Cracking Knuckles: What is That Sound and is it Harmful?

Author: Alex Borja B.S. SPT, HFS

You all have done it. You yank on your fingers and hear that nice satisfying…crrack! Cracking knuckles even has a fancy scientific name that most are unaware of  called, “cavitation”. But we’ll get into that shortly.

Cracking Knuckles: What is That Sound and is it Harmful?

Some of you might crack your knuckles on occasion and others might do it very frequently. Some of you might crack your hands and others might crack your neck. While there are many factors that go into knuckle cracking, a few questions beg to be answered: what exactly is knuckle cracking and is it harmful?

Knuckle Cracking or Cavitation

When you crack your knuckles you are actually separating the joint that makes up two bones. This often happens in a quick and forceful maneuver such as a quick pull of the finger. Joints make up the connection between two bones and is held together by a series of connective tissues and ligaments. All the joints in your body also are encased in a capsule of fluid called, synovial fluid”. (See picture)

Think of it as a sturdy bubble that surrounds the ends of both bones coming together to form a joint. The fluid in the bubble is the synovial fluid. This is sort of a cushioning liquid filled balloon that also allows for increased range of motion.

Cracking Knuckles: What is That Sound and is it Harmful?

A joint with the surrounding capsule and synovial fluid inside

When you crack your knuckles you are actually separating this joint or gapping it apart. This increases the volume of the joint capsule.

What does increase in volume have to do with anything? Think back to chemistry class:

  • 1. Increased volume = decreased pressure
  • 2. Decreased volume = increased pressure

So by gapping or pulling apart the two bones that form a joint, we are increasing the volume and therefore decreasing the pressure.

When the pressure decreases in the synovial fluid, gases are dissolved in the fluid and becomes less soluble. The synovial fluid begins forming bubbles. It is this process that is called cavitation.

When you stretch the joint far enough, you create a bigger drop in pressure in the joint and the bubbles actually burst, producing that satisfying and often relieving cracking sound.

You may have noticed that you have the ability to crack your knuckles multiple times throughout the day, but not twice in a row. This is because it takes roughly 25-30 minutes for the gas bubbles to re-dissolve into the joint fluid. Once this gas is redissolved back in your joints, it is possible to once again cavitate![1][2]

Is Cracking Knuckles Harmful?

Cracking Knuckles: What is That Sound and is it Harmful?Do you ever recall your mom smacking you upside your head when you cracked your knuckles?

Stop that, your going to get arthritis in those joints!

She probably said something along those lines (with or without the slap).

Well lets take a look at the research to see if there is any evidence to back moms claim.

The Research

In a study by Brodeur [1], he examined 300 knuckle crackers to see if there were any harmful effects to the joints. He ended up finding that there was no evidence for knuckle cracking leading to arthritis. He did note, however, that habitual knuckle crackers showed signs of other damage such as to soft tissue and weakened grip strength.

Cracking Knuckles: What is That Sound and is it Harmful?Another interesting finding of this study was that ligament damage was likely due to repeated joint cracking. The idea is that because cracking of the joints requires such a high velocity thrust, the ligaments often take a part in the tension force created.

In other words, think of an golfer who is always swings at high velocities. With this comes repeated tension on the ligaments as they try to hold the joints together during that swing. This can lead to elongated ligaments over time, and thus unstable joints.

Aside from the repeated trauma to ligaments due to joint cavitation, some good does come from infrequent cracking.

For instance, physical therapists often perform high thrust manipulation on the neck, spine, elbow, or any major joint in the body to those in pain with stiffness.

This increases ROM in the joints of those who often are too tight around a joint. You might notice that after you crack your knuckles, neck, or other joint, you feel more loose and free. This is increasing the range you have for that joint temporarily. Physical therapists do it because they combine cracking joints through manipulations for short term relief while fixing the problem (long term) through exercises and manual techniques. We often crack out joints only to be doing again 20-30 minutes later. This habitual cracking can lead to unstable joints years down the road.

So cracking knuckles or joint cavitation does have some benefits when performed in those with restricted motion, combined with other treatments to fix the underlying problem.

In conclusion

Your knuckles cracking are nothing more than this cavitation process of pressure drop, bubble formation, and popping in your joints. It can be a beneficial thing if needed for an acute problem, but should not be habitually done day in and day out. Always remember to see a licensed joint manipulating health care professional before attempting to crack a friends neck or back.

It would be interesting to see how many people out there actually crack their knuckles habitually and if they notice any instability or pain from it.

Are you a habitual knuckle cracker or not? How often do you crack your knuckles and have you noticed anything from it?


1. Brodeur R. The audible release associated with joint manipulation. J Manipulative Physiol Ther. 1995 Mar-Apr;18(3):155-64.

2. A Unsworth, D Dowson, and V Wright. ‘Cracking joints’. A bioengineering study of cavitation in the metacarpophalangeal joint.  Ann Rheum Dis. 1971 July; 30(4): 348–358.

Author: Alex Borja B.S. SPT, HFS

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3 Responses to “Cracking Knuckles: What is That Sound and is it Harmful?”

  1. Dx says : Reply

    Ahhh I’ve always wondered. Thanks for the info!

  2. Steven Bror says : Reply

    Gonna try to stop cracking my knuckles all the tip just in case..

  3. Yohan says : Reply

    Before now, I would do some serious knuckle cracking, yeah , thanks for de info

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