By Adi Narayan, reports on the theory that running, especially when done long-term and over long distances, is bad for the joints.

The publication notes that it would be hard to think otherwise when with each foot strike, a runner’s knee withstands a force equal to eight times his or her body weight—for a 150-pound person, that’s about 1,200 pounds of impact, step after step.

The common wisdom is that regular running or vigorous sport-playing during a person’s youth subjects the joints to so much wear and tear that it increases the person’s risk of developing osteoarthritis later in life. Research has suggested that may be at least partly true: in a study of about 5,000 women published in 1999, researchers found that women who actively participated in heavy physical sports in their teenage years or weight-bearing activities in middle age had a higher than average risk of developing osteoarthritis of the hip by age 50. notes that over the past few years, an emerging body of research has begun to show the opposite, especially when it comes to running. Not only is there no connection between running and arthritis, the new studies say, but running—and perhaps regular vigorous exercise generally—may even help protect people from joint problems later on.

The article points to the well-known long-term study conducted at Stanford University, researchers tracked nearly 1,000 runners (active members of a running club) and nonrunners (healthy adults who didn’t have an intensive exercise regimen) for 21 years. None of the participants had arthritis when the study began, but many of them developed the condition over the next 2 decades. When the Stanford team tabulated the data, published in the Archives of Internal Medicine in 2008, it found that the runners’ knees were no more or less healthy than the nonrunners’ knees. And It didn’t seem to matter how much the runners ran.

The study also found that runners experienced less physical disability and had a 39% lower mortality rate than the nonrunners.

In 2007 a 9-year study of 1,279 elderly residents of Framingham, Mass, resulted in similar findings: that the most active people had the same risk of arthritis as the least active. About 9% of the participants overall developed arthritis over the course of the study, as measured by symptoms reported to their physicians (pain and difficulty walking) as well as x-ray scans. And in the same year, Australian researchers writing in the journal Arthritis and Rheumatism found that people who exercised vigorously had thicker and healthier knee cartilage than their sedentary peers. That suggests the exercisers may have also enjoyed a lower risk of osteoarthritis, which is caused by breakdown and loss of cartilage.

Together, the findings support the theory that osteoarthritis, which affects nearly 20 million Americans, is caused mainly by genes and risk factors such as obesity (obese men and women are at least four times as likely to become arthritic as their thinner peers), rather than daily exercise or wear and tear of joints.

However, the article notes that running can sometimes cause soft-tissue injuries and stress fractures, which result from the compounding of tiny cracks in the bone over time. The tiny cracks may appear in the bones that bear the heaviest loads, such as the tibia, but they usually heal quickly and go unnoticed, according to the article. Stress fractures occur when bone damage happens suddenly, without enough time to heal.

In a second study in the same journal, researchers at Iowa State University used computer modeling to figure out how the length of a runner’s stride might change the force applied to his or her bones and thereby affect the risk of stress fractures. Researchers recruited 10 male participants, each of whom typically ran about three miles per day, and calculated their risk of experiencing a stress fracture — about 9% over 100 days. By observing the participants running at varying stride lengths and recording the amount of force their foot strikes exerted on the ground, researchers were able to estimate the force each runner applied to his shinbone. According to the computer model, if the runners reduced their natural strides about 10%, they could reduce their risk of fracture by a third.

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