Why our joints cracking and popping?

There are two reasons why our joints crack and creak. One is that bones are rubbing together, and the other is that the bones of a joint are fixated. We will examine these one at a time.

Rubbing Bones

Most of the joint sounds we hear are due to bones rubbing. This is "friction popping." When we snap our fingers, we press our thumb and middle finger together hard enough to create 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 rebound violently and vibrate briefly, like tuning forks. This creates the 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, such as 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 popping sound has the same cause as finger snapping: the two bones of the elbow are temporarily in friction, and when they release, they vibrate violently and we hear a "pop." 

A similar but more alarming instance of friction popping takes place in the knee. More specifically, it occurs in our patella, or kneecap. The patella 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 its 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 for our knee to lock up for an instant and then release. At worst, there is a slight twinge to the tendon 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, although they are not as loud here because the forces of friction are not as great. The bones involved are the facets of the cervical vertebrae-typically several of them, which is why the noise sounds "crunchy," like walking on pebbles.

Is It Bad for You?

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, and no harm is done. But there is little value in consciously trying to make these sounds happen. Just as it takes a certain effort to snap our fingers, many people can pop their hips over and over by doing sit-ups or leg lifts. 
Other people can do similar things with their knees. This is not desirable. Even our thumb gets sore if we snap it enough. If a student insists on popping a joint repetitively, the joint may become inflamed and painful. This is because the body is trying to minimize the friction by swelling the fluid sacks that line our joints. These sacks are called bursae, and their inflamed condition is called bursitis. Bursitis most frequently occurs in the small joints of the shoulder and elbow. 

Bursitis is less likely to occur in the patella, but eventually the cartilage can become worn and irritated. This condition is called chondromalacia, and it makes the knee painful to bend.

What to Do?

If a student can pop a hip each time they do leg raises, they should try one of the following variations to avoid creating friction in the socket.

1. Do leg raises with knees bent.
2. Experiment by holding the legs slightly apart.
3. Don't allow the legs to come too close to the 
    floor when bringing the feet down.

Friction in the patella can sometimes be avoided by turning the foot out slightly in Warrior poses and triangles. But due to a unique bone structure in every individual, it sometimes may be more helpful to turn the foot in, rather than out. Frequently, the strain on the patella also can be relieved by stepping backwards into Warrior, rather than forward. 

Stepping backward relaxes the strain on the bent front patella, allowing it to slide as it should with minimal friction.
Friction pops sometimes occur in the elbow or shoulder when practicing Chaturanga or Upward Dog. Asking a student to take their hands wider and their elbows out can help. This variation requires more strength to perform, so beginners may need to hold themselves on the knees, rather than feet. 

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, we hear a popping sound.
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 fixating a glass 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 squeezes out and a temporary vacuum, or fixation, occurs. 

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 fixation that occurs in their fingers. People who "crack" their spines in a spinal twist are doing the same thing. It feels good to them, and 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 it has rested, immobile, for some time. This is because it takes time for fixation to reoccur even when conditions are right. A glass of water, for example, will not instantly fixate to a plate. Releasing joint fixations is actually beneficial, because it allows the free functioning of the joints. 

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 your students are able to repetitively pop a hip, knee, or neck, then it is undesirable friction popping. The occasional friction pop will do no harm, but be mindful that it does not become habit or a nervous twitch.