Pretty much every study that looks at runners and knee problems, whether it's a multi-decade longitudinal study or a massive cross-sectional study, has found that runners have no elevated risk of knee osteoarthritis and perhaps even have lower risk (likely due to running's role in minimizing weight gain). I've written about this many times (including a detailed look in my book), and the evidence is pretty robust -- for the most part.
But there is one gap in the data, relating to the self-selection of runners. Any study that looks at longtime runners is inevitably looking at people who've concluded that their knees can handle running, otherwise they would have stopped. But what if you took a truly random selection of people off the street and "forced" them to run regularly? Are there people for whom running really would cause problems? It's a reasonable question to ask.
An interesting study just published in the catchily-titled journal Knee Surgery, Sports Traumatology, Arthroscopy goes partway (but not all the way) to addressing this question. They took a group of 10 beginner runners and put them through a supervised six-month "beginner marathon" program that concluded with the subjects running a full marathon. Before and after the program, the volume and thickness of cartilage at various places in the knee was measured with highly sensitive 3D MRI analysis. This is the type of runner that is generally considered at highest risk: someone with little training background who ramps up quickly to tackle a long race like a marathon. The study doesn't give much detail about the exact training, but the subjects ended up running between 25 and 60 km per week.
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Here's a graph that shows that resulting changes in cartilage volume at various locations in the knee:
The only change that reached statistical significance was a 3.2% decrease in the lateral femur (marked by an asterisk). On the other hand, you can see that cartilage volume in the medial femur apparently increased a bit (by 2.4%). In both cases, these changes are smaller than the "test-retest" precision of the measurement technique, so the researchers conclude that there are no actual clinically significant changes. The story is the same with cartilage thickness.
Interestingly, previous studies have looked at the cartilage changes immediately after running, and found much bigger changes, presumably because fluid gets temporarily pushed out of the knee. But the cartilage returns to normal within an hour or two -- a reminder that it's a dynamic, living tissue, not just an inert shock absorber. In fact, the researchers note that one thing that definitely does cause cartilage thinning is prolonged inactivity, so there's reason to think that regular compressive loading is essential to maintaining healthy cartilage. Some scientists have suggested that the regular loading provided by running might actually enhance cartilage thickness over time (as an adaptive response), but there's no evidence for that here.
All the volunteers in this study were healthy to start with and didn't have a history of knee problems. So this study still doesn't tell us about the effects of running on the joints of people who have "bad knees," due either to wacky biomechanics or previous acute injuries. Given that about 99% of people I've ever met claim to have self-diagnosed "bad knees," that's certainly a question that still needs to be considered. But this study adds some weight to the idea that even beginners can handle a relatively high load of running with no measurable effect on their knees.
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