Collapsing Bubbles May Make Knuckle Cracks Noisy

A new mathematical model suggests the “popping” sound comes from partial bursts of gas sacs in joint fluid.

The cavity in between the two knuckles is filled with a fluid that is called the synovial fluid suddenly change the pressure in that fluid as a result of increasing the spacing between the knuckles, some of the gases in that fluid can nucleate into a bubble.

The duo’s model revealed that bubble formation did not produce sounds loud enough to match what one would expect to hear when a person cracked her knuckles. Instead, it matched the sounds of bubbles popping within the joints. That led the team to conclude that 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 crackingseemore..