Decoded: What causes knuckle-cracking sound

Next time when your kid asks you to "pull his finger," also tell him the secret behind the distinctive popping sounds that are heard while cracking knuckles.

Settling a a decades-long debate about what happens when you crack your knuckles, an international team of researchers led by the University of Alberta have found that the cause is a cavity forming rapidly inside the finger joint.

"We call it the 'pull my finger study' and actually pulled on someone's finger and filmed what happens in the MRI. When you do that, you can actually see very clearly what is happening inside the joints," explained lead author Greg Kawchuk, professor in the faculty of rehabilitation medicine.

Scientists have debated the cause of joint cracking for decades, dating back to 1947 when British researchers first theorised vapour bubble formation as the cause.

That was put in doubt in the 1970s when another team of scientists instead fingered collapsing bubbles as the cause. To find an answer, the team asked acclaimed chiropractic physician Jerome Fryer to volunteer for the test.

Fryer practices chiropractic care for sports injuries and also conducts spinal research. Fryer's fingers were inserted one at a time into a tube connected to a cable that was slowly pulled until the knuckle joint cracked.

MRI video captured each crack in real time -- occurring 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 super-slippery substance that lubricates the joints.

"It is a little bit like forming a vacuum. As the joint surfaces suddenly separate, there is no more fluid available to fill the increasing joint volume, so a cavity is created and that event is what is associated with the sound," Kawchuk noted.

The ability to crack your knuckles could be related to joint health.

"We can use this new discovery to see when joint problems begin long before symptoms start, which would give patients and clinicians the possibility of addressing joint problems before they begin," the authors concluded.

The study was published in the journal PLOS ONE.