Science Questions

Is cracking your knuckles bad for you?

Tue, 10th Nov 2015

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Question

Eva Muhanguzi asked:

Is cracking your knuckles bad for you?

Answer

We put Eva's cracking question to Chris Smith... Plaster cast on forearm/wrist/hand.

Kat - Let's draw a quick straw poll in the studio. Who can crack their knuckles? I cant Ive got kind of clicky wrists, but anyone?

Max - I can crack my knuckle.

Chris - Go on max.

Kat - Anyone else?

Ginny - I can't crack my knuckles, but I can crack my big toes.

Chris - Go on then.

Ginny - I'd have to take my shoes off.

Kat - The magic of radio. It's not going to work for this one. How about you Chris, you're a knuckle cracker? Is it bad?

Chris - I'm no good at this actually. I can't make my joints crack at all. Very, very occasionally they do it, but I can't actually make them do it. Is it bad? Scientists don't actually know if it's bad. There's anecdotal evidence that it isn't. The anecdotal evidence has won an Ig Nobel Prize so that's probably how reliable the research is. Donald Unger did an n of 1 experiment on himself. He was a gentleman who for 60 years there's scientific dedication for you cracked the knuckles of one hand, but not the other and he wanted to see if there was a difference in his rates of arthritis and there wasn't. So, on the basis of his sample size of one, there doesn't appear to be a problem. Why do we get the cracking noise when you move your fingers and digits? Well actually, that was the work of British scientists in 1947 who first began to speculate what might be going on. But they had no way of proving whether or not they were right at that time. They speculated that what was going on was that when you bend your joint, you drop the pressure in the fluid the synovial fluid inside the joint. The drop in pressure causes gas that's dissolved in the joint fluid to come out of solution and form a bubble, either that the bubble popping into existence or collapsing in on itself would produce the energy that makes the sound. No one knew for sure what was the answer but then Greg Kawchuck who's a researcher at the University of Alberta in Canada, he and his colleagues did an experiment that will probably get another Ig Nobel Prize earlier this year. They published it in PLoS One in April. They persuaded a member of their team to get into the MRI scanner. They put a rope around their index finger and pulled and because they had microphone in there too, they could see when on the time course the pop occurred and they could tally that with what the MRI scan showed at that time. And in sync with the pop occurring is a bubble appearing in the joint. Not disappearing, appearing. So, it is the existence of that bubble popping into existence which takes up lots of space in the joint and therefore pushes all of the ligaments and everything else out of the way as well and that creates this expanding pop that you hear. The bubble then hangs around for a while and takes a while to dissolve naturally into the joints which is why you can't pop your fingers again. They reckon that the collapsing force of those bubbles is about 7 per cent of the energy that would be required to damage cartilage anyway. So we don't think, even if the bubble did then collapse again, it's likely to do you very much damage.

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