Despite what all your concerned non-running friends tell you, runners seem to be less likely to develop osteoarthritis in their knees than their sedentary peers. Really. This is a finding that has been replicated many times over.
There are lots of ways that people try to explain away these results. Maybe it’s selection bias (though the effects show up in randomly selected community samples, not just studies of runners). It almost certainly has something to do with the fact that runners typically weigh less (though the effect persists even if you control for weight).
Alternately, maybe there’s something about running that actually helps make your joints healthier. One theory is that, since exercise helps fight inflammation throughout the body, it may also fight the persistent inflammation that contributes to the progression of osteoarthritis.
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That’s the theory explored by a new study in the European Journal of Applied Physiology, from researchers at Brigham Young University.
The design of the study is pretty cool. They took samples of blood and synovial fluid from the knee—the joint’s lubricating fluid, which they extracted by jamming a needle into the joint. Then the subjects—all healthy recreational runners—ran for 30 minutes (or, in the control condition, sat for 30 minutes) and then gave another sample of blood and synovial fluid.
The advantage of taking synovial fluid is that it gives you a look at what’s happening in the knee itself. If you simply look at levels of inflammatory or anti-inflammatory markers in the blood, you can’t be sure where they originated.
Previous studies of synovial fluid from patients who already had osteoarthritis have found evidence that joint loading (such as running or one-legged knee extension exercises) really does trigger anti-inflammatory markers and reduce markers of cartilage turnover. That's why (or at least one of the reasons why) exercise is so highly recommended to osteoarthritis patients, to the degree they can tolerate it.
But in this study, using healthy patients, there was a problem: healthy knees have very little synovial fluid. As a result, the researchers only managed to collect a full set of usable pre- and post-exercise synovial fluid samples from six of their 15 subjects. The result, unfortunately, is that it’s more or less impossible to draw any firm conclusions from their data. They measured a dozen different markers of inflammation and cartilage turnover—and when you have twice as many outcomes measures as you do subjects, you’re in trouble.
There were a few interesting hints—a decrease in the inflammatory cytokine interleukin-15, which has been associated with increased progression and severity of osteoarthritis; a change in levels of a marker of cartilage turnover. But mostly this just highlights the possibility that interesting findings may await researchers who can figure out how to get sufficient amounts of synovial fluid out of the knees of healthy runners.
In the end, the best things about running (from your knees’ perspective) are probably 1) that it keeps your weight down, and 2) that tackling other runners is heavily frowned upon, since acute sports injuries like ACL tears really do raise your risk of osteoarthritis down the road. But it’s interesting to consider that there may be other, more subtle factors at work.
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