A team of researchers from the United States and Finland has found that rates of knee osteoarthritis (OA) are higher now than in the past – probably not for the reasons you think.
In a study published in the Proceedings of the National Academy of Sciences, the researchers report that knee OA is more than twice as common today, in the post-industrial era, as it was at any time before, going back to prehistoric days. People in the 21st century are also more likely to have arthritis in both knees than were people in the past.
So how did the researchers study arthritis in people who lived hundreds or even thousands of years ago? According to study co-author Daniel Lieberman, PhD, a professor of human evolutionary biology at Harvard, they examined skeletons of people over age 50 that were in museums or had been donated for medical research. In total, 2,576 skeletons from various places around the United States were examined. The oldest skeletons were from hunter-gatherers who died as long as 6,000 years ago; the most recent were people who died in Tennessee and New Mexico between 1976 and 2015. They also examined the skeletons of farmers who died 300 to 800 years ago and the skeletons of people who died in the early 20th century.
The researchers say this big historical perspective was important because it allowed them to study known risk factors, especially age and obesity, under various conditions. Some experts believe the high percentage of people who have OA currently is due to a combination of high obesity rates and longer life expectancy, which create more opportunity for wear-and-tear on joints. So the research team wanted to look specifically at those factors.
While just 1 percent of hunter-gatherers were obese – compared to 25 percent of people today – their knees took a beating because they were always on the move, picking berries and chasing down game. For that reason, the researchers assumed they’d find a lot of OA in our early ancestors as well as in industrial-age workers (1905-1940), who tended to have physically demanding jobs.
But they didn’t. When the researchers took age and weight out of the equation, they still found that 21st century humans had double the rate of OA of their forebears: Only 8 percent of early humans and 6 percent of 20th century workers showed evidence of OA, compared to 16 percent of Americans today.
Lieberman says that means other factors must be driving the increase, but his team didn’t explore what those might be. Their best guess, though, is lack of physical activity. Lieberman points out that the biggest shift between now and even a few decades ago is that adults and children have become extremely sedentary, which can affect joint health. The lack of physical activity can lead to thinner, less resilient cartilage and weaker muscles that support and protect joints. If being sedentary is the culprit, then OA is almost entirely preventable, he and his colleagues note in the article.
Karmela Kim Chan, MD, a rheumatologist at Hospital for Special Surgery and assistant professor at Weill Cornell Medical College, both in New York City, agrees.
“There are many factors that a study like this wouldn’t be able to account for [like diet and injuries]. But it seems fair to assume that we in the post-industrial age are more sedentary than our pre-industrial counterparts. Modernity has made things much easier for us. But that also means we are no longer as active as we evolutionarily were meant to be,” she says.
Like Lieberman, Dr. Chan stresses that activity is good for the knees and for joints in general.
“There is good evidence that in patients with healthy knees, physical activity seems to slow down the development of arthritis. It’s very likely there are biomechanical reasons within the knee that explain this – that is, how cartilage and bone respond to the stimulation activity provides,” she explains. “In addition, exercise improves muscle strength, which probably makes arthritis less symptomatic.”
Dr. Chan recommends biking, either outdoors or on a stationary bike, to help build up the quadriceps (the muscles on the front of the thigh that act as shock absorbers for the knee).
“If [you] don’t enjoy biking, then the elliptical or rowing machine are good proxies,” she says. “In patients with more advanced arthritis who find it difficult to stay upright for extended periods, exercising in the water is a great way to work those muscles without stressing the joint. Even walking back and forth across the length of a pool helps. Not only does the water carry the patient’s weight, it provides resistance [so] the muscles work harder.”
Author: Linda Rath for the Arthritis Foundation
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