Exp Brain Res. 2015 (Apr); 233 (4): 1165–1173 ~ FULL TEXT
Niazi IK1, Türker KS, Flavel S, Kinget M, Duehr J, Haavik H.
Centre for Chiropractic Research,
New Zealand College of Chiropractic,
6 Harrison Rd Mt Wellington,
Newmarket, PO Box 113-044,
Auckland, New Zealand.
This study investigates whether spinal manipulation leads to neural plastic changes involving cortical drive and the H-reflex pathway. Soleus evoked V-wave, H-reflex, and M-wave recruitment curves and maximum voluntary contraction (MVC) in surface electromyography (SEMG) signals of the plantar flexors were recorded from ten subjects before and after manipulation or control intervention. Dependent measures were compared with 2-way ANOVA and Tukey's HSD as post hoc test, p was set at 0.05. Spinal manipulation resulted in increased MVC (measured with SEMG) by 59.5 ± 103.4 % (p = 0.03) and force by 16.05 ± 6.16 4 % (p = 0.0002), increased V/M max ratio by 44.97 ± 36.02 % (p = 0.006), and reduced H-reflex threshold (p = 0.018). Following the control intervention, there was a decrease in MVC (measured with SEMG) by 13.31 ± 7.27 % (p = 0.001) and force by 11.35 ± 9.99 % (p = 0.030), decreased V/M max ratio (23.45 ± 17.65 %; p = 0.03) and a decrease in the median frequency of the power spectrum (p = 0.04) of the SEMG during MVC.
The H-reflex pathway is involved in the neural plastic changes that occur following spinal manipulation. The improvements in MVC following spinal manipulation are likely attributed to increased descending drive and/or modulation in afferents. Spinal manipulation appears to prevent fatigue developed during maximal contractions. Spinal manipulation appears to alter the net excitability of the low-threshold motor units, increase cortical drive, and prevent fatigue.
KEYWORDS: H-reflex · V-wave · Spinal manipulation · Maximal voluntary contraction · Evoked potentials · Neural adaptations
From the Full-Text Article:
Over the past 10 years, several research groups have
demonstrated that spinal manipulation can change various
aspects of nervous system function, including muscle
reflexes, cognitive processing, reaction time, and the speed
at which the brain processes information (Murphy et al.
1995; Herzog et al. 1999; Suter et al. 1999, 2000; Kelly
et al. 2000; Haavik Taylor and Murphy 2007a, b, 2008,
2010a)+. One group has hypothesized that the articular dysfunction
component of the chiropractic clinical construct,
the vertebral subluxation, results in altered afferent input to
the central nervous system that modifies the way in which
the CNS processes and integrates all subsequent sensory
input (Haavik Taylor and Murphy 2007a; Haavik Taylor
et al. 2010). This processing (i.e., sensorimotor integration)
is a central nervous system (CNS) function that appears
most vulnerable to altered inputs.
Over recent years, a series of experiments have been
conducted to further investigate this potential relationship
between the putative vertebral subluxation and altered CNS
function (for recent review see Haavik and Murphy 2012).
Multiple studies have indicated neural plastic changes occur
when spinal manipulation of such dysfunctional spinal segments
is performed. The neural adaptations include altered
sensorimotor integration and altered motor control following
spinal manipulation (Haavik Taylor and Murphy 2007a, b,
2008, 2010a, b). The level of CNS involvement and the exact
mechanisms underlying these neural adaptations following
spinal manipulation remain unclear. This study sought to
investigate possible neural plastic changes with spinal manipulation
by measuring reflex responses such as the H-reflex
and V-wave. Although these evoked responses are affected by
common neural mechanisms, it has been shown in previous
studies that these methods can differentiate between altered
presynaptic inhibition and motoneuron excitability as measured
with the H-reflex (Hultborn et al. 1987; Pierrot-Deseilligny
and Mazevet 2000; Nordlund et al. 2002; Misiaszek
2003; Ekblom 2010), and changes in supraspinal input to the
motor neuron pool as measured with the V-wave (Upton et al.
1971; Aagaard et al. 2002; Duclay and Martin 2005; Vila-Chã
et al. 2012). Therefore, combining measures of the H-reflex
and V-wave may provide a better understanding of the neural
plastic changes that occur with spinal manipulation.
A few previous studies have looked at the effects of
spinal manipulation on the H-reflex recorded from the
medial aspect of the triceps surae of lumbar disk herniation
patients (Floman et al. 1997), and of healthy asymptomatic
individuals (Dishman and Burke 2003), and from the
soleus in both asymptomatic subjects (Murphy et al. 1995;
Suter et al. 2005) and low back pain patients (Suter et al.
2005). Several of these studies showed a decrease in the
H-reflex indicating a transient attenuation of motoneuronal
activity of the lumbosacral spine in asymptomatic subjects
(Murphy et al. 1995; Dishman and Burke 2003) and in low
back pain patients (Suter et al. 2005).
However, there have been advances in the both data collection
and data analysis methodology since the above cited
studies (for example see Tucker et al. 2005; Brinkworth et al.
2007). There is, for example, a known natural variation in the
H-reflex response, which is why it is necessary to record and
average multiple responses (Tucker et al. 2005). It is therefore
not common practice to analyze single H-reflex responses as
has been done in the past (Dishman and Burke 2003).
The purpose of the current project therefore is to take
advantage of the recent discoveries and understanding about
the standardized data collection and analysis methodologies
(Tucker et al. 2005; Brinkworth et al. 2007) related to the
H-reflex and V-waves and explore what effect, if any, spinal
manipulation of vertebral subluxations will have on them.
A total of 18 men took part in the study. Study one
included ten volunteers aged 27.6 ± 5.4 years, and study
two included six volunteers aged 32.6 ± 9.3 years. All
subjects were required to be aged 18–40, have evidence
of spinal dysfunction but have no known contraindications
to spinal manipulation such as recent history of trauma,
known conditions such as metabolic disorders, inflammatory
or infectious arthropathies, or bone malignancies.
All subjects were required to have a self-reported history
of subclinical spinal pain, i.e., recurring, intermittent
low-grade spinal pain, ache, or tension. Participants were
excluded if they reported current pain anywhere in the
body (to remove the confounding effect of current pain),
diagnosed degenerative joint disease, or any medical condition
affecting the sensorimotor system. In keeping with
the definition of subclinical spinal pain, participants were
excluded if they had sought previous treatment for their
intermittent problem. All participants gave their informed
written consent before inclusion in the study. The study
was approved by The Northern Y Regional Ethics Committee
approved this study in accordance with the Declaration
Bipolar surface electrodes (20 mm Blue Sensor Ag/AgCl,
AMBU A/S, Denmark) were used to record the surface
electromyographic (SEMG) activity of the soleus muscle
(SOL) of the right leg for all aspects of the experiments.
Surface electrodes were placed 2 cm distal to the lateral
gastrocnemius muscle and 2 cm apart, and a ground
electrode was placed over the right malleolie at the ankle.
SEMG signals were amplified in custom-built EMG amplifier,
and were recorded with CED Power 1401 MK 2
data acquisition board at 5 kHz and band-pass filtered at
The H-, M-, and V-waves of the SOL muscle were elicited
by stimulation of the tibial nerve. The electrical stimulus
was provided by an isolated stimulator (Digitimer DS7AH,
UK). Stimulating electrodes (32 mm, PALS® Platinum,
Patented Conductive Neurostimulation Electrodes, Axelgaard
Manufacturing Co., Ltd. USA), a custom-built silver
ball with 10-mm diameter was placed on the tibial nerve
(cathode) located in the popliteal fossa of the right leg, and
the other stimulating electrode (PALs platinum rectangular
electrode, 75 × 100 mm; Axelgaard Man) was placed
proximal to the right patella (anode). The position of the
cathode and the intensity of the stimulus were manipulated
until the greatest response with the minimum stimulus
intensity was achieved.
The force recordings were performed using a strain gauge
(10 mV/Nm) attached to a custom-made ankle brace that
signal was recorded by a CED Power 1401 MK 2 Data
Acquisition Board at a sample rate of 1 kHz. The force
measures were recorded, while the subject performed
maximum voluntary ankle dorsiflexion contractions. Three
recordings were made, and the maximum force produced
during these contractions was used for normalization.
During Study one, the ten subjects attended two sessions
each, the control and the experimental (spinal manipulation)
session. The order of these two sessions was randomized
and at least 1 week separated the two sessions. All
experiments for study one were performed on the right leg,
while the volunteers were comfortably lying prone on massage
table with their right leg firmly strapped to the table
with Velcro. The subjects maintained their hips and legs
straight and with the ankle at 90° of plantar flexion. The
right foot was firmly attached to the leg of the table. Particular
care was taken to monitor the posture of the subjects
and ensure their posture and position remained unchanged.
During both the experimental (spinal manipulation) and the
control sessions, the following measures were collected pre
and post the interventions: SEMG signals during MVC; Hand
M-recruitment curves; H-reflex area under curve normalized
to Mmax (Harea/Mmax), H-reflex threshold, V-wave
normalized to Mmax (V/Mmax), M-wave slope, H-reflex
slope and the mean power frequency (MPF) of a fast Fourier
transform (FFT) of the SEMG during MVC.
In Study two, an additional group of eight participants
attended two more sessions each, one control and one
experimental (spinal manipulation) session, where only
force was measured.
Maximal voluntary contractions (MVC)
The subjects performed three progressive MVCs of the
plantar flexors of 5 s of duration, separated by 2-min rest.
Subjects were verbally encouraged to produce maximal
contraction. The highest plantar flexor SEMG activity during
MVC in each experimental or control session was used
for analysis and to compute the submaximal target contraction
levels for H- and M-recruitment curve recordings. In
study two, only the force was measured.
H- and M-recruitment curve recordings
During the H- and M- recruitment curve recordings, the
subject was asked to plantar flex his right leg at around
10 % of his own MVC (rectified and 0.5 Hz low-pass
smoothed SEMG as feedback). Subject was provided with
online feedback of the contraction level exerted on a computer
monitor placed under the table which can be clearly
seen by the subject. While the subject was performing
this low-level tonic contraction, the direct motor response
(M-wave) and the H-reflex of the SOL muscle were elicited
via electrical stimulation of the tibial nerve. To get the
maximum peak-to-peak amplitude of the M-wave, subjects
were stimulated progressively by increasing the current
intensity in 5-mA increments. A total of three trials at
each current intensity were recorded.
Then, at each current
intensity, the preceding M-wave peak-to-peak amplitude
was compared with the new M-wave peak-to-peak amplitude.
Once the preceding M-wave peak-to-peak amplitude
and new M-wave peak-to-peak amplitude had reached
a plateau over the three trials, the current intensity of the
previous stimulation was considered the maximum current
intensity. To construct the M- and H-recruitment curves,
the maximum intensity was divided into 16 segments that
were equally separated. For each randomly chosen current
intensity, a total of five stimuli were delivered at varying
time intervals between 2 and 3 s. To avoid fatigue and mental
distraction of the participants, rest periods of 2 min were
given every 80 stimuli. Moreover, the subjects were given
the possibility to pause the experiment at any time if they
The subjects were asked to perform 7–9 MVCs of 5 s
duration, with 2 min of rest in between prior to and postspinal
manipulation and control session. During each of
the MVC attempts, five supramaximal stimuli (110 % of
the current needed to evoke maximal M-wave; 1-ms square
pulse) were applied to the tibial nerve at the times when
the SEMG was above 90 % of the level achieved during
The development of fatigue in a muscle can be observed by
amplitude and spectral analysis of SEMG recordings (Hagberg
1981). The time-dependent shift in mean power frequency
(MPF) of SEMG signals to lower frequencies during
the fatigue process is a well-established phenomenon.
For this purpose, SEMG recorded during the MVC preand
post-manipulation session or control intervention was
used. MVC data segments epoched offline and processed
in MATLAB using purpose written scripts. A fast Fourier
transform (FFT) was performed, and the mean power frequency
(MPF) was calculated as the frequency (Hz) center
of the spectrum.
The entire spine and sacroiliac joints were assessed for segmental
dysfunction (also known as vertebral subluxation by
the chiropractic profession), and adjusted where deemed
necessary by a registered chiropractor with at least 10-year
clinical experience. 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. All of these biomechanical characteristics are known
clinical indicators of spinal dysfunction (Jull et al. 1988;
Hubka and Phelan 1994; Strender et al. 1997; Hestboek
and Leboeuf-Yde 2000; Fryer et al. 2004; Cooperstein
et al. 2010, 2013).
All of the spinal manipulations carried
out in this study were high-velocity, low-amplitude thrusts
to the spine or pelvic joints. This is a standard manipulation
technique used by chiropractors. The mechanical properties
of this type of CNS perturbation have been investigated;
and although the actual force applied to the subject’s spine
depends on the therapist, the patient, and the spinal location
of the manipulation, the general shape of the force–time
history of spinal manipulations is very consistent (Hessell
et al. 1990) and the duration of the thrust is always <200 ms
(for review see Herzog 1996). The high-velocity type of
manipulation was chosen specifically because previous
research (Herzog et al. 1995) has shown that reflex SEMG
activation observed after manipulations only occurred after
high-velocity, low-amplitude manipulations (as compared
with lower-velocity mobilizations). This manipulation technique
has also been previously used in studies that have
investigated neurophysiological effects of spinal manipulation
(for review see Haavik and Murphy 2012).
The control intervention consisted of passive and active
movements of the subject’s head, spine, and body that are
carried out by the same chiropractor who pre-checks the
subjects for vertebral subluxations and who performs the spinal
manipulations in the experimental intervention session.
This control intervention involved the subjects being moved
into the manipulation setup positions where the chiropractor
would normally apply a thrust to the spine to achieve
the manipulations. However, the experimenter was particularly
careful not to put pressure on any individual spinal segments.
Loading a joint, as is done prior to spinal manipulation,
has been shown to alter paraspinal proprioceptive firing
in anesthetized cats (Pickar and Wheeler 2001) and therefore
was carefully avoided by ending the movement prior to endrange-
of-motion when passively moving the subjects.
spinal manipulation was performed during any control intervention.
This control intervention is not intended to act as a
sham manipulation but to act as a physiological control for
possible changes occurring due to the cutaneous, muscular
or vestibular input that would occur with the type of passive
and active movements involved in preparing a subject/patient
for a manipulation. It also acts as a control for the effects of
the stimulation necessary to collect the dependent measures
of the study and acts as a control for the time required to
carry out the manipulation intervention.
For the evoked potentials, peak-to-peak amplitude of the
H-reflex, M-wave, and V-wave were computed offline from
the unrectified SEMG signals. To reduce inter-subject variability,
H-, M-, and V-waves were normalized to the corresponding
maximal M-wave (Mmax), thus the (Hmax)/Mmax
and V/Mmax ratios were computed. For each recruitment
curve, the current intensity at Hmax and Mmax was identified.
Since the size of the M-wave is affected by contraction
intensity (Pensini and Martin 2004), the Mmax wave
elicited concomitantly either with H-reflex or with V-wave
was used for the respective normalization. The ascending
part of recruitment curve was fit by a general least squares
model, as described by (Brinkworth et al. 2007). From the
curve fit analysis, the following parameters were analyzed:
current intensity at H-reflex threshold (Hthresh); current
intensity at 50 % of the Hmax (50 %Hmax); and the slope
of the ascending limb of the recruitment curve at 50 % of
the Hmax (Hslope). The dependant measures were: H- and
M-recruitment curves; H-reflex area under curve normalized
to Mmax (Harea/Mmax), H-reflex threshold, V-wave normalized
to Mmax (V/Mmax), M-wave slope, H-reflex slope
and the mean power frequency (MPF) of a fast Fourier
transform (FFT) of the SEMG during MVC and absolute
Surface EMG background level
Average rectified value (ARV) of the SOL SEMG was estimated
from each sweep for an epoch of 500 ms before the
stimulation and then averaged. The ARV values were normalized
with respect to the ARV computed from the highest
MVC SEMG and expressed as a percentage.
All pre- to post-intervention changes were evaluated
using two-way ANOVA with factors intervention (spinal
manipulation and control) and time (Pre and Post). Post
hoc pairwise comparison was done using Tukey’s HSD
tests if required. Statistical significance was set at p < 0.05
for all comparisons.
In ten subjects using the two procedures, we obtained 20
sets of results. To be able to fully characterize the possible
changes in the motoneuron excitability and the synaptic
efficacy, the M-wave and H-reflex curves were established.
For that, 16 levels of stimuli were utilized. The stimuli were
adjusted to the stimulus level that induced the 110 % of the
maximal M response (this level was labeled as level 16; see
the “Methods” for details). The curves are shown in Figure 1,
where the M-waves were fitted with the curve so that preand
post-M-wave curves could be superimposed on top of
each other to allow any genuine changes in the H-reflex
curve to be highlighted. As can be seen in the figure, this
made not only the ordinate of the figures normalized (to the
M Max) but the abscissa as well (to the stimulus intensity
that generates the M Max).
Figure 2 illustrates the changes in the H-reflex values,
the maximal voluntary contraction and the fatigue
as a result of spinal and control manipulations. As can be
seen, there are some significant affects from the manipulations.
There was no significant difference between the spinal
manipulation and control group in baseline data (premeasures).
The asterisks highlight the significant changes
as a result of the manipulation or control intervention for
each of the parameters tested against the pre-manipulation
values. The threshold to elicit the H-reflex significantly
decreased by 8.5 % (p = 0.01) as a result of spinal manipulation
and + did not change after the control session (1.5 %
change). The subjects’ SEMGs indicated a significant level
of decrease in the power spectrum as indicated by the
median frequency of the power spectrum (139–124 Hz;
p = 0.04) only after the control session (–9.4 % change),
but fatigue did not develop after the spinal manipulation.
The value of the maximal voluntary contraction as
determined by the SEMG increased significantly by
59.5 ± 103.4 % (p = 0.03) after the spinal manipulation
but not after the control. In fact, the SEMG during
MVC decreased significantly after the control session
by 13.31 ± 7.27 % (p = 0.001). In the follow-up study
with eight subjects, absolute force during MVC measure
increased by 16.05 ± 6.16 4 % (p = 0.0002) for intervention
session and following the control intervention there
was a decrease in force by 11.35 ± 9.99 % (p = 0.030).
The V-wave amplitude (V/Mmax ratio) also changed
dramatically as a result of the interventions in this study
as shown in Figure 3. The change was a significant increase
reaching around 44.97 ± 36.02 % (p = 0.006) after the spinal
manipulation and a significant decrease 23 ± 17.65 %
(p = 0.03) after the control.
This study discovered three original findings: Firstly, that,
the H-reflex pathways can be significantly affected by the
spinal manipulation; and, secondly, that, the cortical drive
as expressed by the size of the V-wave and the SEMG and
force measure during MVC is significantly increased by the
spinal manipulation. Thirdly, the spinal manipulation intervention
appears to have prevented fatigue from occurring in
the SOL, as indicated by a significant decrease in median
frequency in the power spectrum from the control subjects’
H-reflex pathway for the soleus muscle involves the spindle
primary afferent fibers (Ia) originating from the soleus
muscle, single synapses in the spinal cord, and the motoneurons
that innervate the soleus muscle (Pierrot-Deseilligny
and Burke 2005). The excitability of the motoneuron
and the efficacy of the synaptic terminal of the spindle primary
afferents are also affected by the supraspinal and spinal
inputs. The methodology we have used avoids any possible
changes in the position of the stimulating electrodes
relative to the nerve as it not only normalizes the amplitude
of the H-reflex to the M Max but also to the stimuli that
induces the maximal M-wave curve in each trial [“Methods”;
for details refer to (Brinkworth et al. 2007)].
Our results of this study indicate that the H-reflex pathway
has been affected as a result of spinal manipulation.
Since spinal manipulation has lowered the recruitment
threshold of motoneurons to Ia afferent input, it is suggested
that either the low-threshold motoneurons have
become more excitable or the synapses of the Ia primary
afferents became more efficient since a lower stimulus
intensity can now recruit the same motoneuron. Spinal
manipulation therefore appears to alter the net excitability
for the low-threshold motoneurons and/or the efficacy of
the Ia synapse.
Three previous studies have indicated that spinal manipulation
decreases motoneuron excitability in asymptomatic
subjects (Murphy et al. 1995; Dishman and Burke
2003) and in low back pain patients (Suter et al. 2005). Our
results on the other hand indicate an increase in the excitability
of low-threshold motoneurons. None of the previous
studies, however, investigated the H-reflex threshold as
was done in the current study. Murphy et al. (1995) found
a decrease in the peak of the H-reflex curve. We observed
a similar decrease in the peak of the curve (see Fig. 3).
However, according to the currently published standardized
data analysis methodologies (Brinkworth et al. 2007),
the entire area under the curve should be analyzed, not just
the maximum H-reflex. In the current study, there was a
trend toward a decreased area, but this did not reach significance.
Dishman and Burke’s findings (2003) may reflect
the apparent decrease in peak of the H-reflex, as found by
Murphy et al. (1995) and seen in this study, or their finding
may have merely reflected the known natural variation
in the H-reflex response, as they recorded single H-reflexes
only. It has been shown that to ensure robust findings, it is
necessary to record and average multiple responses (Tucker
et al. 2005). Suter et al. (2005) study was conducted in a
low back pain population. It is therefore possible that low
back pain patients have alterations involving the H-reflex
pathway that spinal manipulation effects in a different manner
to that of the healthy young male volunteers that took
part in the current study.
Maximal voluntary contraction
The current results demonstrate that the spinal manipulation
increases maximum voluntary contraction in SEMG
signals of the SOL, which may indicate an increase in drive
to this muscle that lasted for at least 30 min. This improvement
in SEMG signals and absolute measure of force during
MVC following spinal manipulation are likely to be
attributed to an increase in the descending drive and/or
modulation in afferents. This is further evidenced by the
significant increase in the V-wave measurements as a result
of the spinal manipulation. V-wave increase also depends
upon the density of action potentials sent down from the
supraspinal centers that block of the antidromic action
potentials caused by the supramaximal stimulation of the
tibial nerve. Interestingly, these findings following a single
session of spinal manipulation are similar to what has been
observed following strength training (Vila-Chã et al. 2012)
who found that 3 weeks of strength training significantly
increased the V-wave amplitude (as measured by V/Mmax
ratio) by just over 55 %, SEMG during MVC increased
by 14.4 %, and the H-reflex threshold was significantly
decreased by 4.7 %. For comparison, our results from a single
session of chiropractic adjustments demonstrated significant
V-wave amplitude (V/Mmax ratio) increase of 45 %,
an average increase in SEMG during MVC by almost 60 %
and absolute force during MVC by 16 % and a significant
decrease in the H-reflex threshold by 8.5 %.
This is the first study to discover that the spinal manipulation
changes the H-reflex circuitry by increasing the
excitability of the low-threshold motoneurons of the SOL
and/or increasing the efficacy of the synaptic input to lowthreshold
motoneurons from Ia primary afferents originating
from the SOL. Recently, Pavlovian conditioning has
been used to increase in the H-reflex for improving muscles
that have lost their tonus (Chen et al. 2010). Therefore, our
results suggest that chiropractic treatment may also be useful
in such conditions instead of lengthy training of subjects.
However, this study only used a single treatment, and
we are at present studying the effects of long-term treatment
on the H-reflex circuitry. If that induces long-lasting
changes on the H-reflex, it would then be very useful for
the treatment to strengthen muscles that have lost their
tonus due to a variety of reasons.
Changes in fatigue
Although MVC force significantly increased after spinal
manipulation, it decreased significantly after the control
session. Supporting the occurrence of fatigue after control
but not after the spinal intervention as the MPF of the surface
SEMG records fell only after the control manipulation
(Lowery et al. 2002). That is, there was a significant change
in the MPF only in the control condition. This suggests that
spinal manipulation can prevent fatigue in the SOL lasting
for at least 30 min. This may be of relevance for sports performers.
However, this should be interpreted with caution,
as this study was not carried out in a sports population, but
average healthy young male subjects.
Maximal voluntary contraction is a subjective measure,
and unless it is backed up using twitch interpolation studies,
it may be misleading as it is completely depends upon
subject’s participation (Gandevia 2001). Maximal voluntary
contraction can depend not only on the drive that
one can physically achieve, but also the training that he/
she needs to learn to utilize the activation of the muscles’
potential. Therefore, any improvement of the SEMG and
force during MVC can be initially due to the confidence
the subject gains on the procedure and applies more of
the capacity that he/she has. The fact that our subjects
increased their MVCs as a consequence of the spinal
manipulation but not as a result of the control manipulation
suggests that the treatment actually gave our subjects
more confidence and possibly even opened up some of the
neuronal pathways that allow the person to apply more of
the force. These issues have to be further confirmed using
twitch interpolation techniques as well to make a more
This study is the first to indicate that the chiropractic
adjustments of the spine can actually induce significant
changes in the net excitability for the low-threshold motor
units, and/or alters the synaptic efficacy of the Ia synapse
with these low-threshold homonymous motoneurons. The
study also indicates that spinal manipulation can improve
the confidence of the subject to activate his/her muscle as
evidence with the increase in the SEMG signals and force
during MVC, and/or alters motor neuron recruitment patters.
The results suggest that the improvements in MVC
following spinal manipulation are likely attributed to the
increased descending drive and/or modulation in afferents.
They also indicate that spinal manipulation prevents
fatigue. Spinal manipulation may therefore be indicated as
part of the medical treatment for the patients who have lost
tonus of their muscle and or are recovering from muscle
degrading dysfunctions such as stroke or orthopedic operations.
These results may also be of interest to sports performers.
We suggest these findings should be followed up
in the relevant populations.
The authors would like to acknowledge the
following organizations for support and funding Australian Spinal
Research Foundation, Hamblin Chiropractic Research Fund Trust,
New Zealand College of Chiropractic and Koç University. KST is a
Fellow of the Turkish Academy of Sciences Association.
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