• Question: what happens when you stretch and your back or any part of your body cracks? what exactly is going on and why? :)

    Asked by DR.HATSRA to MarthaNari, Jonny on 18 Jun 2015.
    • Photo: Martha Havenith

      Martha Havenith answered on 18 Jun 2015:


      Ha, I had to look this one up, and apparently they just answered the question a few months ago – at least for cracking your fingers. Unless I am very mistaken, whenever something in your body cracks (and doesn’t hurt like hell at the same time), it’s a joint. So it should be the same principle as when you crack your fingers. Your movement pulls the two sides of the joint apart a bit further, and the creates a small bubble of gas in the fluid between them. What you hear is when that bubble ‘pops’. Here is my favourite paragraph from the Guardian article about finger cracking:
      ‘… Kawchuk describes how every crack occurred as the joints suddenly separated, and a gas-filled pocket appeared in the synovial fluid that lubricates the joint. The bubble comes from gas that comes out of the fluid as the pressure in it drops, just as bubbles appear in freshly opened bottles of fizzy drinks. “If you’ve ever washed up glass plates, you’ll know they can be hard to separate when they are wet. The film of water between them creates a tension that needs to be overcome. It’s similar with joints. When you pull on them, they resist at first, and then suddenly give way,” said Kawchuck.’
      If you want to read the whole thing, you can find it here (with some nice pictures of finger pulling devices):
      http://www.theguardian.com/science/2015/apr/15/cracked-it-scientists-solve-puzzle-of-why-knuckles-crack-when-pulled

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