The world's most common joint disease, osteoarthritis (OA) is a major cause of disability among adults over the age of 50. Whether physical activity is beneficial or detrimental to weight-bearing joints, knees in particular, has been open to debate. Some studies implicate physical activity in provoking knee OA, while others suggest that physical activity may actually protect the knee joint from the disease.

Confounding the matter is the fact that knee injury is a known risk factor for knee OA. Then, there's the unclear role of osteophytes in knee OA progression, compounded by the limitations of radiographs for monitoring small yet significant changes in joint structure.

For a clearer picture of the impact of physical activity on the knee joint, a team of researchers in Australia turned to magnetic resonance imaging (MRI). This highly accurate high-tech tool makes it possible to directly visualize joint structures, detect early and pre-disease states of OA, and assess the influence of potential risk factors. Taking advantage of this novel methodology, the researchers studied the effect of physical activity, in various degrees of intensity, frequency, and duration, on knee structures in a total of 257 healthy adults between the ages of 50 and 79, with no history of knee injury or OA. Their findings, presented in Arthritis Care & Research suggest that exercise that is good for the heart is also good for the knee.

Recruited from an established community-based research population, the Melbourne Collaborative Cohort Study, subjects all underwent MRI exams on the tibia bone and tibiofemoral joint of their dominant knee--the one on the leg they first step forward when walking. MRI was used to assess cartilage defects and bone marrow lesions, as well as measure cartilage volume, an indicator of cartilage health and strength.

Loss of knee cartilage is linked to worsening knee symptoms in OA sufferers. Subjects also answered specific questions regarding their exercise and walking habits, as well as routine activity at home and at work, to determine their level of physical activity in both the 6 months and 7 days prior to the study. To create a baseline for each subject, past information on weight, height, body mass index, and physical activity, from questionnaires completed between 1990 and 1994, was obtained. Then, the team performed a series of analyses and comparisons.

Among the notable findings, both baseline and current vigorous physical activity-- exercise that gets the heart pumping and the body sweating--were associated with an increase in tibial cartilage volume, free from cartilage defects. What's more, tibial cartilage volume increased with frequency and duration of vigorous activity. Recent weight-bearing exercise was also linked to increased tibial cartilage volume and reduced cartilage defects. Finally, moderate physical activity, including regular walking, was associated with a lower incidence of bone marrow lesions.

"This is the first study to demonstrate a potentially beneficial effect of walking on the reduction in the risk of bone marrow lesions in the knee," notes the study's leading author, Dr. Flavia M. Cicuttini. "Bone marrow lesions have been associated with pain and radiograph-defined progression of osteoarthritis, type II collagen degradation, and loss of cartilage volume."

Demonstrating a protective effect of past and current vigorous physical activity on knee cartilage in healthy adults, this study strongly supports the benefits of exercise for older individuals at risk for OA. Though both the intensity and duration of physical activity had a significant positive impact on cartilage, the ideal amount of physical activity for joint health remains unclear. "Our data suggest that at least 20 minutes once per week of activity sufficient to result in sweating or some shortness of breath might be adequate. This is similar to, if not somewhat less than, the recommendations for cardiovascular health," Dr. Cicuttini observes.