Heidi Haavik Taylor, BSc (Chiro), PhD , Kelly Holt, BSc (Chiro), PGDipHSc ,
and Bernadette Murphy, DC, PhD 
1. Director of Research, New Zealand College of Chiropractic, Auckland, New Zealand
2. Researcher, Research Department, New Zealand College of Chiropractic, Auckland, New Zealand.
3. Associate Professor, Faculty of Health Sciences, University of Ontario Institute of Technology, Ontario, Canada
This paper was voted best paper at the November 2009 Policy Forum and Scientific Symposium
The elusive vertebral subluxation is the central defining clinical principle of the chiropractic profession. After almost 115 years of discussion there is still little consensus regarding the nature of the vertebral subluxation or its potential associated neurological manifestations. Some authors even deny that the subluxation exists. In this paper a model is presented that assumes that the putative vertebral subluxation represents a state of altered afferent input which is responsible for ongoing maladaptive central plastic changes that over time can lead to dysfunction, pain and other symptoms. A growing body of research that investigates the neuromodulatory effects of chiropractic care supports this model. This paper explores this research and discusses it in light of the vitalistic principles upon which chiropractic was founded. The model outlined in this paper may go some way to explain some of the beneficial effects of chiropractic care on nonmusculoskeletal conditions previously reported in the literature.
The elusive vertebral subluxation is the central defining clinical principle of the chiropractic profession.  A century ago the founder of chiropractic, D.D. Palmer, described vertebral subluxations as “slightly displaced vertebrae which press against nerves causing impingements, the result being too much or not enough functionating.”  Palmer argued that vertebral subluxations cause inflammation which is stressful to the body and viscera and results in “lowered tissue resistance.”  He incorporated these concepts into a metaphysical philosophy of chiropractic which emphasised the supremacy of vital forces and the concept of the body possessing an Innate Intelligence which is compromised in its ability to care for and direct the vital functions of the body by nerve interference caused by vertebral subluxations.  The use of this vitalistic paradigm and metaphysical constructs in his philosophy of chiropractic led detractors to malign his early theories and accuse him of being unscientific.  However, others have recognized that metaphysics plays a very important role in science by being the science of first principles, i.e. the fundamental a priori assumptions that lay the foundation for any research programme. 
Over the past century various authors have described the vertebral subluxation using a multitude of models that range from theories of pinched nerves within the intervertebral foramen (that even D.D. Palmer had discounted by 1910), [2,3] through to more modern developments of Palmer’s early theories that postulate a neurodystrophic hypothesis where the vertebral subluxation leads to lowered tissue resistance by modifying hypothalamic function and ultimately immune responses.  It is interesting to note that after almost 115 years of discussion there is still little consensus regarding the nature of the vertebral subluxation or its associated neurological manifestations. Some authors even deny that they exist.  It is clear therefore that the chiropractic profession needs to continue to invest time and money into subluxation-focussed-research in order to better understand the clinical entity that defines the profession.
Over the past 15 years our research group has been conducting a variety of experiments aimed at testing out the theory that adjusting subluxations improves central nervous system functioning and overall expression of health and well being. To do this the theory was first formulated into a model (Figure 2) that could be scientifically tested with a programme of research studies. This model became the basis for the lead author’s PhD research,  and continues to be a foundational premise that our research group is attempting to elucidate with our work. The model was constructed using early chiropractic research data and a thorough review of the neurophysiology scientific literature. The model assumes that the putative vertebral subluxation represents a state of altered afferent input which is responsible for ongoing maladaptive central plastic changes that over time can lead to dysfunction, pain and other symptoms. Thus a potential mechanism which could explain how chiropractic adjustments improve function is that altered afferent feedback from a vertebral subluxation alters the afferent “milieu” into which subsequent afferent feedback from the spine and limbs is received and processed, thus leading to altered sensorimotor integration of the afferent input, which is then normalised by high-velocity, low-amplitude adjustments of the vertebral subluxation. This theory is plausible considering that it is now well established that the human central nervous system (CNS) retains its ability to adapt to its ever-changing environment, and that both increased (hyperafferentation) and decreased (deafferentation) afferent input leads to changes in CNS functioning. [6-8]
Figure 1: Vertebral Subluxation may lead to altered sensorimotor integration
Figure 2: Chiropractic adjustments may normalise afferent input and therefore promote appropriate sensorimotor integration
It is the aim of this paper to discuss some of the studies conducted by our group over the past 15 years and to consider how this work relates to our model and its a priori vitalistic assumptions. By developing a greater understanding of the effects of the vertebral subluxation on sensorimotor integration researchers may be able to help elucidate some of the many beneficial clinical effects reported by chiropractors in day to day practice.
To attempt to provide any clarification regarding the neurophysiological effects, if any, of adjusting subluxated spinal segments, it was first necessary to explore whether adjusting the spine had any lasting central neural effects at all. Very limited evidence for this existed 15 years ago. [9, 10] According to our model above, adjusting subluxated spinal segments should alter sensorimotor processing, sensorimotor integration and motor control.
Numerous activities of daily living are dependent on appropriate sensorimotor integration. Interactions between sensory and motor systems allow us to engage with our environment, they allow us to reach for and grasp an object, turn towards an auditory stimulus or respond to perturbations of the environment in order to maintain postural stability and balance. 
Figure 3: Sensorimotor integration illustrated by the Action/Perception Cycle
Sensorimotor integration involves strong feedback connections between different brain structures that are associated with numerous, and perhaps all, neuroanatomical subsystems.  These subsystems interconnect to form a dynamic, multimodal, sensorimotor integrative system. This system is dependent on motoric responses to reafferent proprioceptive signals in order to complement and define further sensory input. This adds organisational complexity which results in a higher order sensorimotor integrative system that may be said to have emergent properties.  This is similar to the emergent properties of consciousness, where a ‘thought’ is not made up of a single synapse or active neuron but involves a much higher level of processing and integration and cannot necessarily be explained by its constituent parts.  A breakdown anywhere in these multimodal sensorimotor feedback loops has the potential to influence other interconnected neuroanatomical subsystems, or, perhaps more importantly, the emergent properties of the higher order system.  Therefore, if a vertebral subluxation creates neuroplastic changes (i.e. lasting functional neurophysiological changes) in the CNS due to altered afferent input, its impact on the higher order sensorimotor integrative system may have neurological manifestations that extend well beyond the mechanical site of the lesion. There is, for example, a growing body of evidence that suggests that sensorimotor integration involves highly complex emergent properties that are linked to adaptation and homeostasis  and that chiropractic adjustments influence many of these integrative neural processes such as proprioception, somatosensory processing and feed forward activation. [13-15]
Proprioception is an important component of sensorimotor integration in the CNS. Proprioception includes joint position sense (JPS) and kinaesthesia (the sense of limb movement in the absence of visual cues). [for review see 16] The main source of afferent information for JPS arises from muscle spindles. [for review see 16] However, both mechanoreceptors in joint capsules and cutaneous tactile receptors may also contribute. [for review see 16] Joint position sense has been extensively studied in the ankle, knee and hip joints, [17-29] particularly to investigate the effects of reconstructive surgery, [19, 24, 25] osteoarthritis, [28, 30, 31] joint bracing, [for review see 26] and various exercise or re-training programmes. [17, 18, 21, 32] Recently there has also been an increased focus in the literature on spinal JPS, [33-37] however, much less research has looked at the effect of the spine on limb JPS. [38-40]
Accurate JPS is very important for balance and for the regulation of locomotion.  Impaired balance and locomotion control are known to impact the falls-risk in the elderly, [42, 43] which is a major health concern for this population group. [44-49] Impaired ankle and knee proprioception has been demonstrated in elderly populations compared to younger groups. [50, 51] This is thought to negatively impact balance and locomotor control, leading to more falls. [50, 51] In addition to the sensorimotor system, the vestibular system is vital for good balance and reduced risk of falls. However, the function of the vestibular system is known to decline with age, thus the elderly must rely more heavily on their proprioceptive system to maintain good balance and normal locomotor control.  There are numerous studies that implicate cervical spine impairment in reduced postural control, for example, due to chronic neck pain, [53, 54] neck muscle fatigue,  cervicobrachial pain syndrome,  cervical root compression, [57, 58] head injury or whiplash injury. [55, 59] Therefore, there appears to be a considerable link between cervical function and accurate proprioceptive processing and thus postural control. Although most of these previous studies related to significant cervical problems, one recent study has demonstrated that changes in head and neck position in a group of subjects without any history of neck pain or injury led to reduced accuracy of elbow joint position sense (JPS).  The authors of this study discussed how accurate execution of movement depends on the ability of the CNS to integrate somatosensory, vestibular, and visual information regarding the position of the body.  They argued that placing their subjects’ heads in full flexion and rotation could have led to an overload of the computational capacity of the CNS, thus resulting in increased JPS error.  The same group of researchers also demonstrated that people with whiplash associated disorder (WAD) are affected by smaller angles of neck rotation than individuals who had no history of WAD,  further suggesting that cervical spine dysfunction leads to reduced accuracy of JPS. Recent research, utilising somatosensory evoked potentials  and transcranial magnetic stimulation, [62, 63] has shown that adjusting vertebral subluxations in the cervical spine of patients without frank neck pain, but with a history of some form of subclinical neck pain syndrome (SCNPS) (i.e. individuals with reoccurring neck dysfunction such as minor neck pain, ache and/or stiffness, but asymptomatic between episodes, and who have not considered their symptoms severe enough to seek treatment) [64, 65] can alter cortical somatosensory processing, sensorimotor integration of input from the upper limb, and motor control of upper limb muscles. It is therefore possible that such changes in CNS processing following cervical adjustments could include alterations in proprioceptive processing.
Another important property of the CNS is its ability to gate sensory information. It is thought that this is necessary for the CNS to maintain the internal representation of its current posture and to avoid undesirable reactions to external or internal perturbations. [65, 66] Tinazzi et al have shown that gating of sensory information is distorted in patients with focal hand dystonia.  The authors argued that there was a lack of surround-like inhibition (a neural mechanism that focuses neuronal activity and is considered a fundamental property of retinal ganglion cells and the circuitry of the visual system)  of mainly proprioceptive afferent input in these patients, and that this inefficient integration could give rise to the abnormal motor output and might therefore contribute to the motor impairment present in dystonia patients.  Other groups have also demonstrated that there is a shift in the gain of the sensory signals, i.e. a central re-weighting of proprioceptive input, in patients with spasmodic torticollis  and low back pain patients.  A recent study demonstrated that adjusting dysfunctional cervical segments in SCNPS patients can increase the surround-like inhibition of proprioceptive afferent input,  again suggesting the possibility of central mechanisms of action for high-velocity, low-amplitude spinal adjustments.
Recent research utilising somatosensory evoked potentials (SEPs)  and transcranial magnetic stimulation (TMS) [62, 63] has shown that adjusting dysfunctional segments in the cervical spine can alter somatosensory processing, sensorimotor integration of input from the upper limb, and motor control of upper limb muscles. These studies have shown alterations in the processing of the cortical SEP peaks N20 and N30 following high-velocity, low-amplitude cervical adjustments.  The N20 SEP peak represents processing at the primary somatosensory cortex [72-74] and thus reflects cortical perception. The neural generator(s) of the N30 SEP component remains more controversial. Although some authors suggest this peak is generated in the post-central cortical regions (i.e. S1), [75-77] most evidence suggests that this peak is related to a complex cortical and subcortical loop linking the basal ganglia, thalamus, pre-motor areas, and primary motor cortex. [78-86] The N30 peak is therefore thought to reflect sensorimotor integration.  This means that adjusting cervical vertebral subluxations can alter cortical perception and sensorimotor integration of information from the upper limb. It has also recently been demonstrated that adjusting cervical vertebral subluxations alters cortical upper limb muscular control in a muscle specific manner. [63,88] The TMS experimental measures utilised in these studies, such as short-interval-intracortical-inhibition (SICI), short-interval-intracortical-facilitation (SICF) and the cortical silent period (CSP), all reflect sensorimotor integration and are believed to reflect processing at the level of the cortex. [89-100]
Muscle perception impairments are also present in chronic neck pain patients. Impairment of deep cervical neck flexors and significant postural disturbances during walking and standing have been demonstrated in both insidious-onset and trauma-induced chronic neck pain conditions. [56, 101-106] Altered sensitivity of proprioceptors within the neck muscles has been suggested to be related to the postural (i.e. motor control) disturbances seen in these patients. [102, 106] It has also been argued that the degree to which proprioceptive input to the central nervous system is disturbed and possibly even more importantly how the CNS processes, interprets and transforms this afferent information into motor commands determines the degree to which subjects can successfully execute more challenging balance tasks. [65, 106] It is therefore possible that adjusting vertebral subluxations in patients with sub-clinical or chronic neck pain actually alters the central processing of proprioceptive information, and that this in part is the mechanism by which high-velocity, low-amplitude spinal adjustments reduce pain and improve function in these patient populations. It is possible that the changes in cortical somatosensory processing, [61, 107, 108] sensorimotor integration [61-63] and motor control [62, 63, 109-111] that have been previously documented following high-velocity, low-amplitude spinal adjustments reflect changes in central processing of proprioceptive afferent input.
Centrally Modulated Pain
The CNS utilises peripheral signals continuously to build and maintain an internal reference frame. [112, 113] Motor commands or motor intention (also known as “efference copies”) are also known to interact with afferent signals to generate sensation, and are known to contribute to joint position sense.  Under normal circumstances there is an integration of intention, action and sensory feedback. Furthermore, in a healthy state there is congruence between motor intention and sensory experience (both proprioceptive and visual) when we for example move a limb through space. Thus goal-directed action requires ongoing monitoring of sensorimotor inputs to ensure that motor outputs are congruent with current intentions. This monitoring is automatic but can become conscious if there is a mismatch between expected and realised sensorimotor states. A recent study has demonstrated that providing a sensory–motor conflict, i.e. providing unexpected visual feedback when moving a limb (via hiding a moving limb and/or distorting visual feedback of the movement of that limb) is sufficient to produce additional somaesthetic disturbances, and exacerbation of pre-existing symptoms in a group of fibromyalgia patients.  This suggests that a conflict between our expected and realised sensorimotor states can in some individuals produce or worsen pain sensations. It is therefore possible that a mechanism by which spinal adjustments relieve pain in patients is due to a central effect by improving somatosensory integration processes and removing the conflict between the expected and actual sensorimotor state.
Feed Forward Activation
When performing bodily movements, like throwing a ball for example, the central nervous system will activate a variety of postural muscles prior to any movement of the arm in order to maintain postural stability during the throwing action. This process is known as feed-forward activation (FFA). Delays in FFA are known to occur in individuals suffering from chronic low back pain.  Based on our model, such a delay in muscle activation would be an example of altered motor control.
We were interested to understand what the incidence of delayed feed-forward activation might be in an asymptomatic population and whether this might be related to underlying vertebral subluxations. In order to do this, we selected a uniform population of 90 healthy young males who were evaluated for delays in FFA of the transversus abdominis muscle and internal obliques when undertaking rapid movements of the upper limb. Seventeen subjects had a delay in FFA which was reproducible when retested six months later. These subjects were examined by a chiropractor and were all found to have a sacroiliac joint subluxation on the side of delayed FFA. Following a single chiropractic adjustment of the subluxated sacroiliac joint the FFA activation time improved by an average of 38%.  This study demonstrated an improvement in central nervous system control of muscles associated with the stability of a specific joint due to a chiropractic adjustment. Only one prospective study has investigated the potential role of delayed trunk muscle activation in actually predicting low back pain over a two year period.  The authors found that the odds of sustaining a low back injury increased 2.8-fold when a history of low back pain was present and increased by 3% with each millisecond of abdominal muscle shut-off latency. They found that the latency was an average of 14 milliseconds longer for athletes who sustained low back pain as compared to those who didn’t. Considerably more work needs to be done in this area to determine whether delayed trunk muscle latencies may be a marker of disordered sensorimotor integration, and whether the improvement in activation is sustained following chiropractic care.
Relationship to Observations in Practice
We are currently developing a questionnaire to measure self-reported “body awareness” which might be linked to impaired sensorimotor integration. This questionnaire could be used to assess the incidence of disordered sensorimotor integration in a chiropractic patient group. Some sample questions which reflect the sort of things that a patient with disordered sensorimotor integration may experience include:
- Have you noticed that you have been hitting your head getting out of the car since your neck has been sore?
- Have you been bumping your wrists or elbows more frequently?
- Have you had trouble seeing clearly or focusing on objects since your neck has been sore?
- Have you felt clumsy or uncoordinated since your neck has been sore?
The fact that chiropractic adjustments result in such a plethora of changes to sensorimotor integration is interesting when considering the possible mechanisms associated with the beneficial clinical effects of chiropractic care. Researchers from many diverse fields have recently suggested that sensorimotor integration involves multiple layers of processing that display emergent properties which cannot be explained by individual neurons and pathways. [11, 12, 118] Of interest when considering the vitalistic principles upon which chiropractic was founded is that some researchers are now proposing that emergent signals from optimal sensorimotor integration may underlie appropriate adaptation of respiratory patterns and homeostasis.  This may go some way to explain some of the beneficial effects of chiropractic care on nonmusculoskeletal conditions previously reported in the literature.  Many of the studies discussed in this paper show that chiropractic adjustments result in changes to sensorimotor integration within the central nervous system. What is not yet clear is whether these changes correlate with beneficial clinical outcomes or not. It is also not clear whether these changes are due to the correction of a vertebral subluxation, therefore normalising aberrant afferent input to the CNS, or are they merely due to an afferent barrage associated with the adjustive thrust. These questions remain to be answered and are the focus of our ongoing research efforts.
The authors would like to thank the following organisations for providing funding to help conduct their research over the past fifteen years: Australian Spinal Research Foundation, the Hamblin Chiropractic Research Fund Trust, the University of Auckland Vice Chancellors Fund, New Zealand Tertiary Education Commission Top Achievers Doctoral Scholarship and the Foundation for Chiropractic Education and Research.
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