Children.