The latest piece of evidence on the subject of how the brain drives the muscles pre and post chiropractic adjustment emerges from the New Zealand College of Chiropractic. It appeared in the most recent edition of the Journal of Electromyography and Kinesiology, and was titled “Chiropractic spinal manipulation alters TMS Induced I-wave excitability and shortens the cortical silent period” .
While the title of the study potentially gives away the punch-line, it is worth noting that this study is the latest in a line of research from the NZCC investigating what happens at the brain level when we adjust the spine. This stream of investigation has yielded some encouraging results for the field of chiropractic, as it is becoming increasingly clear that when we adjust, we change the function of the brain.
This is information that first emerged with Dr’s Heidi Haavik and Bernadette Murphy published a piece in the Journal Neural Plasticity revealing a significant (20%) difference in prefrontal cortex activity pre and post-adjustment . Lead researcher Heidi Haavik told Spinal Research :
“This is solid scientific evidence that adjusting the spine changes the way the prefrontal cortex of the brain is processing information from the arm. It demonstrates we change the way the brain works and shows that spinal function impacts brain function. One of the most interesting things about the changes we observed was that the prefrontal cortex is responsible for behaviour, goal directed tasks, decision-making, memory and attention, intelligence, processing of pain and emotional response to it, autonomic function, motor control, eye movements and spatial awareness (Read more of that interview here).
Since that 2016 offering, the team at the NZCC have offered up a number of further additions to that evidence bank. Among them are papers revealing the impacts of chiropractic care on cortical and cerebellar motor processing, and mental response times for complex rotation tasks [4, 5]. There is also evidence that bite force increases post chiropractic spinal manipulation . These studies are in addition to earlier papers revealing altered sensorimotor integration and transient modulation of intra-cortical inhibition following chiropractic care [7, 8].
This latest study took 19 participants (16 male and 3 female) aged between 23 and 39 with a history of reoccurring spinal pain or stiffness, and allocated them to either a spinal manipulation or a control intervention. All participants were pain free at the time of the trial.
They underwent transcranial magnetic stimulation that measured motor evoked potentials (MEPs) for the lower leg muscles (tibialis anterior), including their resting and active motor thresholds. MEP latency and the length of the cortical silent period (or the delay between the initiation of movement in the primary motor cortex to the muscle contraction in the leg) was measured pre and post-intervention for the spinal manipulation group and the control intervention group.
For the spinal manipulation group, the intervention included a full spine and sacroiliac joint assessment for vertebral subluxations. They were adjusted where deemed necessary by an experienced (10 years plus), registered chiropractor.
“The clinical indicators that were used to assess the function of the spine prior to and after each spinal manipulation intervention included assessing for tenderness to palpation of the relevant joints, manually palpating for restricted intersegmental range of motion, assessing for palpable asymmetric intervertebral muscle tension, and any abnormal or blocked joint play and end-feel of the joints. For this study a segment was defined as dysfunctional if at least three of these indicators were present at the same segmental level .”
The control intervention simply consisted of passive movements of the subject’s head, spine and body, after being assessed for vertebral subluxations and moved into spinal manipulation set-up positions.
While the study itself is rich in detail, and certainly warrants a full read, there were some key takeaways. The results for the length of the cortical silent period were significant: “Post hoc analysis revealed significant (P = 0.002) decrease of 19.42 ms in duration of inhibition after spinal manipulation only .” This effect was not present in the control group. According to the researchers, this study also yielded three novel findings:
“Firstly, this study demonstrated in human subjects the existence of individual I-waves using single motor unit recordings. The I-waves were observed in the single unit data as separated entities with significant peaks and are confirmed to be excitatory events as the discharge rate underlying them were higher than the background rate. Secondly, chiropractic manipulation significantly increased the amplitude of the first I-wave. Finally, the CSP [cortical silent period] duration was significantly reduced after spinal manipulation in lower limb muscle .”
It’s a study with clinical relevance as well as prompts for future research. It provides evidence that “spinal manipulation can result in a significant increase in the excitability of the motor pathways to low threshold motor units of human tibialis anterior muscle .”
The study was not segment-specific in its nature. That is, the chiropractor adjusted where the vertebral subluxation was found. Although this piece of research comes with plenty of opportunities for further investigation, it does gives us a hint as to the effect of the chiropractic adjustment on the primary motor cortexes of the brain. When we adjust for vertebral subluxations, it changes the way the brain communicates with the muscles.
Its an exciting development, and we can’t wait to see what else this line of research reveals.
 Haavik H, Niazi I, Jochumsen M, Ugincius P, Sebik O, Yilmaz G, Samran Navid M, Gorkem Ozyurt M and Turker K (2018), “Chiropractic spinal manipulation alters TMS induced I-wave excitability and shortens the cortical silent period,” Journal of Electromyography and Kinesiology, Volume 42 (2018), pp. 24-35
 Lelic, D, Niazi, IK, Holt, K, Jochumsen, M, Dremstrup, K, Yielder, P, Murphy, B, Drewes, A and Haavik, H (2016), “Manipulation of dysfunctional spinal joints affects sensorimotor integration in the pre-frontal cortex: A brain source localization study,” Neural Plasticity, Volume 2016 (2016). Online.
 Staff writer (2016). “Research: Beyond a doubt, adjusting the subluxated spine changes brain function,” Australian Spinal Research Foundation, https://spinalresearch.com.au/research-beyond-doubt-adjusting-subluxated-spine-changes-brain-function/ retrieved 28 June 2018
 Daligadu J, Haavik H, Yielder P, Baarbe J, and Murphy B (2013), “Alterations in Cortical and Cerebellar Motor Processing in Subclinical Neck Pain Patients Following Spinal Manipulation,” JMPT Vol 36, Iss 8, October 2013 pp. 527-537, https://doi.org/10.1016/j.jmpt.2013.08.003
 Baarbe J, Holmes M, Murphy H, Haavik H, Murphy B (2016), “Influence of Subclinical Neck Pain on the Ability to Perform a Mental Rotation Task: A 4-week Longitudinal Study with a Healthy Control Group Comparison,” JMPT Vol. 39, Iss. 1, Jan 2016 pp. 23-30, https://doi.org/10.1016/j.jmpt.2015.12.002
 Haavik H, Ozyurt M, Niazi I, Holt K, Nedergaard R, Yilmaz G, Turker K (2018), “Chiropractic Manipulation Increases Maximal Bite Force in Healthy Individuals,” Brian Sciences, 2018, 8, 76; doi:10.3390/brainsci8050076
 Haavik-Taylor, H., Murphy, B., 2007a. Cervical spine manipulation alters sensorimotor integration: a somatosensory evoked potential study. Clin. Neurophysiol.: Off. J. Int. Feder. Clin. Neurophysiol. 118, 391–402.
 Haavik-Taylor, H., Murphy, B., 2007b. Transient modulation of intracortical inhibition following spinal manipulation. Chiropract. J. Aust. 37, 106–116.