Scientists have announced that they now know why knuckles make a popping sound when they are cracked. Findings for the study confirm a theory that was proposed almost 50 years ago.
A combination of experiments and simulations were used to determine the source of this sound.
Collapsing Bubbles
Researchers published a new paper in Scientific Reports that gets down to the bottom of what causes the sound knuckles make when they're popped. The study says that the sound is caused by the partial collapse of a cavitation bubble that's in the synovial fluid in the joint. What's more interesting is that the bubbles aren't popping — they're just collapsing inside of the finger joint.
Scientists used a mathematical model with a geometrical representation of the joint to simulate what occurs in the joint as the bubble begins to collapse. Abdul Barakat, an author on the study, said that the sound could be caused by multiple bubbles collapsing, but that one bubble was enough to create the cracking sound.
This scenario with bubbles was originally proposed in 1971. Questions arose when other findings showed that the bubbles were still present in the joints after knuckles had already been cracked. Researchers say that the sound is produced by a partial collapse of the bubbles instead of popping.
Previous Research
A 2015 paper on the subject produced data by using MRI scans to show that after cracking knuckles, gas bubbles were still present in the joint fluid. This contradicted the previous theory that proposed that the bubbles popped. This research came to the conclusion that the knuckle popping sound was created by the formation of bubbles in the joint fluid.
Researchers on the study say that the new findings do not contradict previous findings on the matter. They also add that the study is limited because they did not come up with a model for the formation of the bubbles.
Scientists from the 2015 study say that the new study only focuses on one aspect that could cause the knuckle cracking. It focuses on the collapsing of the bubbles.
Is It Bad For You?
One question that the study doesn't answer is whether or not cracking knuckles is detrimental to people's health. Medical professionals have contradicted the old wives' tale that cracking knuckles causes arthritis.
A 2015 study shows that there are no short-term negative effects from cracking knuckles. It did state that there would need to be further research on the subject to determine whether there are long-term effects caused by cracking knuckles.
