What goes 'pop' when we crack our knuckles?

Gregory Kawchuk, a University of Alberta researcher, along with his colleagues, has revealed what makes our knuckles crack by clicking the joints of one of their team inside an MRI scanner.

The researchers captured three images per second inside the scanner as they tugged on a rope attached to the index finger of their volunteer. They were able to capture what happened inside the knuckle joint at the precise moment when it went crack.

The Naked Scientist, Chris Smith, explained to CapeTalk/702's Redi Tlhabi what the research revealed:

They have revealed that the noise, when we crack our knuckles, is a bubble of gas appearing under low pressure, in the joint - and that what makes the sound.

The Naked Scientist, Chris Smith

Previous theories on why our knuckles crack:

  • In 1947, two UK scientists speculated that joints click and crack when low-pressure triggers a bubble to pop into existence inside the joint space.

  • In the 1970s, a second group of British researchers instead claimed that the sound was produced, not by a bubble forming in the joint, but instead when it collapsed in on itself afterwards. This is a process called cavitation, and it became the benchmark explanation for why joints crack.

Scientists had no direct observations to support either theory, according to Smith.

Listen to the full conversation on the Redi Tlhabi Show:


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