At some point in your life, someone's probably told you, "Stop doing that! It's bad for you!" as you cracked your knuckles.
But if you asked that person why, they probably didn't have an answer for you.
Luckily, we have scientists. Gregory Kawchuk at the University of Alberta, Canada, and his colleagues made it their mission to find out exactly what is happening inside our fingers when we "crack" their joints.
In a new study published in PLOS ONE — named, hilariously, the "pull my finger study" — the researchers slowly pull a man's fingers with a cable and used MRI video to capture (in less than 310 milliseconds) what happened inside his joints to cause those distinctive popping sounds.
And, contrary to popular belief, the sound does not come from collapsing air bubbles. Instead, the scans found that the air cavity that formed in the fluid around the separating joints persisted after the noise.
At this point, it's time to turn your volume up and prepare to wince.
“It’s a little bit like forming a vacuum,” 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.”
Do we now know if this habit is bad for us? Not for sure. But research has shown that habitual knuckle-cracking doesn't appear to cause long-term harm. And if my mother, a pediatric rheumatologist, has given up telling me to quit it, it's probably fine.
Screengrab via RehabMedicineUofA/YouTube