Cracked! Why our knuckles click and pop

THE puzzle of why our knuckles make a cracking sound may have finally been … well, cracked.

Scientists have long debated whether the sound came from bubbles being formed in the fluid around our joints, or from them being popped.

Now, a study has found that the microscopic bubbles actually make a popping noise when they only partially collapse.

The researchers also said that a certain pressure is needed to produce the sound. This explains why some of us cannot crack our knuckles at all – our joints need to be close together for the pressure to fall low enough to form bubbles.

A 2015 study using an MRI scanner found that the bubbles did not collapse, suggesting the noise was produced when formed.

But the latest study at Ecole Polytechnique in Paris suggests the MRI video, at a rate of three frames per second, may have missed bubbles partially collapsing in between the snapshots.

Using a geometrical representation of the joint and mathematical equations, it found that pressure changes in the fluid around the joint cause the collapse of the bubble.

But only a partial collapse is needed to produce a sound.

Professor Greg Kawchuk, who led the MRI study, said: ‘What makes this paper interesting is that it suggests other phenomena may occur in between frames of the MRI video.’ The findings still ‘leave the origin of joint cracking sounds open to further study and debate’, he added.