What Makes Your Knuckles Crack

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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.

  • The 'pop' of a cracked knuckle is caused by bubbles bursting in the synovial fluid — the fluid that helps lubricate joints. The bubbles pop when you pull the bones apart, either by stretching the fingers or bending them backward, creating negative pressure.
  • Here’s What Happens When You Crack Your Knuckles Solving the joint-popping puzzle. In a recent study published in PLOS ONE. (Story continues below.) MRI video of one of Fryer’s finger joints popping. Pop goes the knuckle. The researchers observed the same phenomenon occurring in every one.

What Makes Your Knuckles Crack Game

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.

Oct 26, 2001  The cracking or popping sound is thought to be caused by the gases rapidly coming out of solution, allowing the capsule to stretch a little further. The truth of the matter is that after cracking your knuckles there is a greater degree of mobility and a sense of relaxation in the joints. For this reason it develops into a daily habit for many people. On the other hand there are others who often visit a specialist called a chiropractor. The participants were asked crack the knuckle at the base of each finger, known as the metacarpophalangeal joint (MPJ), while being observed through an ultrasound machine. They ended up imaging 400 MPJ cracks, and recorded the sounds so they knew which ones came with a 'pop'.

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.

What is it that makes that popping sound when you crack your knuckles? If you think it's vacuum cavities forming in the synovial fluid of the joint, give yourself a gold star: a team of researchers led by the University of Alberta Faculty of Rehabilitation Medicine have confirmed that that is precisely what it is.

How? By pulling the fingers of a test subject inside an MRI machine.

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'We call it the 'pull my finger study' -- and actually pulled on someone's finger and filmed what happens in the MRI,' said lead author of the study published in PLOS One, Professor Greg Kawchuk of the Faculty of Rehabilitation Medicine. 'When you do that, you can actually see very clearly what is happening inside the joints.'

What happens when knuckles crack

The theory of bubbles in the joint was first floated in 1947: UK researchers JB Roston and R Wheeler Haines hypothesised that cracking the knuckles caused bubbles to form in the synovial fluid; this, they believed, caused the sound. In 1971, however, another study came along that proposed that it was not the formation, but the collapse of the bubble that produced the audible effect -- in other words, that it was the bursting of the bubble that made a noise.

What Makes You Crack Your Knuckles

Other hypothetical sources of the knuckle-cracking noise included stretching ligaments, or the adhesions in the joints snapping -- but the bubble idea has always been the strongest, since X-rays taken directly after cracking a joint show a gas bubble inside that joint. Iomega storcenter software. But whether or not it was the formation or collapse of the bubble had still been something of a mystery.

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The idea for the study came from Nanaimo chiropractor Jerome Fryer, who approached Professor Kawchuk with a theory. Rather than beat around the bush, they decided to take a direct look using magnetic resonance imaging -- with champion knuckle-cracker Fryer as the guinea pig.

'Fryer is so gifted at it, it was like having the Wayne Gretzky of knuckle cracking on our team,' Professor Kawchuk said.

Fryer's fingers were inserted, one at a time, into a tube attached to a cable; this tube slowly pulled on each finger until the knuckle cracked. And, in each instance, it was absolutely the formation of the bubble in the synovial fluid that was associated with the popping sound, occurring within 310 milliseconds.

'It's a little bit like forming a vacuum,' Professor Kawchuk explained. '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.'

Solving a decades-old mystery was far from the team's only focus, though -- as fun as that was. The team believes studying joint cracking could help them better understand joint health -- such as the contradiction between the amount of force required to crack a joint (enough to cause damage to hard surfaces) and the fact that it doesn't appear to do long-term harm.

One thing they found, for instance, was a flash of white in the MRI just before the joint popped -- something no one had ever documented before. Professor Kawchuk believes it was water suddenly being drawn into the joint, and plans to use more advanced MRI to study what happens in the joint just before and after the pop.

'It may be that we can use this new discovery to see when joint problems begin long before symptoms start, which would give patients and clinicians the possibility of addressing joint problems before they begin,' he said.

The 1971 team may have missed the mark on the cause of the sound, but they did get at least one thing correct.

'The data fail to support evidence that knuckle cracking leads to degenerative changes in the metacarpal phalangeal joints in old age,' the study concludes. 'The chief morbid consequence of knuckle cracking would appear to be its annoying effect on the observer.'