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?
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.