More than 30 million Americans have OA and although the spine, hips and knees can be involved, the knee is the most commonly affected joint. It occurs most often in those 50 years and older and usually presents in a slow, progressive manner. Because movement can be painful when you have OA, most people avoid it. However, not only can being active keep knee OA from worsening, it may reduce the risk of getting the disease in the first place.