National Science Health & Safety

Why Knuckles Crack

9:39pm April 21, 2015

by Rob Stein

NPR intern Poncie Rutsch takes a crack at making a big sound. Meredith Rizzo/NPR

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Scientists think they may have solved an old question about the cracking of knuckles: Why does it make that sound?

The crack apparently comes from a bubble forming in the fluid within the joint when the bones separate, according to a study published Wednesday. It's like a tiny air bag inflating.

The findings confirm the original theory about knuckle-cracking, which was first proposed in 1947 but challenged in the 1970s.

According to Greg Kawchuk, a professor of rehabilitation medicine at the University of Alberta, that second group of scientists came along and said, " 'No, no, no — wait! We think what's happening is, the gas bubble forms but then it subsequently collapses. That's what makes the big sound.' "

While many people accepted the bubble-bursting theory, no one was sure. So Kawchuk and a team of scientists figured out a little test, enlisting the help of a pal who is really good at cracking his knuckles.

"We called our colleague the 'Wayne Gretzky of finger-cracking,' " Kawchuck says. "He can make this happen in all 10 of his fingers."

RehabMedicineUofA YouTube

They asked the volunteer to put his hand inside a special type of MRI scanner, and made a movie of the inside of his knuckles as they pulled on the end of each finger to make it crack.

"We've been calling it the 'Pull My Finger Study,' " Kawchuk says.

What they saw was clear: The cracking sound comes when a bubble forms between the bones of the knuckle joint — not when it collapses.

"Our jaws hit floor," he says. "This is the exact answer. It feels pretty great."

Other researchers praised the study, which appears in the online journal PLOS ONE.

"This is the first time we're actually seeing what's going on inside the joint when a knuckle is being cracked," says Dr. Kevin deWeber, who studies sports medicine in Vancouver, Wash. "It's really exciting."

DeWeber says the discovery also reinforces previous research that challenged a common misconception about knuckle-cracking — that it causes arthritis.

"It's mostly an urban myth ... perpetuated by mothers who are sick of hearing their kids crack their knuckles," deWeber says. He thinks cracking knuckles might actually be good for the joints — sort of a massage of the cartilage.

"This study helps us understand the biomechanics of the knuckle-cracking event," deWeber says. "We are reassured that nothing harmful is happening inside. And ... maybe something helpful is happening."

Transcript

DAVID GREENE, HOST:

You know, occasionally we'll warn you if there's something in a story you might find offensive. This is one of those stories. You might want to turn your radio down for a couple minutes if, that is, you can't stand it when someone cracks his or her knuckles, like, say, NPR's health correspondent Rob Stein.

ROB STEIN, BYLINE: First, I should admit something - I'm one of those people who (cracks knuckles) cracks their knuckles. But I had no idea that for decades, there had been two competing theories about what makes that sound. Greg Kawchuk, a bioengineer at the University of Alberta, says the first theory was proposed way back in the 1940s.

GREG KAWCHUK: In 1947, there was the first really serious scientific study into this question that was done by a group in England. And they proposed that the reason why joints crack is that as the joints separate, there's a gas bubble that's formed.

STEIN: A bubble in the fluid that lubricates the knuckle joints.

KAWCHUK: And this gas bubble, they hypothesized, was the source of the sound.

STEIN: As the bubbles suddenly inflated like a tiny airbag. But that theory was challenged in the 1970s.

KAWCHUK: Another group came along and said no, no, no - wait. We think what's happening is the gas bubble forms, but then it subsequently collapses.

STEIN: Like a balloon popping.

KAWCHUK: That's what makes the big sound.

STEIN: Which is the theory a lot of people thought was right ever since then. But no one was ever really sure, so Kawchuk's team decided to find out. They enlisted one of their colleagues who's really good at cracking his knuckles.

KAWCHUK: We called our colleague the Wayne Gretzky of finger-cracking because he can make this happen in all 10 of his fingers.

STEIN: So they put his hand inside a special MRI scanner that let them make a movie of what was happening inside each knuckle as they pulled each finger to make it crack.

KAWCHUK: We've been calling it the pull my finger study (laughter) ever since it was initiated with the same connotations that everyone has around the world for when you say pull my finger.

STEIN: And what they saw was clear - the cracking sound comes when a bubble forms, not when it collapses.

KAWCHUK: Our jaws hit the floor and thought wow, you know, this is the exact answer. And it feels pretty great.

STEIN: To have finally solved the great knuckle-cracking mystery. The results appear in the journal PLOS ONE. Kevin deWeber is a sports medicine doctor in Vancouver, Wash. He says the discovery could help debunk a myth that we've all heard about knuckle-cracking - that it causes arthritis.

KEVIN DEWEBER: It's mostly a urban myth, you know, perpetuated by mothers who are sick of hearing their kids crack their knuckles. And they say, if you crack your knuckles, you'll get arthritis.

STEIN: DeWeber thinks cracking your knuckles could actually be good for you by kind of massaging the cartilage in the joint.

DEWEBER: This is actually the first time we're actually seeing what's going on inside the joint when a knuckle is being cracked, so, you know, it's really exciting.