For some, cracking their knuckles is a daily routine. Others can't stand the sound of it. Regardless of which camp you fall into, there has always been one major, unanswered question: what causes the cracking sound?
The initial theory for why knuckles crack originated in 1947. Scientists proposed that the act of knuckle-cracking caused a gas bubble to form between the joints as they separated. The sound, they claimed, came from the formation of these gas bubbles.
In 1970, this theory was challenged. A different group of scientists—still in agreement that a bubble does form between the joints as they separate—suggested that the cracking sound comes afterward, when bubble collapses (i.e. pops). This idea has been widely accepted as the logical answer since it was proposed.
The Pull My Finger Study
On April 15, 2015, an article was published by a group at the University of Alberta. In what they have joking called the Pull My Finger study, they had one of their colleagues get in an MRI machine for an experiment designed to see what happens when knuckles are cracked.
Each of his 10 fingers were hooked into a finger-pulling harness, one at a time, and pulled as the MRI machine recorded activity between the joints. The video below shows one of these recordings (caution for the squeamish).
By studying the data, they came to a surprising conclusion. While it seemed reasonable that the cracking sound occurs when a gas bubble pops, it was the initial theory from 1947 that proved to be correct. When someone cracks their knuckles, the sound produced comes from the formation of the gas bubbles, not when they collapse.
From the abstract:
As traction forces increased, real-time cine magnetic resonance imaging demonstrated rapid cavity inception at the time of joint separation and sound production after which the resulting cavity remained visible. Our results offer direct experimental evidence that joint cracking is associated with cavity inception rather than collapse of a pre-existing bubble.
It is great to see a study like this being done. These results show that not everything we consider to be common sense is necessarily right, and if it weren't for experiments like this one, we would be none-the-wiser about our misconceptions.