Why do your knuckles pop? - Eleanor Nelsen

Summary

The video explores the science behind the popping sound of joints, commonly heard during activities like knuckle cracking. It explains that synovial fluid, which cushions joints, contains dissolved gases that form bubbles when joints are stretched. The popping sound occurs when these bubbles are created and collapse. While the habit may make joints feel more flexible, myths about it causing arthritis have been debunked, notably by Dr. Donald Unger's 50-year experiment. However, joint cracking may weaken grip strength and annoy others, but there's no significant health risk.

Takeaways

• 🤔 The crackling sound of joints can be seen as relief by some or an unpleasant habit by others.
• 🦴 Joint popping occurs due to various factors, including ligament stretching and, in extreme cases, bones grinding.
• 💥 The most common reason for the popping sound is the formation of gas bubbles in the synovial fluid between joints.
• 👐 Fingers are the easiest joints to crack, but other joints like the neck, back, hips, and wrists can also be cracked.
• 💧 Synovial joints, filled with viscous synovial fluid, cushion bones and help them glide smoothly.
• 🌬️ When joints are stretched, the space between bones expands, causing gas bubbles to form and pop due to lower pressure.
• ⏳ The bubbles collapse, and the gases slowly dissolve back into the synovial fluid over about 20 minutes, explaining why the same joint can't be cracked immediately.
• 🧪 Some scientists believe there may be two popping sounds: one when the bubble forms and another when it bursts.
• 🧐 Dr. Donald Unger's experiment of cracking one hand's knuckles for 50 years found no arthritis, disproving the myth.
• ⚠️ Cracking joints may lead to minor effects like swelling or weaker grip, but the biggest risk is irritating others around you.

Q & A

What is the most common explanation for the popping sound when you crack your joints?

The most common explanation is that the popping sound comes from bubbles in the synovial fluid, which form and then collapse when you stretch the joint, creating the characteristic popping noise.

Why are joints like knuckles and vertebrae easy to crack?

These joints are synovial joints, which are highly flexible and filled with synovial fluid. This fluid contains dissolved gases that form bubbles when the joint is stretched, making them easy to crack.

What is synovial fluid, and what role does it play in joint movement?

Synovial fluid is a viscous liquid found in synovial joints. It lubricates the joints, cushions the bones, and helps them glide smoothly past each other. It also contains molecules like hyaluronic acid and cells that clean up bone or cartilage debris.

How does stretching a joint create a popping sound?

When you stretch a joint, the space between the bones increases, creating a low-pressure zone. This causes dissolved gases in the synovial fluid to form a bubble, which pops and creates the sound.

Why does it take about 20 minutes before you can crack the same joint again?

After the initial pop, the gases from the bubble disperse throughout the synovial cavity and slowly dissolve back into the fluid over about 20 minutes, which is why you need to wait before you can crack the same joint again.

Is cracking your joints harmful or linked to arthritis?

There is no proven link between cracking joints and arthritis. Dr. Donald Unger's experiment of cracking only his left-hand knuckles for 50 years showed no difference between the cracked and uncracked hands in terms of arthritis.

What are some potential negative effects of cracking your joints?

Some studies suggest that joint cracking might cause swelling in the hands and weaken grip strength. However, the most consistent issue seems to be that it annoys people around you.

What are synovial joints, and why are they significant in the context of joint cracking?

Synovial joints are the most flexible type of joints in the body, found in areas like fingers, hips, and shoulders. They are filled with synovial fluid, which allows the bones to move smoothly and is essential for joint cracking due to the presence of dissolved gases.

What did Dr. Donald Unger’s experiment reveal about the long-term effects of joint cracking?

Dr. Unger cracked the knuckles of his left hand for 50 years, leaving his right hand uncracked. After 36,500 cracks, he found no arthritis in either hand, suggesting that joint cracking does not cause arthritis.

What might explain the temporary feeling of flexibility after cracking a joint?

Cracking a joint temporarily enlarges it, which may explain why people feel their joints are looser or more flexible after cracking them.

Outlines

00:00

🤔 What's That Sound?

The opening paragraph raises a common question: why do joints crack or pop when bent in a certain way? Depending on perspective, this sound could either bring relief or seem disturbing. Scientists suggest several reasons for joint popping, ranging from stretched ligaments to bones grinding against one another, but the most widely accepted explanation involves bubbles forming in the joint fluid.

💥 Why Do Joints Pop?

This paragraph explores the most common reason joints pop, focusing on synovial joints, which are the most flexible in the body. The joints are filled with synovial fluid that acts as a lubricant and contains dissolved gases. When joints are stretched, the bones move apart, creating a low-pressure zone that causes gas bubbles to form and pop, resulting in the familiar cracking sound.

🧪 The Science of Bubble Formation

The process of joint popping is compared to the fizzing of soda when opened. Stretching a joint creates a low-pressure area, which causes dissolved gases to form a bubble. This bubble doesn’t last long as the surrounding fluid quickly causes it to collapse, and the gas slowly dissolves back into the synovial fluid, explaining why you can’t crack the same joint immediately.

👨‍🔬 The Two Pop Hypothesis

Some scientists suggest there might be two distinct pops when cracking a joint—one when the bubble forms and another when it bursts. This temporary joint enlargement may explain why habitual joint-crackers often report a feeling of increased flexibility or looseness after popping their joints.

⚠️ Myth of Cracking Joints and Arthritis

A common warning is that cracking joints leads to arthritis. Dr. Donald Unger disproved this myth by cracking the knuckles of only one hand over 50 years. His non-cracking hand and cracking hand were both arthritis-free. For his dedication to this peculiar experiment, Dr. Unger was awarded the Ig Nobel Prize.

📝 Potential Risks and Annoyances

Although joint popping is generally harmless, one study suggests that repetitive joint cracking may cause swelling and weaken grip strength. However, the biggest danger seems to be annoying those around you with the habit!

Keywords

💡Synovial Joints

Synovial joints are flexible joints found in various parts of the body, such as the fingers, neck, and back. These joints are filled with a viscous liquid called synovial fluid, which cushions the bones and allows smooth movement. In the video, synovial joints are central to the explanation of why joints pop when stretched, as these joints contain dissolved gases that form bubbles when manipulated.

💡Synovial Fluid

Synovial fluid is a thick, egg-yolk-like liquid found in synovial joints that serves to lubricate the bones, helping them move smoothly and preventing damage. It contains dissolved gas molecules, phagocytic cells, and lubricating molecules like hyaluronic acid. The video explains how stretching the joints creates a low-pressure zone that pulls gases out of the fluid, leading to the formation of bubbles that 'pop' when joints are cracked.

💡Dissolved Gases

Dissolved gases are molecules like carbon dioxide that are mixed into the synovial fluid in synovial joints. When the joints are stretched, the gases are pulled out of the fluid, forming bubbles that pop. This explains the cracking sound when someone cracks their knuckles or other joints, drawing a comparison to the release of gas in carbonated soda.

💡Bubble Formation

Bubble formation refers to the process that occurs when gases dissolved in synovial fluid are released due to a low-pressure zone created by stretching the joint. These gas bubbles pop, producing the characteristic 'crack' sound. The video suggests that this is the most common explanation for joint cracking, comparing it to how bubbles form and pop in soda when the cap is opened.

💡Low-Pressure Zone

A low-pressure zone is created when the space between bones increases as a joint is stretched. This decrease in pressure allows dissolved gases in the synovial fluid to escape, forming a bubble. The video explains that this low-pressure zone is essential for understanding why joints crack, as it triggers the release of gases from the fluid.

💡Phagocytic Cells

Phagocytic cells are found in synovial fluid and play a role in cleaning up debris from bones and cartilage within the joint. These cells remove potentially harmful particles, ensuring that the joint remains healthy. While the video focuses on the bubble-forming process, it also touches on the importance of these cells in maintaining joint health.

💡Hyaluronic Acid

Hyaluronic acid is one of the lubricating molecules present in synovial fluid that helps cushion the bones and promote smooth movement. Its presence in the synovial fluid helps protect the joint during motion. The video mentions hyaluronic acid as part of the explanation for how synovial fluid keeps joints functioning smoothly, contributing to the body's ability to absorb shocks during movement.

💡Crackling Sound

The crackling sound refers to the noise that occurs when bubbles formed by dissolved gases in synovial fluid are released upon stretching a joint. The video explores how this sound is the result of a low-pressure zone pulling gas out of the synovial fluid. The sound is described as both satisfying and stomach-turning, depending on individual perceptions.

💡Knuckle-Cracking Myths

Knuckle-cracking myths are common misconceptions about the long-term effects of cracking joints, particularly the belief that it can cause arthritis. The video references the experiment of Dr. Donald Unger, who cracked the knuckles of only one hand for 50 years, showing that both hands remained arthritis-free. The story highlights that while knuckle-cracking may annoy others, it has not been proven to cause arthritis.

💡Ig Nobel Prize

The Ig Nobel Prize is a parody of the Nobel Prize, awarded for unusual or humorous scientific research. In the video, Dr. Donald Unger received this prize for his self-experimentation to disprove the myth that knuckle-cracking causes arthritis. His study became a lighthearted example of how even quirky scientific inquiries can produce valuable insights.

Highlights

Cracking joints can either provide relief or cause discomfort depending on who you ask.

The most common explanation for joint popping is the formation of gas bubbles in the synovial fluid.

Synovial joints, the most flexible ones in the body, are found in fingers, neck, back, hips, and other areas.

Synovial fluid is viscous and contains molecules like hyaluronic acid and lubricin to cushion bones and allow smooth movement.

Stretching a joint creates a low-pressure zone that pulls dissolved gases out of the synovial fluid, forming a bubble with a popping sound.

The bubbles collapse after formation, and their gases dissolve back into the synovial fluid within 20 minutes.

Some scientists believe that there may be two pops: one when the bubble forms and another when it bursts.

Joint popping may enlarge the joint temporarily, explaining why frequent crackers feel their joints are looser.

There's a widespread myth that cracking joints can lead to arthritis, though no conclusive evidence supports this.

Dr. Donald Unger tested this myth by cracking the knuckles on only one hand for 50 years, resulting in no arthritis.

For his experiment, Dr. Unger won an Ig Nobel Prize, which celebrates quirky and intriguing scientific achievements.

A study suggests that frequent joint cracking can lead to hand swelling and weakened grip strength.

Synovial fluid not only lubricates joints but also contains phagocytic cells that help clean up cartilage and bone debris.

The popping sound in joints is often compared to the fizz of soda when its cap is twisted open.

The biggest downside to cracking joints seems to be irritating people nearby.

Transcripts

What's that sound?

Depending on whom you ask,

the crackle of popping joints is either the sound of sweet relief

or the noxious tones of a stomach-turning habit.

Really, though. What's that sound?

I mean, why does bending your joints in a certain way make them pop like that?

Scientists have offered several explanations,

including rapidly stretching ligaments,

and in severe cases, actual bones grinding against each other.

But the most common explanation

for why your stretched-out joints sound like bubbles popping

is that, well, there are bubbles in there.

The joints in your fingers are the easiest ones to crack,

but many people also crack the joints between vertebrae in their neck and back,

and even their hips, wrists, shoulders and so on.

All these joints are synovial joints,

and they're the most flexible ones in your body.

The space between the two bones is filled with a viscous liquid,

synovial fluid, which contains long, lubricating molecules,

like hyaluronic acid and lubricin.

Synovial fluid is more or less the texture of egg yolk

and its primary purpose is to cushion the bones

and help them glide past each other.

It also contains phagocytic cells

that help clean up any bone or cartilage debris that ends up in the joint.

But the reason it's important for knuckle cracking

is that, like other fluids in your body,

it contains lots of dissolved gas molecules.

Knuckle-crackers know that to get that satisfying pop,

you stretch the joint farther than it normally goes

by bending your fingers backwards, for example.

When you do that, the bones move away from each other.

The space between bones gets bigger,

but the amount of synovial fluid stays constant.

That creates a low-pressure zone

that pulls dissolved gases out of the synovial fluid,

just like the carbon dioxide that fizzes out of soda

when you twist open the cap.

Inside the joint, the escaping gases form a bubble with a pop.

But the bubble doesn't last long.

The surrounding fluid presses on it until it finally collapses.

The bubble's gases scatter throughout the synovial cavity

and slowly dissolve back into the fluid over the course of about twenty minutes,

which is why it can take a while before you can pop the same joint again.

Some scientists think there may actually be two pops.

One when the bubble forms, and another when it bursts.

Popping a joint temporarily enlarges it,

which may be why dedicated knuckle-, neck- and back-crackers

say the habit makes their joints feel looser and more flexible.

But you may have heard from a concerned relative or annoyed officemate

that cracking your joints will give you arthritis.

A doctor named Donald Unger heard this, too.

So, determined to disprove his mother's warnings,

he cracked the knuckles of his left hand repeatedly for 50 years,

while the right-hand knuckles went unpopped.

36,500 cracks later, both hands were arthritis-free.

For this selfless act of devotion to science,

Dr. Unger received an Ig Nobel Prize,

a parody of the Nobel Prize that recognizes wacky,

but weirdly fascinating, scientific accomplishments.

Unger wrote that his results should prompt investigation into other parental beliefs,

like the importance of eating spinach.

The jury's still out on that one.

As for knuckle-cracking,

one study suggests that all that joint stretching

and bubble bursting can cause your hands to swell

and weaken your grip.

But the biggest proven danger seems to be annoying those around you.