Several studies have confirmed that weak leg muscles are associated with osteoarthritis (OA) of the knee. Studies of body composition in these people have also shown that, despite being weaker, people with OA have significantly greater muscle mass than those without OA, suggesting that those with OA may have the potential to greatly increase their strength. However, research has not clearly shown whether exercises designed to improve leg strength will decrease the severity of pain or slow the progression of OA based on radiographic (x-ray) analysis. To understand the effects of leg strengthening exercise, we will perform a randomized clinical trial of lower extremity strength training using four subgroups of people: (1) OA with knee pain; (2) OA without knee pain; (3) no OA with knee pain; and (4) normal elderly with no OA or knee pain.