A chiropractic adjustment is a precise, manual procedure performed on a specific spinal joint to improve its function. The most common technique is the High-Velocity, Low-Amplitude (HVLA) thrust, which delivers a rapid, short movement to a spinal segment. The primary goal of this controlled input is to restore a normal range of motion to a joint that has become restricted or “locked.” The mechanical and physiological changes that follow this quick thrust are immediate and complex.
The Components of the Spinal Column
The spine is a segmented column built of individual bones called vertebrae, separated by intervertebral discs. Spinal movement is primarily guided and limited by the facet joints, also known as zygapophyseal joints. These paired joints are located at the back of each vertebral level and are the direct target of the HVLA adjustment. Facet joints are synovial joints, meaning they are enclosed by a capsule and contain lubricating synovial fluid. When a joint becomes restricted, the surrounding vertebrae and intervertebral discs provide the necessary structural support for the adjustment to occur.
The Mechanics of Joint Cavitation
The audible “pop” or “crack” associated with an adjustment is known as joint cavitation. This mechanical event results from the rapid separation of the facet joint surfaces during the thrust, causing a sudden drop in pressure within the joint capsule. This pressure drop creates a vacuum effect within the lubricating synovial fluid, pulling dissolved gases like carbon dioxide and nitrogen out of the solution to form a temporary bubble. The sound is believed to be caused by the formation or subsequent collapse of this gas bubble, a process called tribonucleation. For cavitation to occur, the joint must be taken just past its normal physiological limit, but the therapeutic benefit stems from the mechanical gapping and neurological response, regardless of the sound.
The Immediate Neurological Response
The high-velocity thrust triggers an immediate reaction within the nervous system. The joint capsules and surrounding paraspinal muscles are densely populated with sensory nerve endings called mechanoreceptors, which are highly sensitive to changes in movement and pressure. The rapid input from the adjustment stimulates these mechanoreceptors, sending a barrage of signals to the central nervous system. This sensory input is theorized to override or “gate” the pain signals (nociception) traveling from the restricted joint to the brain, similar to the Gate Control Theory of pain. By restoring proper motion, the adjustment also normalizes proprioceptive input, providing clearer sensory information that enhances motor control, coordination, and postural stability.
Changes in Surrounding Soft Tissues
The mechanical and neurological events within the joint have rapid effects on the surrounding soft tissues, primarily the ligaments and muscles. A restricted joint is often guarded by hypertonic, or excessively tight, muscles in a state of protective spasm. The neurological input from the adjustment leads to a reflexive relaxation of these hypertonic muscles, evidenced by studies showing an immediate reduction in the resting electrical activity of the paraspinal muscles. Restoring motion to the facet joint also momentarily deforms the joint capsule and ligaments, which contributes to a temporary increase in the joint’s range of motion. This collective effect—joint gapping, muscle relaxation, and neurological modulation—works to reduce protective splinting and restore more fluid movement to the spinal segment.