Many people worry that running ruins knees. But a new study finds that the activity may in fact benefit the joint, changing the biochemical environment inside the knee in ways that could help keep it working smoothly.
In my many decades as a runner, I have frequently been told by fellow runners and nonrunners alike that I am putting my knees at risk. The widespread argument generally follows the lines that running will slowly wear away the cartilage that cushions the bones in the joint and cause arthritis.
But there is little evidence to support the idea, and a growing body of research that suggests the reverse. Epidemiological studies of long-term runners show that they generally are less likely to develop osteoarthritis in the knees than people of the same age who do not run.
Some scientists have speculated that running may protect knees because it also often is associated with relatively low body mass. Carrying less weight is known to reduce the risk for knee arthritis.
But other researchers have wondered whether running might have a more direct impact on knee joints, perhaps by altering the working of various cells inside the knee.
To find out, researchers at Brigham Young University in Provo, Utah, recruited 15 male and female runners under the age of 30 with no history of knee injury or arthritis. The scientists wished to study people with healthy knees in order to better isolate running's effects on otherwise normal joints.
These volunteers visited a clinic where they had blood drawn from an arm. The researchers also siphoned off a small amount of synovial fluid, a lubricating fluid that reduces friction inside joints, from their right knee. Healthy knees contain only a soupçon of the stuff; arthritic and otherwise unhealthy knees tend to contain much more.
The volunteers next were delivered, in wheelchairs, to the university's nearby biomechanics lab. There, they either sat quietly for 30 minutes or ran on a treadmill for the same 30 minutes at their preferred running pace.
After either running or sitting, they again were wheeled to the clinic and the blood and synovial fluid draws were repeated.
Each volunteer completed both a sitting and running session on separate days.
Then the researchers looked for a variety of substances in the young people's blood and synovial fluid.
In particular, they focused on molecules that are associated with inflammation. Low-grade inflammation in the knee has been shown to contribute to the development and progression of arthritis.
So the researchers looked for changes in the levels of several types of cells that are known to either increase or blunt the amount of inflammation inside the knee.
They also looked at changes in the levels of another substance unpoetically known as cartilage oligomeric matrix protein, or COMP. This substance tends to accumulate in diseased knees and is often used as a marker of incipient or worsening arthritis. People with arthritis can have about five times as much COMP in their synovial fluid as do people with healthy knees.
Unfortunately, because it had turned out to be technically difficult to safely extract much synovial fluid from these healthy knees, the scientists wound up with complete numbers from only six of the runners.
But the data were interesting and consistent. In almost every case, the runners' knees showed substantially lower levels of two types of cells that can contribute to inflammation within the synovial fluid, compared to their baseline levels.
The runners also showed a shift in their COMP levels. After the run, they displayed more of the substance in their blood and less in their synovial fluid. In effect, running seemed to have squeezed the molecules out of the knee and into the blood.
Meanwhile, sitting had slightly increased levels of COMP inside people's knees, and also raised the concentration of one of the inflammatory molecules.
These findings suggest that a single half-hour session of running changes the interior of the knee, reducing inflammation and lessening levels of a marker of arthritis, says Robert Hyldahl, a professor of exercise science at B.Y.U. and lead author of the study, published in the European Journal of Applied Physiology.
But sitting for 30 minutes also changed the knee, he points out, which he and his colleagues had not expected. Sitting seemed to make the knee biochemically more vulnerable to later disease.
Dr. Hyldahl noted that this was a very small and short-term study. He and his colleagues would like to repeat it with much larger numbers, “once we figure out how to get more synovial fluid” safely from healthy knees, he says.
They also hope to study longer running distances and different paces, to see how those variables affect changes within the knee, and to recruit older and injured runners, whose knees might have begun to respond fundamentally differently to the activity than the joints of healthy people in their 20s.
But even with these limitations, the findings suggest that moderate amounts of running are “not likely to harm healthy knees and probably offer protection” against joint damage, Dr. Hyldahl says.