Scientists have used ultrasound to see just what happens when you crack your knuckles, with results showing what looked like a "firework exploding in the joint". The scientists from the University of California, Davis, said this is the first study of its kind and adds to the debate about "one of life's great mysteries".

The team, presenting its findings at the annual meeting of the Radiological Society of North America (RSNA), said the ultrasound showed a bright flash when the knuckle cracked and this, they believe, is related to a gas bubble in the joint.

Researcher Robert D Boutin said: "It's extremely common for joints to crack, pop and snap. We were interested in pursuing this study because there's a raging debate about whether the knuckle-cracking sound results from a bubble popping in the joint or from a bubble being created in the joint."

They made audio and visual recordings of knuckle cracking from 40 healthy adults. Three quarters had a history of habitual knuckle cracking, while the remainder did not. Some said they cracked their knuckles up to 20 times per day for 40 years, while others claimed never to have intentionally cracked their knuckles.

In the study, the participants attempted to crack their knuckles at the base of each finger. They were evaluated for grip strength, range of motion and the laxity of the joint. Using a transducer, they recorded the cracking attempt. Cracks occurred in 62 of the 400 joints studied.

"What we saw was a bright flash on ultrasound, like a firework exploding in the joint," Boutin said. "It was quite an unexpected finding. There have been several theories over the years and a fair amount of controversy about what's happening in the joint when it cracks. We're confident that the cracking sound and bright flash on ultrasound are related to the dynamic changes in pressure associated with a gas bubble in the joint."

As to whether cracking your knuckles is good, bad or inconsequential, the team said this is still unknown. Physical examinations of the participants showed no pain, swelling or disability, nor was there any difference in grip strength or laxity between the knuckle crackers and non-crackers.

Boutin added: "We found that there was no immediate disability in the knuckle crackers in our study, although further research will need to be done to assess any long-term hazard – or benefit – of knuckle cracking."