Ever wonder what causes all those pops and cracks you hear as you move your body? There are lots of urban myths about the cause of these, but usually there are only three causes: fixation, friction, or a release of gas. Most people are aware of only the last two.

Sometimes bubbles of nitrogen form in the synovial fluids of our joints. When these bubbles release they make a popping sound. Once the bubble is gone you won’t be able to pop the joint again for a while. Friction, on the other hand, can be repeated again and again without any waiting. Friction occurs when two surfaces rub against each other. Friction is the noise made when we snap our fingers. Try it now – just before the two fingers release there is pressure building up between them. Once the pressure becomes greater than the friction between the fingers, they release. As finger strikes the base of the thumb, a sound is created. The same thing can happen in a joint; two ligaments, tendons, or pieces of cartilage may get temporarily stuck or rub against another piece of the body. If the pressure builds up enough to overcome the friction, the pieces release, usually with a snapping sound.

Cracking knuckles is a good example of friction-generated noise. With friction you don’t have to wait to recreate a noise; it can be done over and over again quickly. Often people do this as a nervous habit. Many myths, promoted by mothers who can’t stand the sound of their sons cracking their knuckles (yes, it is usually boys who do this), promise that continued cracking of the joints will lead to arthritis. However, no scientific studies have verified this. At worst, continued cracking may loosen the joints minimally. Even snapping your fingers over and over again could be harmful to the tissues in your thumb. But, it would take a lot of snapping for that to happen.

On the other hand, sometimes torn flaps of cartilage or ligaments getting caught in the joint cause the friction. This kind of cracking is not healthy. The key is the presence or absence of pain. Noises accompanied by pain are not good and should be avoided or investigated.

The third cause of joint popping is “fixation.” This is the cause we are really interested in. Fixation is a temporary sticking together of two surfaces. The cracking sound is generated when the surfaces are released. That nice pop you might get in your ribs or lower back when you go into a twist or some other pose is probably caused by releasing fixation. Usually it feels good pressure has been released.

There are three conditions for fixation to occur: first, the two surfaces that are getting stuck together must be smooth; second, there must be some liquid lubricant between the surfaces; finally, the surfaces must be under some pressure that pushes them together.

A good example of fixation is familiar to most readers who aren’t yogis. A frosty glass of beer creates condensation (the liquid lubricant) all over the glass, including the bottom. The bottom of the glass is smooth, just like the surface of the coaster the glass is resting on. The weight of the beer in the mug provides enough weight to press the glass onto the coaster. When we pick up the glass (strictly for experimental purposes mind you yogis don’t drink, right?) the coaster comes along with it.

This is fixation. When you pull the coaster off the bottom of the mug a sound may be audible. When you break the fixation between two bones in the body a sound may be even more noticeable. Even without the sound you will definitely feel the release.

Why do we care about breaking fixation? Well, it feels good for one thing; generally the release is enjoyable. But the main reason to break fixation is to prevent fusion of a joint. Often in older patients who suffer a broken bone, the doctor will fix the two pieces together with a pin to keep the edges in contact. This kind of fixation allows the bones to fuse back into one piece; this use of fixation is good. However, in a joint, if two ends of a bone are held together for a long period of time, they too will fuse together. This use of fixation is not good. The joint becomes useless when the bones are fused.

Fusion can happen to anyone. The joint between our hips (ilium) and our tailbone (sacrum), called the “sacroiliac joint,” can become fused together. A 2006 study in Israel showed that 34.2% of men examined by computer tomography had a bridge formed between their sacrum and ilium. The rate for women was far lower, 4.6%. This incidence of fusion, via the bridge, was age related; the older subjects had a higher incidence of bridging. For some older people the joints of the lumbar spine also start to fuse.[1] Loss of flexibility here is very noticeable and a big problem.

Image courtesy of Spine University in Colorado Springs, Colorado
Fusion begins with fixation; fixation is cured by mobility; and mobility of the joints is one of the big benefits of Yin Yoga.

  1. Sometimes with degenerative joints, a procedure called arthrodesis is used to deliberately fix joints, to allow them to fuse together.

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