Speaker 1: Hey, welcome to side Stuff, a production nuff iHeartRadio. I'm
Speaker 1: Hoory Champ, and today we are taking a crack at
Speaker 1: knuckle cracking. Why do we do it, why does it happen?
Speaker 1: And is it bad for you? We're going to talk
Speaker 1: to a couple of experts about it, including one researcher
Speaker 1: who thinks they figured out the real reason why joins
Speaker 1: made that popping sound. It's not what you think, So
Speaker 1: stop wringing your hands, get cracking with us as we
Speaker 1: snap up and answer to the question why do our
Speaker 1: knuckles crack?
Speaker 2: Hey?
Speaker 1: Everyone, So, one of the fun things about being a
Speaker 1: current is seeing something about yourself end up in your kids.
Speaker 1: It could be your looks, or your personality or even
Speaker 1: your mannerisms. It's usually fun, but sometimes can be a
Speaker 1: little unsettling. Cannot a minute, WHOA, I'm talking me correctly. Yes,
Speaker 1: I am a knuckle cracker, and so is my daughter,
Speaker 1: but my spouse and my son not so much, which
Speaker 1: made me wonder why do some people's knuckles crack and
Speaker 1: some don't? What actually makes that sound? And is it
Speaker 1: bad for you? And if it is bad, why do
Speaker 1: we keep doing it? As it turns out, knuckle cracking
Speaker 1: has to do with the type of joint we have
Speaker 1: in our bodies. To tell us about this type of joint,
Speaker 1: I reached out to doctor Neilima Sharma, a biologist and
Speaker 1: researcher at University College London. Well, thank you so much,
Speaker 1: Taka Sharma for joining us.
Speaker 2: Sure, that's my pleasure.
Speaker 1: Today we're talking about knuckle cracking and here it has
Speaker 1: something to do with something called a sinovial joint. Is
Speaker 1: that how you pronounce this iino you' or sinovio?
Speaker 2: I guess it may depend on whether you're on the
Speaker 2: side of the Atlantic.
Speaker 1: How do you pronounce it?
Speaker 2: I call it a synovial joint?
Speaker 1: Syno jo. We'll go with them. Can you tell us
Speaker 1: what is the sinovial joint?
Speaker 2: So, sinobal joints are the kind of joints that are
Speaker 2: found in your elbow or your hip. They are characterized
Speaker 2: by relative sliding of two surfaces next to each other. So,
Speaker 2: if you want a common example, a door hinge functions
Speaker 2: very similar to how a cynobial join functions. You have
Speaker 2: two metal surfaces in a door hinge that are sliding
Speaker 2: with respect to each other.
Speaker 1: So most of the joints in your body are what
Speaker 1: are called sinobial joints or synovial joints as other people
Speaker 1: pronounce it, And those are the kinds of joints that
Speaker 1: are like door hinges or like ball of socket joints,
Speaker 1: where the surface of one bone curves around the surface
Speaker 1: of the other bone, and they rotate and slide around
Speaker 1: each other. That's one way to join two boats together.
Speaker 1: Another kind of joint that you see in nature and
Speaker 1: in some parts of our body are cartilaginous joints.
Speaker 2: So these are the joints found in your spine. So
Speaker 2: here you have two bony elements which are very stiff,
Speaker 2: and they are connected by a soft cartilage element in
Speaker 2: the middle. Okay, so you can imagine, like the way
Speaker 2: they function is by bending.
Speaker 1: So this kind of join cartilaginous joint don't have any
Speaker 1: moving parts. It's basically like joining the ends of two
Speaker 1: sticks of wood together with a short rubber tube. The
Speaker 1: joint bends by bending the tube. Now, when we were
Speaker 1: evolving as primitive animals, we first evolved bones and cartilage,
Speaker 1: and initially all of our joints were cartilaginous. You can
Speaker 1: imagine we were blobby fish swimming in the ocean and
Speaker 1: we had a skeleton, but it was all put together
Speaker 1: with basically rubber joints. But then at some point we
Speaker 1: started developing this new kind of joint, synovial joint, which
Speaker 1: had two moving, rotating parts. Now, if you have two
Speaker 1: bones rotating and rubbing against each other, after a while,
Speaker 1: they'll wear each other out and get creaky. So a
Speaker 1: feature of these sinovial joints is that they have a
Speaker 1: gap in between the two bones which is filled with
Speaker 1: a lubricant.
Speaker 2: And again, like an adore hinge, you need to keep
Speaker 2: the joint lubricated, so you oil these hinges very often
Speaker 2: to maintain a seamless function. In the same way, our
Speaker 2: sinoval joints are also highly lubricated.
Speaker 1: I see, I'm not very handy around the house, so
Speaker 1: I don't really lubricate my hinges, maybe as much as
Speaker 1: I should. But what's special about a sainovial joint as
Speaker 1: opposed to any other kinds of joints.
Speaker 2: So this substance called sinovial fluet, which exists between these
Speaker 2: two surfaces that gives our sinoval joints that's lubricating properties.
Speaker 1: So snovial joints have a gap between the two surfaces
Speaker 1: of the bones that slide past each other, and this
Speaker 1: gap is filled with a special liquid that is basically
Speaker 1: like a super lubricate.
Speaker 2: So this is a very cool fact about sinoval sewage.
Speaker 2: Particularly the sinoval fluid, is a substance that is known
Speaker 2: to have one of the lowest coefficient of friction in
Speaker 2: both biological materials as well as engineered materials.
Speaker 1: Really, it's super slippery.
Speaker 2: Yeah, it's super slippery. It's more slippery than what ice
Speaker 2: on ice feels like. Really, and ice on ice is
Speaker 2: super slippery. Yeah, whoah, more slippery than oil. Yeah, absolutely,
Speaker 2: And that's why you have to lubricate your door headges
Speaker 2: get a few months now.
Speaker 1: You're making me feel guilty, But.
Speaker 2: You do not have to lubricate your joints as much.
Speaker 2: They function for sixty years of age if you take
Speaker 2: care of them properly.
Speaker 1: Now, these special sinovial joints with the two bones separated
Speaker 1: by gap filled with a super slippery liquid are basically
Speaker 1: the reason our knuckles crack. We'll get into the physics
Speaker 1: of why that happens later in the program. But what
Speaker 1: this type of joint also did is give our vertebrate
Speaker 1: ancestors essentially a superpower.
Speaker 2: This kind of joint. So you can imagine that when
Speaker 2: I'm moving my elbow, I do not really face a
Speaker 2: resistance while I'm moving it. So it's I know, will
Speaker 2: joints allow you to be more mobile and faster. I
Speaker 2: can move my elbow so much faster because there's absolutely
Speaker 2: no resistance here and all I have to do is
Speaker 2: turn my muscle on. This would have allowed animals to
Speaker 2: operate quickly. So if I'm a predator and I have
Speaker 2: a joint that moves early fast, I am definitely able
Speaker 2: to cash more prey and I'm also able to evade predators.
Speaker 2: So they must definitely have given animals and edge over
Speaker 2: other animals who did not have these joints. And maybe
Speaker 2: that's why they spread so fast that most vertebrates that
Speaker 2: we'd know now do have a lot of signe joints.
Speaker 1: Oh, that could have been their secret weapon to take
Speaker 1: over the world.
Speaker 2: Yeah, absolutely, WHOA, this is your speculation, but definitely interesting speculation.
Speaker 1: Yes, so sygnobil joints could possibly be the reason you, me,
Speaker 1: and most vertebrate animals are here. Cinovial joints are easier
Speaker 1: to move and therefore faster, and they have a wider
Speaker 1: range of motion than the cartilaginous type of joint, which
Speaker 1: could have given the vertebrates that had sinobial joints the advantage.
Speaker 1: Now it's interesting to think about what exactly prompted vertebrates
Speaker 1: to develop this type of joint. According to doctor Sharma,
Speaker 1: the conventional wisdom for at least the last hundred years
Speaker 1: or so has been that vertebrates evolved this kind of
Speaker 1: joint when they moved from the ocean to land. The
Speaker 1: idea is that sinovial joints are better for standing and walking.
Speaker 2: People used to think that cinobil joints only evolved because
Speaker 2: animals moved out from water to land, and because on
Speaker 2: land you have to bear larger loads. You need like
Speaker 2: these joints which do not become unstable.
Speaker 1: Because cartilaginist joints are unstable.
Speaker 2: Because they are like a rubber tube. Now, if I
Speaker 2: stand on a rubber tube, it can compress or it
Speaker 2: can buckle out of shape.
Speaker 1: I see.
Speaker 2: It's not that sinovil joints don't become unstable, but they
Speaker 2: are definitely more stable than cartilaginous joints. If you would
Speaker 2: want similar range of motion out of the two joints,
Speaker 2: so there was a conventional wisdom for many years we
Speaker 2: have cygnovial joints because we needed them to walk on land.
Speaker 2: But the reason I reached out to doctor Sharma is
Speaker 2: that she's the lead author in a recent study that
Speaker 2: says this is not true.
Speaker 1: So this is a paper.
Speaker 2: In fact, I was published last year, and I was
Speaker 2: researching the question of when the joints evolved, whether or
Speaker 2: not skates and sharks at sinobile joints.
Speaker 1: What doctor Sharma and her colleagues did, by study current
Speaker 1: animals and studying the fossils of ancient animals, was to
Speaker 1: sort of pinpoint the evolution of sinovial joints and therefore
Speaker 1: when exactly animals started to be able to possibly start
Speaker 1: cracking their knuckles. And they dated it to about four
Speaker 1: hundred million years ago. Now this date is significant according
Speaker 1: to doctor Sharma for two reasons. And they're going with
Speaker 1: me here for a second. I promise it'll be worth it.
Speaker 1: That date four hundred million years ago is before animals
Speaker 1: moved from the ocean to land. So the reason we
Speaker 1: have synovial joints is not so we could stand up.
Speaker 1: But wait, if that's not the reason, then what is well,
Speaker 1: four hundred million years ago. It's also the time when
Speaker 1: jaws were first developed. That's right, before four hundred million
Speaker 1: years ago, fishes just kind of had a circular mouth,
Speaker 1: sort of like lampreeze or leeches. But then four hundred
Speaker 1: million years ago, vertebrates evolved jaws that could open and close,
Speaker 1: and that could bite and chomp and prey. And this
Speaker 1: brings up the crazy idea that maybe the reason we
Speaker 1: evolved synovial joints, and therefore the reason we can crack
Speaker 1: our knuckles, could have been to eat better. Okay, to
Speaker 1: be fair, the evolution of sinnovial joints and the evolution
Speaker 1: of jaws happening at the same time could just be
Speaker 1: a coincidence, and there are sort of examples of old
Speaker 1: fish that maybe have sinobial joints in their fins and
Speaker 1: not their jaws. But still it does point to one
Speaker 1: possible answer to the question why do we crack our knuckles.
Speaker 1: Here's my hypothesis that knuckle cracking is a byproduct of
Speaker 1: developing these kinds of joints so that we could move
Speaker 1: faster and be better at eating and moving and running
Speaker 1: and swimming, and eventually walking and running it's like it's
Speaker 1: something we can't avoid if we wanted to have this
Speaker 1: kind of joint.
Speaker 2: Yeah, I guess not. It is definitely a byproduct of
Speaker 2: the evolution of these kind of joints and an interesting
Speaker 2: vibe product.
Speaker 1: In other words, if we couldn't crack our knuckles, we
Speaker 1: probably wouldn't be here. Okay, I just had one more
Speaker 1: question or doctor Sharma. Okay, last question, do Korsharma? Can
Speaker 1: you crack your knuckles?
Speaker 3: Yes?
Speaker 1: Oh my goodness. Oh well, it's really loud.
Speaker 2: To be honest, I don't think I've done this in
Speaker 2: the last five years. Oh really, Yeah, I do have
Speaker 2: because usually.
Speaker 1: Oh, you have this ability to crack your knuckles, but
Speaker 1: it's not a composion for you. No. That brings us
Speaker 1: the next question we're going to answer in this episode,
Speaker 1: which is, if we can crack our knuckles, why do
Speaker 1: we do it? And why do some of us do
Speaker 1: it compulsively? When we come back, I'm gonna talk Jane
Speaker 1: neuroscientists about this, and we're gonna address whether cracking your
Speaker 1: knuckles is that for you. So stay with us. We'll
Speaker 1: be right cracked, but I mean we'll be right back. Hey,
Speaker 1: we'll come back we're talking about cracking our knuckles, and
Speaker 1: so far we've learned the reason we can crack our
Speaker 1: knuckles is that it's a feature that came with an
Speaker 1: upgrade in the joints of our early vertebrate ancestors. Those
Speaker 1: are the joints in your body are what are called
Speaker 1: sinovial or synovial joints, and they're pretty good. Here. Try this,
Speaker 1: if you can swing your arms around in a circle
Speaker 1: and open and close the fingers in your hand. Isn't
Speaker 1: it incredible how easy and smooth it is to do that,
Speaker 1: and be amazed at what a wide range of motion
Speaker 1: each of your joints has. Well, it's all due to
Speaker 1: signovial joints, but sometimes they do crack. We'll get to
Speaker 1: the physics and the fluid dynamics of why that happens.
Speaker 1: But first I thought we could answer the question of
Speaker 1: why we crack our knuckles from a psychological point of view,
Speaker 1: Why are we compelled to crack our knuckles and why
Speaker 1: does it annoy certain people? To dig into the mind
Speaker 1: of the knuckle cracker, I reached out to doctor Dwayne Godwin,
Speaker 1: a psychologist and neuroscientist at Wake Forest University and the
Speaker 1: co author with me of the book Out of Your Mind. Well,
Speaker 1: thanks doctor Godwin for joining us again.
Speaker 4: Hey or hey, how are you doing.
Speaker 1: I'm great. I'm cracking my knuckles here.
Speaker 4: Oh are you?
Speaker 1: I can't stop. I can't stop. That's the that's the
Speaker 1: question we're trying to answer today. Can you character knuckles?
Speaker 1: You just don't do it. I can.
Speaker 4: It's not very impressive.
Speaker 1: Oh oh boy, that was pretty loud.
Speaker 2: Yeah.
Speaker 4: Oh it's a microphone.
Speaker 1: The first thing I asked doctor Cockwin was what does
Speaker 1: psychologists think of knuckle cracking? And apparently the answer is
Speaker 1: they don't.
Speaker 2: Yeah.
Speaker 4: You know, there's not an extensive literature on knuckle cracking.
Speaker 4: With a lot of the other things we talk about,
Speaker 4: there's usually something like a study, there's scales that you
Speaker 4: can compare with advanced neuroimagery. But knuckle cracking, you know,
Speaker 4: it doesn't really hurt you for the most part. It
Speaker 4: doesn't produce the kind of euphoria that you might associate
Speaker 4: with a drug, so it's harder to study in that way.
Speaker 1: I see, it's not high in the priority list of
Speaker 1: the nih or National Science Foundation.
Speaker 4: You know, if you're sort of balancing things out, Oh am,
Speaker 4: I going to look at brain tumors versus knuckle cracking.
Speaker 1: Yeah, but not everyone can study brain tomors. I mean,
Speaker 1: somebody's got to study the knuckles. Yeah, So knuckle cracking
Speaker 1: is not a high priority in the scientific community, although
Speaker 1: we did find a couple of case reports where psychologists
Speaker 1: have treated people who said they had a problem with
Speaker 1: cracking their knuckles.
Speaker 4: Yeah, there are a few case reports where this joint
Speaker 4: cracking or clicking becomes excessive or distressing or really hard
Speaker 4: to stop. One of the reports described compulsive joint clicking
Speaker 4: driven by an uncomfortable joint sensation that they could ease
Speaker 4: only after repeated clicking. So it would be like, you know,
Speaker 4: popping your neck multiple times to get relief in your neck.
Speaker 4: And then there was another case study that described improvement
Speaker 4: with this dopamine drug and the fidget spinners.
Speaker 1: Yes, you can take a drug to stop cracking your knuckles.
Speaker 1: Look at to why that works in a little bit.
Speaker 1: But first I wanted to ask doctor Godwin what he
Speaker 1: thought was going on in the brain when we crack
Speaker 1: our knuckles.
Speaker 4: So it's not hard to reason out what might be happening.
Speaker 4: A reasonable hypothesis is that knuckle cracking can ride along
Speaker 4: on the same circus that the brain uses for habits
Speaker 4: in our normal lives. The habit system is a feature
Speaker 4: and not a bug. It takes actions that you repeat
Speaker 4: a lot, makes them fast and low effort. You don't
Speaker 4: want to consciously replan every step of tying your shoes
Speaker 4: or unlocking your phone or driving a familiar route, and
Speaker 4: so the brain chunks those behaviors into packages that can
Speaker 4: run with minimal supervision.
Speaker 1: I see, it's like the brain is made to put
Speaker 1: certain things into that category of automatic behavior.
Speaker 4: Yeah, it's very advantageous from an evolutionary perspective. If you
Speaker 4: were thinking about every action that you took, then it
Speaker 4: would be very difficult to navigate the world day to day,
Speaker 4: and it would be difficult if you were performing at
Speaker 4: a very high level in sports. For example, if you
Speaker 4: had to think about every placement of a golf club
Speaker 4: as you were making a swing. If you don't somehow
Speaker 4: make that more automatic, then you get into issues like choking,
Speaker 4: because that is kind of overthinking those kinds of behaviors.
Speaker 1: In other words, our brain is wired to form habits,
Speaker 1: and to form a habit, all you need is for
Speaker 1: you to do it several times and for it to
Speaker 1: feel good.
Speaker 4: So early in learning, the prefrontal cortex is more involved
Speaker 4: because you're deciding and evaluating. But with repetition, the control
Speaker 4: shifts toward the dorsal strainum and motor planning areas and
Speaker 4: the behavior becomes more automatic, and dopamine is involved in that.
Speaker 4: It's one of the signals that helps to stamp in
Speaker 4: a habit if it's producing either a rewarding outcome or
Speaker 4: a relief, and relief in some sense can be rewarding
Speaker 4: related to this sort of knuckle cracking behavior.
Speaker 1: Like if it somehow feels good, then your brain will
Speaker 1: want to keep doing it.
Speaker 4: Yeah, for knuckle cracking, the rewards is kind of this
Speaker 4: relief from stiffness or tension in the joint, and relief
Speaker 4: can be powerful because it teaches the brain that this
Speaker 4: movement fixes the sensation. So over time, the queue can
Speaker 4: be as subtle as a tiny joint feeling a moment
Speaker 4: of stress, or even just seeing your hands, and your
Speaker 4: brain is cued to add activate that loop.
Speaker 1: Yes, I know what you're thinking. If knuckle cracking involves
Speaker 1: the dopamine reward circuit in your brain, does that mean
Speaker 1: that it's like an addiction. Not quite. According to doctor Godwin,
Speaker 1: knuckle cracking doesn't quite rise to the level of an addiction,
Speaker 1: but it can become a compulsion.
Speaker 4: What distinguishes a habit from a compulsion. As a habit,
Speaker 4: you identify it, you know it, and you can stop it.
Speaker 1: Right.
Speaker 4: It's something that you can interrupt or distract yourself from.
Speaker 4: The compulsion is a feeling that's almost overwhelming that you
Speaker 4: must complete the act. It's very much the similar brain circuit,
Speaker 4: but in the case of compulsive behavior, the dopamine stamp
Speaker 4: is more profound.
Speaker 1: I see.
Speaker 4: But for the most part, there's no reports in the
Speaker 4: literature that knuckle cracking fits into a category that would
Speaker 4: identify it as an addiction.
Speaker 1: I see.
Speaker 4: There may be somebody out there that has a knuckle
Speaker 4: cracking addiction. You can't just disclaim that or disprove that,
Speaker 4: but I would say that it would be pretty rare.
Speaker 1: You'd call him a real knucklehead.
Speaker 4: Yeah, or a crackhead. Knuckle crack head.
Speaker 1: I know, I might get in trouble for them. Now.
Speaker 1: The good news for knuckle crackers out there is that,
Speaker 1: as far as anyone knows, popping your joint is not
Speaker 1: that bad for you. In nineteen ninety eight, a doctor
Speaker 1: named Donald Unger from Thousand Oaks, California published the letter
Speaker 1: in the journal Arthritis and Rheumatology, in which he reported
Speaker 1: the longest running experiment on the subject of knuckle cracking
Speaker 1: since he was a kid. Doctor Unger had the habit
Speaker 1: of cracking the knuckles on his left hand, but only
Speaker 1: on his left hand, not his right hand, and he
Speaker 1: did it at least twice a day. Fifty years later,
Speaker 1: in a somewhat tongue in cheek case report in the journal,
Speaker 1: he was happy to report that after about thirty six
Speaker 1: five hundred knuckle crackings, neither of his hands head arthritis,
Speaker 1: and that there was no notice difference in the appearance
Speaker 1: or health of both hands. For that case report, doctor
Speaker 1: Hunger was awarded the two thousand and nine ich Noble Prize,
Speaker 1: which he happily accepted, saying, quote, after about sixty years
Speaker 1: of knuckle cracking to prove that it does not cause arthritis,
Speaker 1: perhaps I deserve some sort of award end quote. Now
Speaker 1: there have been a couple of larger studies. In nineteen
Speaker 1: ninety a couple of doctors from Mount Carmel Mercy Hospital
Speaker 1: in Detroit studied a group of three hundred participants, seventy
Speaker 1: four of which were knuckle crackers. They found that quote,
Speaker 1: there was no increased preponderance of arthritis of the hand
Speaker 1: in either group. However, habitual knuckle crackers were more likely
Speaker 1: to have hand swelling and lower grip strengths. Habitual knuckle
Speaker 1: cracking was associated with manual labor, biting off the nails, smoking,
Speaker 1: and drinking alcohol end quote. And In nineteen seventy five,
Speaker 1: two medical researchers from the University of son Kelvia, Bornia
Speaker 1: published the study in which they surveyed twenty eight elderly
Speaker 1: patients about eighty years old and twenty eight kids about
Speaker 1: eleven years old. They found that just as many kids
Speaker 1: cracked their knuckles as older people, although they do admit
Speaker 1: that some of the elderly patients couldn't remember if they
Speaker 1: cracked their knuckles. But more importantly, they found that the
Speaker 1: knuckles of the elderly patients who cracked their knuckles weren't
Speaker 1: just as healthy as the knuckles of the elderly patients
Speaker 1: who did not crack their knuckles. They concluded, quote, the
Speaker 1: data fails to support evidence that knuckle cracking leads to
Speaker 1: degenerative changes in the metacarpal falangeal joints. In old age,
Speaker 1: the chief morbid consequence of knuckle cracking would appear to
Speaker 1: be its annoying effect on the observer end quote. Yes,
Speaker 1: knuckle cracking can be annoying, And it turns out there's
Speaker 1: actually a name for this. I just have a side question,
Speaker 1: which is when I hear other people crack your knuckles,
Speaker 1: it gives me that sense of like, ew oh, what
Speaker 1: is that.
Speaker 4: Yeah, there's a condition called misphonia, and it's kin to
Speaker 4: the fingers on a chalkboard response, yeah, and sort of
Speaker 4: being grossed out by body horror, that sort of thing.
Speaker 4: And the idea I think is your brain might interpret
Speaker 4: the popping of bones and flesh as being something that is,
Speaker 4: from an evolutionary perspective, something to avoid, uh huh, And
Speaker 4: so you may be having a response to that.
Speaker 1: Oh wow, it's tapping into like a very primitive reflex
Speaker 1: in the brain.
Speaker 4: Maybe again, you know, there's no literature on this, you know,
Speaker 4: it's really interesting that what it does point out there's
Speaker 4: this aspect of the physics of how knuckles pop. There's
Speaker 4: the sound that comes from it.
Speaker 1: And that brings us to the last way we will
Speaker 1: answer the question why do our knuckles cracked, which is
Speaker 1: by digging into the physics of what's going on. What's
Speaker 1: actually making that cracking sound? Is it your bones grinding?
Speaker 1: Is it something actually popping inside of your joints. When
Speaker 1: we come back, we'll talk to a Harvard scientist who
Speaker 1: thinks they finally figured out the real reason our knuckles
Speaker 1: make that sound, and it's not what you think. So
Speaker 1: stay with us. We'll be right back. Hey, welcome back.
Speaker 1: We're cracking the mystery of why we crack our knuckles,
Speaker 1: and so far we've learned it's related to the kind
Speaker 1: of joints we have and that it can be a
Speaker 1: compulsion for some people. Now the question is what actually
Speaker 1: makes that cracking sound. As it turns out, it's been
Speaker 1: a mystery in the scientific community for over one hundred years.
Speaker 1: Here to tell us about it is doctor Vannie Suja,
Speaker 1: a researcher at the Vis Institute at Harvard University who
Speaker 1: specializes in bubble physics. Well, thank you, doctor Susjah for
Speaker 1: joining us.
Speaker 3: First, my pleasure and it's great to see you.
Speaker 1: Okay, my first question is do you crack your knuckles.
Speaker 3: Oh, I do. And one of these people who has
Speaker 3: this compulsive disorder, you know, I cracked my knuckles whenever
Speaker 3: we're I'm frustrated when something doesn't work, and that's more
Speaker 3: days in my life than usual.
Speaker 4: Right.
Speaker 1: Can we hear your knuckles cracking right now?
Speaker 3: Yeah? Let me try.
Speaker 1: Did you hear some Yeah that was pretty fun. Yeah,
Speaker 1: that's really dense. Now. In twenty eighteen, doctor Suja was
Speaker 1: the lead author of the paper titled A Mathematical Model
Speaker 1: of the Sounds Produced by Knuckle Cracking, in which they
Speaker 1: claim to finally settle the debate of what actually makes
Speaker 1: the that happens when we crack our knuckles. It's a
Speaker 1: debate that, according to doctor Suja, has been going on
Speaker 1: for over one hundred years. Tell us the history of
Speaker 1: trying to figure out where the sound comes from.
Speaker 3: Yeah, we were able to trace it back to at
Speaker 3: least the early nineteen hundreds, where there's actual published scientific
Speaker 3: literature on people saying, look, not everybody can crack their knuckles,
Speaker 3: so people kind of like document their way back.
Speaker 1: According to doctor Suja, one breakthrough came in nineteen forty
Speaker 1: seven to doctors from the Saint Thomas Hospital Medical School
Speaker 1: in London decided to do experiments on actual fingers, measuring
Speaker 1: how much tension was needed and how far apart the
Speaker 1: joints needed to be to create a crack, and they
Speaker 1: realized that it might have something to do with how
Speaker 1: sudden the joint snaps.
Speaker 3: They said, like, it has to do with how quickly
Speaker 3: the joints are being separated. There's something there and they
Speaker 3: kind of concluded that when you do this so fast,
Speaker 3: you know, could be vibrations and a tissue and that
Speaker 3: could be the sound.
Speaker 1: But then in nineteen seventy one, another group of British
Speaker 1: scientists for the University of Leads decided to make a
Speaker 1: plastic model of a joint, put some fluid in it,
Speaker 1: and they use the high speed camera to see what
Speaker 1: happens when you pop the joint.
Speaker 3: They actually build a kind of mockup model of the
Speaker 3: tissue and they could see some bubble activity happening as
Speaker 3: they were pulling about this joint and they said it's cavitation.
Speaker 1: And here we get to what scientists think is the
Speaker 1: culprit makes the knuckle cracking sound. Cavitation. Cavitation is what
Speaker 1: happens when you suddenly get bubbles inside of a liquid. Now,
Speaker 1: one way to get bubbles inside a liquid is to
Speaker 1: boil it. When you heat water on a stove, the
Speaker 1: water molecules at the bottom get hot and they turn
Speaker 1: to steam, which is a gas, and so they form bubbles.
Speaker 1: But another way to turn water to gas is to
Speaker 1: lower the pressure. If you put a glass of water
Speaker 1: in a vacuum chamber, you'll start to bubble and boil,
Speaker 1: even if it's a room temperature. It's sort of like
Speaker 1: when you open a can of soda.
Speaker 3: The best anology I can draw is like opening a
Speaker 3: soda bottle. It's pressureized, right, and like you open the cap,
Speaker 3: you're suddenly changing the pressure. And what that leads to
Speaker 3: is sort of bubbles coming out.
Speaker 1: And so here's what those scientists in nineteen seventy one
Speaker 1: thought was happening. When you're cracking your knuckles, you're bending
Speaker 1: your joints, but at some point you can over bend
Speaker 1: them and that pulls the two bones apart. But there's
Speaker 1: fluid in between them, and so that fluid gets stretched,
Speaker 1: which causes the pressure to drop and you form bubbles. Now,
Speaker 1: the scientists in nineteen seventy one saw the bubbles in
Speaker 1: their model and they thought, ah, that's what's making the
Speaker 1: cracking sound. It's the bubbles from cavitation popping.
Speaker 3: This is sort of like the rapid formation and collapse
Speaker 3: of bubbles that had been known to like damage propellers
Speaker 3: with these bubbles, and that ended up being like the
Speaker 3: state of the art for a long time. And remember
Speaker 3: growing up and like it whenever somebody asked me what
Speaker 3: causes the sound of it's a gavitation, because that used
Speaker 3: to be the thing.
Speaker 1: As doctor Suji said, this is what people thought caused
Speaker 1: knuckle cracking for a long time. It's the bubbles from
Speaker 1: cavitation in the joint popping, right, that makes sense. But
Speaker 1: then about forty years later, in twenty fifteen, a group
Speaker 1: of Canadian and Australian engineers and doctors decided to get
Speaker 1: more high tech and they used an MRI machine to
Speaker 1: see what was going on inside of an actual knuckle.
Speaker 3: They actually did the first imaging study. They used MRI
Speaker 3: to look inside the knuckles and as we said before,
Speaker 3: MRA is slow, definitely cannot see what's happening at the
Speaker 3: moment these sounds are coming. But what they could see
Speaker 3: was at slow enough time points, they could actually see
Speaker 3: the bubble there, oh, and it persisted.
Speaker 1: What the Canadian engineers and doctors saw was that even
Speaker 1: after the knuckle had made the cracking sound, there were
Speaker 1: still bubbles. The bubbles hadn't popped. But if the bubbles
Speaker 1: hadn't popped, what made the cracking sound?
Speaker 3: And that gave them an interesting idea. So they said, look,
Speaker 3: there's bubbles still in there. Maybe it's not a collapse,
Speaker 3: but it is a formation of these bubbles that could
Speaker 3: be the source of the sound. So that suddenly opened up,
Speaker 3: you know, more questions, and the flood gates of questions,
Speaker 3: and suddenly, like the knuckle cracking was in the debdfield again.
Speaker 1: Yes, the entire knuckle cracking research field was thrown into
Speaker 1: this array. Okay, to be honest, there aren't that many
Speaker 1: people looking into this. I mean, it's not like the
Speaker 1: government and foundations are pouring billions of dollars to figure
Speaker 1: out knuckle cracking. But still it became a topic of
Speaker 1: debate again. Was knuckle cracking caused by bubbles forming in
Speaker 1: the liquid inside the knuckle joints or was it caused
Speaker 1: by the bubbles popping. That's when doctor Sujah entered the picture.
Speaker 3: So this is more than a decade back. And I
Speaker 3: was in Frans as a master's student.
Speaker 2: You know.
Speaker 3: I was in this class of a wonderful professor by
Speaker 3: the name of Abdel Barka, and he said, I have
Speaker 3: two weeks to figure out an interesting problem in buying mechanics,
Speaker 3: and sought okay. And I'm like sitting in frustration, kind
Speaker 3: of cracking my knuckles, and Sally like, oh, this seems
Speaker 3: to be an interesting problem. You knows buying mechanics in
Speaker 3: there and read more about it, and there's flood mechanics
Speaker 3: in there, there's bubble physics, and I got all excited.
Speaker 1: What doctor Suja did was create the most accurate mathematical
Speaker 1: model of the physics of joint bubble formation that had
Speaker 1: ever been done. Okay, let me through what you did.
Speaker 1: So in a computer you wrote up a simulation of
Speaker 1: what we ended.
Speaker 3: Up modeling three critical events mathematically. So one is how
Speaker 3: the pressure is changing, and there's a so called lubrication
Speaker 3: theory that's in fluid mechanics we can leverage to predict
Speaker 3: how fluid pressure changes when there's motion in a small
Speaker 3: enough gap, so we use that. And second, there's a
Speaker 3: really intriguing theory that basically describes what happens to a
Speaker 3: bubble in the fluid where the pressure is changing. So
Speaker 3: we know how the pressure change is quantitatively, we know
Speaker 3: how this bubble is going to respond to that pressure,
Speaker 3: and finally we wander to link this pressure to the
Speaker 3: sound we hear. So there's another set of equations that
Speaker 3: go from this pressure oscillations to audible sound. So three equations.
Speaker 1: Okay, the details here get a little technical. It turns
Speaker 1: out that there are several ways that the cavitation bubbles
Speaker 1: in the joint can be made, either from the liquid
Speaker 1: being pulled apart or the liquid being stretched by friction.
Speaker 1: And there's also what happens after the joint gets pulled apart,
Speaker 1: which is that the pressure goes up again because flood
Speaker 1: rushes in to fill the gap. But the point is
Speaker 1: that doctor Suja modeled all of this and he simulated
Speaker 1: the sound the bubbles would make, and then he compared
Speaker 1: it to the sound of actual people cracking their knuckles.
Speaker 1: And then you tested this by bringing people into an
Speaker 1: antichoic chamber and having them crack their knuckles. Yes, how
Speaker 1: did you find people who could crack their knuckles?
Speaker 3: A lot of people who love cracking knuckles and like
Speaker 3: helping out for the name of science. So you know,
Speaker 3: it was at the university, and there's a lot of
Speaker 3: frustrated pieces. Yes, we did briefly advertise this study, but
Speaker 3: you know, mostly what of mouth as he put a
Speaker 3: microphone right next to somebody's knuckle.
Speaker 1: Yeah, that's precisely.
Speaker 3: What of those like we had people set in like
Speaker 3: a recording studio and have people come in and crack
Speaker 3: the knuckles close to a microphone.
Speaker 1: Did you have them work on their thesis so that
Speaker 1: they would get frustrated?
Speaker 3: That wou'd have been a good point. I know we
Speaker 3: could have gotten more volunteers that way.
Speaker 1: So doctor Suja compared the frequency signature the sound from
Speaker 1: his mathematical simulation to the signature of real knuckle cracks,
Speaker 1: and they matched. But here's the thing. In doctor Suji's simulation,
Speaker 1: the sound didn't come from either the bubbles forming or
Speaker 1: the bubbles popping.
Speaker 3: And what our modeling work showed was you could still
Speaker 3: have a collapsing bubble and produce sounds, but the bubble
Speaker 3: need not collapse completely for the sounds to happen. You
Speaker 3: don't need the bubble to go to a size of zero,
Speaker 3: but it could be like starting with something big, going
Speaker 3: to ten times smaller and stop there. And still that
Speaker 3: partial change in size was sufficient to explain the sounds. Okay, okay,
Speaker 3: so you can still see the bubble after knuckle cracking.
Speaker 1: What doctor Suji's model showed was that the popping sound
Speaker 1: actually comes from the bubbles shrinking, not when they form,
Speaker 1: not when they pop, when the bubbles shrink, And that
Speaker 1: seems to be the best explanation we have about where
Speaker 1: the knuckle cracking sound comes from. All right, and say,
Speaker 1: you've helped partially resolve this big mystery that's been sitting
Speaker 1: around for over fifty years.
Speaker 3: Partially.
Speaker 1: Yeah.
Speaker 3: The cool thing is we were able to like kind
Speaker 3: of say, maybe both of you guys can be right
Speaker 3: the nineteen seventy one and twenty fifteen, and there's a
Speaker 3: way to go forward from there.
Speaker 1: And as a bonus, doctor Sugis's model actually predicts why
Speaker 1: some people can crack their knuckles and some people can't.
Speaker 3: And a good thing about mathematical moral and a lot
Speaker 3: of marveling for this reasons. You can quantitatively say how
Speaker 3: certain parameters and this problem in fact the sound. For example,
Speaker 3: what if you have a different joint spacing. What if
Speaker 3: you pull the join a little bit harder?
Speaker 2: Right?
Speaker 3: And why can't some people crack their knuckles? The model
Speaker 3: says they are the only spacing is too large. You
Speaker 3: won't be able to generate precious low enough to cause
Speaker 3: the bubbles to form an oslate in a way that
Speaker 3: can produce.
Speaker 1: The sound amazing? Has this changed how you see your
Speaker 1: knuckles or when you crack your nuckles? Do you think
Speaker 1: about it differently now?
Speaker 3: Yes?
Speaker 4: And no.
Speaker 1: Could you go back to your mother and say, see, mom,
Speaker 1: every time I cracked my knuckles, that was good for something? Yeah?
Speaker 3: And the response I would get is like, do you
Speaker 3: have no other, you know, important scientific problem to think about?
Speaker 1: It's never enough, is it right? Exactly? Yeah? It's like,
Speaker 1: why aren't you curring cancer? Why are you trying to
Speaker 1: figure out knuckle cracking?
Speaker 3: Do you have too much time on your plate?
Speaker 1: Stop cracking your knuckles and get to work on something important.
Speaker 2: Yeah?
Speaker 1: All right. Well to recap as annoying as someone cracking
Speaker 1: their knuckles is, it's all because we have these fluid
Speaker 1: filled joints. Let us do amazing movements like walking and
Speaker 1: running and climbing. It's not issarily harmful to crack your knuckles,
Speaker 1: even if you do it compulsively, and hey, it could
Speaker 1: end up getting you a PhD and a job at Harvard.
Speaker 1: You just have to knuckle down. Thanks for joining us,
Speaker 1: see you next time. Hey, a big shout out to
Speaker 1: doctor Gage Crump. We also interviewed about the evolution of
Speaker 1: sinolial joints and to our editor Rose, who can't stand
Speaker 1: people cracking their knuckles, so the fact that she edited
Speaker 1: this episode is a huge active bravery and commitment. Thanks
Speaker 1: to Rose. Oh and also, this week is Brain Awareness Week,
Speaker 1: so if you're interested in learning more about the brain,
Speaker 1: check out my book with doctor Dwayne Godwin, Out of
Speaker 1: Your Mind, available wherever books are sold. You've been listening
Speaker 1: to Science Stuff the production of iHeartRadio, written and produced
Speaker 1: by me or Y Champ, candited by Rose Seguda, executive
Speaker 1: producer Jerry Rowland, audio engineer and mixer Ksey Pegram, and
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Speaker 1: Thanks a lot,