Patients with knee osteoarthritis will be randomized into a control group, a walking exercise group, and a stationary cycling exercise group. The individuals in the exercise groups will be required to exercise three times per week for one year using emerging public health recommendations for aerobic exercise in the adult and aging population. Patient outcome will be assessed using objective gait analysis measurements, knee radiographs to quantify joint space narrowing, magnetic resonance imaging, a general health questionnaire (SF-36), a disease/site specific questionnaire (WOMAC), and a visual-analog pain scale. All subjects will be studied at 0 and 52 weeks.