If you have ever had a spinal adjustment, you probably remember the sound. A quick pop or click, sometimes a few of them, followed by a feeling of release. For a lot of people, that sound is the whole point. No pop, no result. Loud pop, great visit.
It is an understandable way to think, because the sound is the most obvious thing that happens. But the noise and the benefit are two different things, and confusing them can lead you toward chasing the wrong target. Here is what is really going on.
What the sound actually is
The technical term is cavitation. Your joints are surrounded by a capsule filled with synovial fluid, which lubricates the joint and carries nutrients to the cartilage. That fluid has gases dissolved in it, mostly carbon dioxide, nitrogen, and oxygen.
When a joint is stretched quickly past its usual range, the pressure inside the capsule drops suddenly. That pressure drop lets a gas bubble form in the fluid, and the formation of that bubble is what you hear. For decades there was debate about whether the sound came from the bubble forming or collapsing. In 2015, a research group used real-time MRI to watch knuckles crack and showed the noise coincides with a cavity forming inside the fluid, not bursting.1
Once the gas has come out of solution, it takes roughly twenty minutes to dissolve back in. That is why you cannot immediately crack the same joint twice. It is also a hint that the sound has nothing to do with anything "going back into place." A bone that had moved would not need a twenty-minute refractory period to move again.
And no, cracking your knuckles does not appear to cause arthritis. Studies comparing habitual knuckle crackers with non-crackers have not found higher rates of hand osteoarthritis.
Does the sound predict a better outcome?
This has actually been tested. Researchers took people with low back pain who were good candidates for spinal manipulation, treated them, and recorded whether or not an audible pop occurred. Then they compared outcomes. Pain, disability, and range of motion improved whether or not the joint made a sound.2
That matters because it tells you the mechanism of benefit is not the cavitation itself. Current thinking is that manual therapy works through a mix of effects: a brief mechanical input to the joint and surrounding tissue, a change in how the nervous system processes signals from that area, a reduction in local muscle guarding, and a meaningful dose of expectation and reassurance.3 None of those require a noise.
So when a patient tells us "it didn't crack, so I don't think it worked," the honest answer is that the research says otherwise. And when a joint makes a dramatic sound but the person feels no different afterward, that is not a success either.
Why chasing the pop can work against you
There are two practical problems with treating the sound as the goal.
First, it can lead to over-treatment. If the only measure of a good visit is a loud joint, there is pressure on both patient and clinician to keep going until it happens. Some joints are simply quieter than others, and some people have very mobile segments that cavitate with almost no force. The sound tells you about the gas content of the fluid and the stiffness of the capsule that day. It does not tell you whether that joint needed treatment.
Second, it reinforces an unhelpful story about your spine. If you believe the pop means something was "out" and is now "in," then every return of stiffness feels like something has slipped again. That can create dependence on treatment and anxiety about ordinary sensations. Spines are strong, adaptable structures. They do not pop in and out of alignment, and imaging studies of pain-free people show all kinds of so-called abnormalities that cause no symptoms at all.4 We have written more about this in our article on how long an adjustment lasts.
What a good adjustment should be measured by
In our clinic, we treat the sound as incidental. It happens often, it is harmless, and some people enjoy the feeling. But it is not what we are checking for.
What we look at instead is function. Can you turn your head further than you could five minutes ago? Does that movement that was sharp now feel dull or absent? Did your test movement from the screening change? Those are the things that tell us whether the input did something useful, and they are the things we reassess at each visit rather than relying on how a joint sounded.
Manual therapy also works best as a door-opener rather than a destination. A joint that moves more freely after treatment gives you a window to load it, strengthen around it, and teach your nervous system that the movement is safe. Best-practice guidelines for musculoskeletal pain consistently rank education and active exercise ahead of passive treatments, with manual therapy as a useful add-on rather than the centrepiece.5 You can read how that fits into the way we structure care, and why we pair hands-on work with exercise almost every time.
So the next time your adjustment is quiet, do not assume nothing happened. And the next time it is loud, do not assume everything is fixed. Pay attention to how you move and feel afterward. That is the real test.
References
- Kawchuk GN, Fryer J, Jaremko JL, Zeng H, Rowe L, Thompson R. Real-time visualization of joint cavitation. PLoS One. 2015;10(4):e0119470.
- Flynn TW, Fritz JM, Wainner RS, Whitman JM. The audible pop is not necessary for successful spinal high-velocity thrust manipulation in individuals with low back pain. Arch Phys Med Rehabil. 2003;84(7):1057-1060.
- Bialosky JE, Bishop MD, Price DD, Robinson ME, George SZ. The mechanisms of manual therapy in the treatment of musculoskeletal pain: a comprehensive model. Man Ther. 2009;14(5):531-538.
- Brinjikji W, Luetmer PH, Comstock B, et al. Systematic literature review of imaging features of spinal degeneration in asymptomatic populations. AJNR Am J Neuroradiol. 2015;36(4):811-816.
- Lin I, Wiles L, Waller R, et al. What does best practice care for musculoskeletal pain look like? Eleven consistent recommendations from high-quality clinical practice guidelines: systematic review. Br J Sports Med. 2020;54(2):79-86.