Osteoarthritis, the fate runners dread, is caused by degeneration of articular cartilage, a particularly remarkable tissue which covers the ends of bones in the joint. It absorbs shock, distributes loads evenly to avoid a focus of peak stress, and glides smoothly against the opposing articular cartilage in a joint. The ability to run, free of arthritis, for years, even decades, depends on uncompromised function of this cartilage.
Articular cartilage is constructed as a matrix of collagen, a protein that creates sturdy, rope-like fibers. Saturated with water, the collagen matrix has the consistency of a tough, wear-resistant gel. Synovial fluid, a clear, viscous lubricant, bathes the tissue, providing nutrients and removing waste products, since no blood vessels penetrate into articular cartilage.
As osteoarthritis progresses, the surface of articular cartilage becomes roughened. Fissures in the cartilage cut down to the underlying bone, and fragments of cartilage break off causing further damage. The bone, stripped of its protective cartilage, reacts by over-producing bone and forming osteophytes, tiny fingers of bone that extend into the joint space.
Participation in sports can stress and potentially damage articular cartilage, leading many to indict athletic activity as a major cause of osteoarthritis. Baseball pitchers, for example, are reported to have a higher incidence of arthritis in their throwing arms. High impacts, twisting forces, joint injuries, muscle weakness, and joint instability can all lead to abnormal forces and greater peak stresses on certain areas of articular cartilage. All of these influences have been demonstrated to increase the risk of developing osteoarthritis. Nevertheless, a substantial body of research in both animals and humans now demonstrates that for normal joints, moderate levels of physical activity, even over the course of a lifetime, does not cause permanent joint damage or a predisposition to osteoarthritis.
What is the risk of osteoarthritis for runners in particular? Several studies provide evidence that runners, even those who have run competitively for many years, do not have an increased prevalence of osteoarthritis. Some studies have shown that radiographs of runners’ joints are more likely to have osteophytes, the outgrowths of bone often seen in osteoarthritis. The significance of this finding is not clear, however, because many individuals have osteophytes but do not have damaged articular cartilage and do not have symptomatic arthritis.
Confusing the issue for many runners is the misdiagnosis of a reversible joint injury as arthritis. Any component of the muscle, tendon, ligaments and joint can cause pain and injury. The most common cause of knee pain in runners, for example, is not arthritis but patello-femoral dysfunction, caused by abnormal tracking of the kneecap.
Inactivity was once thought to prevent arthritis and protect fragile, arthritic joints from further damage. More recent research has demonstrated the opposite. In the absence of use, joints lose flexibility; without stress, the matrix of articular cartilage is gradually modified and weakened. Furthermore, numerous studies have clearly demonstrated that moderate exercise improves the symptoms of arthritis without causing further joint damage.
Joints are, in fact, strengthened and modified by exercise. These adaptations explain the utility of physical activity in the treatment of arthritis, and the ability of joints to endure years of running without permanent damage. Ligaments and muscles, which support joints, are strengthened and reinforced by the stresses of athletic activity, improving joint mechanics. The flexibility of muscles, encouraged by exercise, can also aid the mechanical function of joints.
Articular cartilage actually has its own feedback mechanism to respond to exercise. Chondrocytes, the living cells that make up just 1% of cartilage, sense the loading of articular cartilage and regulate the production of matrix components to repair and remodel the tissue in response to stress.
So, can running hasten the onset of arthritis? Certainly, in particular circumstances. Abnormal joint stress increases the risk of arthritis and can be caused by running on an injured joint, poor biomechanics, or muscle weakness due to inadequate cross-training.
Does running, however, inevitably cause arthritis? No. Many people run and compete for an entire lifetime without being hindered by arthritis. Aging by itself causes gradual deterioration of articular cartilage, either due to decades of use or because of changes in the cartilage matrix. By the age of 65, over 75% of people show at least minor evidence of osteoarthritis.
Can running help prevent or treat arthritis? Yes, the adaptations of joints to running can help prevent or treat the degeneration associated with inactivity.
The salubrious benefits of exercise are major incentives for many runners, so the thought of incurring permanent damage to a joint is particularly demoralizing. Running is a high impact activity that strains joints to the very limit of their resilience and capabilities. But joints have a wonderfully sophisticated structure that resists damage and adapts to stress. Potential injuries abound, and osteoarthritis is extremely prevalent in both athletes and non-athletes, but permanent joint damage is not an inevitable consequence of running.
Benjamin Ebert, M.D., Ph.D., is a physician and scientist in Boston, MA. His column "Rock Solid" appeared in the July/August 2001 issue of Running Times.