Doctors increasingly are recommending physical activity to help osteoarthritis patients, overturning the more traditional medical advice for people to take it easy to protect their joints.

The new treatment approach comes as osteoarthritis, a degenerative joint disease once considered a problem of old age, has begun showing up in more middle-aged and young adults as a result of obesity and sports injuries. Studies have shown that weight loss, combined with exercises aimed at improving joint function and building up muscles...