The methodology of the study was fascinating. One of the researchers traveled around the country to examine collections of old and new skeletons, looking for signs of “eburnation,” which is a polished surface on the bones of the knee joint that occurs when the bones rub against each other because of the loss of cartilage associated with osteoarthritis.
In total, he examined almost 2,500 skeletons from three distinct time periods:
- “Prehistoric” skeletons from archeological digs in Alaska, California, New Mexico, Kentucky, and Ohio, from hunter-gatherers and early farmers between 300 and 6,000 years ago. All of the people were at least 50 years old when they died.
- Early industrial skeletons from Cleveland and St. Louis, from people who died between 1905 and 1940 and whose bodies were used for medical education and research.
- Postindustrial skeletons from Albuquerque and Knoxville, from people who died between 1976 and 2015 and donated their bodies to medical research.
The results showed that knee osteoarthritis occurred with roughly similar frequency in the prehistoric and early industrial skeletons, but was much more common in the postindustrial skeletons.
Of course, that’s exactly what you’d expect if you subscribe to the old-age-and-obesity theory. Fortunately, in many of the early industrial and postindustrial skeletons, age and body mass index (BMI) at death were recorded, which allowed the researchers include those factors in their analysis. The surprising result: Even accounting for age and BMI, knee osteoarthritis was still roughly twice as common for people born after World War II than it was for people born before it.
So if it’s not obesity or age, what explains the apparent rise in osteoarthritis rates? This study can’t answer that, but the researchers do float a few hypotheses in their discussion.
One possibility is that walking around on hard, paved surfaces all the time isn’t good for our knees. In support of that possibility, they cite a 1982 paper in which sheep spent 2.5 years living either on concrete and tarmac or on wood chips and pastures. The sheep in the concrete jungle had noticeable changes in the cartilage and bone of their knee joints, and maybe we do too.
Another possibility is shoes—and they’re not talking about trainers. They cite a 1998 study showing that high-heeled shoes generate abnormally high forces on the knee joint, and note that, in their analysis, women were about 50 percent more likely to have knee osteoarthritis than men.
But the biggest factor, they suspect, may be physical inactivity. Joints, like muscles, have a use-it-or-lose-it aspect. If you sit at a desk all day, you end up with thinner, lower-quality cartilage in your joints, and weakness in the muscles that would otherwise take some of the load off your joints. The problem, in other words, isn’t too much running; it’s not enough running.
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In real life, of course, things are never that neat and tidy. As the authors are careful to point out, there’s lots of work remaining to explore some of these hypotheses. And even if the theories are confirmed, the fact remains that some runners, despite doing everything “right,” will still get osteoarthritis.
Still, the results are significant because they join a growing body of evidence that argues against osteoarthritis as a wear-and-tear disease, in which your knee are delicate instruments that will wear out if you use them too much. Your knees were made to be used, and are healthiest when used regularly. So use them!
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