Knuckle cracking maths:
Synovial bubbles pop
in partial collapse.
The debate over how knuckles cause a popping sound when cracked has lasted for decades. Now, Chandran Suja and Barakat (2018) have created three equations to mathematically model how the sound is produced. The first equation describes variations in pressure inside the joint, the second describes how pressure variations results in bubble size variations, whilst the third equation links the size variation of bubbles with the production of acoustic pressure waves.
When cracking your fingers the joints are pulled apart, the pressure goes down and bubbles appear in the synovial fluid which lubricates the joint. During knuckle cracking the pressure changes within the joint causing the size of the bubbles to fluctuate quickly resulting in the popping sound. The new model reveals that the bubbles don’t need to completely collapse in order to produce the sound, explaining why bubbles are observed following knuckle cracking.
Original research: http://dx.doi.org/10.1038/s41598-018-22664-4