Why Do Your Knuckles Crack?

For some people the sound of cracking knuckles is cringe-worthy. For others it's a daily, if not hourly habit. Whatever your inclination, new research has revealed why knuckles make that signature "crack" when the joint between the fingers and the hand bones is released.

According to science student Vineeth Chandran Suja, it all boils down to a trio of mathematical equations. Together with his lecturer Dr Abdul Barakat, Chandran Suja devised a model that explains how cracking the knuckles triggers a pressure change which forces tiny air bubbles to collapse in joint fluid.

"The first equation describes the pressure variations inside our joint when we crack our knuckles," explains Chandran Suja in an interview with BBC News. "The second equation is a well-known equation which describes the size variations of bubbles in response to pressure variations," he adds. "And the third equation that we wrote down was coupling the size variation of the bubbles to ones that produce sounds."

A combination of bubbles, fluid and pressure

Together, the three equations offer a mathematical explanation that describes why releasing the knuckle joints is accompanied by such a dramatic sound effect. The study was published in the journal Scientific Reports and explains how a combination of bubbles, fluid and pressure has created what some people consider one of humankind's most annoying habits. 

"When we crack our knuckles we're actually pulling apart our joints," says Chandran Suja. "And when we do that the pressure goes down. Bubbles appear in the fluid, which is lubricating the joint - the synovial fluid. During the process of knuckle cracking there are pressure variations in the joint which causes the size of the bubbles to fluctuate extremely fast, and this leads to sound, which we associate with knuckle cracking."

Bridging the gap between previous studies

While the findings are a breakthrough, it's not the first time scientists have explored the mystery of knuckle cracking. The bubble collapsing theory was first put forward in 1971 and was challenged 40 years later when new research suggested that air pockets remain in synovial fluid long after the joints have been pulled apart.   

The new model bridges the gap between the two conflicting theories and indicates that only a partial bubble collapse is needed to produce the "crack" sound. It also explains why some people are unable to crack their knuckles. If the space between the knuckle bones is too small fluid pressure levels don't drop low enough to trigger a bubble collapse, and hence create a sound.  

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