The Cause of the Knuckle-Cracking Sound | Hacker News Reader

The Cause of the Knuckle-Cracking Sound

journals.plos.org

74 pointsmhb50 comments

I know there isn't anything to worry about, thanks to that one guy popping his knuckles in one hand for 60 years (quick google: http://geekologie.com/2015/03/taking-one-for-the-team-man-cr...) but the description in this article made me not want to pop mine anymore. :(

There's only nothing to worry about if you don't mind reduced grip strength:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1004074/pdf/annr...

Err... that doesn't look like a proven causal relation to me?

That man is a true scientist.

In short, the knuckle popping sound is more akin to quickly pulling your boot out of sucking mud--with the ensuing "schlorp" sound--than pricking a balloon with a pin and its more acoustically similar "pop". When joints pull apart, the joint fluid is viscous enough that gases cannot immediately rush in to fill the gap. That creates a pressure differential, and when the fluid finally gives way, the rapid formation of a gas bubble makes an audible popping or cracking noise.

Very interesting. In my "old age" (38 next month) I've found that my ankles and knees have begun popping or cracking with regularity. I can rotate my right foot in a circular motion and it will pop just below the top of the ankle on every single downstroke no matter how many times I do it. My knees have begun popping every time I stand up and usually when I stretch them out under a desk or table. Five years ago this didn't happen, and I'm actually more active today than I was back then.

I've often wondered if the "gas bubble" theory was only partially correct, as there are different kinds of pops and cracks with different sounds and sensations (including pain).

In my old age also (36), one of my ankles pops every time going up (not down) stairs. I also, crack my knuckles, fingers, neck numerous times throughout the day. It's more of a habit now, versus "feeling" anything.

I'd love to know, in laymans terms, the repercussions to doing this. As far as I could gather from the article, it doesn't sound like any long-term effects.

One other question I have, though, is why is it "easier" to crack these parts over time? The more I do it, the easier it is to repeat in the future. Am I just creating this space, and making it easier to "fill" with every "distraction", as the article called it?

Dr. Donald Unger won the 2009 Ig Nobel for demonstrating (N=1) that knuckle-cracking doesn't lead to arthritis, by cracking the knuckles of one hand daily for 60 years, and never cracking those of the other.

http://tech.mit.edu/V129/N41/ignobels.html

Like I said, for someone that does this NUMEROUS times a day, that is some EXTREME self-control to be able to only do it to one hand that long.

When I don't crack my knuckles regularly, they start to feel extremely stiff and experience a dull pain. There is no way I'd be able to stop myself from cracking one of my hands for that long. That really is extraordinary self-control.

There was an Ig Nobel winner who did some work on this[1]. A doctor that cracked the knuckled on one hand and one hand only for decades, and found no discernible difference.

1: http://www.scientificamerican.com/article/crack-research/

Something that might be enlightening: taking an mucokinetic—a drug that increases and dilutes mucus, e.g. guaifenesin—increases cracking. Presumably sinovial fluid qualifies as "mucus."

There is some evidence that lack of sideways flexibility in the joints contributes to synovitis in climbers. A common recommendation is to regularly twist and stretch the finger joints like this https://www.youtube.com/watch?v=IMO__OrzM_U . Perhaps cracking the joints has a similar effect.

My ankle does the exact same. My understanding is that for this sort of cracking, where there is no "refractory period" between cracks, the gas bubble thing is not the cause. Rather it might be a tendon "snapping" across a bone, like some sort of bistable system.

I also have another sort of cracking where the refractory period is weeks, rather than ~20 minutes. I suspect this is another phenomenon as well, though I don't know what it might be. I experience this in my wrists, sometimes sternum (perhaps snapping cartilage in the xiphoid process? I really try to avoid this one.)

My elbows also crack when I hyperextend them. The refractory period there is probably somewhere between 20 minutes and an hour, but I'm not convinced that is the gas bubble thing either.

I pop my knuckles, all 3 thumb joints, wrists (by pronation of hand and supination of forearms, or vice versa), elbows (by hyperextension), shoulders, and cervical vertebrae on a regular basis, and occasionally thoracic vertebrae, toes, and hips. I can very rarely manage to pop one knee or the other, or lumbar vertebrae. I have never been able to pop anything in my ankle; there's just too much joint stabilization there. But I can manage the metatarsal-cuboid joints sometimes by twisting the foot from the toes.

If you can snap your ankle every time you move it in a particular way, it is probably not the same thing. I have never been able to pop the same joint twice in rapid succession. Indeed, that's why I pop so many different joints. Once you've done them all, there's no choice but to wait.

I'd certainly be wary of pops emanating from joints without a great deal of natural range of motion, or those so wrapped up in muscles and ligaments it is practically impossible to isolate any one joint in particular. If you experience any pain while joint-popping, stop doing it, immediately.

Wow, how did you manage to do the thoracic vertebrae? I have been trying to achieve that one for years! On the plus side, I was able to master repeated ankle cracking without refractory period - So I haven't exactly wasted my life.

There are two reliable ways to do it. First, you cross your arms over your chest and put your hands on the opposite shoulders. A brawny accomplice then stands behind you and picks you up by your elbows. Your spine is stretched by your own body weight and your accomplice's chest. That is truly satisfying, but I haven't had it done in many years.

Second method, you put your back against a hard, flat surface and push yourself into it. This one may only pop one or two at a time. And you might break your furniture.

this started for me at 14 :( i feel old

> I can rotate my right foot in a circular motion and it will pop just below the top of the ankle on every single downstroke no matter how many times I do it.

I've been doing that, also with my right foot, since high school.

Me too! I thank the fact I played basketball since I was in 3rd grade.

Neat, my right ankle has also done that for years, but on the top of the upstroke! It's done it for years, though lately my left ankle also does it. (I'm 30) once again the internet brings together the mutants that once believed themselves alone.

Mini Ask HN: Do you find it rude to crack one's knuckles in business meetings? I sometimes do this and wonder if it's bad form.

It's kind of rude when talking to someone or in a meeting, otherwise not a big deal.

"Rude" things are 100% superficial, pretty much by definition of rude. (As soon as something becomes an issue in terms of actual consequences, we stop using the word "rude" and starting saying other things: unacceptable, not right, fucked up)

If you do it once or twice then no big deal, but if you do it repeatedly, or a bunch of fingers one right after the other then it's rude.

Some people hate it, so you might want to be aware of that.

Some people find it distracting, like tapping a pen, so you might want to be aware of that too.

I know a woman that cannot stand it. Doing it in internal meetings will get you a beating. If you were to do it in a business meeting where she was the client, she would probably never speak to you again and take the business elsewhere.

In my own experience joints clicking is strongly related to chronic dehydration (abbusive gren teas/strong coffee drinking) coupled with physical activity.

When I avoid coffeinated drinks and related excessive water loss but continuing some active physical exercise the effect is almost disappear. It is "lubrication" issue.)

Unfortunately my girlfriend still hates the sound...

I crack in my back and neck before sleeping. It helps. It may go pretty bad if I already have some kind of pain so I don't do that anymore. I wish I could teach my girlfriend to do it properly so she could fix me but well...she hates it ;)

Given that the cracking is cavitation bubbles, it's kinda surprising that it wouldn't be doing damage. You just have to look at metal ship propellers that are worn from cavitation damage to see how cavitation bubbles can eat away at things.

Consider how many cavitation bubbles a propeller has to deal with in its lifetime, versus a typical knuckle.

I wouldn't discount it so fast, tuna have damage to their tail because of cavitation.

http://www.newscientist.com/article/dn13553-dolphins-swim-so...

Current theory is that cavitation damage is caused by low pressure bubbles imploding in a high-pressure medium.

The pop in your joint is coming from the shockwave of bubble formation and expansion. That energy is diffused around the surface of the bubble. The shockwave of an imploding bubble is focused on a much smaller area. It's like the difference between trying to burn ants in sunlight using a concave versus a convex lens.

Human bodies may have the advantage of being able to heal damage that would normally otherwise slowly accumulate over time. I am not sure what sort of tissue exactly would be potentially damaged by this though, and how well it can heal.

It does do damage:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1004074/pdf/annr...

Crack your knuckles if you cracked your knuckles while reading this story. I cracked mine.

So what does it mean if your joints don't pop? I'm unable to induce it, and it happens very rarely by chance. However, my wife pops her knuckles (and other joints) all the time.

Seems like it would be slightly loser ligaments which allow you to move your joints apart enough to create the empty spaces. Or, possibly, a different tension profile which causes your wife's joints to suddenly open while yours open more gradually.

After trying for a while, I finally 'managed' to do it any time I wanted to, now that I stopped it doesn't happen anymore. I'd second the stretched ligament theory.

tl;dr it's from creating cavities not popping bubbles.

Not creating and popping them, but rather that's the sound of creation.

Thanks. Can you explain it in simpler English please? I'm not native and googling for "cavity" results in pictures of teeth with holes not ankles. If I transfer "hole" to the topic of ankle I don't see the difference between the bubble and the hole theory.

Bubble theory: bubble collapses (hole was there, hole stops being there after collapse).

Hole theory: hole appears, (no hole before cracking, hole forms after cracking).

However, "hole" is usually used when the empty space goes entirely through the object, whereas a cavity means that there is an end/depth to it. For example, digging a hole in a sand is really also (more accurately) digging a cavity.

"Cavity" literally means "an empty space". Bones are hollow but filled with marrow. To create a cavity, you have to stretch the bone so that there isn't enough marrow to fill it up.

;-) Pretty sure we're talking about cavities forming _between_ bones. Stretching a bone out enough to to create a cavity "within the marrow" does not sound appealing in the least (yikes)

Oh yeah? I was wondering about that bit! ;) Must have misread something.

Pretty sure the crack from that would be much louder.

Think about bubbles in a bottle of carbonated soda. Before you open the bottle, the carbonation is a liquid inside of the water because of the pressure inside the bottle. When you open the bottle, the pressure drops and the carbonation converts to a gas and becomes bubbles.

But, imagine if instead of opening the bottle, you just somehow reduced the pressure then quickly increased it back to what it originally was. The carbonation would go from liquid -> gas -> liquid very quickly. CO2 bubbles would form then collapse.

Converting from liquid to gas is not a special property of CO2. There are liquids in your joints that will convert to gas if you drop the pressure. That's what happens for an instant when you crack your knuckles. What you are hearing is the bubbles of gas collapsing as the gas converts back to liquid.

What they actually found in this MRI study is that the bubbles stick around for several minutes, slowly dissolving back into the fluid. They don't immediately collapse as you would expect. The sound seems to come from the very rapid expansion.

The time it takes for the bubbles to dissolve again seems to be responsible for the "cool-down" period that you experience between cracks.

For more awesomeness have a look at this:

http://en.wikipedia.org/wiki/VA-111_Shkval

This will sound weird, but... Have you ever stabbed a big piece of meat with a thick knife? And you know the sort of sucking sound it makes when you extract your knife from the meat? This is similar to what's happening inside your knuckles. The sudden in-rushing of gas into the space between your knuckles.

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