Snap, Crackle, Pop!
What cause those snaps, crackles, and pops?
Adapted from the book Your Spine, Your Yoga by Bernie Clark
If you move, you may crinkle and click, creak and crack. You may groan and complain about getting old, but noises are a normal part of life and for many people, a part of yoga!, A technical term for joint noise is crepitus, which is Latin for “rattle”. The exact cause of these noises is uncertain.
There are several possibilities.
The first is simple friction; when two things come together, get stuck and then finally release, a sound is often created. An example is when you snap your fingers: your thumb and finger are initially stuck together, the pressure between them builds, and finally there is a release of tension with an audible click. This can happen between our tissues as well. A tendon or ligament may get temporarily caught up on another ligament or on a bone’s surface, perhaps due to a bony growth (an osteophyte or “bone spur”); as we move, the ligament tries to slip over the obstacle but can’t, until a high enough stress builds up and then finally the ligament releases with an loud snap.
Something similar happens in many knees: the kneecap is embedded in the patellar tendon, which is a continuation of the quadriceps tendon. Sometimes when the knee flexes, such as during squatting or Chair Pose (Utkatasana), the patella and tendon slide up the side of the femoral condyle, the valley at the end of the femur, where it doesn’t feel comfortable. Finally it slips back into the bottom of the groove with an audible snap and a sense that things are normal again.
(Contrary to many urban legends, constant repetition of this form of cracking joints, such as cracking your knuckles constantly, does not lead to arthritic damage, but it can lead to a loss of strength in associated muscles and to soft-tissue swelling.)
Alternately, any crinkling noises you may hear from the knees may be due to the friction between pieces of cartilage that have broken lose and are lying around in the knee joint, grinding against each other as the knee moves. All friction sources of sounds can be repeated over and over again right away, like cracking your knuckles or snapping your fingers; no resting (refractory) period is needed.
Friction can create many of the sounds in our knees and neck as we move our legs and head, but most sounds from our sacrum or lower back are more mysterious. One often described source of sounds is gas bubbles collapsing inside a joint capsule. The synovial fluid in our joints has a slightly lower pressure than the surrounding tissues of the body or the air pressure outside our body. Why this is so is not well understood; perhaps it keeps things inside the joint capsule. The lower pressure does serve to create a bit of a vacuum between the joints’ surfaces, keeping them together. Within this low-pressure environment, various gases can precipitate out of the synovial fluid, creating a small bubble of vapor. Common gases can include nitrogen or oxygen, but 80% of the gas is carbon dioxide.
These bubbles are not able to escape the joint capsule, but sometimes, because the joint is under pressure or being moved, the bubbles collapse back into the synovial fluid. That collapse creates a popping sound. Once popped, there is no bubble, so this source of sound cannot be quickly repeated over and over again; it takes time for a new bubble to form, an interval referred to as a refractory period.
There is another version of this theory, which proposes that the sound comes from the creation of the gaseous bubble, not its collapse. This bubble formation is often called cavitation, which literally means the creation of little cavities. In the world of engineering and dentistry, cavitation is a bad thing, because cavities weaken structures or cause metal fatigue. But in living joints, the collapse of cavities can feel rather nice. Many people pay good money to get their cavities cracked.
Another possible source of the sounds we hear is the breaking of the relative vacuum that exists in a joint. Since the joints have lower internal pressure than the outside environment, when a joint is distracted and pulled apart, a release of the vacuum creates a popping sound. This is like when you stick a suction cup against a smooth surface and then quickly pull it off; a sound is created. If a joint lubricated with synovial fluid is fixated, the bones’ smooth cartilaginous surfaces are held together via a vacuum, and when that fixation is broken, we hear a pop. This is often the cause of the sounds we hear from our spine’s facet joints.
So far we have found three possible sources for sounds: friction, cavitation and fixation.
But there are other possibilities too. Our softer tissues also make noises. Muscles can create sounds when they contract! Our fascia may also speak to us; when layers of fascia—which have fluid-filled pouches occupying the spaces between fascial layers—separate, they too can make noises, much like the sounds of popping bubble wrap, only in this case, the bubbles in the fascia contain water, not air.
So what causes those noises that you hear when a chiropractor adjusts your SI joints, or when you roll into a finishing twist at the end of your yoga practice? We’re not sure.
There is debate over whether any sound can come from an SI joint itself, as the amount of separation possible there may be simply too small. Sounds from that region are more likely to be coming from the facet joints between L5 and S1, or from any accessory joints around the SI joint. Some studies have shown that where we, or our therapists, think the sound comes from may not be the actual source.
A therapist may target your L4/L5 joint for a good crack, but the sound created often comes from other joints; indeed, some studies have shown the actual sound can be up to 14 cm away from the expected source. Since each lumbar vertebra has four facet joints, it is possible to get several cracks from one vertebra. One clue that a cavitation sound is being generated is the refractory period; it often takes between 40 and 95 minutes before a cracked joint can be cracked again.
But the question remains: did the adjustment create the sound in the targeted joint? And if not, was the adjustment useful?
You body talks all the time, but is getting cracked good for you? Some researchers do not believe there is any significance to the sound; the sound is neither good nor bad, simply benign. The adjustment may feel good for some people, but few studies have correlated sounds with pain reduction. An adjustment that a therapist induces may help with a pathology or with reducing pain, but not all adjustments create a crack, and not all cracks are at the targeted location of the adjustment, so it is not surprising that cracking and pain reduction are not highly correlated.
Some researchers believe that after a cavitation has been heard, the joint is more mobile, but whether this mobility was caused by the noise or not is not certain. And, even if this is a quick and easy way to increase our range of motion, the increased mobility does not last. Before long, the pressure in the joint renormalizes and we crave another crack. In the end, sometimes these noises are not all they are cracked up to be.
 Noises are not just a part of life; they are also a part of death. Corpses can create noises when they are moved.
 There is a connection between the words crepitus and decrepit. As we age we may become decrepit, which is the Latin for “broken down” or “noisy”.
 See R. Brodeur, “ The Audible Release Associated with Joint Manipulation,” Journal of Manipulative and Physiological Therapeutics 18.3 (1995): 155–64.
 The pressure inside a joint is about 3 mm of mercury less than the outside atmospheric pressure, so the difference is slight; see “Synovial Fold -Noise- and Suction,” Dynamic Disc Designs, retrieved November 6, 2016, from https://dynamicdiscdesigns.com/synovial-fold-noise-suction.
 See Brodeur, “ The Audible Release.”
 See Brodeur, “ The Audible Release.”
 See M.J. Stokes and R.G. Cooper, “Muscle Sounds During Voluntary and Stimulated Contractions of the Human Adductor Pollicis Muscle,” Journal of Applied Physiology 72.5 (1992): 1908–13.
 Dr. Jean Claude Guimberteau called these bubbles “the Multimicrovacuolar Collagenic Absorbing System,” or simply “the microvacuolar system”; see his book Architecture of Human Living Fascia: Cells and Extracellular Matrix as Revealed by Endoscopy (Pencaitland, UK: Handspring Publishing, 2015).
 See D.E. Harrison, D.D. Harrison, and S.J. Troyanovich, “ The Sacroiliac Joint: A Review of Anatomy and Biomechanics with Clinical Implications,” Journal of Manipulative and Physiological therapeutics 20.9 (1997): 607–17.
 See Gregory D. Cramer et al., “Distribution of Cavitations as Identified with Accelerometry During Lumbar Spinal Manipulation,” Journal of Manipulative and Physiological Therapeutics 34.9 (2011): 572–83.
 See J. Kim Ross, David E. Bereznick, and Stuart M. McGill, “Determining Cavitation Location During Lumbar and Thoracic Spinal Manipulation: Is Spinal Manipulation Accurate and Specific?” Spine 29.13 (2004): 1452–57, doi:10.1097/01.BRS.0000129024.95630.57.
 See David E. Bereznick, et al., “ e Refractory Period of the Audible ‘Crack’ After Lumbar Manipulation: A Preliminary Study,” Journal of Manipulative and Physiological Therapeutics 31.3 (2008): 199–203
 See G.P. Grieve, Common Vertebral Joint Problems, 2nd ed. (New York: Churchill Livingstone, 1989), 787.
 See T.W. Flynn, J.M. Fritz, R.S. Wainner, and J.M. Whit- man, “ e Audible Pop is Not Necessary for Successful Spinal High-Velocity rust Manipulation in Individuals with Low Back Pain,” Archives of Physical Medicine and Rehabilitation, 84.7 (2003): 1057–60.
 See R. Sandoz, “Some Physical Mechanisms and Effects of Spinal Adjustments,” Annals of the Swiss Chiropractors’ Association 6 (1976): 91–138.
 See R. Be a and R. Matthews, “Does the Adjustment Cavitate the Targeted Joint? An investigation into the Location of Cavitation Sounds,” Journal of Manipulative and Physiological Therapeutics 27.3 (2004): e2, doi:10.1016/j. jmpt.2003.12.014.