Scientists have found a way to investigate and explain one of the most annoying sounds a person can make – cracking the knuckles.
The distinctive popping sound that knuckles can make can be described by three mathematical equations, according to researchers in France and the United States. This model confirms the theory that the knuckle-cracking is caused by tiny bubbles collapsing in the fluid of finger joints as pressure changes, the BBC reports.
This is a phenomenon that has been debated for a century now.
Vineeth Chandran Suja, a postgraduate student at Stanford University in California, was cracking his knuckles in class when he decided to look into the matter. Together with his lecturer, Abdul Barakat of École Polytechnique, they came up with a series of equations to explain the sound made when the joint between the fingers and hand bones are released.
Chandran Suja said,
The first equation describes the pressure variations inside our joint when we crack our knuckles. The second equation is a well-known equation which describes the size variations of bubbles in response to pressure variations. And the third equation that we wrote down was coupling the size variation of the bubbles to ones that produce sounds.
These equations make up a mathematical model that shows how knuckles crack, Chandran Suja said. “When we crack our knuckles we’re actually pulling apart our joints. And when we do that the pressure goes down. Bubbles appear in the fluid, which is lubricating the joint – the synovial fluid. During the process of knuckle cracking there are pressure variations in the joint which causes the size of the bubbles to fluctuate extremely fast, and this leads to sound, which we associate with knuckle cracking.”
This new mathematical model appears to resolve two conflicting theories on knuckle-cracking. One states that the collapse of said bubbles causes the cracking, while another states that bubbles continue to persist in the fluid. According to this model, there is a partial collapse of bubbles, so more bubbles can actually continue to exist even after a knuckle has been cracked.
The study was published in Scientific Reports.