There are many myths and rumors about joint cracking. The two most common being our knuckles will get bigger if we crack them or we will get arthritis. Neither of these is likely but there is some truth to the idea that some forms of cracking are undesirable.

Two types of cracking

There are two reasons why our joints crack and creak. 1. Bones are rubbing together. 2. The bones of a joint are fixated. We will examine these one at a time.

Bones rubbing

Most of the sounds we hear are due to bones rubbing. We call this “friction popping”. When we snap our fingers to keep time or make a point we press our thumb and middle finger together hard enough to create a lot of friction. Then we try to overpower this friction with other muscles of the hand. This opposition of forces slightly bends the bones of the finger and thumb. When the two fingers finally slip past one another the bones violently rebound and briefly vibrate like tuning forks. This creates a snapping sound.

The snapping of our fingers is not at all painful or harmful but sometimes we inadvertently create these popping sounds in other joints like our elbows. When our elbow briefly “catches” and then pops it can be quite surprising and even slightly painful if the vibrating bones press a nerve. The reasons for the popping sound are the same as snapping the thumb and finger – the two bones of the elbow are temporarily frictioned so the bones are minutely bent and when they release they vibrate violently and a “pop” is heard.

A similar but more alarming instance of “friction popping” is our knee. More specifically it is our knee cap. The patella or knee cap sometimes rides up on the side of the groove it glides in and temporarily sticks there. It is being held on the lip of the groove by the pull of the thigh muscles.

This is much like snapping our thumb and finger but this moment is very brief because as the knee bends and moves the patella loses it precarious balance of forces and “pops” violently back down into the groove where it belongs. There is nothing really harmful in this.

The patella is not injuring the ligaments or cartilage but it can be alarming and surprising for our knee to “lock up” for an instant and then release. At worst there is a slight twinge to the tendon of around the patella because it was stretched briefly.

The most common place to hear friction popping is in our neck. Most of us can roll our heads and hear these sounds. This is due to bones briefly sticking and then releasing. The principle is the same as snapping our fingers but the sounds are not as loud because the forces of friction are not as great. The bones involved are the facets of the cervical vertebrae. Several of these small joints are typically involved which is why it sounds “crunchy”, like walking on pebbles of rock.

Is it good for you?

The archetype of friction popping is snapping our thumb and middle finger. If our elbow or knee inadvertently pops there is nothing to worry about. There is just enough slack in our joints that these twinges are inevitable. No harm is being done. But there is little value in consciously trying to make these sounds happen.

Just as it takes a certain effort to snap our thumb many people can pop their hips over and over by doing sit ups or leg raises. Other people can do similar things with their knees. This is not desirable. Even our thumb gets sore if we snap it enough. A worst case scenario is a painful wearing of the cartilage of the joints. Bicyclists sometimes develop this condition in their knee caps due to the friction of pedaling. The occasional pop is nothing to worry about but to habitually create these sounds is inadvisable.

Joint Fixation

The second cause of joint popping is fixation. The bones of a fixated joint are temporarily stuck together due to suction, not friction. When this vacuum is broken a popping sound is heard. An everyday example of fixation is when the bottom of a glass of water sticks to the surface it is resting on.

When two hard, smooth surfaces have a film of fluid between them they can create a vacuum by forcing the fluid out to the edges. As long as the seal of fluid remains unbroken the vacuum remains. If we are careful we can lift quite a heavy plate by holding a glass fixated to it.

Most of the joints of the body are ideally shaped for fixation to occur. The ends of the bones are lined with hard, smooth cartilage and the joint itself is filled with synovial fluid. This fluid is necessary to lubricate the joints and minimize friction but if a joint is immobile long enough then some of the fluid between the bones is squeezed out and a temporary vacuum or fixation is created.

The most common places for fixation to occur are the fingers, toes and joints of the spine and ribs. When fixation occurs we typically feel “stuck” or “tight”. This is because are joints are not moving. People who crack their knuckles are breaking the fixations that occur in their fingers. People who “crack” their spines in a spinal twist are doing the same thing. It feels good to them. There is no harm in it.

Know the difference

There is an important difference between releasing fixation and friction popping. Once a fixation has been released the joint will not pop again until has rested immobile for some time. This is because it takes time for fixation to re-occur even when conditions are right. A glass of water will not instantly fixate to a plate.

Friction popping is not like fixation. It can be created at will. We can snap our finger and thumb as often as we like. If you or a student are able to repetitively pop a hip, knee or neck then it is friction popping and undesirable. The occasional friction pop will do no harm but it shouldn’t become a nervous twitch.

Friction popping is not necessarily harmful but is not therapeutic. Releasing joint fixations is actually beneficial. It allows the free functioning of the joints.