One of the many forms of treatment that physical therapists use is spinal manipulation. This involves placing the joints of the spine in a safe position and performing a quick stretch typically eliciting a “pop”. This “pop” is what you as the patient and we as the physical therapist love so much…that crisp sound of pain blasting from your neck or back. At that moment of bliss there are many things happening at the level of the musculoskeletal and nervous systems. That is what we are going to discuss today… What could possibly cause that much pain relief? Could it be the restoration of joint mobility? Could it be the renewed alignment of the spine? Let’s discuss.
When the tissues of the spine and surrounding structures become chemically irritated/inflamed there is a change that occurs in the nervous system. This chemical change causes a sensitization in the afferent (sensory) pathways leading to increased pain sensation and decreased localization of pain, central sensitization. This also leads to increased tightness present in the paraspinal musculature. These muscles tend to be very tender with decreased pain/pressure threshold, or the amount of pressure tolerated until it is perceived as pain. This tightening is thought to be related to the body’s reflexes, specifically the H-reflex. These reflexes are protective mechanisms in the body to reduce the risk of injury and are triggered by mechanoreceptors in the spinal joints and surrounding muscle. Though this could be a major contributing factor to the sensation of pain it also seems to be a window into pain relief and improved function. Physical therapists can utilize these receptors to elicit an inhibitory response from the same reflex causing relaxation and the blocking/gating of the sensation of pain.
One study, performed by Vernon, to assess the effects of spinal manipulation on pain/pressure threshold found that following spinal manipulation to the cervical spine patients were able to tolerate 50% more pressure before it was perceived as pain. This indicates that spinal manipulation has an inhibitory effect in the sensory pathways. In another study by Terret and Vernon, patients had electrical current applied to paraspinal tissue in a graded fashion until pain was perceived. The patients then underwent spinal manipulation and their tolerance to the stimulus increased up to 2.4 times the original tolerance. This indicates, once again, that there is some effect in the sensory pathways that can be elicited by spinal manipulation. This seems to be occurring due to mechanical input into the facet joints/capsules and muscle spindles and GTO of surrounding musculature. This mechanical input elicits a response from afferent pathways triggering a decrease in sensitivity, an increase in pain pressure threshold, inhibition to the H-reflex leading to decreased resting muscle tone/tightness all working to decrease central sensitization and decrease pain.
No, physical therapists are not moving joints back to their original location or improving joint motion so significantly with one pop as to improve your range of motion 30 degrees. Physical therapists are utilizing the mechanisms put in place by the human body to protect itself to our advantage. Manipulations are utilized to elicit a neurophysiological reset decreasing muscle spasm, decreasing pain, and increasing pain threshold through stimulation of mechanoreceptors. This then allows the patient to perform corrective exercise and load the associated tissues to make meaningful change.
- Pickar JG. Neurophysiological effects of spinal manipulation. The Spine Journal. 2002;2(5):357-371. doi:10.1016/s1529-9430(02)00400-x