It is well known that exercise, alone or combined with weight reduction, reduces pain and improves function in patients with osteoarthritis. The knowledge of the effects of exercise on cartilage is limited and needs to be improved however. It seems as cartilage adapts to loading as other biological tissues like bone and muscle, and moderate loading seems to be beneficial both for prevention and treatment of osteoarthritis. Too high loads, like elite soccer or repeated knee bendings several hours daily, are associated with increased risk of osteoarthritis. Too high or too low mechanical load decrease the proteoglycan content of the cartilage, indicating not only elite sports but also physical inactivity being a possible risk factor for osteoarthritis development. Muscle weakness may proceed osteoarthritis. Prevention and treatment of osteoarthritis should include regular loading of the cartilage, keeping muscles strong and maintaining normal body weight.