What Is An Adjustment Actually Doing? - The Physiology of Spinal Manipulation

Spinal manipulative therapy (SMT), commonly known as a "chiropractic adjustment," is a widely used therapeutic intervention for musculoskeletal conditions. Despite its prevalence, the precise physiological mechanisms underlying its effects remain an area of active scientific investigation. This article explores the current evidence regarding what happens physiologically during and after spinal manipulation.

Joint Mechanics During Manipulation

When a chiropractor performs an adjustment, they typically apply a high-velocity, low-amplitude (HVLA) thrust to a specific joint, typically taking it slightly beyond its normal physiological range of motion (the range you can move without external force) but within its anatomical limits (the total range of motion that can occur in a joint before injury occurs).

This creates several simultaneous mechanical effects:

1. Joint Cavitation: The quick separation of joint surfaces creates a temporary vacuum within the synovial fluid, causing dissolved gases to form bubbles that then collapse or "pop" - creating the characteristic sound associated with adjustments (Kawchuk et al., 2015).

2. Gap Formation: Studies using magnetic resonance imaging have confirmed that SMT creates a temporary increase in joint space, with measurable separation of articulating surfaces (Cramer et al., 2012).

3. Tissue Strain: The applied force creates measurable stress across multiple tissue types including the joint capsule, ligaments, and muscle fibers (Herzog, 2010).

Effects on Surrounding Structures

Muscles

SMT appears to have several effects on surrounding musculature:

• Reflexive Inhibition: Evidence suggests manipulation can reduce muscle hypertonicity (how tight a muscle is) through reflexive pathways. EMG studies show decreased electrical activity in paraspinal muscles following SMT (Pickar & Bolton, 2012).

• Muscle Spindle Response: The high-velocity thrust stimulates muscle spindles, which may reset aberrant firing patterns and normalize proprioceptive input (Reed et al., 2014). This means it can re-regulate muscle function and tone.

Ligaments and Tendons

Connective tissues experience mechanical deformation during manipulation:

• Viscoelastic Response: The quick stretch applied to ligaments and tendons during SMT appears to temporarily alter their viscoelastic properties, potentially reducing stiffness (Solomonow, 2009).

• Mechanoreceptor Stimulation: Ligaments contain numerous mechanoreceptors, nerve that tell the brain what your joints and muscles are doing and how they are positioned at any given time. When these nerves are stimulated during SMT, they send signals to the central nervous system that may help normalize proprioceptive function (Cao et al., 2013). In this way, the adjustment improves your brain-body communication and improves feedback.

Somatosensory System

Your somatosensory nerves are nerves that send information from the body to the brain. You have a region in your brain called the somatosensory cortex where this information gets processed.

SMT creates profound input to the somatosensory system:

• Altered Afferent (Sensory) Input: The mechanical stimulation caused by an adjustment activates a type of sensory nerve we talked about previously: mechanoreceptors. These nerves are found n the joint capsules, muscles, and ligaments. When movement occurs, they are in charge of sending a barrage of sensory information to the spinal cord and brain (Boal & Gillette, 2004). They tell your brain what your body is doing. The adjustment improves the accuracy and speed of this feedback.

• Proprioceptive Recalibration: Evidence suggests manipulation may help "reset" aberrant proprioceptive signaling, improving joint position sense and movement coordination (Haavik & Murphy, 2012).

Pain Modulation

Multiple mechanisms appear to contribute to SMT's analgesic effects:

• Gate Control Mechanism: The adjustment provides a mechanical stimulation of sensory nerves may inhibit pain transmission. (Bialosky et al., 2009). This is a good thing, since it can shut off inaccurate pain signals that can occur when sensory nerves are not functioning optimally

• Central Pain Processing: Functional MRI studies have demonstrated that SMT influences central pain processing networks, including reduced activity in pain-related brain regions (Gay et al., 2014). "Central pain processing networks" is a fancy way of saying, things happening in your brain and spinal cord itself, as opposed to the nerves directly in your trunk and limbs.

This means that adjustments modulate the pain response both in the joints themselves (peripherally), and in our brain (centrally).

The "Popping" Sound Explained

The characteristic cracking sound associated with joint manipulation has been extensively studied:

• Cavitation Phenomenon: The sound results from tribonucleation - the formation and subsequent collapse of gas bubbles within the synovial fluid when joint surfaces rapidly separate (Kawchuk et al., 2015).

• Refractory Period: Following cavitation, the joint enters a refractory period of 15-30 minutes during which another cavitation is difficult to elicit as the gases gradually redissolve into the synovial fluid (Cramer et al., 2011)

This means the "crack" or "pop" you may hear during adjustment isn't your bones, it's just gas.

Does an Adjustment Release Endorphins? - Evidence vs. Myth

The claim that SMT causes endorphin release has some supporting evidence:

• Beta-Endorphin Levels: Several studies have detected transient increases in serum beta-endorphin levels following SMT (Vernon et al., 1986; Christian et al., 1988), though not all studies show consistent results.

• Opioid Receptor Involvement: Some analgesic effects of SMT can be partially blocked by naloxone (an opioid antagonist), suggesting endogenous opioid involvement in pain reduction (Vicenzino et al., 2000).

Current Assessment: While some evidence supports endorphin release following SMT, the clinical significance remain uncertain. Multiple pain-inhibitory mechanisms likely work in concert rather than endorphins alone (Coronado et al., 2012).

Other Affects of SMT on the Body

Adjustments Can Decrease Inflammation

Research on SMT's effects on inflammation shows complex results:

• Cytokine Response: Cytokines are a pro-inflammatory substance our bodies create. Some studies report decreased levels of cytokines after SMT (Teodorczyk-Injeyan et al., 2006), while others show a more complex pattern of inflammatory modulation.

• Substance P Reduction: Substance P is a neuropeptide involved in pain and inflammation signaling. There is evidence that levels of Substance P may be reduced after spinal manipulation. (Molina-Ortega et al., 2014).

Adjustments Can Improve Blood Flow

SMT appears to influence local and regional blood flow:

• Muscle Perfusion: Studies using near-infrared spectroscopy and Doppler ultrasound have detected increased blood flow to paraspinal muscles following SMT (Budgell & Polus, 2006).

• Sympathetic Nervous System Effects: SMT may influence sympathetic outflow, potentially affecting vascular tone and blood flow regulation (Kingston et al., 2014).

• Tissue Oxygenation: Increased local blood flow may enhance oxygen and nutrient delivery while improving metabolic waste removal, potentially aiding tissue healing (Cramer et al., 2010).

Conclusion

The physiological effects of spinal manipulative therapy operate through multiple interconnected mechanisms involving mechanical, neurological, and possibly biochemical pathways. Current evidence suggests SMT influences joint mechanics, muscle function, proprioception, pain processing, and potentially local tissue biochemistry. While our understanding continues to evolve, the multifaceted physiological response to SMT helps explain its observed clinical effects on pain reduction and function improvement in certain musculoskeletal conditions.

References

Bialosky, J. E., Bishop, M. D., Price, D. D., Robinson, M. E., & George, S. Z. (2009). The mechanisms of manual therapy in the treatment of musculoskeletal pain: a comprehensive model. Manual Therapy, 14(5), 531-538.

Boal, R. W., & Gillette, R. G. (2004). Central neuronal plasticity, low back pain and spinal manipulative therapy. Journal of Manipulative and Physiological Therapeutics, 27(5), 314-326.

Brodeur, R. (1995). The audible release associated with joint manipulation. Journal of Manipulative and Physiological Therapeutics, 18(3), 155-164.

Budgell, B., & Polus, B. (2006). The effects of thoracic manipulation on heart rate variability: a controlled crossover trial. Journal of Manipulative and Physiological Therapeutics, 29(8), 603-610.

Cao, D. Y., Reed, W. R., Long, C. R., Kawchuk, G. N., & Pickar, J. G. (2013). Effects of thrust amplitude and duration of high-velocity, low-amplitude spinal manipulation on lumbar muscle spindle responses to vertebral position and movement. Journal of Manipulative and Physiological Therapeutics, 36(2), 68-77.

Christian, G. F., Stanton, G. J., Sissons, D., How, H. Y., Jamison, J., Alder, B., ... & Fullerton, J. (1988). Immunoreactive ACTH, β-endorphin, and cortisol levels in plasma following spinal manipulative therapy. Spine, 13(12), 1411-1417.

Coronado, R. A., Gay, C. W., Bialosky, J. E., Carnaby, G. D., Bishop, M. D., & George, S. Z. (2012). Changes in pain sensitivity following spinal manipulation: a systematic review and meta-analysis. Journal of Electromyography and Kinesiology, 22(5), 752-767.

Cramer, G. D., Gregerson, D. M., Knudsen, J. T., Hubbard, B. B., Ustas, L. M., & Cantu, J. A. (2011). The effects of side-posture positioning and spinal adjusting on the lumbar Z joints: a randomized controlled trial with sixty-four subjects. Spine, 36(11), E430-E437.

Cramer, G. D., Cambron, J., Cantu, J. A., Dougherty, P., Buehler, D., & Gregerson, D. (2010). Magnetic resonance imaging zygapophyseal joint space changes (gapping) in low back pain patients following spinal manipulation and side-posture positioning: a randomized controlled mechanisms trial with blinding. Journal of Manipulative and Physiological Therapeutics, 33(4), 279-287.

Cramer, G. D., Henderson, C. N., Little, J. W., Daley, C., & Grieve, T. J. (2012). Zygapophyseal joint adhesions after induced hypomobility. Journal of Manipulative and Physiological Therapeutics, 35(1), 45-54.

Gay, C. W., Robinson, M. E., George, S. Z., Perlstein, W. M., & Bishop, M. D. (2014). Immediate changes after manual therapy in resting-state functional connectivity as measured by functional magnetic resonance imaging in participants with induced low back pain. Journal of Manipulative and Physiological Therapeutics, 37(9), 614-627.

Haavik, H., & Murphy, B. (2012). The role of spinal manipulation in addressing disordered sensorimotor integration and altered motor control. Journal of Electromyography and Kinesiology, 22(5), 768-776.

Herzog, W. (2010). The biomechanics of spinal manipulation. Journal of Bodywork and Movement Therapies, 14(3), 280-286.

Kawchuk, G. N., Fryer, J., Jaremko, J. L., Zeng, H., Rowe, L., & Thompson, R. (2015). Real-time visualization of joint cavitation. PloS One, 10(4), e0119470.

Kingston, L., Claydon, L., & Tumilty, S. (2014). The effects of spinal mobilizations on the sympathetic nervous system: a systematic review. Manual Therapy, 19(4), 281-287.

Molina-Ortega, F., Lomas-Vega, R., Hita-Contreras, F., Plaza Manzano, G., Achalandabaso, A., Ramos-Morcillo, A. J., & Martínez-Amat, A. (2014). Immediate effects of spinal manipulation on nitric oxide, substance P and pain perception. Manual Therapy, 19(5), 411-417.

Pickar, J. G., & Bolton, P. S. (2012). Spinal manipulative therapy and somatosensory activation. Journal of Electromyography and Kinesiology, 22(5), 785-794.

Reed, W. R., Long, C. R., Kawchuk, G. N., & Pickar, J. G. (2014). Neural responses to the mechanical parameters of a high-velocity, low-amplitude spinal manipulation: effect of preload parameters. Journal of Manipulative and Physiological Therapeutics, 37(2), 68-78.

Solomonow, M. (2009). Ligaments: a source of musculoskeletal disorders. Journal of Bodywork and Movement Therapies, 13(2), 136-154.

Teodorczyk-Injeyan, J. A., Injeyan, H. S., & Ruegg, R. (2006). Spinal manipulative therapy reduces inflammatory cytokines but not substance P production in normal subjects. Journal of Manipulative and Physiological Therapeutics, 29(1), 14-21.

Vernon, H. T., Dhami, M. S., Howley, T. P., & Annett, R. (1986). Spinal manipulation and beta-endorphin: a controlled study of the effect of a spinal manipulation on plasma beta-endorphin levels in normal males. Journal of Manipulative and Physiological Therapeutics, 9(2), 115-123.

Vicenzino, B., Collins, D., & Wright, A. (2000). The initial effects of a cervical spine manipulative physiotherapy treatment on the pain and dysfunction of lateral epicondylalgia. Pain, 83(3), 435-441.​​​​​​​​​​​​​​​​