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

"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.

Comments

Reinventing the (square) wheel

[Anonymous Coward]

I won't bother to look up the original reference(s). This has been known for decades, I read about it at least two decades ago. And this kind of "modeling" is, er, crude. Among other problems, it sorta oversimplifies the strain field in the surrounding fluid. Typical of would-be physicists.

Re:Reinventing the (square) wheel

[ShanghaiBill]

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.

Re:Reinventing the (square) wheel

[Anonymous Coward]

What I said is perfectly understood by any proper scientist.

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, 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?

Re:Reinventing the (square) wheel

[Mister Liberty]

Never seen such an attitude. You're a disgrace, and you know it, hence the An. Coward.

Re: Reinventing the (square) wheel

[danperc7]

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Re:Reinventing the (square) wheel

[Anonymous Coward]

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

[Anonymous Coward]

I was feeling a time variant strain field in my abdomen so I ran to the toilet as a bowel movement was very obviously necessary to occur. Thanks AC!

Re:Reinventing the (square) wheel

[Anonymous Coward]

Or, possibly, I am just fed up with "scientific" papers that litter the field, clog the real scientific communication, and undeservedly bump up the number of publications in many resumes. A large proportion of accepted papers in my field are nothing but nonsense churned out by people who have no real skills in it, and that is a major impediment to progress. Not to mention whole careers built out of thin air.

Re:Reinventing the (square) wheel

[Anonymous Coward]

And you do think ShanghaiBill is at fault. Re-read that cunt's post then shut the fuck up.

Re: Reinventing the (square) wheel

[Anonymous Coward]

You are definitely building yourself up with hot air.

Re:Reinventing the (square) wheel

[Anonymous Coward]

Grow a sack, snowflake. ShanghaiBill's attitude sucks also, but hey, that's ok. Fuck off.

Re:Reinventing the (square) wheel

[tomxor]

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

[Opportunist]

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.

Re:Reinventing the (square) wheel

[Anonymous Coward]

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.

Re:Reinventing the (square) wheel

[Anonymous Coward]

It's interesting to see that so far, I've been the only one actually saying anything specific wrt the science involved. Beyond posturing, would _you_ be able to write anything relevant to the scientific problem at hand?

> Strain fields are not relevant to every scientist's field
I was of course referring to the general problem of "scientists" not fulfilling their due diligence duty by looking for prior art. In these times of easy googling it's all the more damning.

> How convenient of you to exclude yourself from your own rules.
Huh? This is Slashdot, mate, not a formal academic assignment. Do you really expect me to spend time rummaging through old cardboard boxes?

> 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?

Re:Reinventing the (square) wheel

[Anonymous Coward]

Flat out wrong. The burden of proof is obviously on the people who publish their "discovery" to show that they did their homework in the first place. And the fact is, they didn't, both because they haven't searched for prior art (I'm not the only one here stating that this study has already been done, btw) and because they do not seem to bother checking that they have the relevant skills (in fluid mechanics or rheology, at least, and possibly acoustics as well) to begin with.

Re:Reinventing the (square) wheel

[Anonymous Coward]

And you do think ShanghaiBill is at fault.

No. In fact, considering your consistent belligerent attitude it was downright polite of him to even say your attitude "may be unjustified" instead of "is unjustified". You are at fault. Shut the fuck up.

Re:Reinventing the (square) wheel

[tomxor]

> 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.

Re:Reinventing the (square) wheel

[Anonymous Coward]

> your current attitude is not welcome in the scientific or academic world

As opposed to well-behaved ignoramuses or even crooks who make a living out of false or nonsensical science, in many cases with taxpayers' money? How am I supposed to put up with them? Be nice, clap hands each time I've heard one lecture too many on a topic the lecturer has not contributed a whit to? And knowing that (s)he teaches the nonsense to unsuspecting students?

BTW: I'm not claiming this is true of all scientific fields; I can only judge mine, but there the camel's back was broken long ago. Just look at the sheer number of journals and papers, wonder at the inflation, and ask yourself if we do really make so much progress so fast, or if the standards have simply gone awry.

Re:Reinventing the (square) wheel

[lengel]

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.

Re:Reinventing the (square) wheel

[Ol Olsoc]

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 required to find the old papers.

Well lookie here! A "proper scientist" who refuses to provide citations! There was a word for people like you around campus.

Unemployed.

I have always maintained that a good scientist or researcher should be able to explain to regular people (reasonably intelligent but ignorant of the matter at hand) what it is they are trying to say, to educate them to at least a minimal level of the subject.

But here you come in like the Grand Maximus lord of science, waving your massive science cock around like a bludgeon, slaying anyone who dares to disagree or even ask for clarification. It is useless, even self destructive as the immediate result is to look upon you as a mental masturbating douchenozzle who is first ridiculed, then laughed at, then ignored.

Re:Reinventing the (square) wheel

[Opportunist]

You honestly expect them to argue for both sides of the debate?

Re:Reinventing the (square) wheel

[Anonymous Coward]

Awww come on, this is Slashdot. Do you expect people to read the referenced article? :-))
And a a general response to various "penis size" comments, sorry to the /. crowd for talking about a topic I know, and whose deep understanding is required to even express any meaningful opinion :-)))

That said, I did read the article, and I stand to my points. I sort of half-retract on the first point, because the authors did read and reference previous literature, but you are right to mention that they acknowledge it was indeed simplistic; if only they woud draw the conclusion that it was maybe not the right kind of approach altogether, or at least not guaranteed to be sufficient.

Hell, I won't spend an hour on their "model" to eg check whether it even keeps balance of mass, since this is a common problem with some studies. They don't develop their own fluid mechanics approach, but just use an old "axisymmetric" solution as a given. Assuming this is even correct (it possibly is), the boundary conditions for those are often problematic (ie they have a major influence on the result, and finding the "right" ones can be tricky, even for people really specialized in fluid mechanics, which they don't seem to be). And they immediately reduce it to a "zero-dimensional" model, as with the rest of the study. As a counterexample, this approach is perfectly fine (provably) with Hertz contact laws, but then it's compressible elasticity. So from there, I usually grow tired and stop reading. There really is too much to say about the propensity of some physicists to systematically reduce everything to a neat zero-dimensional formula.

P.S. I like your comparison with classical mechanics and relativity. Irrelevant, but funny.

Re:Reinventing the (square) wheel

[Anonymous Coward]

Yes. This is standard. It's called "related work".
If you can't explain in your paper why you don't have any related work, and people point out that yes, this is nothing new and has been known for decades, it's your own job to fix your article with the correct related work.

Re:Reinventing the (square) wheel

[kamapuaa]

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.

Re:Reinventing the (square) wheel

[sexconker]

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.

Re:Reinventing the (square) wheel

[Anonymous Coward]

I won't bother to look up the original reference(s). This has been known for decades, I read about it at least two decades ago. And this kind of "modeling" is, er, crude. Among other problems, it sorta oversimplifies the strain field in the surrounding fluid. Typical of would-be physicists.

The previous work is clearly pointed out in the paper, but of course, refusing to read it, you wouldnt know. Apparently (as described in the intro of the paper) this explanation for knuckle popping was known and widely accepted for a long time, but has more recently become contested. The mathematical modelling might be crude, but it predicts the spectrum to a degree which can decide between this old explanation and the newer other explanation, so it does the job. To do that, you mostly need to have a good handle on the short-time-scale dynamics of the cavitation. So as you are obviously an expert, here are two questions:
1) Not having read the paper, but only a summary based on a BBC article about the actual scientific article, how can you be sure strain fields have not been taken into account?
2) Assuming the liquid to be aqeuous, do you expect strain fields to be at all significant?

Re:Reinventing the (square) wheel

[Anonymous Coward]

1) I _did_ read the article. As I stated above, it's typical physicist oversimplification. They shop for old literature in a field they I am not certain they master *ahem* to grab a zero-dimensional "model" for liquid flow between two spheres, sort of Reynolds lubrication. That's fine if you do have the skills to check if it even makes sense in the precise context, and that kind of skill requires PhD level in Mechanics. There is way too much to say here for a random /. comment, but as a mere example the "solution" they use might very well be unstable in various ways in this context, depending eg on boundary conditions or the precise geometry. I've seem so many papers ignoring that (in the sense of mathematical ignorance), and developing sophisticated models around a flow that cannot exist in the sense that any infinitesimal perturbation drives the flow away from it, fast. Really too much to say.

2) Aqueous or otherwise, it's certainly mosty dissipative, let's assume more or less complicated viscous. Given the probably very high rate of strain around a bubble whose diameter is rapidly evolving in the vicinity of zero, yes, the damping influence of viscosity might be important, and other neglected factors might as well. And again, I won't check the old reference they provide for the maximum pressure formula, but it wouldn't surprise me much if it didn't pass, say, balance of mass.
You mileage may vary, mine is close to reset :-))

Re:Reinventing the (square) wheel

[Opportunist]

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.

So much tetarded judgment in the first sentences..

[Anonymous Coward]

Yeah. Retarded.

How does it feel?

This confirms it: The most annoying dateless stupidity-favoring thing here is ... you.

Last Date

[mentil]

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

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

Re:Last Date

[93 Escort Wagon]

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

You forgot to mention the restraining order.

Bit what CAUSES them???

[Anonymous Coward]

* 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.

Re:Bit what CAUSES them???

[MayeulC]

* 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 resistance as it increases, and then it suddenly decreases as the bubbles collapses, so this is definitely related (plus, the sound feels nice)

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

I know, right? This can happen everywhere you have some synovial fluid, so basically at every joint.

Of course, always take with a grain of salt the sayings of a random internet stranger who barely knows a thing or two on the topic :)

Re:Bit what CAUSES them???

[MayeulC]

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, 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 :)

Re:Bit what CAUSES them???

[MrKaos]

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.

* 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.

Re:Bit what CAUSES them???

[Anonymous Coward]

LOL did not read. Asshole.

Re:Bit what CAUSES them???

[Mister Liberty]

In my case lower back, i.e. the vertebrae.

Re:Bit what CAUSES them???

[thegarbz]

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???

[Anonymous Coward]

"Miss-attribution."

Who is she?

Re:Bit what CAUSES them???

[swillden]

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.

Re:Bit what CAUSES them???

[Anonymous Coward]

Stimulating the sensory nerve fibers inhibits activity in the pain fibers, so it feels good.

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.

Regarding the spine, I can tell you from personal experience that it's better to let someone else do it. (That should be someone who actually knows how to do it and how not to do it.)

Re:Bit what CAUSES them???

[lgw]

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???

[Anonymous Coward]

I always wonder if there is a purpose to the pressure build-up. Maybe the pressure inside the joint acts as a cushion?

Re:Bit what CAUSES them???

[Anonymous Coward]

* What causes the bubbles?

Cavitation. The Wikipedia page mentions it on cracking joints mentions it.

Re:Bit what CAUSES them???

[dgatwood]

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?

Re:Bit what CAUSES them???

[swillden]

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.

Talk about bewaring the idles of March

[Anonymous Coward]

this is boring stuff.

Re: Talk about bewaring the idles of March

[Anonymous Coward]

Huh? I'm not sure how this is going to help build a tunnel.

Re: Talk about bewaring the idles of March

[Anonymous Coward]

Eric Idle?

I know a knuckle

[Anonymous Coward]

that creimer never cracked... the moose knuckle!

CROFLOLFL!!!!

More Visual Explanation

[Kunedog]

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?...

How the principle explains another real world phenomenon:
https://www.youtube.com/watch?...

Re:More Visual Explanation

[dublin]

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...

[CriticalYetLazy]

I've seen numerous results of research regarding this subject at least a decade ago already drawing the same conclusions.

Re:Not really news...

[Anonymous Coward]

That's why this finding is published in Scientific Reports ;)

Re:Not really news...

[Anonymous Coward]

I've seen numerous results of research regarding this subject at least a decade ago already drawing the same conclusions.

Agreed. I remember seeing a demonstration using a high resolution functional MRI that showed the bubble/vacuum phenomenon. Not news.

Re:Not really news...

[TheDarkener]

I remember hearing about this explanation in elementary school, 25 years ago.

Re:Not really news...

[johnwfran]

Hah! I remember the same explanation from my Latin instructor in high school 36 years ago. What do I win?

I cracked my knuckles while reading this!

[Anonymous Coward]

I bet some of you cracked your knuckles after reading this post...

lol

[Anonymous Coward]

wrong sound , more like a crunch ...proof see roommates face

In case you were wondering

[Glarimore]

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?...

Which scientists?

[sjbe]

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?

[Translation Error]

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."

Re:Really?

[hcs_$reboot]

These guys must have had some time travelling engine

Re:Really?

[Anonymous Coward]

I totally read this in Cliff Claven's voice.

Re:Really?

[Quirkz]

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!"

Not all that new

[XXongo]

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

Re:Not all that new

[lgw]

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.

And

[hcs_$reboot]

did scientists explain also why this sound is unbearable?

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Sigh

[Anonymous Coward]

We have understood this phenomena for decades and without the use of algorithms or mathematical formulas. I'm sorry you weren't born yet, and that your parents and teachers never bothered to, you know, teach you anything (especially humility). Millennials and their 'science' are just flipping broken, man. Thanks a lot, helicopters of the world. Here is the fruit of both your labors and your loins.

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[Anonymous Coward]

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[Anonymous Coward]

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Old, OLD news.

[Anonymous Coward]

This is nowhere near new information.

Come on

[Impy the Impiuos Imp]

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.

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[Anonymous Coward]

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Where do the BUBBLES come from?

[nadass]

So where do the original bubbles come from? They surely cannot remain within the synovial fluid pockets if they are bursting (thus causing the sounds which were mathematically modeled... because that's what they wanted to model, supposing that there are tiny bubbles in the first place).

What about the possibility that ligaments are also involved? And that the displacement of ligaments are 'slapping' the bubbles around? Or what about all of the other things in the human body?

Weird

[Anonymous Coward]

How are they discovering this now?
This was the explanation given to me more than a decade ago when I was still in school.

"Science"

[Anonymous Coward]

And then "scientists" wonder why no one believes there shit any more. Coal workers are out of jobs, homosexuals are getting "married" and these idiots are wasting taxpayer dollars on researching useless bullshit.

This is already known!!!

[Anonymous Coward]

We've known for at least a decade that it's micro-cavitation that causes the sound!

This is not news! Olds perhaps...

What?

[Anonymous Coward]

You have to be kidding! I read about this being discovered back in the 90s.

Fallacies everywhere

[Anonymous Coward]

What I said is perfectly understood by any proper scientist.

The No True Scotsman defense.

First, it it those people's duty to look up the possible references

It falls onto the one making claims.

Ever heard about Navier-Stokes? Or any kind of mathematics?

Is that an attempt of Falacia ad Verecundiam? (appeal to authority)

Okay, but

[93 Escort Wagon]

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

Re:Okay, but

[Areyoukiddingme]

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

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Re:Okay, but

[93 Escort Wagon]

I said "clapping", not "fapping".

bin there

[Anonymous Coward]

The same principal is used to explain the inflation which accompanied the big bang. The nitrogen liquid (or initial elementary whatever) changes state under decompression, which is a phase change which has more energy in its ground state than in its previous state, which energy is released as inflation in the big bang, or a cracking sound in the joints.