Cracking joints is manipulating one's joints to produce a distinct cracking or popping sound. It is sometimes performed by physical therapists, chiropractors, osteopaths, and masseurs in Turkish baths.
For many decades, the physical mechanism that causes the cracking sound as a result of bending, twisting, or compressing joints was uncertain. Suggested causes included:
- Formation of bubbles of joint air as the joint is expanded.
- Cavitation within the joint—small cavities of partial vacuum form in the synovial fluid and then rapidly collapse, producing a sharp sound.
- Rapid stretching of ligaments.
- Intra-articular (within-joint) adhesions being broken.
There were several theories to explain the cracking of joints. Synovial fluid cavitation has some evidence to support it. When a spinal manipulation is performed, the applied force separates the articular surfaces of a fully encapsulated synovial joint, which in turn creates a reduction in pressure within the joint cavity. In this low-pressure environment, some of the gases that are dissolved in the synovial fluid (which are naturally found in all bodily fluids) leave the solution, making a bubble, or cavity, which rapidly collapses upon itself, resulting in a "clicking" sound. The contents of the resultant gas bubble are thought to be mainly carbon dioxide, oxygen and nitrogen. The effects of this process will remain for a period of time known as the "refractory period," during which the joint cannot be "re-cracked," which lasts about twenty minutes, while the gases are slowly reabsorbed into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.
In 2015, research showed that bubbles remained in the fluid after cracking, suggesting that the cracking sound was produced when the bubble within the joint was formed, not when it collapsed. In 2018, a team in France created a mathematical simulation of what happens in a joint just before it cracks. The team concluded that the sound is caused by bubbles' collapse, and bubbles observed in the fluid are the result of a partial collapse. Due to the theoretical basis and lack of physical experimentation, the scientific community is still not fully convinced of this conclusion.
The common claim that cracking one's knuckles causes arthritis is not supported by evidence. A study published in 2011 examined the hand radiographs of 215 people (aged 50 to 89) and compared the joints of those who regularly cracked their knuckles to those who did not. The study concluded that knuckle-cracking did not cause hand osteoarthritis, no matter how many years or how often a person cracked their knuckles. A 1990 study also concluded that there was no increased preponderance of arthritis of the hand of chronic knuckle-crackers but that habitual knuckle-crackers were more likely to have hand swelling and lowered grip strength. It claimed further that habitual knuckle-cracking was associated with manual labour, biting of the nails, smoking, and drinking alcohol and suggested it resulted in functional hand impairment. This early study has been criticized for not taking into consideration the possibility of confounding factors, such as whether the ability to crack one's knuckles is associated with impaired hand functioning rather than being a cause of it.
- Crepitus—sounds made by joints
- Richard Boggs, Hammaming in the Sham: A Journey Through the Turkish Baths of Damascus, Aleppo and Beyond, 2012, ISBN 1859643256, p. 161
- Shmerling, Robert H. (14 May 2018). "Knuckle cracking: Annoying & harmful, or just annoying?". How do we know that knuckle cracking is harmless?. health.harvard.edu. Retrieved 19 July 2019.
One study published in 1990 found that among 74 people who regularly cracked their knuckles, their average grip strength was lower and there were more instances of hand swelling than among 226 people who did not crack their knuckles. However, the incidence of arthritis was the same in both groups.
- Castellanos, Jorge; Axelrod, David (May 1990). "Effect of habitual knuckle cracking on hand function". Annals of the Rheumatic Diseases. 49 (5): 308–9. doi:10.1136/ard.49.5.308. PMC 1004074. PMID 2344210.
- Dvorsky, George. "Simulation May Finally Explain Why Knuckle Cracking Makes That Awful Sound". Gizmodo. Retrieved 30 March 2018.
- Gregory N. Kawchuk; Jerome Fryer; Jacob L. Jaremko; Hongbo Zeng; Lindsay Rowe; Richard Thompson (2015). "Real-Time Visualization of Joint Cavitation". PLoS ONE. 10 (6): 384–390. Bibcode:2015PLoSO..1019470K. doi:10.1371/journal.pone.0119470. PMC 4398549. PMID 25875374.
- Knapton, Sarah (15 April 2015). "Why knuckle cracking makes a popping sound, and why it might be beneficial". The Daily Telegraph. London. Retrieved 17 December 2016.
- Sample, Ian; editor, science (15 April 2015). "Cracked it! Scientists solve puzzle of why knuckles pop when pulled". The Guardian. London. Retrieved 20 September 2016.
- Protopapas M, Cymet T, Protapapas M (1 May 2002). "Joint cracking and popping: understanding noises that accompany articular release". J Am Osteopath Assoc. 102 (5): 283–7. PMID 12033758. Archived from the original on 27 September 2007. Retrieved 2 March 2007.
- Brodeur R. (1995). "The audible release associated with joint manipulation". J Manipulative Physiol Ther. 18 (3): 155–64. PMID 7790795.
- Maigne, Jean-Yves; Vautravers, Philippe (September 2003). "Mechanism of action of spinal manipulative therapy". Joint Bone Spine. 70 (5): 336–341. doi:10.1016/S1297-319X(03)00074-5.
- Unsworth A, Dowson D, Wright V (1971). "'Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint". Ann Rheum Dis. 30 (4): 348–58. doi:10.1136/ard.30.4.348. PMC 1005793. PMID 5557778.
- Fryer, Gary; Jacob Mudge & McLaughlin, Patrick (2002). "The Effect of Talocrural Joint Manipulation on Range of Motion at the Ankle" (PDF). Journal of Manipulative and Physiological Therapeutics. 25 (6): 384–390. doi:10.1067/mmt.2002.126129. PMID 12183696.
- "Why Does Cracking Your Knuckles Make So Much Noise? Science Finally Has an Answer". Time. Retrieved 30 March 2018.
- Chandran Suja, V.; Barakat, A. I. (29 March 2018). "A Mathematical Model for the Sounds Produced by Knuckle Cracking". Scientific Reports. 8 (1): 4600. Bibcode:2018NatSR...8.4600C. doi:10.1038/s41598-018-22664-4. ISSN 2045-2322. PMC 5876406. PMID 29599511.
- Rizvi, Asad; Loukas, Marios; Oskouian, Rod J.; Tubbs, R. Shane (August 2018). "Let's get a hand on this: Review of the clinical anatomy of "knuckle cracking"". Clinical Anatomy. 31 (6): 942–945. doi:10.1002/ca.23243. ISSN 0897-3806. PMID 30080300.
- Deweber K, Olszewski M, Ortolano R (2011). "Knuckle cracking and hand osteoarthritis". J Am Board Fam Med. 24 (2): 169–174. doi:10.3122/jabfm.2011.02.100156. PMID 21383216.
- Simkin, Peter (November 1990). "Habitual knuckle cracking and hand function". Annals of the Rheumatic Diseases. 49 (11): 957. doi:10.1136/ard.49.11.957-b. PMC 1004281. PMID 2256753.