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Researchers at l’X decode knuckle cracking

Today, at Stanford University, a researcher from the l’X Hydrodynamics Laboratory and his student were able to explain the origin of this rather peculiar cracking sound: the collapse of bubbles within the fluid in our joints.

Everyone knows the characteristic noise of cracking joints. In order to shed light on the source of this noise, Abdul Barakat, researcher at the l’X Hydrodynamics Laboratory (LadHyX), and one of his students, Vineeth Chandran Suja—who is currently completing a PhD at Stanford—embarked on a research project. They found that the sound is produced when microbubbles collapse in synovial fluid, a lubricant found at the juncture of the joints.

To carry out their research, Abdul Barakat and Vineeth Chandran Suja worked together on a mathematical model correlated with acoustic chamber experiments, in order to reproduce the phenomena that lead to the cracking of joints. Their results published in Scientific Reports show that the collapsing of cavitation bubbles gives us the cracking sound. The simulations produced the same frequency and amplitude of the sound emitted as those recorded in experimental reports in existing literature and in the experiments done by the researchers.

A knuckle-cracking debate that has raged for 60 years

The origin of this sound has been a subject of debate in the scientific community for 60 years, due to limited imaging technology unable to observe this event occurring in an extremely short space of time. As early as 1971, it was posited that these bubbles collapse in on themselves, but there was no experimental or mathematical proof to strengthen this hypothesis. Subsequently, an imaging study in 2015 suggested that the sound was produced by the formation of bubbles, which were visible after the crack.

Opinions remain divided on whether cracking one’s knuckles can cause arthritis. The American doctor, Donald Unger, cracked the knuckles of his left hand for his whole life, but never his right hand, without ever showing any signs of arthritis.