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.
"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.
Re:Reinventing the (square) wheel (Score:4, Insightful)
I won't bother to look up the original reference(s).
Noted.
And this kind of "modeling" is, er, crude. Among other problems, it sorta oversimplifies the strain field in the surrounding fluid.
How do you have a "strain field" in a liquid?
Typical of would-be physicists.
Considering that you think quoting your sources is beneath you, and you seem to be using terms you don't understand, it is possible that your condescending attitude may be unjustified.
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Not all that new (Score:4, Informative)
The entire thread replying to the first comment posted here seems to be ignoring the content of that comment: this is not new. Some of the detailed analysis might or might not be new, but the main result is known.
Here are some popular articles from years back:
https://www.telegraph.co.uk/news/science/science-news/11539913/Why-knuckle-cracking-makes-a-popping-sound-and-why-it-might-be-beneficial.html : Quote: "When muscle joints are pulled apart there forms a tiny cavity filled with gas which then collapses, creating a popping noise."
https://health.howstuffworks.com/human-body/systems/musculoskeletal/question437.htm
https://articles.mercola.com/sites/articles/archive/2014/11/08/cracking-knuckles-harmful.aspx
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this is not new. Some of the detailed analysis might or might not be new, but the main result is known.
You've just described almost all published results in any scientific endeavor.
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Fed up or not, the second you start to act like an a-hole when posting then other people stop caring about what you have to say, regardless of it's accuracy.
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Nah. I listen more to assholes than non assholes, because assholes are more often correct and fed up with the bullshit. Also, I'm an asshole myself and typically find myself agreeing when another asshole takes the time to rant about bullshit.
The original AC is correct, of course. This has been known for decades. Nothing new here, except maybe a crappy mathematical model for the sound, which is utterly pointless. It's a fucking pop.
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Most proper scientists don't actually speak condescending-douche fluently. You'll need to translate a bit, since you are clearly a native speaker.
Re:Reinventing the (square) wheel (Score:4, Insightful)
What I said is perfectly understood by any proper scientist.
Strain fields are not relevant to every scientist's field (it's more relevant to continuum mechanics just as you noted), but then by applying the term "proper" perhaps you are referring to those more properly learned scientists by some canonical reference of "science" - care to share with us filthy commoners?
First, it it those people's duty to look up the possible references, if they want to style themselves as, you know, scientists; since this is very old news, and I'm not the one at fault, I won't spend the time required to find the old papers.
Second, if you had any kind of skill in the field
How convenient of you to exclude yourself from your own rules.
you would have noted that the paper talks about "fluid", not "liquid". That's not the same thing. Learn elementary rheology, or better, continuum Mechanics. I won't start a basic course here (although I could, having taught this at PhD level), but for the record, a liquid does undergo strain; but by definition of a _liquid_, only its time rate involves dissipation. And by the way, a change with time in the strain field does very obviously necessarily occur here. Then, the behavior of a general fluid can involve the strain field proper, such as occurs in say, viscoelasticity.
Do you even know what a field is in PDEs, anyway? Or PDEs? Ever heard about Navier-Stokes? Or any kind of mathematics?
You are saying all the right things as far as tooting your knowledge of fluid dynamics (at least as far as name dropping can get you), but in all the wrong ways. I assure you, you could not have taught this at PhD level, and you will probably never understand why. My guess is you are an undergrad with second order ignorance.
Re:Reinventing the (square) wheel (Score:4, Insightful)
> you could not have taught this at PhD level Pathetic. I did precisely that about 25 years ago, for PhD students in Mathematics. Remember, I'm still the only one writing anything specific to mechanical models here. Care to show your skills by, say, telling us a thing or two about, say, properly dealing with the incompressibility constraint in CFD?
As I suspected, you have completely missed why everyone here has issue with you: your current attitude is not welcome in the scientific or academic world because it is self serving, you may or may not have filled the role of a professor but you did not and clearly currently cannot _play_ it to anyone else's benefit. People will be interested in what you have to bring to a discussion when you stop trying to measure the length of your penis and compare it to everyone. You are not necessarily unique in this aspect, it's just that most people grow out of it, you appear to have much growing to do.
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First, it it those people's duty to look up the possible references, if they want to style themselves as, you know, scientists;
No. Sorry, but no. You make a claim, you have the burden of proof.
I've seen people trying to reverse the burden of proof (i.e. "I claim and if you don't believe me, prove me wrong"), but asking people to prove your statement if they want to believe it, that's ... new.
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You honestly expect them to argue for both sides of the debate?
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So I guess I'm not part of the "in" crowd. I didn't know it for decades. Any chance I could still get some kind of proof?
Li'l hint why I'd insist: We used to know for centuries, not just decades, that the Earth is the center of the universe.
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I've seen people trying to reverse the burden of proof (i.e. "I claim and if you don't believe me, prove me wrong")
Wow, you're aware of the scientific method! Pretty impressive. Thanks for sharing.
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What I said is perfectly understood by any proper scientist.
Well Brenda - not everyone here is a scientist. And those of us who are find you to be a tad lacking. As well as making an egregious error in assuming what scientists do or do not know. They are not all fountains of identical knowledge.
But enough of calling you out - let's delve into your writing.
First, it it those people's duty to look up the possible references, if they want to style themselves as, you know, scientists; since this is very old news, and I'm not the one at fault, I won't spend the time
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If you bothered to read the article you would learn they specifically discuss this. Their model is more advanced and actually realized the previous understanding was too naive and simplistic. The bubbles do not totally collapse but break into a wide variety of sizes and it is the collapse of bubbles in a specific range of sizes that creates the sound.
I suppose when Einstein published his theory of relativity you would have commented that this has been known for hundreds of years by Newton.
Last Date (Score:3)
something sure to impress your next date -- and possibly your last
Not bloody likely, my last date stopped answering my calls.
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Not bloody likely, my last date stopped answering my calls.
You forgot to mention the restraining order.
Bit what CAUSES them??? (Score:3, Interesting)
* 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.
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* 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
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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 :)
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Cavitating the joint also increases range of motion in the joint. This might be another source of feeling good.
A certain health care profession that gets little respect around here has known these things for a long time.
Paging Doctor Ruby. Doctor Ruby, please pick up the green phone.
Re:Bit what CAUSES them??? (Score:5, Interesting)
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.
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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.
Re:Bit what CAUSES them??? (Score:4, Informative)
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.
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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
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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.
Re: Talk about bewaring the idles of March (Score:1)
Eric Idle?
More Visual Explanation (Score:2)
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]
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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)
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I remember hearing about this explanation in elementary school, 25 years ago.
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In case you were wondering (Score:2)
Which scientists? (Score:2)
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)
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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!"
And (Score:2)
Come on (Score:2)
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.
Okay, but (Score:2)
Can these scientists say explain what is the sound of one hand clapping?
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Can these scientists say explain what is the sound of one hand clapping?
*wank* *wank* *wank* *wank*
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I said "clapping", not "fapping".