There is no easy explanation of what happens during spinal manipulation. However, there are many theories and some evidence, according to a systematic literature review from 2024. In this review, a group of researchers evaluated and summarized peer-reviewed literature on the current evidence regarding anatomical changes in response to spinal manipulation.
As examples of the theories, the researchers refer to various papers claiming that spinal manipulation alters the structures of the spine—for instance, by an increase in facet joint space post-SM or changing the resting muscle thickness. It is suggested that these changes contribute to the patient feeling better.
The systematic review screened nearly 20,000 scientific papers and identified eight credible studies about anatomical changes related to spinal manipulation. The findings indicated that facet joint space increases temporarily and spinal stiffness decreases. However, resting muscle thickness does not appear to change.
Based on their review, the researchers suggest the following explanation, which you can adapt for your practice and patients. Regarding anatomical or positional changes, you could say:
"There is no simple answer, because the spine is a difficult area to study. It seems likely that the manipulation/adjustment causes some physical changes, but it is not known exactly how. Presently, though, we are fairly confident that the facets, i.e., the small joints at the back of the spine, open up a little bit. There also seems to be a measurable change in the stiffness of the spine immediately after manipulation. We assume that these changes are part of what helps you feel better."
Additionally, the researchers call for more studies on the anatomical mechanisms activated by spinal manipulation to better understand how the treatment works.
Kenneth J. Young et al. Mechanisms of manipulation: a systematic review of the literature on immediate anatomical structural or positional changes in response to manually delivered high-velocity, low-amplitude spinal manipulation. Chiropractic & Manual Therapies 2024.