When You Crack Your Knuckles

If you’re a knuckle-cracker, you know how it goes: Sometimes you just don’t feel right until you pull, bend, or push your fingers until they pop and you’re flooded with satisfaction. On the other hand, if you’re not prone to cracking your knuckles, the urge might seem bizarre or even a little gross. Regardless of where you stand, you've no doubt been curious at one point or another about your (or your boyfriend's or your sister's) knuckling-cracking habit and how it's even possible.

What Pops When You Crack Your Knuckles
What Pops When You Crack Your Knuckles
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What S Happening When You Crack Your Knuckles
Is It Bad To Crack Your Knuckles

What Causes That Popping Sound?
For decades, scientists have been trying to figure out what goes on physically to elicit that signature popping noise. A recent study in PLOS ONE has cracked, so to speak, the code behind this weird bodily function, revealing that the sound happens as a result of an air bubble that forms when a joint is pulled apart. The process is technically called “tribonucleation,” or the quick separation of two surfaces followed by a cavity formation, say the researchers in the study.

What Pops When You Crack Your Knuckles

PLOS ONE / University of Alberta

A team of University of Alberta researchers had a study participant place his fingers into a tubular finger trap one at a time. A cable attached to the finger’s tip then slowly pulled until a knuckle cracked. The cracks were caught on MRI video so researchers could investigate what was going on, and each happened in the space of one frame (a.k.a. in 310 milliseconds).

Studies have suggested that cracking your knuckles can actually lead to arthritis, instability in the joints, and a loss of strength or mobility in the hands. From a scientific point of view, cracking your knuckles over the course of many years can damage the cartilage that protects the joint. The Truth About Cracking Your Knuckles. Cracking joints can be divided into three categories. The first is the accidental cracking or popping joints you hear when you move around normally. The second is cracking the knuckles in your hand. And the third category is when you intentionally crack other parts of your body, like your neck and back. Now, the question is: What happens when you crack your knuckles? What happens with you crack your joints? A joint is where two bones meet. These two bones are held together with connective tissue and ligaments. Surrounding the joint is a lubricant called the synovial fluid, which is a thick and clear liquid. The first detailed study about cracking joints and knuckles was performed by the researchers J. Rotson and R. Wheeler Haines.

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While research from 1971 suggested the popping sound was due to the collapse of pre-existing bubbles in joints’ synovial fluid, this finding confirms a 1947 study that found it’s actually the creation of bubbles in the synovial fluid that causes the noise. Think of synovial fluid as the lubricant that exists between two joints. It’s necessary for proper joint and bone movement and comfort.

In the video below of the action in progress, the joint separates, a dark bubble appears in the intervening fluid, and then everything settles back into place. Although the joint looks like it’s back to normal, it has to undergo a refractory period before it can crack again. “The fluid takes time to refill and create the same dynamics it had before,' says Michael Suk, M.D., chairman of the department of orthopedic surgery at Geisinger Health System. 'It’s like pouring oil through a very small hole in an hourglass—it takes time for it to fill up again.'

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Why Do People Even Crack Their Knuckles in the First Place?
“There’s both a mental and physical component,” says Suk. “From a mental standpoint, it’s almost a nervous habit for some people, much like drumming their fingers or biting their nails. I think to some degree, there’s a feeling associated with doing it as a mental stress reliever.” Meanwhile, “from a physical standpoint, I think what this study shows is as you create space in the knuckle, you’re decompressing the joint,' says Suk. 'In many cases, that can result in greater fluid movement in the joint itself, so your finger feels less constrained.”

Are Some People Just Not Capable of Doing It?
Although it seems like some people can crack their knuckles without an issue and others can’t no matter what, that’s likely not the case. “If we understand joints to be what they are, everyone has the potential to crack their knuckles,” says Suk. “The difference is that some people have a lower threshold of pressure for separating them, but others require much more force to create the separation.” Don’t take that as license to apply a ton of pressure just to hear the pop, though. “There have been some reports that people can tear or stretch tendons based on how they crack their knuckles,” says Suk. “Some people pull, while others bend their fingers. Depending on how forcefully you do so, you can injure your hand.”

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Can Cracking Your Knuckles Really Cause Arthritis?
As for that rumor that you’re going to pay for your knuckle-cracking ways with arthritis, there’s not much truth to it, says Suk. “There’s no scientific merit to that,” he says. “A couple studies have looked at habitual knuckle-crackers and discovered there’s no difference in the quality or quantity of arthritis in their hands.” So even though it’s not the prettiest of habits, contrary to what your parents said when you were little, it likely won’t cause any long-term damage if you’re gentle.

Be aware, though: Although all joints share some common characteristics, they’re not all the same. “It’s probably hard to extrapolate from this study about the safety of cracking all joints across the board,” says Suk. You hear that, habitual back-crackers?

Gif courtesy of giphy.com

Cracking finger joints makes a distinct cracking or popping sound.

Cracking joints is manipulating one's joints to produce a distinct cracking or popping sound. It is sometimes performed by physical therapists, chiropractors, osteopaths, and masseurs in Turkish baths.^[1]

The cracking of joints, especially knuckles, was long believed to lead to arthritis^[2]^[3] and other joint problems. However, medical research has not demonstrated such a connection.

The cracking mechanism and the resulting sound is caused by carbon dioxide cavitation bubbles suddenly partially collapsing inside the joints.^[4]

Causes[edit]

MRI of a cracking finger joint, visualizing cavitation.

For many decades, the physical mechanism that causes the cracking sound as a result of bending, twisting, or compressing joints was uncertain. Suggested causes included:

Formation of bubbles of joint air as the joint is expanded.^[5]
Cavitation within the joint—small cavities of partial vacuum form in the synovial fluid and then rapidly collapse, producing a sharp sound.^[6]^[7]
Rapid stretching of ligaments.^[8]
Intra-articular (within-joint) adhesions being broken.^[8]

What Pops When You Crack Your Knuckles

There were several theories to explain the cracking of joints. Synovial fluid cavitation has some evidence to support it.^[9] When a spinal manipulation is performed, the applied force separates the articular surfaces of a fully encapsulated synovial joint, which in turn creates a reduction in pressure within the joint cavity. In this low-pressure environment, some of the gases that are dissolved in the synovial fluid (which are naturally found in all bodily fluids) leave the solution, making a bubble, or cavity, which rapidly collapses upon itself, resulting in a 'clicking' sound.^[10] The contents of the resultant gas bubble are thought to be mainly carbon dioxide.^[11] The effects of this process will remain for a period of time known as the 'refractory period,' during which the joint cannot be 're-cracked,' which lasts about twenty minutes, while the gases are slowly reabsorbed into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.^[12]

In 2015, research showed that bubbles remained in the fluid after cracking, suggesting that the cracking sound was produced when the bubble within the joint was formed, not when it collapsed.^[5] In 2018, a team in France created a mathematical simulation of what happens in a joint just before it cracks. The team concluded that the sound is caused by bubbles' collapse, and bubbles observed in the fluid are the result of a partial collapse. Due to the theoretical basis and lack of physical experimentation, the scientific community is still not fully convinced of this conclusion.^[4]^[13]^[14]

The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound.^[8]

Effects[edit]

The common claim that cracking one's knuckles causes arthritis is not supported by evidence.^[15] A study published in 2011 examined the hand radiographs of 215 people (aged 50 to 89) and compared the joints of those who regularly cracked their knuckles to those who did not.^[16] The study concluded that knuckle-cracking did not cause hand osteoarthritis, no matter how many years or how often a person cracked their knuckles.^[16] A 1990 study also concluded that there was no increased preponderance of arthritis of the hand of chronic knuckle-crackers but that habitual knuckle-crackers were more likely to have hand swelling and lowered grip strength.^[3] It claimed further that habitual knuckle-cracking was associated with manual labour, biting of the nails, smoking, and drinking alcohol and suggested it resulted in functional hand impairment.^[3] This early study has been criticized for not taking into consideration the possibility of confounding factors, such as whether the ability to crack one's knuckles is associated with impaired hand functioning rather than being a cause of it.^[17]