The mystery as to what happens to make a knuckle crack has been solved, thanks to a simple test of “pull my finger”.

In a new study published by the University of Alberta, a team of researchers used magnetic resonance imaging to determine what happens inside a finger joint to make it “pop”.

They observed that the sound is caused by a rapidly forming cavity inside the joint when pulled. Placing a habitual knuckle-cracker’s fingers into tubes which slowly pulled the knuckles apart, researchers captured the cracking moments – which occur in less than 310 milliseconds. In every instance, the cracking and joint separation was associated with the rapid creation of a gas-filled cavity within the synovial fluid, a natural substance that lubricates the joints.

However, the findings have not brought researchers any closer to determining why only some people can crack their joints on demand.

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