What Is Happening When I Crack My Knuckles And What Is The Cause Of The Cracking Sound?

If you crack your knuckles you are among millions of people around the world who enjoy doing so. Many do this to relieve pressure, especially in the back. But when we crack our knuckles, what is going on inside and what causes that cracking sound we hear?

Most believe the cracking sound is cause by two joints or bones clicking against each other. 

Scientists have debated since the mid 1040's, and since no one could make up their minds, a group of scientists from the University of Alberta decided to perform a detailed study that would finally settle the argument and mystery. Their study/ investigation, was nicknamed the “pull my finger study.”  In real-time using cine magnetic resonance imaging (MRI) they captured joint cracking. In order to accomplish this, one of the study authors was placed  inside an MRI machine with cables attached to his fingers which were slowly pulled until they cracked.

They found that the sound was associated with the rapid formation of a gas-filled cavity within the lubricating fluid surrounding the joint, which remained visible after the noise was produced.

“It’s a little bit like forming a vacuum,” explains lead author Greg Kawchuk. “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’s associated with the sound.”

Watch the video below to observe knuckle cracking in real time: