The idea is that when you run, the flexible, tough tissue that cushions your knee joints, called cartilage, takes a pounding.

And over time, that cartilage supposedly wears down, which makes your bones rub together, and leads to the pain, stiffness, and swelling of osteoarthritis.

But is this actually true?

Past studies that compared runners to non-runners, sometimes collecting data over a couple of decades, have found mixed results.

A few have found that there's more risk of knee osteoarthritis in specific groups of runners, like in men younger than 50 who run more than 32 km a week.

But lots of papers comparing long-term runners to swimmers, non-runners, or sedentary people didn't turn up evidence that running will doom you to arthritic misery.

In some cases runners even seemed to be less likely to get arthritis.

So could running actually protect your joints somehow?

To find out, a study published in 2016 looked at what's happening inside your knees when you run.

Scientists gathered a group of male and female runners, younger than 30 with no history of knee problems.

And brought them in for two experiment sessions over two days.

They started each session by collecting a blood sample and a bit of synovial fluid, the natural lubricant inside the knee joint.

Then they had the participants spend 30 minutes running on a treadmill one day, and 30 minutes sitting on another day.

Afterward they took more samples.

Getting a useful amount of fluid from healthy knees, turned out to be pretty tricky.

And the researchers only ended up with complete data for 6 people.

But the results are still interesting.

The scientists were looking for molecules related to inflammation, because extra inflammation has been linked to the development of arthritis.

They were also measuring a compound called cartilage oligomeric matrix protein, or COMP, which can be a marker of arthritis if there's a bunch of it in your synovial fluid.

After running, the subjects had less of these molecules in their knees, and more in their blood, spread out in their bodies.

Sitting, on the other hand, slightly increased concentrations of COMP and some other inflammatory molecules in their knees.

It's hard to tell exactly what this means.

But it seems like running might squeeze inflammatory compounds out of your knee, which could reduce cartilage damage and arthritis.

But this study did have a pretty tiny sample size, so... Jury's still out!

But if you like running and it makes you feel healthy, the next time someone says, "But you'll ruin your knees!"

Take it with a grain of salt.

Thanks for asking and thanks especially to all our patrons who keep these answers coming.

For more running-related science, check out this other SciShow video, where Hank explains why you get that sharp pain in your sides sometimes.