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Today I was walking past a door, when I accidentally hit my hand on the frame. All my knuckles popped instantly. I got curious and did the same thing but with my knuckles popped and this time the impact felt much more pronounced. Is the function of the popping of knuckle to retard the hand slowly, hence reducing the force when hit?

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    $\begingroup$ How do you propose that anyone knows whether that’s the purpose (if indeed there is a ‘purpose’, which it seems likely there isn’t)? (Genuine Q) $\endgroup$
    – user438383
    Commented Sep 12, 2023 at 18:09

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Is the function of the popping of knuckle to retard the hand slowly, hence reducing the force when hit?

No. The only physiological effect of popping the knuckles is to temporarily stretch the joint capsule improving range of motion slightly, which is why people often wiggle their fingers afterwards, or pop them before using their hands for something requiring a lot of dexterity (like safe-cracking in movies.) It may also cause the release of a small amount of oxytocin, which makes one feel good/less stressed/happy, accounting for its propensity to be habitual.

It doesn't seem to have a purpose. It's a phenomenon which can happen in many joints, including the neck and back. Some joints can click (e.g. the ankle) with certain movements, which is different.

It can be harmful to the joint capsule if forced.


What happens when you crack your knuckles?

For centuries, people have wondered about how knuckle popping works, and for decades people (usually older adults) tried to discourage their kids from popping/cracking their knuckles, warning them of future arthritis. However, it does not cause arthritis, and the mysterious phenomenon may have an explanation at last.

"Cracking" one's knuckles (or popping if you prefer; it depends on where you grew up) refers to the sound made when certain kinds of stretching are applied to the joints. One of my kids was addicted to it, so knowing if it could cause arthritis was important to me. In fact, there aren't any studies linking habitual knuckle popping to osteoarthritis. Knuckle-poppers are no more likely to develop osteoarthritis than non-poppers.

The [lack of] evidence for the association of knuckle cracking and osteoarthritis comes mainly from observational studies that have failed to show an association. [bracketed words mine to make statement clearer]

This has been born out by large retrospective studies and by small ones.

But what makes the noise? It's related to tribonucleation defined by Wikipedia as

a mechanism that creates small gas bubbles by the action of making and breaking contact between solid surfaces immersed in a liquid containing dissolved gas.

Knuckles, the joints between the flat part of your hand and your fingers, and those between the nearest and the middle finger bone) are solid cartilagenous surfaces surrounded by a fibrous joint capsule, and filled with a lubricating (synovial) fluid containing mostly proteins and dissolved gasses. When you pull the surfaces apart, the joint capsule stretches to the point that the dissolved gasses come out of solution and form a bubble (cavitation). The formation of that bubble produces a pop.

I don't understand why it produces a noise; a physicist might know. It has been compared to the noise that occurs when a suction cup is pulled off of a solid surface and air rushes in. Or my analogy would be clicking your tongue: placing the tongue to the roof of your mouth, creating a vacuum there, and then breaking contact produces a pop, how loud it is is proportional to the strength of the vacuum produced. It can be a soft "tsk, tsk" sound or if both surfaces are wet with saliva, a loud POP! I don't know why that causes a noise either, but it happens.

The formation of a bubble in the joint was first shown on xray in 1947. The noise produced was shown by MRI to be produced when the bubble formed by Kawchuk et al. in 2014, not when the gas was resorbed. You cannot pop your knuckle again until the gas bubble is resorbed into the synovial fluid.

On the left, no stretching action yet (normal joint); on the right, stretching applied to the joint with the formation of a bubble. Note how much more area the joint now occupies due to the stretching and gas.

Upon stretching finger, air (black) bubble forms[Upon stretching, air bubble forms2

Real-Time Visualization of Joint Cavitation

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  • $\begingroup$ Do habitual knuckle poppers produce more sound than non-poppers? I have observed that people with that habit can crack it easier and louder than others. I also observe that they can do it more frequently than others. $\endgroup$
    – Aurelius
    Commented Sep 17, 2023 at 19:47
  • $\begingroup$ @Aurelius - It's possible that habitual knuckle poppers have loosened the joint capsule a bit, so there might be a bit more synovial fluid in their joint space, thus more gas, thus a bigger bubble, thus a louder pop. (I don't know this for a fact.) I don't think they can pop their knuckles more frequently; it would take longer for the larger gas bubble to be resorbed, if my suspicion is correct. $\endgroup$ Commented Sep 18, 2023 at 12:40
  • $\begingroup$ (+1) Couple of doubts: 1. "It may also cause the release of a small amount of oxytocin, which makes one feel good/less stressed/happy, accounting for its propensity to be habitual" --> Is it actually? I always thought it is just placebo effect acting and there is no physiology involved. 2. "It doesn't seem to have a purpose" --> If that is the case, why did humans picked up the habit? Can it be considered an evolutionary habit? $\endgroup$ Commented Sep 19, 2023 at 10:35
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    $\begingroup$ @NilayGhosh re 1: "It may..." (emphasis mine). re 2: I don't know what an evolutionary habit is, so I googled "evolutionary habit" (scholar) and didn't get any hits. That piece is erroneous. Lamarck was wrong. $\endgroup$ Commented Sep 19, 2023 at 10:56
  • $\begingroup$ I asked a follow up question on this topic: biology.stackexchange.com/questions/113071/… $\endgroup$ Commented Sep 24, 2023 at 7:50

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