One of the the most common questions I get as a chiropractor is “what is that popping sound that usually happens with an adjustment?” Whether you hate or love that popping sound and resultant release of the nearby muscular tension, it is a question that has a very scientific answer.
The popping or cracking sound is related to a specific process: Cavitation. The term cavitation refers to the formation and collapse of air bubbles in a fluid or fluid-like space. The sound is simply a by-product of that collapse, similar to the snapping of chewing gum or popping of bubbles.
So how does this happen in your joints with a chiropractic adjustment? When a joint is “adjusted” or “unlocked”, there is a rapid increase in the space in that joint, with a resulting decrease in the built-up pressure. This increase of space and decrease of pressure is what reduces the soreness around a stuck or locked joint.
When this joint pressure suddenly decreases, some normal gases (Oxygen, Nitrogen, Carbon Dioxide, just to name a few) in the synovial fluid (the normal fluid in a your joints that has a honey-like consistency) leave the fluid, creating little air bubbles, or “cavities”. These bubbles then collapse in on themselves, making a “popping” sound.
It is important to remember that the cavitation is not a “breaking” sound. In fact, it is a releasing sound, and one that should not be feared. Of course, there are some people who do not receive manual chiropractic adjustments, whether by their preference or because of specific medical reasons. There are always other ways to unlock those stuck joints!
References: 1) (1995) Brodeur. JMPT. 18(3): 155-64. 2) (2002) Protopapas. JAOA. 102(5): 283-7.