March 11, 2026 • 37 mins
Why do our joints snap, crackle and pop? And why can't we stop doing it? Jorge investigates the evolution, psychology and physics of this annoying habit.
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Speaker 1 (00:01):
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
(00:24):
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?
Speaker 2 (00:39):
Hey?
Speaker 1 (00:39):
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,
(01:12):
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
bad for you? And if it is bad, why do
we keep doing it? As it turns out, knuckle cracking
(01:33):
has to do with the type of joint we have
in our bodies. To tell us about this type of joint,
I reached out to doctor Neilima Sharma, a biologist and
researcher at University College London. Well, thank you so much,
Taka Sharma for joining us.
Speaker 2 (01:50):
Sure, that's my pleasure.
Speaker 1 (01:51):
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?
Speaker 2 (02:01):
I guess it may depend on whether you're on the
side of the Atlantic.
Speaker 1 (02:06):
How do you pronounce it?
Speaker 2 (02:08):
I call it a synovial joint?
Speaker 1 (02:10):
Syno jo. We'll go with them. Can you tell us
what is the sinovial joint?
Speaker 2 (02:13):
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
(02:34):
two metal surfaces in a door hinge that are sliding
with respect to each other.
Speaker 1 (02:39):
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.
(03:01):
Another kind of joint that you see in nature and
in some parts of our body are cartilaginous joints.
Speaker 2 (03:08):
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.
Speaker 1 (03:23):
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
(03:45):
imagine we were blobby fish swimming in the ocean and
we had a skeleton, but it was all put together
with basically rubber joints. But then at some point we
started developing this new kind of joint, synovial joint, which
had two moving, rotating parts. Now, if you have two
bones rotating and rubbing against each other, after a while,
(04:07):
they'll wear each other out and get creaky. So a
feature of these sinovial joints is that they have a
gap in between the two bones which is filled with
a lubricant.
Speaker 2 (04:19):
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.
Speaker 1 (04:33):
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.
Speaker 2 (04:46):
So this substance called sinovial fluet, which exists between these
two surfaces that gives our sinoval joints that's lubricating properties.
Speaker 1 (04:57):
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.
Speaker 2 (05:10):
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.
Speaker 1 (05:26):
Really, it's super slippery.
Speaker 2 (05:28):
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.
Speaker 1 (05:46):
You're making me feel guilty, But.
Speaker 2 (05:48):
You do not have to lubricate your joints as much.
They function for sixty years of age if you take
care of them properly.
Speaker 1 (05:55):
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.
Speaker 2 (06:17):
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
operate quickly. So if I'm a predator and I have
(06:40):
a joint that moves early fast, I am definitely able
to cash more prey and I'm also able to evade predators.
So they must definitely have given animals and edge over
other animals who did not have these joints. And maybe
that's why they spread so fast that most vertebrates that
we'd know now do have a lot of signe joints.
Speaker 1 (07:00):
Oh, that could have been their secret weapon to take
over the world.
Speaker 2 (07:05):
Yeah, absolutely, WHOA, this is your speculation, but definitely interesting speculation.
Speaker 1 (07:12):
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.
(07:33):
Now it's interesting to think about what exactly prompted vertebrates
to develop this type of joint. According to doctor Sharma,
the conventional wisdom for at least the last hundred years
or so has been that vertebrates evolved this kind of
joint when they moved from the ocean to land. The
idea is that sinovial joints are better for standing and walking.
Speaker 2 (07:54):
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.
Speaker 1 (08:08):
Because cartilaginist joints are unstable.
Speaker 2 (08:11):
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.
Speaker 1 (08:18):
I see.
Speaker 2 (08:19):
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
(08:40):
that she's the lead author in a recent study that
says this is not true.
Speaker 1 (08:46):
So this is a paper.
Speaker 2 (08:47):
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.
Speaker 1 (08:57):
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
(09:20):
to doctor Sharma for two reasons. And they're going with
me here for a second. I promise it'll be worth it.
That date four hundred million years ago is before animals
moved from the ocean to land. So the reason we
have synovial joints is not so we could stand up.
But wait, if that's not the reason, then what is well,
(09:43):
four hundred million years ago. It's also the time when
jaws were first developed. That's right, before four hundred million
years ago, fishes just kind of had a circular mouth,
sort of like lampreeze or leeches. But then four hundred
million years ago, vertebrates evolved jaws that could open and close,
(10:03):
and that could bite and chomp and prey. And this
brings up the crazy idea that maybe the reason we
evolved synovial joints, and therefore the reason we can crack
our knuckles, could have been to eat better. Okay, to
be fair, the evolution of sinnovial joints and the evolution
of jaws happening at the same time could just be
(10:24):
a coincidence, and there are sort of examples of old
fish that maybe have sinobial joints in their fins and
not their jaws. But still it does point to one
possible answer to the question why do we crack our knuckles.
Here's my hypothesis that knuckle cracking is a byproduct of
developing these kinds of joints so that we could move
(10:46):
faster and be better at eating and moving and running
and swimming, and eventually walking and running it's like it's
something we can't avoid if we wanted to have this
kind of joint.
Speaker 2 (10:55):
Yeah, I guess not. It is definitely a byproduct of
the evolution of these kind of joints and an interesting
vibe product.
Speaker 1 (11:04):
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?
Speaker 3 (11:16):
Yes?
Speaker 1 (11:17):
Oh my goodness. Oh well, it's really loud.
Speaker 2 (11:24):
To be honest, I don't think I've done this in
the last five years. Oh really, Yeah, I do have
because usually.
Speaker 1 (11:31):
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
(11:53):
knuckles is that for you. So stay with us. We'll
be right cracked, but I mean we'll be right back. Hey,
we'll come back we're talking about cracking our knuckles, and
(12:15):
so far we've learned the reason we can crack our
knuckles is that it's a feature that came with an
upgrade in the joints of our early vertebrate ancestors. Those
are the joints in your body are what are called
sinovial or synovial joints, and they're pretty good. Here. Try this,
if you can swing your arms around in a circle
(12:36):
and open and close the fingers in your hand. Isn't
it incredible how easy and smooth it is to do that,
and be amazed at what a wide range of motion
each of your joints has. Well, it's all due to
signovial joints, but sometimes they do crack. We'll get to
the physics and the fluid dynamics of why that happens.
(12:59):
But first I thought we could answer the question of
why we crack our knuckles from a psychological point of view,
Why are we compelled to crack our knuckles and why
does it annoy certain people? To dig into the mind
of the knuckle cracker, I reached out to doctor Dwayne Godwin,
a psychologist and neuroscientist at Wake Forest University and the
co author with me of the book Out of Your Mind. Well,
(13:23):
thanks doctor Godwin for joining us again.
Speaker 4 (13:25):
Hey or hey, how are you doing.
Speaker 1 (13:27):
I'm great. I'm cracking my knuckles here.
Speaker 4 (13:29):
Oh are you?
Speaker 1 (13:31):
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.
Speaker 4 (13:38):
It's not very impressive.
Speaker 1 (13:40):
Oh oh boy, that was pretty loud.
Speaker 2 (13:42):
Yeah.
Speaker 4 (13:42):
Oh it's a microphone.
Speaker 1 (13:46):
The first thing I asked doctor Cockwin was what does
psychologists think of knuckle cracking? And apparently the answer is
they don't.
Speaker 2 (13:56):
Yeah.
Speaker 4 (13:56):
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
(14:18):
with a drug, so it's harder to study in that way.
Speaker 1 (14:21):
I see, it's not high in the priority list of
the nih or National Science Foundation.
Speaker 4 (14:26):
You know, if you're sort of balancing things out, Oh am,
I going to look at brain tumors versus knuckle cracking.
Speaker 1 (14:33):
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.
Speaker 4 (14:53):
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,
(15:15):
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.
Speaker 1 (15:27):
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.
Speaker 4 (15:39):
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
a lot, makes them fast and low effort. You don't
(16:02):
want to consciously replan every step of tying your shoes
or unlocking your phone or driving a familiar route, and
so the brain chunks those behaviors into packages that can
run with minimal supervision.
Speaker 1 (16:16):
I see, it's like the brain is made to put
certain things into that category of automatic behavior.
Speaker 4 (16:22):
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
as you were making a swing. If you don't somehow
(16:44):
make that more automatic, then you get into issues like choking,
because that is kind of overthinking those kinds of behaviors.
Speaker 1 (16:53):
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.
Speaker 4 (17:05):
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
(17:28):
a relief, and relief in some sense can be rewarding
related to this sort of knuckle cracking behavior.
Speaker 1 (17:34):
Like if it somehow feels good, then your brain will
want to keep doing it.
Speaker 4 (17:39):
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.
Speaker 1 (18:03):
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.
Speaker 4 (18:22):
What distinguishes a habit from a compulsion. As a habit,
you identify it, you know it, and you can stop it.
Speaker 1 (18:29):
Right.
Speaker 4 (18:29):
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.
Speaker 1 (18:46):
I see.
Speaker 4 (18:46):
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.
Speaker 1 (18:54):
I see.
Speaker 4 (18:55):
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.
Speaker 1 (19:04):
You'd call him a real knucklehead.
Speaker 4 (19:05):
Yeah, or a crackhead. Knuckle crack head.
Speaker 1 (19:08):
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
(19:30):
the longest running experiment on the subject of knuckle cracking
since he was a kid. Doctor Unger had the habit
of cracking the knuckles on his left hand, but only
on his left hand, not his right hand, and he
did it at least twice a day. Fifty years later,
in a somewhat tongue in cheek case report in the journal,
(19:50):
he was happy to report that after about thirty six
five hundred knuckle crackings, neither of his hands head arthritis,
and that there was no notice difference in the appearance
or health of both hands. For that case report, doctor
Hunger was awarded the two thousand and nine ich Noble Prize,
which he happily accepted, saying, quote, after about sixty years
(20:14):
of knuckle cracking to prove that it does not cause arthritis,
perhaps I deserve some sort of award end quote. Now
there have been a couple of larger studies. In nineteen
ninety a couple of doctors from Mount Carmel Mercy Hospital
in Detroit studied a group of three hundred participants, seventy
four of which were knuckle crackers. They found that quote,
(20:36):
there was no increased preponderance of arthritis of the hand
in either group. However, habitual knuckle crackers were more likely
to have hand swelling and lower grip strengths. Habitual knuckle
cracking was associated with manual labor, biting off the nails, smoking,
and drinking alcohol end quote. And In nineteen seventy five,
(20:57):
two medical researchers from the University of son Kelvia, Bornia
published the study in which they surveyed twenty eight elderly
patients about eighty years old and twenty eight kids about
eleven years old. They found that just as many kids
cracked their knuckles as older people, although they do admit
that some of the elderly patients couldn't remember if they
(21:18):
cracked their knuckles. But more importantly, they found that the
knuckles of the elderly patients who cracked their knuckles weren't
just as healthy as the knuckles of the elderly patients
who did not crack their knuckles. They concluded, quote, the
data fails to support evidence that knuckle cracking leads to
degenerative changes in the metacarpal falangeal joints. In old age,
(21:42):
the chief morbid consequence of knuckle cracking would appear to
be its annoying effect on the observer end quote. Yes,
knuckle cracking can be annoying, And it turns out there's
actually a name for this. I just have a side question,
which is when I hear other people crack your knuckles,
it gives me that sense of like, ew oh, what
(22:05):
is that.
Speaker 4 (22:05):
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,
(22:26):
from an evolutionary perspective, something to avoid, uh huh, And
so you may be having a response to that.
Speaker 1 (22:32):
Oh wow, it's tapping into like a very primitive reflex
in the brain.
Speaker 4 (22:38):
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.
Speaker 1 (22:52):
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
(23:13):
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.
(23:43):
We're cracking the mystery of why we crack our knuckles,
and so far we've learned it's related to the kind
of joints we have and that it can be a
compulsion for some people. Now the question is what actually
makes that cracking sound. As it turns out, it's been
a mystery in the scientific community for over one hundred years.
(24:05):
Here to tell us about it is doctor Vannie Suja,
a researcher at the Vis Institute at Harvard University who
specializes in bubble physics. Well, thank you, doctor Susjah for
joining us.
Speaker 3 (24:18):
First, my pleasure and it's great to see you.
Speaker 1 (24:21):
Okay, my first question is do you crack your knuckles.
Speaker 3 (24:24):
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.
Speaker 4 (24:35):
Right.
Speaker 1 (24:35):
Can we hear your knuckles cracking right now?
Speaker 3 (24:37):
Yeah? Let me try.
Speaker 1 (24:41):
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
(25:03):
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.
Speaker 3 (25:13):
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.
Speaker 1 (25:26):
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
(25:47):
sudden the joint snaps.
Speaker 3 (25:51):
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.
Speaker 1 (26:03):
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.
Speaker 3 (26:18):
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.
Speaker 1 (26:31):
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
(26:53):
to steam, which is a gas, and so they form bubbles.
But another way to turn water to gas is to
lower the pressure. If you put a glass of water
in a vacuum chamber, you'll start to bubble and boil,
even if it's a room temperature. It's sort of like
when you open a can of soda.
Speaker 3 (27:13):
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.
Speaker 1 (27:26):
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,
(27:47):
the scientists in nineteen seventy one saw the bubbles in
their model and they thought, ah, that's what's making the
cracking sound. It's the bubbles from cavitation popping.
Speaker 3 (28:00):
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.
Speaker 1 (28:19):
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
(28:41):
see what was going on inside of an actual knuckle.
Speaker 3 (28:46):
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
(29:06):
the bubble there, oh, and it persisted.
Speaker 1 (29:10):
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?
Speaker 3 (29:26):
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.
Speaker 1 (29:48):
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
(30:09):
the liquid inside the knuckle joints or was it caused
by the bubbles popping. That's when doctor Sujah entered the picture.
Speaker 3 (30:18):
So this is more than a decade back. And I
was in Frans as a master's student.
Speaker 2 (30:23):
You know.
Speaker 3 (30:23):
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
(30:44):
in there, there's bubble physics, and I got all excited.
Speaker 1 (30:48):
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.
Speaker 3 (31:04):
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
(31:26):
bubble in the fluid where the pressure is changing. So
we know how the pressure change is quantitatively, we know
how this bubble is going to respond to that pressure,
and finally we wander to link this pressure to the
sound we hear. So there's another set of equations that
go from this pressure oscillations to audible sound. So three equations.
Speaker 1 (31:47):
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
rushes in to fill the gap. But the point is
(32:07):
that doctor Suja modeled all of this and he simulated
the sound the bubbles would make, and then he compared
it to the sound of actual people cracking their knuckles.
And then you tested this by bringing people into an
antichoic chamber and having them crack their knuckles. Yes, how
did you find people who could crack their knuckles?
Speaker 3 (32:29):
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.
Speaker 1 (32:48):
Yeah, that's precisely.
Speaker 3 (32:49):
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.
Speaker 1 (32:56):
Did you have them work on their thesis so that
they would get frustrated?
Speaker 3 (33:00):
That wou'd have been a good point. I know we
could have gotten more volunteers that way.
Speaker 1 (33:05):
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.
Speaker 3 (33:26):
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
(33:48):
partial change in size was sufficient to explain the sounds. Okay, okay,
so you can still see the bubble after knuckle cracking.
Speaker 1 (33:57):
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,
(34:18):
you've helped partially resolve this big mystery that's been sitting
around for over fifty years.
Speaker 3 (34:23):
Partially.
Speaker 1 (34:24):
Yeah.
Speaker 3 (34:24):
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.
Speaker 1 (34:32):
And as a bonus, doctor Sugis's model actually predicts why
some people can crack their knuckles and some people can't.
Speaker 3 (34:40):
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?
Speaker 2 (34:55):
Right?
Speaker 3 (34:56):
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.
Speaker 1 (35:09):
The sound amazing? Has this changed how you see your
knuckles or when you crack your nuckles? Do you think
about it differently now?
Speaker 3 (35:17):
Yes?
Speaker 4 (35:17):
And no.
Speaker 1 (35:17):
Could you go back to your mother and say, see, mom,
every time I cracked my knuckles, that was good for something? Yeah?
Speaker 3 (35:23):
And the response I would get is like, do you
have no other, you know, important scientific problem to think about?
Speaker 1 (35:32):
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?
Speaker 3 (35:38):
Do you have too much time on your plate?
Speaker 1 (35:40):
Stop cracking your knuckles and get to work on something important.
Speaker 2 (35:43):
Yeah?
Speaker 1 (35:45):
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
(36:05):
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
doctor Gage Crump. We also interviewed about the evolution of
sinolial joints and to our editor Rose, who can't stand
people cracking their knuckles, so the fact that she edited
(36:28):
this episode is a huge active bravery and commitment. Thanks
to Rose. Oh and also, this week is Brain Awareness Week,
so if you're interested in learning more about the brain,
check out my book with doctor Dwayne Godwin, Out of
Your Mind, available wherever books are sold. You've been listening
(36:50):
to Science Stuff the production of iHeartRadio, written and produced
by me or Y Champ, candited by Rose Seguda, executive
producer Jerry Rowland, audio engineer and mixer Ksey Pegram, and
you can follow me on social media. Just search for
PhD Comics and the name of your favorite platform. Be
sure to subscribe to Sign Stuff on the iHeartRadio app,
(37:11):
Apple Podcasts or wherever you get your podcasts, and please
tell your friends we'll be back next Wednesday with another episode. Hey,
please take a second and leave us a review on
Apple Podcasts, Spotify, or wherever you listen to the podcast.
(37:33):
Thanks a lot,
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
(00:24):
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?
Speaker 2 (00:39):
Hey?
Speaker 1 (00:39):
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,
(01:12):
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
bad for you? And if it is bad, why do
we keep doing it? As it turns out, knuckle cracking
(01:33):
has to do with the type of joint we have
in our bodies. To tell us about this type of joint,
I reached out to doctor Neilima Sharma, a biologist and
researcher at University College London. Well, thank you so much,
Taka Sharma for joining us.
Speaker 2 (01:50):
Sure, that's my pleasure.
Speaker 1 (01:51):
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?
Speaker 2 (02:01):
I guess it may depend on whether you're on the
side of the Atlantic.
Speaker 1 (02:06):
How do you pronounce it?
Speaker 2 (02:08):
I call it a synovial joint?
Speaker 1 (02:10):
Syno jo. We'll go with them. Can you tell us
what is the sinovial joint?
Speaker 2 (02:13):
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
(02:34):
two metal surfaces in a door hinge that are sliding
with respect to each other.
Speaker 1 (02:39):
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.
(03:01):
Another kind of joint that you see in nature and
in some parts of our body are cartilaginous joints.
Speaker 2 (03:08):
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.
Speaker 1 (03:23):
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
(03:45):
imagine we were blobby fish swimming in the ocean and
we had a skeleton, but it was all put together
with basically rubber joints. But then at some point we
started developing this new kind of joint, synovial joint, which
had two moving, rotating parts. Now, if you have two
bones rotating and rubbing against each other, after a while,
(04:07):
they'll wear each other out and get creaky. So a
feature of these sinovial joints is that they have a
gap in between the two bones which is filled with
a lubricant.
Speaker 2 (04:19):
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.
Speaker 1 (04:33):
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.
Speaker 2 (04:46):
So this substance called sinovial fluet, which exists between these
two surfaces that gives our sinoval joints that's lubricating properties.
Speaker 1 (04:57):
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.
Speaker 2 (05:10):
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.
Speaker 1 (05:26):
Really, it's super slippery.
Speaker 2 (05:28):
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.
Speaker 1 (05:46):
You're making me feel guilty, But.
Speaker 2 (05:48):
You do not have to lubricate your joints as much.
They function for sixty years of age if you take
care of them properly.
Speaker 1 (05:55):
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.
Speaker 2 (06:17):
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
operate quickly. So if I'm a predator and I have
(06:40):
a joint that moves early fast, I am definitely able
to cash more prey and I'm also able to evade predators.
So they must definitely have given animals and edge over
other animals who did not have these joints. And maybe
that's why they spread so fast that most vertebrates that
we'd know now do have a lot of signe joints.
Speaker 1 (07:00):
Oh, that could have been their secret weapon to take
over the world.
Speaker 2 (07:05):
Yeah, absolutely, WHOA, this is your speculation, but definitely interesting speculation.
Speaker 1 (07:12):
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.
(07:33):
Now it's interesting to think about what exactly prompted vertebrates
to develop this type of joint. According to doctor Sharma,
the conventional wisdom for at least the last hundred years
or so has been that vertebrates evolved this kind of
joint when they moved from the ocean to land. The
idea is that sinovial joints are better for standing and walking.
Speaker 2 (07:54):
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.
Speaker 1 (08:08):
Because cartilaginist joints are unstable.
Speaker 2 (08:11):
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.
Speaker 1 (08:18):
I see.
Speaker 2 (08:19):
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
(08:40):
that she's the lead author in a recent study that
says this is not true.
Speaker 1 (08:46):
So this is a paper.
Speaker 2 (08:47):
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.
Speaker 1 (08:57):
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
(09:20):
to doctor Sharma for two reasons. And they're going with
me here for a second. I promise it'll be worth it.
That date four hundred million years ago is before animals
moved from the ocean to land. So the reason we
have synovial joints is not so we could stand up.
But wait, if that's not the reason, then what is well,
(09:43):
four hundred million years ago. It's also the time when
jaws were first developed. That's right, before four hundred million
years ago, fishes just kind of had a circular mouth,
sort of like lampreeze or leeches. But then four hundred
million years ago, vertebrates evolved jaws that could open and close,
(10:03):
and that could bite and chomp and prey. And this
brings up the crazy idea that maybe the reason we
evolved synovial joints, and therefore the reason we can crack
our knuckles, could have been to eat better. Okay, to
be fair, the evolution of sinnovial joints and the evolution
of jaws happening at the same time could just be
(10:24):
a coincidence, and there are sort of examples of old
fish that maybe have sinobial joints in their fins and
not their jaws. But still it does point to one
possible answer to the question why do we crack our knuckles.
Here's my hypothesis that knuckle cracking is a byproduct of
developing these kinds of joints so that we could move
(10:46):
faster and be better at eating and moving and running
and swimming, and eventually walking and running it's like it's
something we can't avoid if we wanted to have this
kind of joint.
Speaker 2 (10:55):
Yeah, I guess not. It is definitely a byproduct of
the evolution of these kind of joints and an interesting
vibe product.
Speaker 1 (11:04):
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?
Speaker 3 (11:16):
Yes?
Speaker 1 (11:17):
Oh my goodness. Oh well, it's really loud.
Speaker 2 (11:24):
To be honest, I don't think I've done this in
the last five years. Oh really, Yeah, I do have
because usually.
Speaker 1 (11:31):
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
(11:53):
knuckles is that for you. So stay with us. We'll
be right cracked, but I mean we'll be right back. Hey,
we'll come back we're talking about cracking our knuckles, and
(12:15):
so far we've learned the reason we can crack our
knuckles is that it's a feature that came with an
upgrade in the joints of our early vertebrate ancestors. Those
are the joints in your body are what are called
sinovial or synovial joints, and they're pretty good. Here. Try this,
if you can swing your arms around in a circle
(12:36):
and open and close the fingers in your hand. Isn't
it incredible how easy and smooth it is to do that,
and be amazed at what a wide range of motion
each of your joints has. Well, it's all due to
signovial joints, but sometimes they do crack. We'll get to
the physics and the fluid dynamics of why that happens.
(12:59):
But first I thought we could answer the question of
why we crack our knuckles from a psychological point of view,
Why are we compelled to crack our knuckles and why
does it annoy certain people? To dig into the mind
of the knuckle cracker, I reached out to doctor Dwayne Godwin,
a psychologist and neuroscientist at Wake Forest University and the
co author with me of the book Out of Your Mind. Well,
(13:23):
thanks doctor Godwin for joining us again.
Speaker 4 (13:25):
Hey or hey, how are you doing.
Speaker 1 (13:27):
I'm great. I'm cracking my knuckles here.
Speaker 4 (13:29):
Oh are you?
Speaker 1 (13:31):
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.
Speaker 4 (13:38):
It's not very impressive.
Speaker 1 (13:40):
Oh oh boy, that was pretty loud.
Speaker 2 (13:42):
Yeah.
Speaker 4 (13:42):
Oh it's a microphone.
Speaker 1 (13:46):
The first thing I asked doctor Cockwin was what does
psychologists think of knuckle cracking? And apparently the answer is
they don't.
Speaker 2 (13:56):
Yeah.
Speaker 4 (13:56):
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
(14:18):
with a drug, so it's harder to study in that way.
Speaker 1 (14:21):
I see, it's not high in the priority list of
the nih or National Science Foundation.
Speaker 4 (14:26):
You know, if you're sort of balancing things out, Oh am,
I going to look at brain tumors versus knuckle cracking.
Speaker 1 (14:33):
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.
Speaker 4 (14:53):
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,
(15:15):
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.
Speaker 1 (15:27):
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.
Speaker 4 (15:39):
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
a lot, makes them fast and low effort. You don't
(16:02):
want to consciously replan every step of tying your shoes
or unlocking your phone or driving a familiar route, and
so the brain chunks those behaviors into packages that can
run with minimal supervision.
Speaker 1 (16:16):
I see, it's like the brain is made to put
certain things into that category of automatic behavior.
Speaker 4 (16:22):
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
as you were making a swing. If you don't somehow
(16:44):
make that more automatic, then you get into issues like choking,
because that is kind of overthinking those kinds of behaviors.
Speaker 1 (16:53):
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.
Speaker 4 (17:05):
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
(17:28):
a relief, and relief in some sense can be rewarding
related to this sort of knuckle cracking behavior.
Speaker 1 (17:34):
Like if it somehow feels good, then your brain will
want to keep doing it.
Speaker 4 (17:39):
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.
Speaker 1 (18:03):
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.
Speaker 4 (18:22):
What distinguishes a habit from a compulsion. As a habit,
you identify it, you know it, and you can stop it.
Speaker 1 (18:29):
Right.
Speaker 4 (18:29):
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.
Speaker 1 (18:46):
I see.
Speaker 4 (18:46):
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.
Speaker 1 (18:54):
I see.
Speaker 4 (18:55):
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.
Speaker 1 (19:04):
You'd call him a real knucklehead.
Speaker 4 (19:05):
Yeah, or a crackhead. Knuckle crack head.
Speaker 1 (19:08):
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
(19:30):
the longest running experiment on the subject of knuckle cracking
since he was a kid. Doctor Unger had the habit
of cracking the knuckles on his left hand, but only
on his left hand, not his right hand, and he
did it at least twice a day. Fifty years later,
in a somewhat tongue in cheek case report in the journal,
(19:50):
he was happy to report that after about thirty six
five hundred knuckle crackings, neither of his hands head arthritis,
and that there was no notice difference in the appearance
or health of both hands. For that case report, doctor
Hunger was awarded the two thousand and nine ich Noble Prize,
which he happily accepted, saying, quote, after about sixty years
(20:14):
of knuckle cracking to prove that it does not cause arthritis,
perhaps I deserve some sort of award end quote. Now
there have been a couple of larger studies. In nineteen
ninety a couple of doctors from Mount Carmel Mercy Hospital
in Detroit studied a group of three hundred participants, seventy
four of which were knuckle crackers. They found that quote,
(20:36):
there was no increased preponderance of arthritis of the hand
in either group. However, habitual knuckle crackers were more likely
to have hand swelling and lower grip strengths. Habitual knuckle
cracking was associated with manual labor, biting off the nails, smoking,
and drinking alcohol end quote. And In nineteen seventy five,
(20:57):
two medical researchers from the University of son Kelvia, Bornia
published the study in which they surveyed twenty eight elderly
patients about eighty years old and twenty eight kids about
eleven years old. They found that just as many kids
cracked their knuckles as older people, although they do admit
that some of the elderly patients couldn't remember if they
(21:18):
cracked their knuckles. But more importantly, they found that the
knuckles of the elderly patients who cracked their knuckles weren't
just as healthy as the knuckles of the elderly patients
who did not crack their knuckles. They concluded, quote, the
data fails to support evidence that knuckle cracking leads to
degenerative changes in the metacarpal falangeal joints. In old age,
(21:42):
the chief morbid consequence of knuckle cracking would appear to
be its annoying effect on the observer end quote. Yes,
knuckle cracking can be annoying, And it turns out there's
actually a name for this. I just have a side question,
which is when I hear other people crack your knuckles,
it gives me that sense of like, ew oh, what
(22:05):
is that.
Speaker 4 (22:05):
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,
(22:26):
from an evolutionary perspective, something to avoid, uh huh, And
so you may be having a response to that.
Speaker 1 (22:32):
Oh wow, it's tapping into like a very primitive reflex
in the brain.
Speaker 4 (22:38):
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.
Speaker 1 (22:52):
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
(23:13):
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.
(23:43):
We're cracking the mystery of why we crack our knuckles,
and so far we've learned it's related to the kind
of joints we have and that it can be a
compulsion for some people. Now the question is what actually
makes that cracking sound. As it turns out, it's been
a mystery in the scientific community for over one hundred years.
(24:05):
Here to tell us about it is doctor Vannie Suja,
a researcher at the Vis Institute at Harvard University who
specializes in bubble physics. Well, thank you, doctor Susjah for
joining us.
Speaker 3 (24:18):
First, my pleasure and it's great to see you.
Speaker 1 (24:21):
Okay, my first question is do you crack your knuckles.
Speaker 3 (24:24):
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.
Speaker 4 (24:35):
Right.
Speaker 1 (24:35):
Can we hear your knuckles cracking right now?
Speaker 3 (24:37):
Yeah? Let me try.
Speaker 1 (24:41):
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
(25:03):
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.
Speaker 3 (25:13):
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.
Speaker 1 (25:26):
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
(25:47):
sudden the joint snaps.
Speaker 3 (25:51):
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.
Speaker 1 (26:03):
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.
Speaker 3 (26:18):
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.
Speaker 1 (26:31):
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
(26:53):
to steam, which is a gas, and so they form bubbles.
But another way to turn water to gas is to
lower the pressure. If you put a glass of water
in a vacuum chamber, you'll start to bubble and boil,
even if it's a room temperature. It's sort of like
when you open a can of soda.
Speaker 3 (27:13):
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.
Speaker 1 (27:26):
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,
(27:47):
the scientists in nineteen seventy one saw the bubbles in
their model and they thought, ah, that's what's making the
cracking sound. It's the bubbles from cavitation popping.
Speaker 3 (28:00):
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.
Speaker 1 (28:19):
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
(28:41):
see what was going on inside of an actual knuckle.
Speaker 3 (28:46):
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
(29:06):
the bubble there, oh, and it persisted.
Speaker 1 (29:10):
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?
Speaker 3 (29:26):
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.
Speaker 1 (29:48):
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
(30:09):
the liquid inside the knuckle joints or was it caused
by the bubbles popping. That's when doctor Sujah entered the picture.
Speaker 3 (30:18):
So this is more than a decade back. And I
was in Frans as a master's student.
Speaker 2 (30:23):
You know.
Speaker 3 (30:23):
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
(30:44):
in there, there's bubble physics, and I got all excited.
Speaker 1 (30:48):
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.
Speaker 3 (31:04):
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
(31:26):
bubble in the fluid where the pressure is changing. So
we know how the pressure change is quantitatively, we know
how this bubble is going to respond to that pressure,
and finally we wander to link this pressure to the
sound we hear. So there's another set of equations that
go from this pressure oscillations to audible sound. So three equations.
Speaker 1 (31:47):
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
rushes in to fill the gap. But the point is
(32:07):
that doctor Suja modeled all of this and he simulated
the sound the bubbles would make, and then he compared
it to the sound of actual people cracking their knuckles.
And then you tested this by bringing people into an
antichoic chamber and having them crack their knuckles. Yes, how
did you find people who could crack their knuckles?
Speaker 3 (32:29):
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.
Speaker 1 (32:48):
Yeah, that's precisely.
Speaker 3 (32:49):
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.
Speaker 1 (32:56):
Did you have them work on their thesis so that
they would get frustrated?
Speaker 3 (33:00):
That wou'd have been a good point. I know we
could have gotten more volunteers that way.
Speaker 1 (33:05):
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.
Speaker 3 (33:26):
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
(33:48):
partial change in size was sufficient to explain the sounds. Okay, okay,
so you can still see the bubble after knuckle cracking.
Speaker 1 (33:57):
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,
(34:18):
you've helped partially resolve this big mystery that's been sitting
around for over fifty years.
Speaker 3 (34:23):
Partially.
Speaker 1 (34:24):
Yeah.
Speaker 3 (34:24):
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.
Speaker 1 (34:32):
And as a bonus, doctor Sugis's model actually predicts why
some people can crack their knuckles and some people can't.
Speaker 3 (34:40):
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?
Speaker 2 (34:55):
Right?
Speaker 3 (34:56):
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.
Speaker 1 (35:09):
The sound amazing? Has this changed how you see your
knuckles or when you crack your nuckles? Do you think
about it differently now?
Speaker 3 (35:17):
Yes?
Speaker 4 (35:17):
And no.
Speaker 1 (35:17):
Could you go back to your mother and say, see, mom,
every time I cracked my knuckles, that was good for something? Yeah?
Speaker 3 (35:23):
And the response I would get is like, do you
have no other, you know, important scientific problem to think about?
Speaker 1 (35:32):
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?
Speaker 3 (35:38):
Do you have too much time on your plate?
Speaker 1 (35:40):
Stop cracking your knuckles and get to work on something important.
Speaker 2 (35:43):
Yeah?
Speaker 1 (35:45):
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
(36:05):
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
doctor Gage Crump. We also interviewed about the evolution of
sinolial joints and to our editor Rose, who can't stand
people cracking their knuckles, so the fact that she edited
(36:28):
this episode is a huge active bravery and commitment. Thanks
to Rose. Oh and also, this week is Brain Awareness Week,
so if you're interested in learning more about the brain,
check out my book with doctor Dwayne Godwin, Out of
Your Mind, available wherever books are sold. You've been listening
(36:50):
to Science Stuff the production of iHeartRadio, written and produced
by me or Y Champ, candited by Rose Seguda, executive
producer Jerry Rowland, audio engineer and mixer Ksey Pegram, and
you can follow me on social media. Just search for
PhD Comics and the name of your favorite platform. Be
sure to subscribe to Sign Stuff on the iHeartRadio app,
(37:11):
Apple Podcasts or wherever you get your podcasts, and please
tell your friends we'll be back next Wednesday with another episode. Hey,
please take a second and leave us a review on
Apple Podcasts, Spotify, or wherever you listen to the podcast.
(37:33):
Thanks a lot,
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