Researchers have shown that noise caused by finger crunches comes from the bursting of micro-bubbles in the joints. A phenomenon that has nothing to do with cartilage lesions and is not likely to lead to osteoarthritis.
"Stop cracking your fingers, you'll have osteoarthritis later." Who has not heard this sentence, but reassure parents and their children: there is no real problem in cracking his fingers, unless it becomes an obsessive and compulsive disorder.
Machinary habit for some, horrifying mania for others, cracking his fingers was so far surrounded by uncertainty about the causes. Where does the famous "crack" come from when you pull on your joints? Scientists have finally shown that this is the noise corresponding to the bursting of a gas bubble in the joint.
These are the conclusions of a study conducted by researchers at the École Polytechnique in Palaiseau, Essonne, and Stanford University in California and published in the journal Scientific Reports. Crunches are due to the hyperextension of the joint that creates the vacuum inside: the gas in solution in the joint fluid then passes into the gas phase and forms a microscopic bubble in the joints of the fingers that will burst.
Why do the fingers crack?
This phenomenon was mentioned in the 1920s: Donald Unger, a young American, had the habit of cracking his fingers. His mother kept telling him, "Stop Donald, you're going to have arthritis! Becoming a doctor, he wanted to verify the statements of his mother by conducting a scientific study.
He only cracked the knuckles of his left hand for 60 years! To be precise, the joints of his left hand have cracked at least 36,500 times, while those of the right have cracked only rarely and spontaneously. Result: no more osteoarthritis in his left hand than in his right hand. His two hands were identical and the hypothesis of cartilage damage was abandoned.
Since then, more work has been done on this topic and overall, there is no more osteoarthritis in crackers than in others. In short, cracking his fingers is not dangerous for articulation as long as it is reasonable.
The demonstration is mathematical
This is a mathematical formula developed by Vineeth Chandran Suja, a former Polytechnic student, and his professor, Dr. Abdul Barakat, who led to the demonstration of this bursting of a micro-bubble in the joint fluid, which lubricates the joints of the hand, when the vacuum is made in the joint.
Together, they developed a series of equations that explain this typical sound that accompanies the release of bubbles in the joint.
"The first equation describes the pressure variations inside our articulation when we crack it," Vineeth Chandran Suja told BBC News. "The second equation is a well-known equation that describes the variations in bubble size in response to pressure changes. And the third one we wrote was to couple the variation in the size of the bubbles to those that produce these sounds, "says the student, who is currently in postgraduate at Stanford University.
"During the process of joint cracking, there are pressure variations in the joint that cause extremely rapid fluctuation of the size of the bubbles, which leads to a sound that we associate with cracking joints."
One bubble is enough to produce the noise
This is not the first time scientists have tried to understand where the noise from the joints comes from when they are cracked. A hypothesis similar to that of Vineeth Chandran Suja and Dr. Abdul Barakat was even formulated in the early 1970s before being questioned by several studies.
The novelty here is that the mathematical formula shows "that the bursting of only one of these bubbles is enough to produce the noise," says AFP Dr. Abdul Bakarat. The work of both researchers also calls into question the conclusion of a 2015 study, which argued that the crack came from the formation of bubbles rather than their bursting. The mathematical formula they developed, on the contrary, asserts that it is the bursting of microscopic bubbles that produces the sound. Mystery solved.
The crack of the fingers is not necessarily bad for the joints, unless it is too frequent or too supported. The crack is not limited to the fingers. There are toe crackers and neck crackers. For the neck, it is more annoying because it is generally always the same vertebral stage that is solicited. Little by little, the ligaments relax and the floor in question becomes unstable. To control this instability, the muscles contract and in the long run, it can be painful! Then there are joints that crack without you want to do it. This is especially the case for the hip. The crack is also a noise that is found regularly when you go to an osteopath, even if it is not essential for the session to be successful.
Conclusion: Whole generations of parents have lied to their offspring, threatening them with osteoarthritis.