Here's why popping sounds occur when we crack our knuckles

In a new study, scientists have unraveled the mystery as to why the popping sounds occur when we crack our knuckles.

An international team of researchers led by the University of Alberta used MRI video to determine the act inside finger joints, and observed for the first time, that the cause is a cavity forming rapidly inside the joint.

Lead author, Professor Greg Kawchuk, said that the study they call "pull my finger study" shows very clearly in the MRI what happens when someone's finger is pulled.

Fingers of Nanaimo chiropractor Jerome Fryer, who had approached Kawchuk about the new knuckle-cracking theory and could continuously crack his knuckles, 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, like forming a vacuum, within the synovial fluid, a super-slippery substance that lubricates the joints.

Kawchuk said "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."

The findings pave the way for new research into the therapeutic benefit or harm of joint cracking.

Scientists have calculated that the amount of force at work when you crack your knuckles has enough energy to cause damage to hard surfaces, yet research also shows that habitual knuckle cracking does not appear to cause long-term harm.

The study is published in PLOS ONE.