Why Do Our Knuckles Crack?

Hey, welcome to side Stuff, a production nuff iHeartRadio. I'm Hoory Champ, and today we are taking a crack at knuckle cracking. Why do we do it, why does it happen? And is it bad for you? We're going to talk to a couple of experts about it, including one researcher who thinks they figured out the real reason why joins made that popping sound. It's not what you think, So stop wringing your hands, get cracking with us as we snap up and answer to the question why do our knuckles crack?

Hey?

Everyone, So, one of the fun things about being a current is seeing something about yourself end up in your kids. It could be your looks, or your personality or even your mannerisms. It's usually fun, but sometimes can be a little unsettling. Cannot a minute, WHOA, I'm talking me correctly. Yes, I am a knuckle cracker, and so is my daughter, but my spouse and my son not so much, which made me wonder why do some people's knuckles crack and some don't? What actually makes that sound? And is it

Sure, that's my pleasure.

Today we're talking about knuckle cracking and here it has something to do with something called a sinovial joint. Is that how you pronounce this iino you' or sinovio?

I guess it may depend on whether you're on the side of the Atlantic.

How do you pronounce it?

I call it a synovial joint?

Syno jo. We'll go with them. Can you tell us what is the sinovial joint?

So, sinobal joints are the kind of joints that are found in your elbow or your hip. They are characterized by relative sliding of two surfaces next to each other. So, if you want a common example, a door hinge functions very similar to how a cynobial join functions. You have two metal surfaces in a door hinge that are sliding with respect to each other.

So most of the joints in your body are what are called sinobial joints or synovial joints as other people pronounce it, And those are the kinds of joints that are like door hinges or like ball of socket joints, where the surface of one bone curves around the surface of the other bone, and they rotate and slide around each other. That's one way to join two boats together. Another kind of joint that you see in nature and in some parts of our body are cartilaginous joints.

So these are the joints found in your spine. So here you have two bony elements which are very stiff, and they are connected by a soft cartilage element in the middle. Okay, so you can imagine, like the way they function is by bending.

So this kind of join cartilaginous joint don't have any moving parts. It's basically like joining the ends of two sticks of wood together with a short rubber tube. The joint bends by bending the tube. Now, when we were evolving as primitive animals, we first evolved bones and cartilage, and initially all of our joints were cartilaginous. You can imagine we were blobby fish swimming in the ocean and we had a skeleton, but it was all put together

And again, like an adore hinge, you need to keep the joint lubricated, so you oil these hinges very often to maintain a seamless function. In the same way, our sinoval joints are also highly lubricated.

I see, I'm not very handy around the house, so I don't really lubricate my hinges, maybe as much as I should. But what's special about a sainovial joint as opposed to any other kinds of joints.

So this substance called sinovial fluet, which exists between these two surfaces that gives our sinoval joints that's lubricating properties.

So snovial joints have a gap between the two surfaces of the bones that slide past each other, and this gap is filled with a special liquid that is basically like a super lubricate.

So this is a very cool fact about sinoval sewage. Particularly the sinoval fluid, is a substance that is known to have one of the lowest coefficient of friction in both biological materials as well as engineered materials.

Really, it's super slippery.

Yeah, it's super slippery. It's more slippery than what ice on ice feels like. Really, and ice on ice is super slippery. Yeah, whoah, more slippery than oil. Yeah, absolutely, And that's why you have to lubricate your door headges get a few months now.

You're making me feel guilty, But.

You do not have to lubricate your joints as much. They function for sixty years of age if you take care of them properly.

Now, these special sinovial joints with the two bones separated by gap filled with a super slippery liquid are basically the reason our knuckles crack. We'll get into the physics of why that happens later in the program. But what this type of joint also did is give our vertebrate ancestors essentially a superpower.

This kind of joint. So you can imagine that when I'm moving my elbow, I do not really face a resistance while I'm moving it. So it's I know, will joints allow you to be more mobile and faster. I can move my elbow so much faster because there's absolutely no resistance here and all I have to do is turn my muscle on. This would have allowed animals to

Oh, that could have been their secret weapon to take over the world.

Yeah, absolutely, WHOA, this is your speculation, but definitely interesting speculation.

Yes, so sygnobil joints could possibly be the reason you, me, and most vertebrate animals are here. Cinovial joints are easier to move and therefore faster, and they have a wider range of motion than the cartilaginous type of joint, which could have given the vertebrates that had sinobial joints the advantage. Now it's interesting to think about what exactly prompted vertebrates

People used to think that cinobil joints only evolved because animals moved out from water to land, and because on land you have to bear larger loads. You need like these joints which do not become unstable.

Because cartilaginist joints are unstable.

Because they are like a rubber tube. Now, if I stand on a rubber tube, it can compress or it can buckle out of shape.

I see.

It's not that sinovil joints don't become unstable, but they are definitely more stable than cartilaginous joints. If you would want similar range of motion out of the two joints, so there was a conventional wisdom for many years we have cygnovial joints because we needed them to walk on land. But the reason I reached out to doctor Sharma is that she's the lead author in a recent study that says this is not true.

So this is a paper.

In fact, I was published last year, and I was researching the question of when the joints evolved, whether or not skates and sharks at sinobile joints.

What doctor Sharma and her colleagues did, by study current animals and studying the fossils of ancient animals, was to sort of pinpoint the evolution of sinovial joints and therefore when exactly animals started to be able to possibly start cracking their knuckles. And they dated it to about four hundred million years ago. Now this date is significant according to doctor Sharma for two reasons. And they're going with me here for a second. I promise it'll be worth it.

Yeah, I guess not. It is definitely a byproduct of the evolution of these kind of joints and an interesting vibe product.

In other words, if we couldn't crack our knuckles, we probably wouldn't be here. Okay, I just had one more question or doctor Sharma. Okay, last question, do Korsharma? Can you crack your knuckles?

Yes?

Oh my goodness. Oh well, it's really loud.

To be honest, I don't think I've done this in the last five years. Oh really, Yeah, I do have because usually.

Oh, you have this ability to crack your knuckles, but it's not a composion for you. No. That brings us the next question we're going to answer in this episode, which is, if we can crack our knuckles, why do we do it? And why do some of us do it compulsively? When we come back, I'm gonna talk Jane neuroscientists about this, and we're gonna address whether cracking your knuckles is that for you. So stay with us. We'll

Hey or hey, how are you doing.

I'm great. I'm cracking my knuckles here.

Oh are you?

I can't stop. I can't stop. That's the that's the question we're trying to answer today. Can you character knuckles? You just don't do it. I can.

It's not very impressive.

Oh oh boy, that was pretty loud.

Yeah.

Oh it's a microphone.

The first thing I asked doctor Cockwin was what does psychologists think of knuckle cracking? And apparently the answer is they don't.

Yeah.

You know, there's not an extensive literature on knuckle cracking. With a lot of the other things we talk about, there's usually something like a study, there's scales that you can compare with advanced neuroimagery. But knuckle cracking, you know, it doesn't really hurt you for the most part. It doesn't produce the kind of euphoria that you might associate with a drug, so it's harder to study in that way.

I see, it's not high in the priority list of the nih or National Science Foundation.

You know, if you're sort of balancing things out, Oh am, I going to look at brain tumors versus knuckle cracking.

Yeah, but not everyone can study brain tomors. I mean, somebody's got to study the knuckles. Yeah, So knuckle cracking is not a high priority in the scientific community, although we did find a couple of case reports where psychologists have treated people who said they had a problem with cracking their knuckles.

Yeah, there are a few case reports where this joint cracking or clicking becomes excessive or distressing or really hard to stop. One of the reports described compulsive joint clicking driven by an uncomfortable joint sensation that they could ease only after repeated clicking. So it would be like, you know, popping your neck multiple times to get relief in your neck. And then there was another case study that described improvement with this dopamine drug and the fidget spinners.

Yes, you can take a drug to stop cracking your knuckles. Look at to why that works in a little bit. But first I wanted to ask doctor Godwin what he thought was going on in the brain when we crack our knuckles.

So it's not hard to reason out what might be happening. A reasonable hypothesis is that knuckle cracking can ride along on the same circus that the brain uses for habits in our normal lives. The habit system is a feature and not a bug. It takes actions that you repeat

I see, it's like the brain is made to put certain things into that category of automatic behavior.

Yeah, it's very advantageous from an evolutionary perspective. If you were thinking about every action that you took, then it would be very difficult to navigate the world day to day, and it would be difficult if you were performing at a very high level in sports. For example, if you had to think about every placement of a golf club

In other words, our brain is wired to form habits, and to form a habit, all you need is for you to do it several times and for it to feel good.

So early in learning, the prefrontal cortex is more involved because you're deciding and evaluating. But with repetition, the control shifts toward the dorsal strainum and motor planning areas and the behavior becomes more automatic, and dopamine is involved in that. It's one of the signals that helps to stamp in a habit if it's producing either a rewarding outcome or a relief, and relief in some sense can be rewarding related to this sort of knuckle cracking behavior.

Like if it somehow feels good, then your brain will want to keep doing it.

Yeah, for knuckle cracking, the rewards is kind of this relief from stiffness or tension in the joint, and relief can be powerful because it teaches the brain that this movement fixes the sensation. So over time, the queue can be as subtle as a tiny joint feeling a moment of stress, or even just seeing your hands, and your brain is cued to add activate that loop.

Yes, I know what you're thinking. If knuckle cracking involves the dopamine reward circuit in your brain, does that mean that it's like an addiction. Not quite. According to doctor Godwin, knuckle cracking doesn't quite rise to the level of an addiction, but it can become a compulsion.

What distinguishes a habit from a compulsion. As a habit, you identify it, you know it, and you can stop it.

Right.

It's something that you can interrupt or distract yourself from. The compulsion is a feeling that's almost overwhelming that you must complete the act. It's very much the similar brain circuit, but in the case of compulsive behavior, the dopamine stamp is more profound.

I see.

But for the most part, there's no reports in the literature that knuckle cracking fits into a category that would identify it as an addiction.

I see.

There may be somebody out there that has a knuckle cracking addiction. You can't just disclaim that or disprove that, but I would say that it would be pretty rare.

You'd call him a real knucklehead.

Yeah, or a crackhead. Knuckle crack head.

I know, I might get in trouble for them. Now. The good news for knuckle crackers out there is that, as far as anyone knows, popping your joint is not that bad for you. In nineteen ninety eight, a doctor named Donald Unger from Thousand Oaks, California published the letter in the journal Arthritis and Rheumatology, in which he reported the longest running experiment on the subject of knuckle cracking

Yeah, there's a condition called misphonia, and it's kin to the fingers on a chalkboard response, yeah, and sort of being grossed out by body horror, that sort of thing. And the idea I think is your brain might interpret the popping of bones and flesh as being something that is, from an evolutionary perspective, something to avoid, uh huh, And so you may be having a response to that.

Oh wow, it's tapping into like a very primitive reflex in the brain.

Maybe again, you know, there's no literature on this, you know, it's really interesting that what it does point out there's this aspect of the physics of how knuckles pop. There's the sound that comes from it.

And that brings us to the last way we will answer the question why do our knuckles cracked, which is by digging into the physics of what's going on. What's actually making that cracking sound? Is it your bones grinding? Is it something actually popping inside of your joints. When we come back, we'll talk to a Harvard scientist who thinks they finally figured out the real reason our knuckles make that sound, and it's not what you think. So stay with us. We'll be right back. Hey, welcome back.

First, my pleasure and it's great to see you.

Okay, my first question is do you crack your knuckles.

Oh, I do. And one of these people who has this compulsive disorder, you know, I cracked my knuckles whenever we're I'm frustrated when something doesn't work, and that's more days in my life than usual.

Right.

Can we hear your knuckles cracking right now?

Yeah? Let me try.

Did you hear some Yeah that was pretty fun. Yeah, that's really dense. Now. In twenty eighteen, doctor Suja was the lead author of the paper titled A Mathematical Model of the Sounds Produced by Knuckle Cracking, in which they claim to finally settle the debate of what actually makes the that happens when we crack our knuckles. It's a debate that, according to doctor Suja, has been going on for over one hundred years. Tell us the history of trying to figure out where the sound comes from.

Yeah, we were able to trace it back to at least the early nineteen hundreds, where there's actual published scientific literature on people saying, look, not everybody can crack their knuckles, so people kind of like document their way back.

According to doctor Suja, one breakthrough came in nineteen forty seven to doctors from the Saint Thomas Hospital Medical School in London decided to do experiments on actual fingers, measuring how much tension was needed and how far apart the joints needed to be to create a crack, and they realized that it might have something to do with how sudden the joint snaps.

They said, like, it has to do with how quickly the joints are being separated. There's something there and they kind of concluded that when you do this so fast, you know, could be vibrations and a tissue and that could be the sound.

But then in nineteen seventy one, another group of British scientists for the University of Leads decided to make a plastic model of a joint, put some fluid in it, and they use the high speed camera to see what happens when you pop the joint.

They actually build a kind of mockup model of the tissue and they could see some bubble activity happening as they were pulling about this joint and they said it's cavitation.

And here we get to what scientists think is the culprit makes the knuckle cracking sound. Cavitation. Cavitation is what happens when you suddenly get bubbles inside of a liquid. Now, one way to get bubbles inside a liquid is to boil it. When you heat water on a stove, the water molecules at the bottom get hot and they turn to steam, which is a gas, and so they form bubbles. But another way to turn water to gas is to

The best anology I can draw is like opening a soda bottle. It's pressureized, right, and like you open the cap, you're suddenly changing the pressure. And what that leads to is sort of bubbles coming out.

And so here's what those scientists in nineteen seventy one thought was happening. When you're cracking your knuckles, you're bending your joints, but at some point you can over bend them and that pulls the two bones apart. But there's fluid in between them, and so that fluid gets stretched, which causes the pressure to drop and you form bubbles. Now, the scientists in nineteen seventy one saw the bubbles in their model and they thought, ah, that's what's making the

This is sort of like the rapid formation and collapse of bubbles that had been known to like damage propellers with these bubbles, and that ended up being like the state of the art for a long time. And remember growing up and like it whenever somebody asked me what causes the sound of it's a gavitation, because that used to be the thing.

As doctor Suji said, this is what people thought caused knuckle cracking for a long time. It's the bubbles from cavitation in the joint popping, right, that makes sense. But then about forty years later, in twenty fifteen, a group of Canadian and Australian engineers and doctors decided to get more high tech and they used an MRI machine to see what was going on inside of an actual knuckle.

They actually did the first imaging study. They used MRI to look inside the knuckles and as we said before, MRA is slow, definitely cannot see what's happening at the moment these sounds are coming. But what they could see was at slow enough time points, they could actually see the bubble there, oh, and it persisted.

What the Canadian engineers and doctors saw was that even after the knuckle had made the cracking sound, there were still bubbles. The bubbles hadn't popped. But if the bubbles hadn't popped, what made the cracking sound?

And that gave them an interesting idea. So they said, look, there's bubbles still in there. Maybe it's not a collapse, but it is a formation of these bubbles that could be the source of the sound. So that suddenly opened up, you know, more questions, and the flood gates of questions, and suddenly, like the knuckle cracking was in the debdfield again.

Yes, the entire knuckle cracking research field was thrown into this array. Okay, to be honest, there aren't that many people looking into this. I mean, it's not like the government and foundations are pouring billions of dollars to figure out knuckle cracking. But still it became a topic of debate again. Was knuckle cracking caused by bubbles forming in the liquid inside the knuckle joints or was it caused by the bubbles popping. That's when doctor Sujah entered the picture.

So this is more than a decade back. And I was in Frans as a master's student.

You know.

I was in this class of a wonderful professor by the name of Abdel Barka, and he said, I have two weeks to figure out an interesting problem in buying mechanics, and sought okay. And I'm like sitting in frustration, kind of cracking my knuckles, and Sally like, oh, this seems to be an interesting problem. You knows buying mechanics in there and read more about it, and there's flood mechanics in there, there's bubble physics, and I got all excited.

What doctor Suja did was create the most accurate mathematical model of the physics of joint bubble formation that had ever been done. Okay, let me through what you did. So in a computer you wrote up a simulation of what we ended.

Up modeling three critical events mathematically. So one is how the pressure is changing, and there's a so called lubrication theory that's in fluid mechanics we can leverage to predict how fluid pressure changes when there's motion in a small enough gap, so we use that. And second, there's a really intriguing theory that basically describes what happens to a

Okay, the details here get a little technical. It turns out that there are several ways that the cavitation bubbles in the joint can be made, either from the liquid being pulled apart or the liquid being stretched by friction. And there's also what happens after the joint gets pulled apart, which is that the pressure goes up again because flood

A lot of people who love cracking knuckles and like helping out for the name of science. So you know, it was at the university, and there's a lot of frustrated pieces. Yes, we did briefly advertise this study, but you know, mostly what of mouth as he put a microphone right next to somebody's knuckle.

Yeah, that's precisely.

What of those like we had people set in like a recording studio and have people come in and crack the knuckles close to a microphone.

Did you have them work on their thesis so that they would get frustrated?

That wou'd have been a good point. I know we could have gotten more volunteers that way.

So doctor Suja compared the frequency signature the sound from his mathematical simulation to the signature of real knuckle cracks, and they matched. But here's the thing. In doctor Suji's simulation, the sound didn't come from either the bubbles forming or the bubbles popping.

And what our modeling work showed was you could still have a collapsing bubble and produce sounds, but the bubble need not collapse completely for the sounds to happen. You don't need the bubble to go to a size of zero, but it could be like starting with something big, going to ten times smaller and stop there. And still that partial change in size was sufficient to explain the sounds. Okay, okay, so you can still see the bubble after knuckle cracking.

What doctor Suji's model showed was that the popping sound actually comes from the bubbles shrinking, not when they form, not when they pop, when the bubbles shrink, And that seems to be the best explanation we have about where the knuckle cracking sound comes from. All right, and say, you've helped partially resolve this big mystery that's been sitting around for over fifty years.

Partially.

Yeah.

The cool thing is we were able to like kind of say, maybe both of you guys can be right the nineteen seventy one and twenty fifteen, and there's a way to go forward from there.

And as a bonus, doctor Sugis's model actually predicts why some people can crack their knuckles and some people can't.

And a good thing about mathematical moral and a lot of marveling for this reasons. You can quantitatively say how certain parameters and this problem in fact the sound. For example, what if you have a different joint spacing. What if you pull the join a little bit harder?

Right?

And why can't some people crack their knuckles? The model says they are the only spacing is too large. You won't be able to generate precious low enough to cause the bubbles to form an oslate in a way that can produce.

The sound amazing? Has this changed how you see your knuckles or when you crack your nuckles? Do you think about it differently now?

Yes?

And no.

Could you go back to your mother and say, see, mom, every time I cracked my knuckles, that was good for something? Yeah?

And the response I would get is like, do you have no other, you know, important scientific problem to think about?

It's never enough, is it right? Exactly? Yeah? It's like, why aren't you curring cancer? Why are you trying to figure out knuckle cracking?

Do you have too much time on your plate?

Stop cracking your knuckles and get to work on something important.

Yeah?

All right. Well to recap as annoying as someone cracking their knuckles is, it's all because we have these fluid filled joints. Let us do amazing movements like walking and running and climbing. It's not issarily harmful to crack your knuckles, even if you do it compulsively, and hey, it could end up getting you a PhD and a job at Harvard. You just have to knuckle down. Thanks for joining us, see you next time. Hey, a big shout out to