Chiropractic Journal of Australia 2018; 45 (4): 338–358 ~ FULL TEXT
Amy Louise Haas, PhD, DC, David Russell, BSc (Psych), BSc (Chiro), Cert TT
Nashua, NH, USA
Objective: The purpose of this study was to report the sustained changes in
heart rate variability (HRV) observed in 6 patients undergoing continuous
chiropractic care for the correction of vertebral subluxations.
Clinical Features: Six patients between 25 to 55 years of age all presented with
primarily musculoskeletal complaints for chiropractic care in a private practice
setting. All patients were nonsmokers with no reported cardiac pathology. All
patients were initially assessed for indicators of vertebral subluxation before
being accepted for chiropractic care, and were monitored for changes in HRV
scores over time.
Intervention and Outcomes: Chiropractic care, using Diversified and Thompson
techniques to correct vertebral subluxations, was provided for an initial period of
10 to 52 weeks at a frequency of 2 to 3 visits per week. HRV, measured by
SSDN, increased over the early part of their course of chiropractic care, and
these increases were sustained whilst the patient remained under long term
continuous care in all 6 patients. Improvements in SDNN ranged from 50% to
greater than 300% as compared to pre-care values.
Conclusion: Patients receiving continuous chiropractic care to correct vertebral
subluxation demonstrated a sustained improvement in heart rate variability (HRV). This novel finding
objectively demonstrates long-term change consistent with improved
neurophysiological regulation, adaptability and resilience in patients undergoing
chiropractic care, and suggests the utility of chiropractic care for outcomes
greater than only musculoskeletal improvements.
Key Indexing Terms: Chiropractic; Heart Rate Variability
The primary objective of chiropractic care is to optimize health and wellbeing
through the enhancement of nervous system function by reducing nerve
interference caused by vertebral subluxations. [1–3] The Australian Spinal
Research Foundation defines vertebral subluxation as “a diminished state of
being, comprising of a state of reduced coherence, altered biomechanical
function, altered neurological function and altered adaptability.”  A vertebral
subluxation has been recognised as a complex of functional and/or structural
changes in the articulations of the spine and pelvis that compromise neural
integrity and may influence organ system function and general health.  The
correction of vertebral subluxations is achieved through chiropractic adjustments
that are a typically manually performed. [1, 2, 6] Research over the past 2
decades has shown that the chiropractic adjustment (also referred to as
chiropractic spinal manipulation in the chiropractic research literature) results in
changes in spinal biomechanics and structure [7–11], central nervous system
function [12–16], motor output [17–20], and autonomic output. [21, 22]
Using objective measures of spinal and neurological function provides the means
to quantitatively observe the effects of the chiropractic adjustment. Objective
measures used in chiropractic clinical practice to identify the site of intervention
and/or to measure outcomes often include both musculoskeletal assessments
such as pre- and post-adjustment x-ray, leg length inequality, posture or gait
changes, sEMG, algometry, range of motion, motion and static palpation ,
and non-musculoskeletal metrics such as paraspinal thermal balance, heart rate,
blood pressure, respiration, reaction time, head repositioning sense, and balance
testing. [23, 24] While each measure represents a unique and targeted view on
structural and/or physiological changes associated with the chiropractic
adjustment, employing alternative outcomes assessment technology allows for
the opportunity to expand our understanding of the effects of the adjustment in a
way that reflects global changes in autonomic nervous system function and
adaptability, and also interfaces with multiple healthcare disciplines. 
Measurement of heart rate variability (HRV) has been documented as an
effective method to objectively measure improvement in nervous system
function. Originally conceived as an assessment tool for cardiac physiology
, HRV reflects the influence of the Vagus nerve and the sympathetic nervous
system on intrinsic heart rhythm [25–27] and therefore HRV monitoring
represents a unique window into autonomic nervous system function.
Studies of HRV in the past 2 decades have established that decreased HRV, or
"vagal tone", correlates and/or predicts pathological conditions such as
cardiovascular disease [26–28],
diabetic neuropathy ,
emotional dysregulation and post-traumatic stress disorder [33–37],
sleep disorders , and
Conversely, high HRV has been associated
with healthy longevity [42–44], and is used by elite athletes to predict their ability
to function optimally in an upcoming workout. 
According to McCraty et al
, an optimal level of HRV within an organism reflects healthy function and an
inherent self-regulatory capacity, adaptability or resilience. Too little variation
indicates age-related system depletion, chronic stress, pathology or inadequate
functioning in various levels of self-regulatory control systems. McCraty’s
assessment echoes the ideas of Hans Selye, the Nobel laureate who described
the General Adaptation Syndrome , the body’s mal-adaptation to sustained
emotional, physical or chemical stressors. HRV is increasingly emerging as a
way to assess and predict multiple health outcomes, from disease to thriving
health.  Accordingly, interventions that increase HRV "vagal tone" would be
expected to improve health outcomes and promote vitality and wellness.
The chiropractic profession, as well as the professions of acupuncture and
osteopathy, have increasingly used HRV for research purposes [47–58];
however, none of these studies have approached the effect of long-term
chiropractic care on HRV. The current study chronicles the improvements in HRV
in 6 patients undergoing long-term continuous chiropractic care (of 3 months to 3
years’ duration) for the correction of vertebral subluxation.
This retrospective case series compares recorded baseline and ongoing
progress HRV measurements of 6 patients following a program of chiropractic
care for the correction of vertebral subluxation. The patients (3 female and 3
male) were 25 to 55 years of age, nonsmokers with no cardiac, hypertensive, or
frank pathological conditions reported. As per criteria recommended by Nunan et
al , patients whose data were included for the study reported no use of of
statins, beta blockers, drugs that affect baroreceptor activity, neuroactive drugs,
or SSRIs. Patient data were excluded if the patient reported unusual physical,
chemical, or emotional stress in the day preceding testing.
Assessment for Vertebral Subluxation Complex
All patients were assessed for eligibility for chiropractic care using multiple
objective methods to identify the locations and components of vertebral
subluxation [23, 25], including posture or movement asymmetry, supine or prone
leg length inequality, tissue texture/tonal/temperature changes or tenderness,
pain or sensation changes, static and motion palpation findings, sEMG and
paraspinal thermography, x-ray analysis of weightbearing spinal integrity and
structure [7–11], biotensegrity-based posture and movement assessment ,
cerebellar testing including Rhomberg’s and/or Mittelmeyer’s test,
proprioceptive/balance deficit and reaction time via the BESS (Balance Error
Scoring System) based mobile SWAY® app [61, 62] (SWAY Medical, LLC, Tulsa,
Outcome Measure: HRV
HRV data were collected using Pulse Wave ProfilerTM (PWP) instrument.
Patients were assessed for baseline HRV at their initial appointment, prior to the
start of care, and reassessed every 12 visits for 3–months, at 6–months, and
every 6–months thereafter throughout their program of chiropractic care. HRV
test-test reliability was confirmed by recording a set of 3 consecutive HRV
measurements on non-adjusted control patients. For each HRV assessment
during the course of a patient’s care program, the following protocol was
followed. Patients were seated comfortably in a closed room. No breathing
instructions were given, and patients were instructed to refrain from movement,
conversation, or using digital devices. The left hand was used for data collection,
which consisted of 1 minute of resting measurement followed by 5 minutes of
data collection. After data collection, the resultant heart rate graph was visually
inspected and SDNN data were accepted unless anomalies such as random
spikes or missed data capture were noted (less than 10% of samples). Results
for each case were compared to normal HRV scores for 5’ SDNN measurements
per age and gender as per Voss et al , though conflicting reports suggest that
normative 5’ SDNN values may be higher [44, 59] or lower. 
Initial chiropractic care programs ranged from 2 to 3 visits weekly for 10 to 52
weeks based on patient presentation. [64, 65] Visit frequency transitioned to
weekly after improvement or normalization of objective testing as well as posture
and movement patterns. Exercises to facilitate neuromuscular re-education were
selected per patient and included Pettibon Cervical TractionTM, use of a Pettibon
Therapeutic Wobble ChairTM, home use of Pettibon HeadweightsTM or a
Chiropractic BioPhysics® Denneroll, “pointer” spinal stabilizing exercise, and
isometric head retraction exercises against a wall or car headrest.
All patients were managed using Diversified and Thompson Terminal Point
techniques. Chiropractic adjustments were primarily manual full-spine or drop-assisted,
with some instrument-assisted adjusting using an ActivatorTM
instrument. Diversified is the most widely used chiropractic technique and
system of adjusting that uses primarily motion and static palpation to locate
levels of vertebral subluxation, and focuses on the restoration of proper
biomechanics within the spine  and improved nervous system function. The
Thompson Terminal Point Technique is a full-spine analysis and adjusting
technique that utilizes a drop table to assist in the delivery of high-velocity, low
amplitude chiropractic adjustments.  Pre- and post-adjustment assessments
of each level of vertebral subluxation were noted, as well as subjective
statements made by the patient. The force administered during a chiropractic
adjustment was modified individually to a patient’s size, frame and spinal
Patient Responses to Chiropractic Care
A 52–year-old female presented with a chief complaint of pain in her right lower
back and a concern with lack of progress in recovering from multiple
pneumonias. Her baseline HRV recording indicated an SDNN of 21.6 msec,
below the normal SDNN of 36.9 ± 13.8 msec reported for her gender and age.
An initial program of chiropractic care consisted of 3 visits a week for 10–weeks,
and progressed to 2 scheduled visits per week when muscle activity and balance
improved as measured by sEMG. Subjective outcomes included improvement of
lung and low back pain, improved immune system function as reflected by
reported reduced frequency of respiratory infections, and improvement of athletic
Objective radiographic improvements, [64, 65] after 4–months of
care, included reduction of forward head posture from 2.1 cm to 0.7 cm, a 66.7%
improvement, as well as improvement of C2–C7 sagittal curve (from 30° to 38.7°
as measured by X-ray, representing a 22.5% improvement) and atlas plane to
horizontal (from 22.8° to 29.9°, representing a 31.1% improvement). Her
baseline SDNN of 21.6 msec improved to 125.2 msec after 6–weeks of care (a
479.6% increase from initial testing) and to 143.2 msec after 9–months of care (a
563% increase from her initial testing). After 21–months of care, her SDNN
settled to 87.5 msec, an increase of 305.1%, and after another 13–months of
weekly visits, her SDNN measured 88.9 msec (an increase of 311.6%) while also
dealing with increased work and family stressors.
A 43–year-old, athletic female presented with a chief complaint of right
arm/elbow pain that inhibited her from lifting heavy weights in the gym, and goals
of being active and fit. Her baseline HRV recording indicated an SDNN of 86.2
msec, above the normal of 45.4 ± 20.5 msec for her gender and age  but
consistent with an SDNN for an aerobically-trained athlete. [69–71]
An initial program of chiropractic care consisted of 2 visits per week for 52–weeks. Outcomes of care included elimination of arm and elbow pain with the
first 12 visits, improved muscle balance and activity as measured by sEMG after
8–months of care, reported improved weight lifting and athletic performance
through the course of her care, and improved measured sagittal cervical spine
structure (atlas plane to horizontal improved from 24° to 32.8°) after 18–months of
care. [64, 65] HRV improved after 2–months of care to an SDNN of 152.4 msec,
and after 15–months her SDNN was 143.6 msec. The patient continued ongoing
care at a frequency of weekly visits. Her HRV recordings showed an SDNN of
142.5 after 28 continuous months of care, a 72.5% improvement from her initial
pre-care HRV measurement.
A 31–year-old, athletic male presented with a chief complaint of left upper back
and shoulder pain, and goals of increased fitness, strength, and endurance. His
baseline HRV recording indicated an SDNN of 69.6 msec, within the reported
normal SDNN of 50.0 ± 20.9 msec for his gender and age. 
An initial program of chiropractic care consisted of 28 visits over 3–months, and
progressed to 2 scheduled visits per week when muscle activity and balance
improved as measured by sEMG, and once per week when the patient’s
symptoms fully abated, at which time he terminated care. His baseline SDNN of
69.6 msec improved over 8–months to a sustained measurement of greater than
116 msec, a 69.4% improvement over his initial HRV measurement.
A 48–year-old male presented with occasional headaches, poor sleep patterns,
fatigue, and right shoulder pain, with goals of increased cardiovascular
fitness. His baseline HRV recording indicated an SDNN of 24.3 msec, at the low
end of the normal SDNN of 36.8 ± 14.6 msec reported for his gender and age.
An initial program of chiropractic care consisted of 3 visits a week for 12–weeks,
and was ongoing at the time of manuscript preparation. Subjective outcomes
include improvement of sleep and decreased fatigue, and elimination of shoulder
pain. Objective outcomes include increased cervical range of motion and
improved overall posture as assessed by visual observation, as well as an
improvement of reaction time as measured by SWAY® testing from a baseline of
427 ms before care to 309 ms after 3–months of continuous care. Normal
reaction time for a patient of this age is 280 ± 40 msec.  His baseline SDNN
of 24.3 msec improved to 30.9 msec within the first month of care, with a further
improvement to 90.2 msec after 3–months of care, an improvement sustained at 4.5 months of care
with an SDNN of 88.0 representing a 262.1% increase in HRV as compared to
his pre-care score.
A 25–year-old female referred by her gastroenterologist presented for
chiropractic care seeking improvement of lower back pain, gastrointestinal
dysfunction, and anxiety. Her baseline HRV recording indicated an SDNN of
50.6 msec, within the reported normal range of 48.7 ± 19.0 msec for her gender
and age. 
An initial program of chiropractic care consisted of 3 visits a week for 12–weeks,
and was ongoing at the time of manuscript preparation. Subjective outcomes
after 3–months of care include resolution of anxiety, improvement of elimination
function from 4 times per week to twice daily, and reduction of low back
pain. Objective outcomes include improvement of muscle activity and balance as
measured by sEMG, and improvement in single leg raise balance as measured
by SWAY® (baseline proprietary SWAY score 78.2/100 before care to 92.7/100
after 3–months of care.  Her pre-care HRV baseline SDNN of 50.6 msec
improved to 62 msec after one month of care, and to 110.6 after 2–months of
care, and slightly diminished to 75.9 msec after 3–months of care (a sustained
50% increase over her pre-care score), possibly due to reported life stressors.
A 42–year-old male presented with right neck and shoulder pain, with goals of
maintaining strength and increasing flexibility. His baseline HRV recording
indicated an SDNN of 59.3 msec, within the reported normal range of 44.6 ± 16.8
msec for his age and gender. 
An initial program of chiropractic care consisted of 3 visits per week for 12–
weeks, and was ongoing at the time of manuscript preparation. Subjective
outcomes include reduced neck and shoulder pain, improved athletic
performance, and improvement of gait and flexibility. Objective improvements
include an improvement of proprietary SWAY balance score from 60.1/100
before care to 95.2/100 after 3–months of care.  His baseline SDNN of 59.3
msec increased to 93.6 msec after 1–month of care, and was maintained at 95.6
msec after 3–months of care, a 61.2% increase from his pre-care score.
This work was approved by the IRB of the Foundation for Vertebral Subluxation.
Data from unadjusted patients were collected to confirm protocol test-test
reliability. While the individual baseline SDNN differed between individuals as
predicted by Pinna’s HRV reliability analysis , the HRV trials for each
unadjusted individual in Figure 1 demonstrate good test-test reliability, with
standard deviations of less than 10%. The test-test reliability observed indicates
that changes observed in Figures 2 and 3 are due to physiological change rather
than random chance. Previous studies have demonstrated that HRV is not
affected in non-adjusted controls and is not affected by the placebo effect. 
Figure 2 shows 6 retrospective case studies of patients whose baseline SDNN
measured lower at their initial intake, prior to administration of care, and
progressively increased over the course of their care, with a sustained
improvement. HRV assessment for these data points was performed prior to any
adjustment performed during that day's visit. Trend lines on each graph
represent the progressive increase of baseline HRV in these patients over time,
which is indicated on the X-axis in number of months.
Figure 3 contains comparative PWP representations of autonomic activity and
balance for the 6 case studies, prior to initiation of chiropractic care and at their
most recent assessment.
This case series chronicles HRV changes of 6 adult patients receiving
chiropractic care using Diversified and Thompson adjusting techniques for the
correction of vertebral subluxation. The data described here are consistent with
immediate and long-lasting neurophysiological changes effected by chiropractic
case management. While the absolute values of baseline and post-care HRV
differed between individuals, each individual’s outcome improved to values more
consistent with younger or athletic individuals [69–71], and the improvements in
HRV of 2 of these individuals are consistent with a transition from predicted
worse health outcomes to greatly improved health outcomes .
One concern central to the use of HRV in clinical outcomes data is that HRV data
is reported using many different units of measure as time domain versus
frequency domain, power spectrum, high-frequency, low-frequency, very-lowfrequency,
and ratios of these various units, which are intended to represent
balance and coherence within the autonomic nervous system. Of the data
analysis methods available, SDNN is thought to reflect both parasympathetic and
sympathetic activity and to provide an index of total HRV . Multiple studies
have shown that of the data set derived from HRV measurement, SDNN has
stronger reliability  and decreases linearly with age [44, 55], and therefore is
simplest and most useful for cross-study comparison. Further, age normalized
standards for SDNN obtained using 5–minute sampling times are readily
available from multiple sources [59, 63] making SDNN data readily comparable
across healthcare disciplines. Therefore, while all HRV metrics are reported in
appendix 1, this study focuses on changes in SDNN in particular.
Pinna’s review of reliability of HRV measurements states that “short-term HRV
measures are subject to large day-to-day variations…[and that] differences
between individuals mostly reflect differences in the subject’s error-free values
rather than random error”.  Intra-subject variability is also due to an intrinsic
lability of HRV parameters, probably because they are under the influence of
such factors as mood, alertness and mental activity, which are very difficult to
control for in any study, as well as changes associated with frequency and depth
of respiration.  Intrinsic intra-individual HRV variation complicates crossgroup
comparison and may indeed underlie the variation and wide standard
deviation in SDNN ranges reported. [43, 45, 56, 60, 63] Since individual physiology
can vary according to the effect of various stressors on dynamic physiology
[37, 45], the magnitude of response to various interventions, including
chiropractic adjustment, may vary per individual per day. Therefore, we
hypothesized that while any individual data point may not hold conclusive data, a
collection of data points over time may show a trend indicative of HRV
change. Furthermore, since changes in HRV 5 minute readings after
interventions such as exercise are known to be transient , changes observed
pre- versus post-adjustment cannot be interpreted to signify that the individual
adjustments performed confer permanent physiological change. The sustained
nature of the HRV improvements reported in these 6 cases may therefore reflect
neuroplastic changes associated with long-term chiropractic care.
Sustained improvement of HRV over a course of chiropractic care may have
implications for prediction of multiple health outcomes. For cardiac cases, data
from 24–hour sampling times demonstrate a 5.3 times higher (34%) mortality for
individuals with abnormally low SDNN.  Strikingly, in 1 report, patients with
moderate 24–hour sampling time SDNN values (50–100 msec) have a 400%
lower risk of mortality than those with low values (0–50 msec).  For the
purposes of comparison of SDNN obtained under the conditions used in the
current study, Bilchick et al  suggest a "cutoff" of 30 msec for 5–minute
sampling-time SDNN for separation of better or worse health outcomes in cardiac
cases. It is of interest that cases 3 and 4 in the current case series show a preadjustment
SDNN value close to 30 msec and a post-adjustment SDNN value
greatly increased past this "cutoff." Abnormally low SDNN has implications for
non-cardiac health outcomes as well: low HRV has been shown to be predictive
of worsened prognosis in cancer cases [39, 40], and conversely, higher HRV may
increase resilience to cancer.  The case studies presented here are
consistent with immediate and lasting improvement in health outcomes, and
these data suggest the possibility that sustained improvement of HRV after a
course of chiropractic care may have positive implications for the body’s ability to
overcome pathologies such as cancer and cardiovascular disease.
Several of the post-care SDNN values in these case studies described herein are
above the normative values established in the literature. [59, 63] Though clear
lower limits have been established for 5’ HRV measurements, no clear upper
limits have been identified as yet, therefore the significance of these findings is
unclear. One possible explanation may be extrapolated from Stein et al’s study
on 24–hour Holter monitoring of older adults, which suggested that some values
of HRV may be mildly exaggerated by erratic rhythms found more prevalently in
this age group. However, SDNN in particular was less affected than the other
HRV metrics reported.  Without the ability or expertise to evaluate the
individual PQRST waveforms used to generate the data in these case studies,
the authors cannot exclude the possibility of an undetected erratic rhythm in
cases 1 and 3 in this study generating an artificially elevated
SDNN. Alternatively, because SDNN values used as comparable normative data
per age and gender for the 6 case studies were generated from a “normal”
population [59, 63], it is possible that a higher reference range would be
identifiable for highly functional and healthy individuals, and that the SDNN
values observed for cases 1 and 3 may simply be reflective of physiology
improved above what is expected from a “normal” population. Indeed, several
studies have shown that trained aerobic athletes have a higher baseline SDNN
than their untrained counterparts. Aubert et al  reported elevated 10’ SDNN
values of 97.9 ± 15.7 msec for aerobic trained athletes as compared to 65.4 ±
38.9 for sedentary individuals; similarly, Martinelli et al reported elevated 5’
SDNN values of 89.9 ± 24.8 msec for endurance-trained cyclists as compared to
59.1 ± 36.5 for their untrained counterparts , and for young (ages 18–25)
individuals, Corrales et al report elevated 5’ SDNN values for male athletes of
101.2 ± 37.4 msec, and for female athletes of 106.6 ± 38.1 msec, significantly
higher than for their untrained male (83.1 ± 31.7) or female (71.8 ± 24.5)
counterparts.  The observation of significantly higher reference ranges for
aerobically trained athletes suggests the possibility that the unexplained higher
SDNN values reported for cases 1 and 3 could be consistent with improved
cardiovascular physiology, as is found in trained athletes. Further study will be
necessary to explore this possibility.
Improvement of HRV may reflect a means by which the chiropractic adjustment
affects human physiology and is not exclusive of other potential physiological
effects of the chiropractic adjustment as theorized by Kent  and Pickar ,
or Ingber . Rather, improvement of HRV may represent readout of the
physical and physiological effects that is related to and interconnected with these
theories and others.
While the neurophysiological basis for the effect of the chiropractic adjustment on
HRV remains to be further elucidated, recent research suggests several plausible
mechanisms by which incoming sensory information from joints in the spine,
especially of the head and neck, may affect cardiac regulation.  The fastigial
nucleus (FN), an evolutionarily-conserved structure, receives input from
spinocerebellar tracts, in particular somatosensory information from the spinal
joints of the head and upper body.  Projections have been identified reaching
from the FN of the cerebellum to several different structures that may affect
cardiac regulation, including the amygdala, the hypothalamus, and medullary
nuclei including the cardiovascular centre.  Further, the FN sends projections
to the nucleus tractus solitarius (NTS), which contains the intermedius nucleus of
the medulla (InM). The InM contains neurochemically diverse neurons and sends
both excitatory and inhibitory projections to the NTS. [79, 80]. These data provide
a novel pathway that may underlie possible reflex changes in autonomic
variables after neck muscle spindle afferent activation. Incoming information from
either the spinocerebellar tracts or directly from cervical spine afferents may
therefore relay somatosensory input through the FN, the NTS, and the InM, both
of which may in turn affect sympathetic as well as cardiac control centers.
Alternatively, input created via a chiropractic adjustment relayed through the FN
to the amygdala could influence the integration of signals from inside and outside
the body, thus affecting the body’s adaptive capacity. Thayer’s neurovisceral
integration model  holds that a core set of neural structures provides an
organism with the ability to integrate signals from inside and out the body and
adaptively regulate cognition, perception, action, and physiology. Thayer’s
recent meta-analysis of fMRI and PET data has demonstrated association of
activation of the the amygdala and the prefrontal cortex with changes in HRV.
 Consistent with this hypothesis, Lelic et al.  have demonstrated that
manipulation of dysfunctional spinal joints affects sensorimotor integration in the
prefrontal cortex. Therefore, reduced or abnormal vertebral motion or position,
intersegmentally or globally, could result in alteration of somatosensory input
through the spinocerebellar tracts to structures that influence neurovisceral
integration, as measured by HRV. Much research will be needed to test this
hypothesis; however, the existence of neural pathways leading from the spine
through the brain to the modulators of cardiac activity is a promising start.
Thayer’s neurovisceral integration model is reminiscent of modulation of
physiology via a central pattern generator (CPG), a cooperative set of neurons
that generate rhythmic patterns such as gait, breathing, and swallowing.  A
particular property of a CPG-modulated system is that sensory input to CPGs
leads to adaptive changes , such a pebble in a shoe will lead to a limping gait
pattern. A third possible underlying neurophysiological mechanism by which
chiropractic care may affect HRV is that changes in sensory input as generated
by a chiropractic adjustment, when perceived by any of the neural apparati
discussed above, may affect a CPG that modulates heart rate. If this is the case,
changes in HRV with chiropractic care could be viewed as evidence of adaptive
changes executed by a CPG in response to sensory input. Consistent with this
possibility, Senzon et al have demonstrated that sensory input generated by
Network Spinal Analysis care results in generation of rhythmic muscle
contraction that when measured by sEMG shows mathematical properties of
CPG-modulated activity.  Indeed, if vertebral subluxation was to create
afferent sensory changes delivered to neural structures that either comprise or
influence a CPG, reduction or resolution of vertebral subluxation by the
chiropractic adjustment may represent a means by which adaptive capacity may
be modulated. Much further study will be necessary to explore this intriguing
The 6 case studies presented here contradict multiple studies that have clearly
established a linear decline in SDNN with age. [44, 55, 63] The observed
improvement in SDNN in the 6 cases presented suggests the possibility that the
cumulative effect of regular chiropractic care may reverse a diminished HRV, and
indeed, may be protective against the predicted age-related decline in HRV.
Further longitudinal study will be necessary to confirm this observation and to
explore the physiology that may underpin these improvements.
As with any case series there are a number of inherent limitations. Although all
patients demonstrated objective improvements in HRV, the inability to define
whether these improvements were due to natural progression, pain reduction,
unreported home care and self-medication, adjunct therapies administered
during the program of care, or vertebral subluxation based chiropractic care
makes these factors limitations to the study and causal effect cannot be
determined. It’s clear that further clinical research is required to evaluate the
relationship between chiropractic care for the correction of vertebral subluxation
and improvement in HRV in adult patients.
The data presented demonstrate a sustained improvement in HRV over a course
of chiropractic care that is consistent with improved health outcomes. While no
definitive conclusion can be made from this study, these data show objective,
non-musculoskeletal outcomes that are consistent with neurophysiological
effects associated with reduction or resolution of vertebral subluxation including
improvements in coherence, spinal biomechanical function, neurological function,
resilience, and adaptability. Future directions for HRV research in the
chiropractic research arena should include expanding upon the current research
using a sample size large enough for statistical analysis and longitudinal study,
exploration of potential effects of the chiropractic adjustment on EEG activity of
cardiac and cardiac-related nuclei, and exploration of whether sensory input
generated by the chiropractic adjustment may affect nuclei in the medulla in a
way that directs HRV changes.
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