Physics: What causes the sound of knuckle cracking?

Knuckle cracking is very common, but the origin of the typical sound that accompanies the release of the joint between the finger and hand bones has been debated by scientists since the early 1900s. Now a mathematical model developed by V. Chandran Suja and Abdul Bakarat, described in Scientific Reports, offers a possible explanation.

The authors combined a geometrical representation of the joint and a series of mathematical equations to simulate the events leading to the generation of the sound. Their results suggest that pressure variations in the joint fluid resulting from the joint motion (during knuckle cracking) cause the collapse of microscopic bubbles present in the fluid. It is the collapse of these bubbles that causes the cracking sound - a theory first put forward in 1971 but challenged 40 years later when new experiments showed that bubbles persisted in the fluid long after the knuckles had cracked. The authors’ mathematical model appears to resolve this contradiction by showing that only a partial collapse of the bubbles is needed to produce the sound, allowing for microbubbles to persist in the joint fluid after knuckle cracking.

The authors also show that the pressure generated by the collapse of the bubbles produces acoustic waves that can be predicted mathematically, as well as measured experimentally. The acoustic pressure waves calculated by the model are very similar to the acoustic signature of knuckle cracking presented in the literature and actual knuckle cracking recorded by the authors from three test subjects. The similarities between the mathematical simulation and experimental results - visually represented as similar-looking graphs - suggest that the authors’ model can not only characterize the sound of knuckle cracking but also show that bubble collapse is a plausible reason for the cracking sound.