The staccato pop of a cracked knuckle might sound like a balloon bursting — but it’s actually triggered by the formation of gas bubbles, rather than their disappearance.

Researchers at the University of California, Davis, used ultrasound imaging to look into the yellowish, viscous fluid that lubricates our joints. They watched as volunteers flexed their fingers and found that the cracking noise occurred a split-second before a visible burst of gas, which appeared as a bright flash on the sonogram.

“It looks like fireworks going off in the joint,” said Dr. Robert Boutin, the radiologist who led the project.

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It’s still unclear exactly why the sound was audible around 10 milliseconds before the appearance of the gas. Dr. Richard Thompson, a medical imaging specialist at the University of Alberta who was not involved in the study, hypothesized that the bubbles could begin developing when you hear the audible pop, but only become visible on ultrasound once they are fully formed.

What is clear, though, is that the sound is produced around the same time that the bubbles are. When you crack your knuckles, you pull apart the bones that form the joint, creating a kind of vacuum that allows for the appearance of bubbles. That’s when you hear the noise.

The bubbles then stick around for about 20 to 30 minutes, and you won’t be able to crack your knuckles again until they are gone — a sure sign that the sound is not caused by bursting bubbles, as was the leading hypothesis for many years.

Boutin and his colleagues presented their findings Tuesday at the annual meeting of the Radiological Society of North America in Chicago.

The results build on those of Thompson, who used magnetic resonance imaging to study one person’s hand. However, MRI doesn’t produce as many images per second as ultrasound and his study, published in April, couldn’t break down which came first, the sound or the bubble.

“That’s a big improvement with this ultrasound study,” Thompson said.

Boutin’s team looked inside the hands of 40 people, ranging from those who never cracked their knuckles to those who had done it 20 times a day for 40 years. When hand injury experts examined them before and after cracking, they found no difference between the two groups, suggesting that knuckle cracking does not put your joints in any immediate danger.

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