When muscle joints are pulled apart there forms a tiny cavity filled with gas which then collapses, creating a popping noise.
The wince-inducing sound of knuckles cracking is caused by a small bubble building up in the fluid of the fingers then ‘popping ‘, scientists believe, and it could even be beneficial to health.
For decades researchers have debated what causes the unpleasant sound and argued about whether knuckle cracking could cause joint problems like arthritis.
Now a new study from the University of Alberta suggests that when muscle joints are pulled apart there forms a tiny cavity filled with gas which then collapses, creating a popping noise.
It takes a while for the gas to be re-dissolved in the slippery synovial fluid in the joints which explains why knuckles cannot be “re-cracked” immediately.
After watching cracking joints under an MRI scanner, the team also saw an unexpected white flash, which they believe could be water being drawn to the joint, which could even have a beneficial effect.
Previously scientists have calculated that the amount of force at work when you crack your knuckles has enough energy to cause damage to hard surfaces like bone, yet research also shows that habitual knuckle cracking does not appear to cause long-term harm.
Those conflicting results are something the researchers are planning to investigate next.
“The ability to crack your knuckles could be related to joint health,” said said Greg Kawchuk, a professor in the Faculty of Rehabilitation Medicine.
“Some people can crack their joints and others cannot and we’d like to know why.
“It’s a little bit like forming a vacuum. 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.”
To work out what was happening when knuckles are cracked, the team looked at ten finger joings, inserting them 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, in less than 310 milliseconds.
In every instance, the cracking and joint separation were associated with rapid creation of a gas-filled cavity within the synovial fluid, a slippery substance that lubricates the joints.
Image A shows the finger in a resting phase, B just prior to joint cracking, C immediately after joint cracking and D in the aftermath as the joint returns to its usual position
Prof Kawchuk is hoping to use even more advanced MRI technology to understand what happens in the joint after the pop, and what it all could mean for health.
The authors suggest the findings may pave the way for new research into the potential therapeutic benefits or harms of joint cracking.
Although there is no evidence that knuckle cracking causes arthritis, there have been reports that it can injure ligaments and dislocated tendons.
The research was published in the journal PLOS One.