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When you get up in the morning, do you hear a snap, crackle, pop? We're not talking about cereal here. We are talking about your knees. Well, if you reach for the painkiller before that cup of coffee, you are in good company. There's been a noticeable increase in knee arthritis in recent decades. NPR's science correspondent Richard Harris explains why.
RICHARD HARRIS, BYLINE: Arthritis in human beings is hardly a new disease.
DANIEL LIEBERMAN: There's a famous Neanderthal that has arthritis.
HARRIS: Daniel Lieberman, who studies human evolution at Harvard University, says that ancient specimen was diagnosed when anthropologists saw evidence of bone wearing on bone minus the usual protection of cartilage, a classic case of osteoarthritis.
Now, a few years ago, Lieberman was putting together a list of diseases that modern humans weren't well-adapted to cope with like heart disease, near-sightedness and lower back pain.
LIEBERMAN: I wanted to include arthritis in the list but realized that there wasn't any really good data.
HARRIS: So Lieberman asked his research fellow Ian Wallace to fly around the country and study human skeletons that had ended up in museums or had been donated to medical schools for scientific research. Wallace says the individuals died as long as 6,000 years ago.
IAN WALLACE: The oldest specimens that we looked at were some skeletons of prehistoric Inuit hunter gatherers from Alaska.
HARRIS: The most recent were people who died in Tennessee in 2015. Conventional wisdom is that knee arthritis results from wear and tear, which is why it's more common among older people and those who stress their knee joints due to excess body weight.
WALLACE: So going into it, I suppose my expectation was that people in the past, especially hunter gatherers and early farmers, would have had a much higher prevalence of osteoarthritis than people do today.
HARRIS: But their study results published in the Proceedings of the National Academy of Sciences found that's not at all the case.
WALLACE: And so, yeah, I was actually extremely surprised to find that it is more common today.
LIEBERMAN: The incidence of arthritis has more than doubled for people who were born essentially after World War II.
HARRIS: Again, Daniel Lieberman at Harvard.
LIEBERMAN: And that's an incredible difference.
HARRIS: Lieberman says that's after the scientists corrected for body mass and age. So there's apparently something else at work driving the increase in knee arthritis. The current study doesn't explore that question.
LIEBERMAN: If I were a betting man, I would guess physical activity is really especially important.
HARRIS: Lieberman notes the trend toward sedentary lifestyles since World War II.
LIEBERMAN: You know, one of the things that's really shifted in our world today is that we, well, we sit all the time and kids sit all the time. And that may be affecting how our joints are forming and how our joints are aging.
HARRIS: This makes sense to Richard Loeser, a rheumatologist at the University of North Carolina Chapel Hill.
RICHARD LOESER: Your joints aren't like your automobile tires that just wear out as you use them.
HARRIS: He says exercise helps nutrients diffuse into cartilage in the knee and keep it strong and healthy.
LOESER: If that is formed and more healthy when you're younger, then your joints are more likely to be functioning better and have less osteoarthritis when you get older.
HARRIS: If you're already fully grown, is it too late?
LOESER: No, no, it's not too late because exercise by strengthening your muscles and by stimulating your cartilage can still, you know, improve the health of your joints.
HARRIS: That's not to say that exercise fully explains the trend that the Harvard researchers have noted.
LOESER: There may be dietary factors that are also important, sports injuries that have become, you know, more and more common are probably contributing to this as well.
HARRIS: Whatever the case, it appears that knee arthritis is potentially preventable, so the upward trend could eventually be held in check. Richard Harris, NPR News.
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