Question

Jenny Lee asked:

I have a class of year five students and we’ve been learning how muscles work. Some of the kids noticed how they can make their joints crack. They didn’t seem to be able to do it when they’d been doing exercise. We want to know what happens when your fingers crack and why might exercise affect it?

Answer

The reason you get joint cracking is because there is fluid inside your joints.  When you move a joint the pressure in this fluid drops and this can cause a bubble to form made up of the gas dissolved in the liquid.  The bubble pops into existence and it will take up about 15% of the space inside the joint.  This causes all the little ligaments and supporting structures around the joints to pop outwards, so that’s the first crack you hear.  
When you carry on moving the joint it goes click again as the bubble disappears.  This happens when you put the pressure back up in the joint and it the bubble collapses on itself.  That releases even more energy and that’s the second sound that you hear.  It only releases about 5% of the energy that would be able to damage cartilage, so there’s no link to arthritis if you crack your joints. Although there were some interesting studies done in Germany, where they looked at someone who cracked the knuckles of their left hand for 35 years but never those of their right hand.  There was no difference in arthritis but the muscles on the left hand were much weaker than the muscles on the right hand. Explain that one, if you will.

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