Correlation between Cranio-Vertebral Angle and Muscle Activity
According to Body Movements in Forward Head Posture
KYUNGWOO KANG
Department of Physical Therapy,
Yeungnam University Collage,
Daegu,
SOUTH KOREA
Abstract: - The purpose of this study is to determine the correlation between FHP and how it may affect muscle
activity during raising the arm and loss of body balance, 40 young people participated. All subjects will have
their cranio-vertebral angle (CVA) and muscle activity of the serratus anterior (SA), upper trapezius (UT), and
lower trapezius (LT), static balance index, and central pressure excursion index (CPEI) measured. Muscle
activity was measured using TeleMyo 2400(Noraxon U.S.A., Inc., Scottsdale, AZ, USA), and static balance
indicators and CPEI were measured using the MatscanVersaTek system (Tekscan Inc., MA). Spearman
correlation analysis was used to determine the correlation between variables. CVA and SA, UT, and LT all
showed significant correlation, positive correlation with SA(r=0.429/p=0.006) and LT(r=0.377/p=0.017), and
negative correlation with UT(r=-0.473/p=0.007) (Table 2). CVA showed a moderate level of negative
correlation with AREA(r=0.-0.420 /p=0.007) and L-R distance(r=-0.508 /p=0.000) among balance indicators,
and did not show a significant correlation with CPEI (Table 3). In people with more severe FHP, SA, and LT
muscle activity tended to be lower, UT muscle activity tended to be higher, and static balance ability was
lower. According to the results of this study, FHP can have a negative effect on various factors of the body,
such as arm movement and static balance, suggesting that establishing correct posture is necessary to prevent
secondary physical problems.
Key-Words: - Forward head posture, Cranio-vertebral angle, Muscle activity, Static balance,
Electromyography, Serratus anterior, Upper trapezius, Lower trapezius.
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1 Introduction
A forward head posture is defined as a posture in
which the head is positioned excessively forward
beyond a vertical reference line, [1]. FHP causes
various side effects such as neck pain, kyphosis,
reduced mobility of the spine, rounder shoulder,
etc, [2], [3], [4]. In particular, FHP is often
observed in people who are spending a long time in
a sitting position, [5]. prolonged sitting can also
lead to weakness in the trunk, and because the
muscles around the neck muscles are connected to
the trunk, it can affect overall musculoskeletal
functions by causing changes in the posture of the
trunk or even the tilt of the pelvis, [6].
The FHP is the most common cervical postural
deviation occurring in the sagittal plane. Forward
movement of the head and neck due to a prolonged
flexed head position can cause sagittal postural
defects of the cervical spine. The head moved
forward increases the mechanical load, which can
adversely affect muscle imbalance, functional
impairment, and head posture control, [7], [8]. FHP
can cause posture defects on the sagittal plane,
which can reduce the ability to control the posture
of the head making it difficult to maintain balance,
[2]. This is supported by a few studies that people
who have lost the ability to adjust their heads have
a reduced balance ability, [9], [10], [11]. And FHP
can change the position of the scapula. The
movement of the scapula is essential for body
movements such as raising the arm. A person who
has changed the position of the scapula due to FHP
can experience pain, and the functional movement
of the arm can be adversely affected by the
imbalance of the muscles around the scapula, [12],
[13], [14].
Cranio-vertebral angle (CVA) is the most widely
known measurement method used to determine
how forward the head is tilted in a person in FHP,
[15]. Studies have shown that abnormal forward
head postures and increased kyphosis postures lead
to an imbalance in related muscles, providing a
framework for body movement and increasing
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imbalance in the spine and surrounding soft tissues
that maintain body posture against gravity, [13],
[16]. The numbers that quantify CVA are objective
indicators that can express the degree of FHP and
can be used to identify the correlation between
body imbalance and loss of balance caused by FHP,
[17].
As mentioned earlier, FHP creates a rounded
shoulder posture, which can affect the activation of
muscles around the scapula, [2]. Depending on the
degree of FHP, it can be assumed that it is related
to body movements such as raising the arm, [14].
This is because the movement of fully raising the
arm (180 °flexion or abduction of shoulder)
requires 60 °upward rotation of the
scapulothoracic joint, [18]. The muscles around the
scapula that affect upward rotation include the
serratus anterior, upper trapezius, and lower
trapezius, and these are muscles that can be
affected by the non-ideal position of the scapula in
FHP or the resulting rounded shoulder posture,
[12]. According to scapulohumeral rhythm, upward
rotation of the scapulothoracic joint rarely occurs
during the initial 30-degree raising of the arm, [19].
During the subsequent arm-raising movement, it
has been revealed that the movement of the
shoulder joint (flexion or abduction) and the
upward rotation of the scapulothoracic joint move
at a ratio of 2:1, [18].
Surface EMG (sEMG) is a beneficial device that
can measure the muscle activity of superficial
muscles, [19]. A few research results show that
CVA affects muscle activity depending on its
degree. In these studies, EMG equipment is useful
in providing numerical data on muscle activity, so
it is widely used as a research device for muscle
activity. However, studies on CVA and muscle
activity mainly involve muscles around the neck,
and there are not many studies that measure muscle
activity related to movements in other parts of the
body, [20], [21]. In the case of people who show
more severe FHP due to a smaller CVA angle,
there is a change in the muscle activity around the
scapula. Since the movement of the scapulothoracic
joint is an essential element in the movement of
raising the arm, it can be affected by the CVA. The
primary upward rotators of the scapulothoracic
joint are the serratus anterior, upper trapezius, and
lower trapezius. There have been several studies
measuring the activity of these muscles during
raising the arm or in people with FHP, [12], [22],
[23].
Serratus anterior is a muscle that greatly
contributes to the upward rotation of the
scapulothoracic joint, [23]. According to previous
research results, during shoulder joint abduction of
more than 140 degrees, the muscle activity of the
serratus anterior was much higher than that of the
upper trapezius or lower trapezius, [23]. This fact
may be a factor to be considered in the design of
this study to measure EMG while raising the arm.
In previous studies, there were studies on various
side effects on the body by FHP, but there have
been few studies investigating the correlation
between FHP and the activity of muscles around
the scapula during raising the arm and the body's
balance ability. Therefore, the purpose of this study
is to find out through correlation how physical
imbalance caused by FHP affects muscle activities
for arm movement and static balance ability and to
consider the causes.
2 Methods
2.1 Subjects
The subjects were 40 adults in their 20s living in
Daegu, South Korea. The subjects were excluded
from the study, including those who suffered back
pain or had been treated for back pain six months
before the study, those with abdominal or spinal
surgery, nervous or cognitive disorders, trunks, and
back pain stabilization exercises. All subjects were
informed of the purpose of the study, the method,
content, and procedures of the experiment, and
those who agreed to participate in the experiment
were selected as subjects. The study is approved by
the Bioethics Committee (YNC IRB/2021-R-0005-
001).
2.2 Measurement Tools and Measurement
Methods
2.2.1 Cranio-vertebral Angle
For CVA measurement, the digital camera (Canon
650D, Canon, Japan) was fixed and mounted at a
distance of 1m, and the side of the subject was
photographed, for accurate measurement of CVA, a
plumb line suspended from the ceiling was allowed
to descend directly next to the subject. Subjects were
instructed to stand comfortably with both arms
relaxed on the side of the trunk and maintain a
natural head posture. CVA was defined as the angle
between the horizontal line and the line from the
spinous process of the seventh cervical vertebra to
the ear tragus. The CV angle means that the smaller
the angle, the greater the flexion of the lower
cervical spine (Figure 1).
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Fig. 1: Measurement method for cranio-vertal angle
2.2.2 Muscle Activities
Surface electromyography (sEMG) was measured
using TeleMyo 2400 (Noraxon U.S.A., Inc.,
Scottsdale, AZ, USA) to collect data on muscle
activity. EMG signal data were converted to digital
signals using the software MyoResearch XP 1.07
(Noraxon U.S.A., Inc., Scottsdale, AZ, USA) for
statistical analysis, and a notch filter of 60 Hz was
used. the raw EMG signal was sampled at 1000Hz
and the signal was full-wave rectified. And band-
pass filtering in the 30-500 Hz was performed.
EMG amplitudes were analyzed by Root Mean
Square (RMS) and the collected EMG data were
calculated as percentages of RVC (%RVC).
sEMG of the upper trapezius (UT), lower
trapezius (LT), and serratus anterior (SA) were
measured while raising the arm. After a preliminary
description of the test procedure to the subject, the
skin around the electrode area was shaved and
polished then cleaned with alcohol to lower the
skin impedance. And the electrode was securely
attached. The EMG electrode locations for each
muscle were as follows. (1) UT: The midpoint of
an imaginary line connecting the spinous process of
the 7th cervical vertebrae and the posterior sides of
the scapular acromion. (2) LT: The point 5cm
inferomedial from the root of the spine of the
scapula (3) SA: The point of the muscle located in
front of the latissimus dorsi under the axillary and
parallel to the inferior angle of the scapula, [23].
Participants were instructed to take
measurements while standing in a comfortable
position and looking straight ahead. Since previous
research has shown that after the shoulder joint is
flexed to 140 degrees, muscle activity in LT
becomes smaller and UT becomes larger, EMG
was measured at a flexion angle of 140 degrees to
minimize this effect, [23]. Participants were
instructed to hold a 1kg dumbbell and perform the
exercise at a comfortable speed using their
dominant arm. A stop sign was given when they
reached 140 degrees using a goniometer, and EMG
data was collected after stopping. Care was taken
not to rotate the torso while lifting the arms. EMG
data were collected for 5 seconds while maintaining
the posture to obtain the RMS value for the
intermediate 3-second interval. This was repeatedly
measured three times to obtain the RVC value
through the average value.
2.2.3 Static Balance
A MatscanVersaTek System (Tekscan Inc., MA)
was used to measure static balance and center
pressure excursion index (CPEI). Matscan
components consisted of an HR mat, cuff, USB, and
two hubs. The HR mat is 0.18mm thick and contains
2288 sensors, [24], [25]. The analysis was
performed using the Tekscan program, and static
balance was measured for 30 seconds at 30 frames
per second in one leg standing with the hip and knee
joints flexion at 90 degrees, [26]. The measured
static balance values were the total area(Area) where
the body sway occurred, the total distance(Total
distance), the maximum distance anterior and
posterior(A-P distance), and the maximum distance
left and right(L-R distance).
CPEI had the HR mat walk in a comfortable
walking condition. CPEI was the measured value of
the distance between the line connecting the points
starting from the heel and passing to the tip of the
toe and the actually moved COP excursion path,
divided by the foot width, and converted into a
percentage and converted into data, [27]. Matscan is
a valid and reliable screening tool for measuring
sway and COP excursion, [25].
2.3 Statistical Analyses
SPSS 22.0 (IBM, Chicago, IL, USA) was used for
all statistical analyses. The Shapiro-Wilk test was
used to test the normality of all variables, but the
normality test was not satisfied. Accordingly,
Spearman correlation analysis was used to
determine the correlation between CVA and muscle
activities and between CVA and balance indicators.
The statistical significance level was set to 0.05.
3 Results
A total of 40 subjects participated in the experiment,
and the average and standard deviation for general
characteristics such as age, height, weight, CVA,
muscle activities, Area, Total distance, A-P
distance, L-R distance, and CPEI of the subjects are
shown in Table 1.
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Table 1. Means ± SDs of general characteristics and cranio-vertebral angle, muscle activity, balance indicators,
CPEI of subjects
Total participants (n=40)
Age(year)
22.28 ± 2.72
Weight(kg)
58.20 ± 10.29
Height(cm)
164.38 ± 6.06
Cranio-vertebral angle (°)
53.92 ± 4.42
Serratus anterior(%RVC)
91.01± 3.52
Upper trapezius(%RVC)
135.34±3.72
Lower trapezius(%RVC)
83.29±1.90
Area(cm2)
5.01 ± 1.48
Total distance(cm)
117.66 ± 28.45
A-P distance(cm)
3.88 ± 0.69
L-R distance(cm)
3.16 ± 0.58
Center pressure excursion index(%)
17.54 ± 9.42
Table 2. Correlation between cranio-vertebral angle and muscle activities during shoulder flexion
CVA
Area
Total distance
A-P distance
r
p-value
r
p-value
r
p-value
r
p-value
1
0.429
0.006**
1
-0.473
0.002**
-0.333
0.036*
1
0.377
0.017*
0.323
0.042*
-0.203
0.208
1
CVA: cranio-vertebral angle, SA: serratus anterior, UT: upper trapezius, LT: lower trapezius * p<0.05, ** p<0.01
* CVA showed a moderate positive correlation with SA.
* CVA showed a moderate negative correlation with UT.
* CVA showed a weak positive correlation with LT.
Table 3. Correlation between cranio-vertebral angle, balance indicators, CPEI of subjects
CVA
Area
Total distance
A-P distance
L-R distance
CPEI
r
p-value
r
p-value
r
p-value
r
p-value
r
p-value
r
p-value
CVA
1
Area
-0.420
0.007**
1
Total
Distance
-0.198
0.219
0.496
0.001**
1
A-P
Distance
-0.230
0.154
0.790
0.000**
0.437
0.005**
1
L-R
Distance
-0.508
0.001**
0.698
0.000**
0.548
0.000**
0.353
0.025*
1
CPEI
-0.151
0.353
0.111
0.496
0.097
0.553
-0.045
0.781
0.238
0.139
1
CVA: cranio-vertebral angle, CPEI: center pressure excursion index
* p<0.05, ** p<0.01
* CVA showed a moderate negative correlation with Area.
* CVA showed a moderate negative correlation with L-R Distance.
As a result of the experiment on the correlation
between CVA and muscle activities during
shoulder flexion, CVA and SA, UT, and LT all
showed significant correlation, positive correlation
with SA(r=0.429/p=0.006) and
LT(r=0.377/p=0.017), and negative correlation
with UT(r=-0.473/p=0.007) (Table 2).
As a result of the experiment on the correlation
between CVA and balance indicator, CVA and
Area, L-R distance showed a significant correlation,
negative correlation with Area (r=-0.420/p=0.007)
and negative correlation with L-R distance (r=-
0.508 /p=0.000) (Table 3).
4 Discussion
In this study, we tried to find out how much FHP
affects muscle activities for arm movement and
static balance ability. It is generally known that the
smaller the CVA angle, the more severe the FHP.
As a result of this study, CVA showed a moderate
positive correlation with SA and a weak positive
correlation with LT. This means that the more
severe the FHP, the lower the SA and LT muscle
activity. In addition, CVA showed a moderate
negative correlation with UT, meaning that the
more severe the FHP, the higher the muscle activity
of UT. Regarding static balance, CVA showed a
moderate negative correlation with L-R distance
and AREA among the static balance indicators,
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meaning that people with smaller CVA angles
experienced greater body sway when maintaining
static balance. This means that people with more
severe FHP have poorer static balance abilities.
Upward rotation of the scapulothoracic joint is
essential during the movement of raising the arm
above the head (Whether it is shoulder flexion or
abduction). While fully raising the arm, the
scapulothoracic joint must be upward rotated by
about 60 degrees, and the muscles involved in
upward rotation are SA, UT, and LT, [28].
Typically, as FHP is more severe, a round shoulder
posture also develops, and this non-ideal posture
causes a change in the position of the scapula. In
the FHP, the scapula is protracted, rotated
downward, tilted forward, and rotated internally.
This position of the scapula will inevitably affect
muscle activity by causing changes in the length of
the SA, UT, and LT muscles around the scapula,
[23], [28].
The results of this study showed that the activity
of UT was higher in people with more severe
FHP(smaller CVA angles). UT is known to be an
upward rotator of the scapulothoracic joint and
antagonist of the levator scapula muscle. In one
previous study, it was mentioned that people with
severe FHP always have increased tension in the
levator scapula muscle, so muscle activity of UT is
increased as acting as an antagonist to offset the
increased tension of FHP. Also in that study, it was
said that the role of the upward rotator was added
while raising the arm, resulting in higher activity of
UT, [29]. In another study, in the case of FHP, the
torque of cervical spine flexion due to gravity may
increase, and there was also a view that the muscle
activity of UT would increase because the cervical
extensors must show more activity to counteract
this imbalance, [30].
The results of this study showed that people with
more severe FHP showed weaker SA and LT
muscle activity. One of the clinical assumptions
associated with these weakened muscle activities is
that non-ideal changes in scapula position increase
the passive length of the muscle. Because SA and
LT become lengthened at FHP, it appears that they
bring biomechanical disadvantages from the
perspective of the length-tension relationship and
muscle activity also decreases, [30]. In addition, it
is known that during upward rotation of the
scapulothoracic joint, UT and LT act as a couple of
forces to cause movement. In this study, a tendency
for UT activity to increase during arm flexion in
people with FHP was found. So when there is
increased UT activity during upward rotation of the
scapula, the involvement of LT, which is involved
as a pair force, may be relatively become smaller,
[28].
SA contributes to the 3D stereoscopic movement
of the scapula during arm raising. It contributes to
upward rotation along with UT and LT, but can
also cause lateral rotation and posterior tilt of the
scapula, [31]. Therefore, in addition to pain caused
by hyperactivation of UT, people with FHP may
also experience muscle imbalance problems caused
by the inactivity of SA. Less activated SA causes
internal rotation and anterior tilt of the scapula,
which accelerates the shortening of the pectoralis
minor muscle, which can entrench the round
shoulder posture. In addition, raising the arm while
anterior tilt of the scapula leaning forward can put
pressure on the tissue in the space under the
acromion of the scapula, which can lead to
additional problems such as impingement
syndrome, [28].
There have not been many studies in which the
forward head posture or CVA angle directly affects
the balance ability, but various studies have found
evidence that it can indirectly affect it. First of all,
posture with increased CVA, such as the forward
head posture, can affect not only the imbalance of
the muscles around the neck but also the muscle
condition of the trunk. Prolonged sitting posture
and excessive use of computers or smartphones are
among the main factors that cause increased CVA,
and a number of studies have revealed the
relationship between these factors and the trunk
muscle, [7], [8], [32], [33].
Trunk muscle endurance may be a major factor
influencing the decrease in balance, [34].
Prolonged sitting posture that causes an FHP
eventually leads to a decrease in physical activity
and a decrease in muscle activity to maintain the
posture, which can lead to a decrease in endurance
of the core muscle strength that maintains the
posture in the long run, [22], [35]. In particular, it
has been reported that trunk extensor fatigue has an
effect on increasing the postural sway of static
balance, [36]. It has already been shown by several
studies that weakness in trunk muscles or core
muscles can lead to a decrease in balance, [34],
[37], [38], [39]. There may be various reasons for
the decrease in balance ability of a person with a
smaller CVA angle, among which a decrease in
trunk muscle endurance due to prolonged sitting
posture may be the main factor.
The percentage value calculated by CPEI
indicates that the smaller the number, the more
inward the foot arch collapses, which is often used
as an indicator to measure the degree of flat feet,
[24], [25]. Static balance can be greatly influenced
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by the feet touching the ground. In this study, CPIE
values were also checked to determine whether the
degree of the arch of the foot, which varies from
person to person, affected the balance. The value of
CPEI did not show a significant relationship with
CVA or any other balance indicators. This means
that the decrease in balance ability in subjects who
showed a smaller CVA angle was not due to at
least problems such as the arch of the foot. At least,
according to the results of this study, the decrease
in balance ability could be interpreted as being
caused by the degree of the CVA rather than the
foot. Similarly, there are several studies that
strengthen the core is more important than the leg
or foot to enhance static balance ability, [40], [41],
[42].
5 Conclusion
In this study, we investigated the correlation
between CVA and muscle activities during raising
the arm and static balance to determine the causes
of these results. In our finding, people with more
severe FHP(those with smaller CVA) tended to
develop an imbalance in muscle activity while
raising their arms and decreased balance ability. In
general, many researchers focused and investigated
only on pain or the muscles around the neck in
research on FHP. However, our findings
contributed to confirming that people with FHP can
be adversely affected by balance or body
movement. The balance of the muscles around the
scapula reduces the efficiency of the arm-raising
motion, can cause pain, and can also lead to
secondary dysfunction such as impingement
syndrome. Our findings suggest that it can be
beneficial for people with FHP to suppress UT and
strengthen SA or LT when raising their arms.
Furthermore, our results found that the more severe
the FHP, the greater sway can occur in a static
standing position. These results may serve as a
reference for future studies on the association
between FHP and elderly falls. This study has
limitations in that the subjects were young, the
sample number was small, and the relationship
between CVA and other direct body imbalance
factors was not considered. In the future, there is a
need to further investigate this relationship so that
it can serve as basic data for improving posture and
balance in CVA patients such as FHP.
Acknowledgement:
This research was supported by the Yeungnam
University College Research Grants in 2021.
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Contribution of Individual Authors to the
Creation of a Scientific Article (Ghostwriting
Policy)
This paper was solely authored by KyungWoo
Kang, who was responsible for conceiving and
developing the research ideas, designing
experiments, collecting and analyzing data,
interpreting the results, and drafting the paper.
Sources of Funding for Research Presented in a
Scientific Article or Scientific Article Itself
This research was supported by the Yeungnam
University College Research Grants in 2021.
Conflict of Interest
The authors have no conflicts of interest to declare.
Creative Commons Attribution License 4.0
(Attribution 4.0 International , CC BY 4.0)
This article is published under the terms of the
Creative Commons Attribution License 4.0
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