Arthritis is the most common chronic disease and affects almost 70 million Americans.

There are more than 100 different types of arthritis and each is treated somewhat differently from the others. The most common form of arthritis is osteoarthritis (OA).

Osteoarthritis (OA) is a major cause of disability among adults over the age of 50.

OA affects articular cartilage, the gristle that caps the end of long bones that interact with each other inside a joint.

Cartilage consists of a compressible matrix inside of which are cells called chondrocytes- much like grapes sitting inside a gelatin mold. Under normal circumstances, chondrocytes produce substances that help nourish the matrix. During the development of OA, chondrocytes begin to produce materials that are detrimental to cartilage. Over a period of time, small cracks or fissures develop in cartilage. At the same time, chemicals that stimulate inflammation are produced by cells that line the joint. These chemicals further weaken cartilage. When cartilage damage reaches a certain point, mechanical factors come into play since cartilage no longer is capable of maintaining joint integrity.

Exercises consisting of stretching, strengthening, and non-impact aerobic types are essential for the comprehensive treatment of osteoarthritis, particularly OA involving the knee. This has been the topic of other articles I've written).

However, whether physical activity is beneficial or detrimental to weight-bearing joints such as the knees has been the subject of 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.

What makes it particularly difficult to sort out is the fact that knee injury is a known risk factor for knee OA. Also, older clinical trials that have sought to answer the exercise question have used x-ray. X-rays are not sensitive enough for detecting small changes in cartilage.

To more clearly delineate the impact of physical activity on the knee joint, researchers in Australia used magnetic resonance imaging (MRI). This highly accurate 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.

The researchers studied the effect of physical activity, in various degrees of intensity, frequency, and duration, on the knee in a total of 257 healthy adults between the ages of 50 and 79, with no history of knee injury or OA. Their findings suggest that exercise that is good for the heart is also good for the knee.

Subjects all underwent MRI exams on the tibia (lower leg bone) and 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 to measure cartilage volume, an indicator of cartilage health and strength. Loss of knee cartilage is linked to worsening knee symptoms in OA.

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.

Baseline past information on weight, height, body mass index, and physical activity, were obtained from questionnaires completed between 1990 and 1994. Then, the team performed a series of analyses and comparisons.

What the researchers found was that vigorous physical activity, past and present-- exercise that increases heart rate and causes sweating--were associated with an increase in tibial cartilage volume and no cartilage defects. In addition, 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.

The study's senior author, Dr. Flavia M. Cicuttini stated, "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. Bone marrow lesions have been associated with pain and radiograph-defined (x-ray) progression of osteoarthritis, type II collagen degradation, and loss of cartilage volume."

This demonstration of a protective effect of past and current vigorous physical activity on knee cartilage 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," the authors concluded.

(Racunica TL, et al. Effect of Physical Activity on Articular Knee Joint Structures in Community-Based Adults. Arthritis Care & Research. 2007; 57).