Spinal Manipulation Therapy: What Happens During an Adjustment

Most people who have had a chiropractic adjustment remember the sound — that sharp pop as the practitioner applies a quick thrust to the spine. Fewer can explain what actually happened. The mechanics of spinal manipulation are well understood in biomechanical terms, but they remain poorly communicated outside clinical and research settings. What follows is a plain account of the procedure, its physiological effects, and the evidence behind it.

The Mechanics of an Adjustment

What a High-Velocity, Low-Amplitude Thrust Actually Involves

An HVLA thrust — high-velocity, low-amplitude — is a controlled manual force applied to a single spinal segment. The name is descriptive: the practitioner delivers a quick impulse (high velocity) through a very short distance (low amplitude), typically no more than a few millimetres beyond the joint’s passive range of motion. The force is directed along a specific vector, chosen to match the anatomy of the target joint.

The speed matters more than the strength. A well-executed thrust is fast — completed in under 150 milliseconds — but requires surprisingly little force. Most of the practitioner’s effort goes into positioning the patient and isolating the correct segment. The thrust itself is brief and precise, closer to a flick than a push. The joint is taken just past the point where its own passive structures would normally stop it, into what biomechanists call the paraphysiological space. This is a small window of additional movement that exists beyond the normal passive range but well short of the anatomical limit where injury would occur.

For someone who has never experienced one, the speed is usually the most surprising element. Patients often expect something gradual. The reality is closer to a single, sharp input — over before the nervous system has fully registered it.

The Sound: Why Joints Pop

The audible pop that frequently accompanies an adjustment is joint cavitation, and it has nothing to do with bones. Synovial joints — including the facet joints of the spine — are enclosed in a capsule filled with synovial fluid. When the joint surfaces are separated rapidly during a thrust, the pressure inside the capsule drops. Dissolved gases in the synovial fluid, primarily carbon dioxide, come out of solution and form a bubble. The pop is the sound of that bubble forming and collapsing almost instantaneously.

This mechanism was confirmed in a widely cited 2015 MRI study by Kawchuk and colleagues at the University of Alberta, which captured real-time imaging of the gas cavity forming during metacarpophalangeal joint manipulation. The study put to rest a long-running debate about whether the sound came from bubble formation or collapse — the imaging showed it was the creation of the cavity that produced the acoustic signal.

Two points worth noting. First, the pop does not indicate that anything has been “put back in place.” It is a byproduct of the mechanical event, not the therapeutic mechanism. Second, an audible release is not required for an effective adjustment. Research has shown that the neurophysiological effects of manipulation occur whether or not cavitation is audible.

How Practitioners Choose Where to Apply Force

Before any force is applied, the practitioner conducts a hands-on assessment of spinal motion. Static palpation identifies areas of tenderness, muscle guarding, or altered tissue texture along the paraspinal muscles. Motion palpation — where the practitioner moves individual segments through their range while feeling for resistance — is the primary tool for identifying joints that are restricted or hypomobile.

The goal is to locate a specific segmental level where normal joint play is reduced. A thoracic vertebra that does not rotate freely to one side, or a lumbar segment that resists extension, becomes a candidate for intervention. The practitioner cross-references what they feel with the patient’s history, reported symptoms, and any imaging that is available. In New Zealand, chiropractors can refer directly for X-rays and, in some cases, MRI.

This assessment is not a search for bones that are “out of place.” Modern chiropractic assessment is concerned with functional restriction — joints that are not moving through their expected range — and with the neurological and muscular patterns that accompany that restriction. The decision about where to adjust is clinical, informed by the same orthopaedic and neurological reasoning used across manual therapy disciplines.

Positioning and Setup Before the Thrust

The setup is where most of the skill lies. For a lumbar adjustment, the patient typically lies on one side with the upper knee drawn forward to introduce rotation in the lower spine. The practitioner positions themselves to control the patient’s pelvis and shoulder girdle, using body position rather than arm strength to generate leverage. A cervical adjustment usually has the patient supine, with the practitioner cradling the head and introducing a precise combination of rotation and lateral flexion to isolate the target segment.

What the patient feels during setup is steady, deliberate pressure. The practitioner is taking up the slack in the tissues surrounding the target joint — a phase called pre-load. This is the tension the patient notices just before the thrust: a firm, sustained contact that feels as though the practitioner is leaning in. It is not painful, but it is unfamiliar, and first-time patients sometimes tense up in anticipation.

Good practitioners recognise that tension and pause. They may ask the patient to breathe out, or simply wait until the muscles relax. The pre-load phase is critical because the thrust will only be effective if the surrounding tissues are not bracing against it. The entire sequence — positioning, pre-load, thrust — typically takes between ten and thirty seconds.

What the Evidence Says About How It Works

The Neurophysiological Model

For much of the twentieth century, the working theory was straightforward: joints become misaligned or restricted, manipulation restores their position, and symptoms resolve. The evidence has moved considerably past that explanation. The current consensus among researchers leans toward a neurophysiological model, in which the primary therapeutic effects of spinal manipulation are mediated by the nervous system rather than by mechanical repositioning of vertebrae.

When the thrust is applied, it stimulates mechanoreceptors embedded in the joint capsule, ligaments, and paraspinal muscles. These receptors — particularly the Type I and Type III afferents — send rapid signals into the spinal cord, where they modulate activity in the dorsal horn. The dorsal horn is the processing centre for incoming sensory information, including pain signals. The influx of mechanoreceptor input appears to temporarily alter the way the central nervous system processes nociceptive (pain-related) signals from the same spinal segment.

There is also evidence that manipulation affects muscle spindle behaviour, reducing the resting tone of paraspinal muscles that have been in a guarded or hypertonic state. This helps explain the immediate sense of relaxation or “release” that patients often report. The picture is one of a manual input triggering a cascade of neural responses, rather than a mechanical correction of joint position.

Effects on Range of Motion and Pain

The clinical evidence for spinal manipulation is best established for acute low back pain. Systematic reviews, including those conducted for Cochrane, have found that spinal manipulation produces short-term improvements in pain and function that are comparable to other recommended therapies. The effect sizes are statistically significant but clinically modest — meaningful to the individual patient, but not dramatically superior to alternatives such as exercise therapy or non-steroidal anti-inflammatory medication.

Immediate changes in segmental range of motion following an adjustment are well documented and relatively uncontroversial in the research literature. A restricted joint that receives a thrust typically shows measurable improvement in its passive range, at least in the short term. Whether that mechanical change is the source of symptom relief, or whether it is a secondary effect alongside the neurophysiological response, remains a subject of active investigation.

For neck pain, the evidence is more mixed but generally supportive of manipulation as one component of a multimodal approach. For other conditions — headaches, thoracic pain, extremity complaints — the evidence base is thinner, and clinical guidelines tend to be more cautious in their recommendations. Honest reading of the literature suggests that spinal manipulation is a well-supported intervention for certain musculoskeletal conditions, not a universal treatment.

What Remains Uncertain

Several important questions remain unresolved. The dose-response relationship — how many treatment sessions are optimal, how frequently they should occur, and when diminishing returns set in — is not well established. Most clinical trials compare manipulation to a control over a fixed number of sessions, but the evidence does not yet provide clear guidance on treatment scheduling for individual patients.

Long-term outcomes are harder to attribute specifically to manipulation because most patients receive it as part of a broader management plan that includes exercise, ergonomic advice, and sometimes concurrent care from other providers. Isolating the contribution of the manipulation component from the overall treatment package is methodologically challenging. Studies that follow patients for twelve months or longer tend to show convergence between treatment groups — most people with acute low back pain improve regardless of the specific intervention.

There is also the persistent difficulty of blinding in manual therapy trials. A patient knows whether they have received a thrust. Sham manipulation protocols exist but are imperfect — the patient may recognise the difference. This makes it difficult to fully separate the specific effects of manipulation from the non-specific effects of hands-on care, therapeutic attention, and patient expectation. These are legitimate methodological challenges, not reasons to dismiss the evidence that does exist.

How Spinal Manipulation Compares to Other Approaches

Spinal manipulation sits within a family of manual therapy techniques, and understanding its neighbours helps clarify what distinguishes it. Mobilisation uses slower, repetitive, oscillatory movements applied within the joint’s passive range of motion — no thrust, no cavitation. It is gentler and more gradual, and it is used by physiotherapists, osteopaths, and chiropractors alike. The evidence suggests similar clinical outcomes for mobilisation and manipulation in many conditions, though manipulation may produce faster short-term relief.

Physiotherapy-led manual therapy often combines mobilisation with exercise prescription and movement retraining. Chiropractic care typically centres the adjustment more prominently, though contemporary practice increasingly incorporates rehabilitation exercises and soft tissue work. The boundaries between professions have blurred considerably.

What has shifted the policy landscape is the evidence on pharmacological management of low back pain. Clinical guidelines in the United Kingdom, the United States, and Denmark now recommend non-pharmacological approaches — including spinal manipulation — as first-line treatments for acute low back pain, ahead of paracetamol (which has been shown to be no more effective than placebo for this condition) and opioids (which carry significant risks). This repositioning reflects not only the evidence for manipulation but the growing recognition of the harms associated with over-reliance on medication for musculoskeletal complaints.

What to Expect as a Patient

The Initial Consultation and Assessment

The first appointment is longer than subsequent visits, typically forty-five minutes to an hour. Most of that time is clinical assessment. The practitioner takes a detailed case history: the nature and duration of the complaint, aggravating and relieving factors, any relevant medical history, and medications. This is standard clinical intake, indistinguishable in structure from what a physiotherapist or sports medicine physician would conduct.

The physical examination follows. It includes orthopaedic testing — specific movements and positions designed to reproduce symptoms and narrow the diagnosis — and a neurological screen that checks reflexes, sensation, and muscle strength in the relevant limbs. The purpose is to identify the likely source of the complaint and, critically, to rule out conditions that would contraindicate manipulation.

In New Zealand, chiropractors are registered health professionals regulated by the Chiropractic Board under the Health Practitioners Competence Assurance Act 2003. The qualification requires a minimum of five years’ university-level study, including clinical training. ACC recognises chiropractic as a funded treatment provider, which means patients can be referred or self-refer without a GP intermediary. The regulatory framework is the same structure that governs physiotherapists, dentists, and medical practitioners.

During the Adjustment Itself

The thrust itself takes less than a second. What precedes it — the positioning and pre-load described earlier — takes considerably longer and is the part that most first-time patients find unfamiliar. The sensation of being precisely positioned, the firm contact as the practitioner takes up slack, and the knowledge that something fast is about to happen can produce a moment of apprehension that is entirely normal.

The thrust feels like a quick, firm push. There is often a pop, sometimes more than one. Most patients describe the sensation as surprising rather than painful. The force is highly localised — the surrounding joints and tissues are largely unaffected. Immediately after, there is often a sense of increased mobility in the area, sometimes accompanied by a feeling of warmth as local blood flow responds to the tissue disturbance.

Some patients feel immediate relief. Others notice a gradual change over the following hours. A minority experience a brief increase in local soreness, similar to the feeling after unaccustomed exercise. The practitioner typically checks the patient’s response immediately — asking about the sensation, retesting range of motion, and noting any change in the presenting complaint. The entire treatment portion of a follow-up visit usually takes between ten and twenty minutes, though this varies with the complexity of the presentation.

Safety Profile and Contraindications

The safety profile of spinal manipulation has been extensively studied. Mild adverse effects — localised soreness, stiffness, or fatigue — occur in roughly half of patients and typically resolve within 24 to 48 hours. These are considered normal tissue responses to manual loading, comparable to post-exercise soreness.

Serious adverse events are rare. The most discussed risk is vertebral artery dissection following cervical manipulation, but large-scale epidemiological studies have struggled to establish a causal relationship. A 2017 systematic review in the journal Spine found no excess risk of vertebral artery stroke following chiropractic visits compared to primary care visits for the same complaint, suggesting that the association may reflect patients seeking care for symptoms of an already-developing dissection rather than the manipulation causing the event.

Absolute contraindications to spinal manipulation include fracture at the target site, active malignancy affecting the spine, severe osteoporosis, spinal cord compression, and signs of vascular compromise. These are screened for during the initial assessment. In New Zealand, chiropractors are required to maintain competence in patient safety screening as a condition of their annual practising certificate issued by the Chiropractic Board. The profession operates under the same complaints and disciplinary framework as other registered health professions.

After the First Visit

The 24 to 48 hours following a first adjustment are when the body responds to what was, mechanically, a novel input. Most patients report feeling looser or more mobile in the treated area. Some experience mild soreness at the site of the adjustment — a dull ache that fades within a day. Both responses are within normal range.

At the next visit, the practitioner reassesses. They repeat the motion palpation and orthopaedic tests from the initial examination, comparing the findings to the baseline. If the restricted segment has improved, the treatment plan may shift to adjacent areas or to rehabilitation exercises. If the response was limited, the practitioner may modify the approach — different positioning, a different technique, or a referral for further investigation.

Treatment frequency varies. Acute complaints might warrant two to three visits over a week or two, tapering as symptoms resolve. There is no evidence supporting indefinite maintenance adjustments for patients who are symptom-free, and New Zealand’s ACC guidelines reflect an expectation that treatment should be outcome-focused and time-limited. A responsible practitioner sets clear expectations about the likely number of sessions and the criteria for discharge or onward referral.

Spinal manipulation is a specific manual skill with a growing body of research behind it, though that research is still filling in significant gaps. The biomechanics are well characterised. The therapeutic mechanism is increasingly understood in neurophysiological rather than mechanical terms. And the clinical evidence, while not dramatic in its effect sizes, is strong enough to have shifted international guidelines toward recommending it. For someone considering a first visit, the procedure is considerably less mysterious than its reputation suggests.