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Last sync:2023-01-27 10:45
SciShow explains what really causes those popping sounds your joints make -- fluid dynamics, people! -- and what you should watch out for if you're a habitual knuckle-popper.
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Are you ready? (crack) Ahhh...

I've been cracking my knuckles since I can remember and I do it so often that I've been called "Crunchy." I've gotten so good at it that I can pop almost every joint in my body.

But like, why do they do that? Cause even though it feels awesome, it does sound rather alarming.

Well, a joint is just where two bones in your body come together, but they don't actually touch because if they did, the friction would grind them into a bone powder which I think most people would find unpleasant.

So instead, your bones are capped by cushions of articular cartilage which are kept lubricated by thick, clear mucousy stuff called synovial fluid. And that fluid is produced by the synovial membrane, which surrounds the entire joint.

When you stretch or bend a joint, those bones pull away from each other and that causes the synovial membrane to stretch, which increases the amount of space inside it, in turn lowering the pressure.

And this is important because your synovial fluid is full of dissolved gasses, mostly carbon dioxide and oxygen. And when the pressure of a fluid drops, any gasses trapped within it become less soluble. Basically, they undissolve, and this means they form bubbles.

So that pop you hear is actually the sound of a bubble forming inside your synovial fluid.

What's really cool is if you take an x-ray of a joint right after cracking it, you can actually see the bubble. It increases the size of the joint cavity by about 15%.

And it takes about twenty to thirty minutes for that gas to dissolve back into the fluid, which is why you generally can't crack the same joint over and over again.

There's weirdly not a lot of data on whether knuckle popping is dangerous, but a doctor named Donald Unger was awarded an Ig Nobel Prize in 2009 for habitually popping his joints on his left hand, but not his right over the course of sixty years. His left hand didn't develop any issues. It's not super hardcore peer reviewed science, but it's still pretty interesting.

So, while knuckle popping doesn't appear to cause arthritis, it apparently can lead to weaker grip strength. This might be the result of stretching out your synovial membranes or your tendons.

The most dangerous part of cracking your joints is probably that people nearby don't always find the noise enjoyable. I beg to differ. (crack)

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