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it creates a sound that sets teeth on edge and, we're often told, raises the risk of 1. but people who can crack their 2 may actually have healthier 4 than others.
掰指关节的声音让人很不舒服，而且我们常被告知，掰指关节会增加患关节炎的风险。但实际上，那些“咔咔”地掰指关节的人比其他人拥有更健康的关节。
scientists say that the force involved in making fingers 'pop' is more than enough to cause damage.
yet, studies show that regular 3-cracking doesn't seem to do any harm.
this suggests it is actually a sign of very healthy joints.
the theory comes from canadian researchers who used mri scanners to peer deep inside the fingers and record just what happens when knuckles are cracked.
one of the team, a 'gifted knuckle-cracker', put one finger at a time into a tube connected to a cable.
this wire was then slowly pulled until the 5 in large knuckle in the middle of each finger popped.
after about six seconds pulling the cable, the two bones suddenly shot apart, creating a cavity, or bubble, within the fluid that bathes the inside of the joint.
the cracking - which took less than a third of a second - occurred at the precise moment the bubble was formed, the journal plos one reports.
lead author professor greg kawchuk, from the university of alberta, said: 'we call it the "pull my finger study" - and actually pulled someone's finger and filmed what happens in the mri.
'when you do that, you can see very clearly what is happening inside the joints.
'it is a bit like forming a vacuum.
'as the joint surfaces suddenly separate, there is no more fluid available to fill the increasing joint volume, so a cavity is formed and that event is what is associated with the sound.'
the scans also showed a mysterious white flash occurs just before the noise is heard.
further research is needed into this 6 of light - and why the cracking sound is so loud.
however, the researchers are happy to have shown that the noise is created when a bubble forms inside the knuckle.
although this was first suggested more than 100 years ago, recent thinking has been that the crack is created when a bubble bursts.
previous studies have shown that between 25 and 54 per cent of us crack our knuckles - with men doing it more than women.
and despite the common wisdom that the habit raises the risk of arthritis, previous studies have failed to prove this.
one comparison of los angeles nursing home residents found those who had regularly cracked their knuckles were actually less likely to have damaged joints.
the us researchers behind that study concluded: 'the chief 7 consequence of knuckle cracking would appear to be its annoying effect on the observer.'
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