When you crack your knuckles, gas is released. Where does this gas come from, and more importantly where does it “go” when cracked?
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Cracking your knuckles/joints is where nitrogen within synovial fluid (fluid found in the cavities of joints) is compressed and the joint is essentially stretched. This stretching causes the dissolved gas to release from the fluid due to the pressure from the joint stretching, causing the ‘crack’ or ‘pop’ we hear.
IIRC, the nitrogen gas dissolves back into the synovial fluid and the process repeats itself over and over for our enjoyment!
The bubbles are made of oxygen, nitrogen and carbon dioxide. They build up in the fluid that cushions bones and lubricates your joints. It’s like your body’s motor oil.
Scientists thought that the sound was the bubble popping but more recently there’s been research showing the sound may be that of the bubble forming.
This is clearly something we care a great deal about:
https://arstechnica.com/science/2018/04/new-model-offers-snappy-explanation-of-knuckle-cracking/
There are several theories, however, we don’t know definitively what happens when we crack out knuckles.
This bubble process is called [cavitation](https://en.m.wikipedia.org/wiki/Cavitation). Basically when you suddenly extend the space inside the joint by stretching it, the same amount of synovial fluid takes up less space, and the pressure inside the joint drops; as that happens the gases (mostly nitrogen and hydrogen) that were dissolved into the joint fluid can no longer stay dissolved, so they spontaneously turn into bubbles. As the knuckle comes back, the pressure goes up, the bobble pops, and you get noise. At the end of which, the gases are back in an environment which has enough pressure, so over the next few minutes they dissolve back into the fluid. So it’s almost entirely a closed system, the gases were always there, and they go back afterwards.
As a fun aside, cavitation is *very* important in submarines. Propellers of ships and submarines reduce the pressure of water just forward of the propeller, and if they’re going fast enough, the propeller drops low enough for air bubbles to form in the same way they do in your knuckles. When they go pop, they make a very loud and very detectable noise. If you’re a sub trying to stay silent and hidden underwater, this is very bad. So submarines have to go slow when they don’t want to be heard by other ships; billions of dollars gets spent in designing propellers that have less cavitation.
Cotton-Eye Joe, is that you?
Where does gas come from, where does it go, where does gas come from Cotton Eye Joe?
The bubbles are made of oxygen, nitrogen and carbon dioxide. They build up in the fluid that cushions bones and lubricates your joints. It’s like your body’s motor oil.
Scientists thought that the sound was the bubble popping but more recently there’s been research showing the sound may be that of the bubble forming.
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