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Scientists Explain the Sound of Knuckle Cracking (bbc.com) 86

"The BBC reports on something sure to impress your next date -- and possibly your last -- when you explain it," writes Slashdot reader dryriver. From the report: Scientists have turned their attention to investigating that most annoying of human habits -- the sound made when you crack your knuckles. The characteristic pop can be explained by three mathematical equations, say researchers in the US and France. Their model confirms the idea that the cracking sound is due to tiny bubbles collapsing in the fluid of the joint as the pressure changes. Surprisingly, perhaps, the phenomenon has been debated for around a century. Science student Vineeth Chandran Suja was cracking his knuckles in class in France when he decided to investigate.

"The first equation describes the pressure variations inside our joint when we crack our knuckles," he told BBC News. "The second equation is a well-known equation which describes the size variations of bubbles in response to pressure variations. And the third equation that we wrote down was coupling the size variation of the bubbles to ones that produce sounds." The equations make up a complete mathematical model that describes the sound of knuckle cracking, said Chandran Suja, who is now a postgraduate student at Stanford University in California. "When we crack our knuckles we're actually pulling apart our joints," he explained. "And when we do that the pressure goes down. Bubbles appear in the fluid, which is lubricating the joint -- the synovial fluid. "During the process of knuckle cracking 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 cracking.''
The study has been published in the journal Scientific Reports.
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Scientists Explain the Sound of Knuckle Cracking

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  • by mentil ( 1748130 ) on Friday March 30, 2018 @06:12AM (#56352099)

    something sure to impress your next date -- and possibly your last

    Not bloody likely, my last date stopped answering my calls.

  • by Anonymous Coward on Friday March 30, 2018 @06:17AM (#56352115)

    * Why does this uncomfortable feeling happen in the first place that goes away when you crack your knuckles?
    * What causes the bubbles?
    * Are the bubbles related to the feeling, or not or merely a side-effect?

    And it's not just knuckles. Toes, shins, knees, elbows, neck, ... it seems it can be everything, depending on the person.

    • * Why does this uncomfortable feeling happen in the first place that goes away when you crack your knuckles?

      Well, that's probably because of habit, but that's a wild guess at this point. Maybe this has some unknown benefit as well? I however read somewhere that it was advised against cracking one's spine, as it could cause microlesions.

      * What causes the bubbles?

      As far as I know, there is always an equilibrium between different states of matter, and this is no different. Changes in pressure changes the equilibrium dynamics (as well as the gas pressure).

      * Are the bubbles related to the feeling, or not or merely a side-effect?

      Well, this causes a non-linear pressure response, whereby you feel more and more resista

      • As far as I know, there is always an equilibrium between different states of matter, and this is no different. Changes in pressure changes the equilibrium dynamics (as well as the gas pressure).

        To elaborate a bit on this, you can find some more information there [wikipedia.org], and in the detailed article. It's written there that cavitation in this case arises from the dissolved carbon dioxide, not from the vaporization of the synovial fluid itself, but it basically works the same :)

    • by MrKaos ( 858439 ) on Friday March 30, 2018 @07:44AM (#56352297) Journal

      For context, I've spent the last 4 years doing *extreme* physiotherapy to repair all the accumulated damage I've done to my body from years of the sports and martial arts I've done. I've collected a lot of data on this subject and, annoying sound aside, I think I'm coming to some conclusions.

      * Why does this uncomfortable feeling happen in the first place that goes away when you crack your knuckles?

      You injured yourself and after the injury healed scar tissue formed and caused the fascia tissue, which is rich in nerves, to stick to a place in your muscles. I call them adhesions for want of a better term. Can you feel that knot in your back that is driving you crazy, that is what your body is trying to resolve. As far as I can tell your body is trying to get you to move through a full range of motion to break up the scar tissue.

      * What causes the bubbles?

      I'm not sure, however the more you extend the range of motion the less you 'crack'. That cracking is called Crepitus. [wikipedia.org]

      * Are the bubbles related to the feeling, or not or merely a side-effect?

      I can only conclude they must be, however I'm not sure how. I'm glad this study came out, I thought the cracking was the joint cavitating. The cracking is the engine of the change. If you manually manipulate the joint you will generally find some range of motion causes pain, you may even feel bone grinding. Manipulate the joint through that pain and it will crack a lot more until it finally releases and then it will be silent and smooth.

      I also discovered the cracking releases endomorphines in the brain and jokingly asked the physiotherapist if that makes me a 'crack addict' ;)

      And it's not just knuckles. Toes, shins, knees, elbows, neck, ... it seems it can be everything, depending on the person.

      Shoulders, wrists, thumbs, hips, ankle, arches of feet, chest, all of these I cavitated until they resolved. The extreme physiotherapy is dry needling, where a needle about 7cm long is stuck into the adhesions and knots all around the body. Well over 6000 of these needles have been stuck into me now. The most at one time is 50 (legs) and I've had four inserted into my feet at the same time while I squeeze my feet. All of this effort resulted in the intense feeling of needing to move and cavitate these joints. This has resolved 28 major injuries I accumulated over the years.

      I took measurements and drew diagrams of the movements that would produce the cavitation. For example my left ankle cavitated approximately 90 times per minute (three cavs per rotation, one in the opposite direction) over an exhausting 5 hour session for 4 sesions (IIRC), measured temperature of the ankle joint peaked a 34C from 27C at the beginning of the sessions. It has now settled down to maybe 10 per minute over a 30 minute session for 18months of work. I've been in shock twice from how severe some of the releases are. Elbows, by far, are the most painful joint to release (240 cavs , 2 massive cavs per day over approx 4 months). My right shoulder moved in a single, giant cavitation. My hips have almost been re-aligned in two planes - due to the ankle I suspect.

      It has been totally worthwhile, I feel amazing - for the true meaning of the word. I only need 6 hours sleep and it is very deep, like someone took all of the marbles out of my bed. I think more clearly, I feel stronger, more flexible that I was in my 20's. I've had some x-rays done before that I plan to compare with new x-rays when I am done. Still working on the ankle and once that is all done I'm keen to start training again.

      Most of all, my joints barely crack at all anymore.

    • In my case lower back, i.e. the vertebrae.
    • Miss-attribution. The pressure variations both cause the bubble and cause them to collapse suddenly. It's called cavitation. We don't normally have bubbles in our joints and we don't build up any over time either.

      • by swillden ( 191260 ) <shawn-ds@willden.org> on Friday March 30, 2018 @09:21AM (#56352639) Homepage Journal

        Miss-attribution. The pressure variations both cause the bubble and cause them to collapse suddenly. It's called cavitation. We don't normally have bubbles in our joints and we don't build up any over time either.

        What does build up in the sinovial fluid is the nitrogen gas which is pulled out of solution (forming bubbles) and then released. After the gas is released, you have to wait a while for gas to migrate from surrounding tissues (by osmosis) before you can crack again.

        I've noticed that after loading up a significant amount of nitrogen from repeated deep SCUBA dives, my joints become considerably more "crackly". It takes less motion to pop them, and they can be popped again in a shorter amount of time than normal. Some people tell me they get the same effect just by taking a plane ride that starts at sea level, so the 8000-foot cabin pressure creates enough ppN2 differential to increase cracking. Doesn't do that for me, though.

        • by dgatwood ( 11270 )

          I've noticed that after loading up a significant amount of nitrogen from repeated deep SCUBA dives, my joints become considerably more "crackly". It takes less motion to pop them, and they can be popped again in a shorter amount of time than normal.

          Shorter time? Hmm. The joints in my right hand won't crack for a long time after I've cracked them once, but the joints in my left hand can be cracked immediately after cracking them, and I've done this at least thirty times in a row while reading this discussi

          • I've noticed that after loading up a significant amount of nitrogen from repeated deep SCUBA dives, my joints become considerably more "crackly". It takes less motion to pop them, and they can be popped again in a shorter amount of time than normal.

            Shorter time? Hmm. The joints in my right hand won't crack for a long time after I've cracked them once, but the joints in my left hand can be cracked immediately after cracking them, and I've done this at least thirty times in a row while reading this discussion thread. Perhaps there are multiple causes of cracking joints, some of which don't involve bubbles?

            I suppose that's possible. It's the only explanation I've ever heard, other than tendon snapping, but that's usually only on large joints with big tendons going past them.

  • I think these videos show the same principle at work (at least as I understand them). It seems to be important that the formed bubbles are vacuums (i.e. formed by pulling joints apart rather than evaporating gas) because that's what allows them to collapse.

    Collapsing vacuum bubbles in a fluid:
    https://www.youtube.com/watch?... [youtube.com]

    How the principle explains another real world phenomenon:
    https://www.youtube.com/watch?... [youtube.com]
    • by dublin ( 31215 )

      A vacuum doesn't explain why is there a recovery period required before the joint can be "cracked" again. The explanation in the article seems to leave this well-known aspect unexplained. If it's a simple cavitation bubble (unliklely), then you should be able to crack the joint again immediately.

  • Not really news... (Score:3, Insightful)

    by CriticalYetLazy ( 5026193 ) on Friday March 30, 2018 @07:58AM (#56352321)
    I've seen numerous results of research regarding this subject at least a decade ago already drawing the same conclusions.
  • In case you were wondering what it sounds like when an auditorium full of people crack their knuckles simultaneously, here you go: https://www.youtube.com/watch?... [youtube.com]
  • Scientists have turned their attention to investigating that most annoying of human habits -- the sound made when you crack your knuckles

    It annoys the crap out of me when the media talks about "scientists" as if they are some sort of monolithic entity. WHICH SCIENTISTS are they talking about? What are their names and specialties?

    Anyway this sounds like a candidate for an IgNobel prize if I ever heard one.

  • Really? (Score:4, Informative)

    by Translation Error ( 1176675 ) on Friday March 30, 2018 @09:42AM (#56352735)
    I saw this explained in an episode of Cheers from 1989 ("Hot Rocks" s07e17)...

    "The phenomenon of knuckle cracking is relatively harmless. But in fact, the sound you're hearing is not a cracking at all, but rather a popping of tiny gas bubbles imploding in the sinovial fluid of the metacarpal phalangeal joint."

    • These guys must have had some time travelling engine
    • by Quirkz ( 1206400 )

      I remember that episode, too. If I recall correctly (it's been 30 years), Woody had just said, "Ah, knuckles cracking, I hate that sound!"

      Then the explanation is given.

      Woody: "Ah, joint fluid imploding, I hate that sound!"

  • did scientists explain also why this sound is unbearable?
  • I've known this for over 20 years -- pressure drops as the ends of the bones are levered apart and it passes some boil pressure point for some liquid or dissolved gas in there, which violently gassifies causing the pop.

    They must have just defined these equations.

  • Can these scientists say explain what is the sound of one hand clapping?

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