What the Popping Sound Actually Tells You — and What It Doesn’t

Quick Answers, Before the Details

What the sound actually is

For most of the last century, the prevailing explanation was that a gas bubble formed inside the joint and then collapsed, with the collapse producing the popping sound.

In 2015, researchers at the University of Alberta tested that explanation directly. They placed a finger joint inside an MRI scanner and slowly applied traction through a cable while imaging the joint at about three frames per second until it cracked.

What they observed challenged the older explanation. As traction increased, the joint surfaces initially resisted separating, held together in part by the fluid between them. Then they separated suddenly. At that same moment, a dark, gas-filled cavity appeared within the joint fluid.

The key finding was the timing: the popping sound occurred as the cavity formed, not when it collapsed. In fact, the cavity remained visible on MRI for as long as traction was maintained.

In other words, the pop is associated with the rapid formation of a gas cavity as the joint surfaces suddenly separate. It is not the sound of a bubble collapsing, and it is not the sound of bones being put “back into place.”

The process is called tribonucleation. Two joint surfaces separated by a thin layer of fluid initially resist being pulled apart. When the pulling force becomes great enough to overcome that resistance, the surfaces separate rapidly. This causes a sudden drop in pressure within the joint fluid, allowing dissolved gas to come out of solution and form a gas-filled cavity.

A familiar, though imperfect, comparison is opening a bottle of soda. The drop in pressure allows gas that was dissolved in the liquid to come out of solution. In a joint, however, the pressure change occurs when the joint surfaces rapidly separate, and the resulting cavity forms within the joint fluid.

Once a joint has cracked, it usually cannot be made to crack again immediately. It enters what is commonly called a refractory period, during which another pop is difficult or impossible to produce. This period is often described as lasting roughly 20 minutes, although its exact duration varies. Anyone who habitually cracks their knuckles will recognize the effect.

One question remains unresolved: why is the sound so loud? The researchers noted that tribonucleation in other settings produces relatively little sound, yet a cracking joint can sometimes be heard across a room. The mechanism that makes joint cracking so acoustically pronounced has not been definitively established.

What most people think the sound means

Research suggests that many patients interpret the popping sound in ways that do not match its known physiology.

In one survey of 100 people, roughly half believed the sound meant that vertebrae were being repositioned. About one quarter thought it came from bones rubbing against each other. Fewer than one in ten attributed it to gas coming out of solution in the joint, which is consistent with the current physiological explanation. About four in ten also regarded the sound as evidence that the manipulation had been successful.

A separate, larger survey of nearly 500 people in Italy found a similar pattern. Many participants believed that a successful manipulation should produce an audible pop.

Importantly, in the first survey, these beliefs did not differ significantly between people who had previously received manipulation and those who had not. That suggests the misconception is not simply something patients acquire through treatment. Many already expect the pop to mean that something has been put back into place, and that hearing it confirms the treatment worked.

What the research found about the sound and the result

Several studies have asked whether hearing a pop during manipulation predicts a better clinical result. So far, the answer has been consistent: it does not.

A 2022 systematic review examined the available research on audible pops and pain outcomes, including four prospective studies and one randomized trial. It found no evidence that experiencing an audible pop during manipulation was associated with greater pain relief.

One of the most frequently cited studies followed 71 patients with low back pain who received thrust manipulation. Some manipulations produced an audible pop and others did not. When the researchers compared the patients afterward, they found no meaningful differences in pain, disability, or range of motion. The likelihood of a successful outcome was essentially the same whether a pop occurred or not.

That conclusion is specific and should not be stretched beyond what the studies tested. It does not mean that manipulation itself has no clinical effect. A large review combining 47 randomized trials found that spinal manipulative therapy produced modest improvements in pain and function for chronic low back pain, with outcomes generally comparable to those of other recommended treatments. More broadly, guidelines commonly place manual therapy within a treatment plan that includes active care such as exercise rather than treating the sound of an adjustment as a therapeutic goal.

The important distinction is between the manipulation and the noise it may produce. Manipulation can have clinical effects, but the available evidence does not show that an audible pop is necessary for those effects or predicts a better result. The sound therefore should not be used as a scorecard for whether the treatment worked.

Loudness is an even simpler question: it has not been studied as an outcome predictor. Researchers have examined whether a pop occurred and, in some studies, how many pops occurred, but there is no evidence showing that louder pops produce better outcomes.

A loud crack tells you that a loud crack occurred. It does not tell you that the treatment was more effective.

What the sound cannot tell you

It cannot tell you exactly which joint produced it. Sound can travel through surrounding tissues, and the joint that cavitates is not necessarily the joint the clinician intended to target. Hearing a pop indicates that cavitation occurred somewhere in the manipulated region. The sound alone does not identify the specific joint that produced it.

It cannot tell you whether the problem was identified correctly. A joint can cavitate whether or not it is responsible for the patient’s symptoms. The pop results from the mechanical conditions that produce rapid joint separation and cavitation, not from the accuracy of the diagnosis or the appropriateness of the treatment.

It cannot tell you what happens next. Cavitation provides no information about the other factors that may have contributed to the problem or may affect recovery, such as strength, movement capacity, training or activity load, previous injuries, and other relevant physical or behavioral factors.

In short, the pop tells you something about what happened mechanically at that moment. It does not tell you whether the correct structure was treated, why the person was hurting, or whether the treatment will lead to a better outcome.

What actually determines the result

The first requirement is identifying the problem accurately. Pain is not always coming from the place where it is felt. Shoulder pain, for example, can sometimes originate in the neck. Pain near the hip may actually involve the proximal hamstring. Knee symptoms can be influenced by how the foot and the rest of the lower limb load during movement. If the source or contributing factors are misidentified, treatment can be directed at the wrong problem. Even a technique performed skillfully cannot correct a problem it was never designed to address.

The second is matching the treatment to the clinical findings. Manipulation may be useful for some presentations and unnecessary for others. The same is true of soft-tissue treatment and progressive resistance exercise. Rather than assuming that one technique is best for everyone, treatment should be selected according to the individual presentation and adjusted according to the patient’s response. Manual therapy can be one part of that plan, while exercise and progressively increasing physical capacity often address what passive treatment alone cannot.

The third is what happens after the pain improves. Pain relief and recovery are not necessarily the same thing. A person may feel better before strength, mobility, tolerance to load, or confidence in the previously painful activity has fully recovered. Rehabilitation should therefore continue far enough to restore the capacities the person needs for daily life, work, training, or sport. That can also help reduce the risk of recurrence, although no treatment can guarantee that a problem will not return.

The larger point is that no single technique is a cure-all. The evidence does not support treating manipulation, soft-tissue work, chiropractic care, or exercise as universally superior solutions. Good care depends on identifying the relevant problem, choosing interventions that fit the findings and the patient’s goals, monitoring the response, and progressing toward restored function rather than using a particular technique, or the sound it produces, as the measure of success.

That is what the initial evaluation is for: not to decide how to produce a pop, but to determine what needs to be treated and what combination of care is most appropriate.

Four questions worth asking before any manual treatment

These questions are useful with any provider, including this one. If you have received treatment before but the improvement did not last, they are especially worth asking before beginning another course of care.

1. What did the examination find?

There should be a specific, understandable answer. That might involve a particular structure, movement, functional limitation, or pattern of symptoms rather than simply naming the part of the body that hurts.

2. Why are you using this technique for that finding?

The provider should be able to explain the reasoning that connects the examination findings to the proposed treatment. You should understand what the technique is intended to accomplish and why it is appropriate in your case.

3. What are we doing about the factors that contributed to the problem?

Treating the painful area may help with symptoms, but symptoms can have multiple contributing factors. Depending on the problem, those might include strength, mobility, activity or training load, work demands, movement tolerance, previous injury, or other relevant factors. A complete plan should address the important contributors that can actually be identified and modified.

4. What should improve, and by when?

There should be some way to judge whether treatment is working. The provider should explain what changes you can reasonably expect, such as less pain, greater movement, improved strength, or better tolerance of a specific activity, and approximately when the plan should be reassessed if those changes are not occurring.

None of these questions asks whether a joint made a noise. That is because the sound is not the outcome. Improvement in the problem you came in with is.