Influence of forward head posture on muscle activation pattern of the trapezius pars descendens muscle in young adults

Abstract

Forward head posture (FHP) is a serious problem causing head and neck disability, but the characteristics of muscle activity during long-term postural maintenance are unclear. This study aimed to investigate a comparison of electromyography (EMG) activation properties and subjective fatigue between young adults with and without habitual FHP. In this study, we examined the changes in the spatial and temporal distribution patterns of muscle activity using high-density surface EMG (HD-SEMG) in addition to mean frequency, a conventional measure of muscle fatigue. Nineteen male participants were included in the study (FHP group (n = 9; age = 22.3 ± 1.5 years) and normal group (n = 10; age = 22.5 ± 1.4 years)). Participants held three head positions (e.g., forward, backward, and neutral positions) for a total of 30 min each, and the EMG activity of the trapezius pars descendens muscle during posture maintenance was measured by HD-SEMG. The root mean square (RMS), the modified entropy, and the correlation coefficient were calculated. Additionally, the visual analogue scale (VAS) was evaluated to assess subjective fatigue. The RMS, VAS, modified entropy, and correlation coefficients were significantly higher in the FHP group than in the normal group (p < 0.001). With increasing postural maintenance time, the modified entropy and correlation coefficient values significantly decreased, and the mean frequency and VAS values significantly increased (p < 0.001). Furthermore, the forward position had significantly higher RMS, correlation coefficient, modified entropy, and VAS values than in the neutral position (p < 0.001). The HD-SEMG potential distribution patterns in the FHP group showed less heterogeneity and greater muscle activity in the entire muscle and subjective fatigue than those in the normal group. Excess muscle activity even in the neutral/comfortable position in the FHP group could potentially be a mechanism of neuromuscular conditions in this population.

Full text

1
Vol.:(0123456789)
Scientic Reports | (2022) 12:19484 | https://doi.org/10.1038/s41598-022-24095-8
www.nature.com/scientificreports
Inuence of forward head posture
on muscle activation pattern
of the trapezius pars descendens
muscle in young adults
Yuichi Nishikawa1*, Kohei Watanabe2, Takanori Chihara1, Jiro Sakamoto3,
Toshihiko Komatsuzaki1, Kenji Kawano4, Akira Kobayashi4, Kazumi Inoue4, Noriaki Maeda5,
Shinobu Tanaka1 & Allison Hyngstrom6
Forward head posture (FHP) is a serious problem causing head and neck disability, but the
characteristics of muscle activity during long-term postural maintenance are unclear. This study
aimed to investigate a comparison of electromyography (EMG) activation properties and subjective
fatigue between young adults with and without habitual FHP. In this study, we examined the changes
in the spatial and temporal distribution patterns of muscle activity using high-density surface EMG
(HD-SEMG) in addition to mean frequency, a conventional measure of muscle fatigue. Nineteen male
participants were included in the study (FHP group (n = 9; age = 22.3 ± 1.5 years) and normal group
(n = 10; age = 22.5 ± 1.4 years)). Participants held three head positions (e.g., forward, backward, and
neutral positions) for a total of 30 min each, and the EMG activity of the trapezius pars descendens
muscle during posture maintenance was measured by HD-SEMG. The root mean square (RMS), the
modied entropy, and the correlation coecient were calculated. Additionally, the visual analogue
scale (VAS) was evaluated to assess subjective fatigue. The RMS, VAS, modied entropy, and
correlation coecients were signicantly higher in the FHP group than in the normal group (p < 0.001).
With increasing postural maintenance time, the modied entropy and correlation coecient values
signicantly decreased, and the mean frequency and VAS values signicantly increased (p < 0.001).
Furthermore, the forward position had signicantly higher RMS, correlation coecient, modied
entropy, and VAS values than in the neutral position (p < 0.001). The HD-SEMG potential distribution
patterns in the FHP group showed less heterogeneity and greater muscle activity in the entire muscle
and subjective fatigue than those in the normal group. Excess muscle activity even in the neutral/
comfortable position in the FHP group could potentially be a mechanism of neuromuscular conditions
in this population.
Forward head posture (FHP) is a head and neck exion posture that is associated with cervical neck disease
and due to several environmental/behavioral factors, it is seen increasingly in young adults. In recent years,
the frequency of working from home and attending meetings online has increased rapidly with the spread of
novel coronavirus disease1, and it has been observed that people are spending more time in a seated position2.
Prolonged sitting has been suggested as a risk factor for neck pain3, and a previous study reported that there is
an association between sitting time in total per day and the intensity of neck pain4. Furthermore, there has been
a potentially harmful increase in the use of smartphones for texting, especially among young people, combined
with the increasing prevalence of neck pain3,5. e prolonged use of smartphones and personal computers could
cause musculoskeletal problems. In a previous study, it was reported that screen viewing time is associated with
an increased posture of exion of the neck and head in children, especially in a sitting position6. Knowledge in
OPEN
1Faculty of Frontier Engineering, Institute of Science & Engineering, Kanazawa University, Kanazawa,
Kakuma-Machi, Kanazawa, Ishikawa 920-1192, Japan. 2Laboratory of Neuromuscular Biomechanics, School of
Health and Sport Sciences, Chukyo University, Nagoya, Japan. 3Faculty of Advanced Manufacturing Technology
Institute, Kanazawa University, Kanazawa, Kanazawa, Ishikawa, Japan. 4Division of Seat Evaluation & Engineering,
Toyota Boshoku, Toyota, Aichi, Japan. 5Division of Sports Rehabilitation, Graduate School of Biomechanical and
Health Sciences, Hiroshima University, Hiroshima, Hiroshima, Japan. 6Department of Physical Therapy, Marquette
University, Milwaukee, WI, USA. *email: yuichi@se.kanazawa-u.ac.jp
Content courtesy of Springer Nature, terms of use apply. Rights reserved

2
Vol:.(1234567890)
Scientic Reports | (2022) 12:19484 | https://doi.org/10.1038/s41598-022-24095-8
www.nature.com/scientificreports/
this area is clinically relevant, as the long-term eects of FHP during adolescence have been suggested to pre-
dispose adults to headaches and neck pain7,8, and the identication of abnormalities in subjective fatigue and
muscle activity in asymptomatic FHP is important in the prevention of head and neck orthopedic problems.
FHP can be associated with key abnormalities in neuromuscular function, such as a lower endurance of
the deep neck extensors and exors, as well as a higher activity of the supercial muscles in adults with neck
pain9. However, several studies that examine FHP using surface electromyography (EMG) have focused on
the standing position in which the head and neck do not touch a pillow or head rest of a seat in people with
neck symptoms10,11, and no reports have been made in a sitting position, especially the resting position (head
leaning against a pillow or other object) in people with asymptomatic FHP. It is important to study this posi-
tion because it is necessary to maintain the same posture for long periods of time in the resting posture while
working at home or while using an economy class seat in airplane travel. Furthermore, several previous studies
have examined the assessment of neuromuscular function using surface EMG during several postures in people
with FHP. However, a pair of small electrodes is generally used to record surface EMG signals from a muscle of
interest, and the detected surface EMG signals can only provide information about a very small portion of the
muscle. As a method to estimate motor unit activation or to provide more detailed physiological data, a high-
density surface EMG (HD-SEMG) technique has been developed recently that records surface EMG signals
from large areas of muscle using multiple two-dimensionally oriented electrodes12. Previous studies reported
region-specic muscle activity and muscle fatigue in the upper trapezius muscle13,14. Consequently, HD-SEMG
could be useful to understand neuromuscular function and/or fatigue in people with FHP. However, there are
no reports of HD-SEMG applications to head and neck extensor muscles, and the neuromuscular function and/
or fatigue properties of FHP remain unclear.
Here, we compared EMG properties and subjective fatigue between young adults with and without FHP. Due
to weakness in the deep neck extensors, we hypothesized that the FHP group would show greater subjective
fatigue and activity in the muscle that maintains the neck position, i.e., trapezius pars descendens muscle, in a
head-leaning sitting posture than the normal group. e results of this study identify early muscle abnormalities
in people with asymptomatic FHP and provide some mechanistic insight with regard to FHP-related neck pain,
and provide insight into some of the factors contributing to head and neck disorders in FHP. To help us interpret
our results, measurements of EMG distribution patterns were performed using HD-SEMG. Other HD-SEMG
measures, such as entropy, will be used to provide mechanistic insight.
Materials and methods
Participants. Nineteen young adults were enrolled in this study aer signing an informed consent form.
All experimental protocols of this study were approved by the Ethics Committee of the Institute of Science
and Technology, Kanazawa University (No. 2021-8), and all methods were carried out in accordance with the
requirements of the Declaration of Helsinki. e inclusion criteria were age ≥ 20years old and no neck or shoul-
der pain. e exclusion criteria were a history of neck and back injury and neurological diseases (Parkinson’s
syndrome, dementia, myositis, spinal muscular atrophy, and dystonia). e craniovertebral angle was calcu-
lated as the angle between the horizontal line passing through C7 and a line extending from the tragus of the
ear to C7 for all participants (Fig.1)15. e FHP group included those with a craniovertebral angle < 53°, n = 9
(age, 22.3 ± 1.5years; height, 171.6 ± 3.6cm; weight, 60.6 ± 5.2kg) and the normal group had a craniovertebral
angle > 53°, n = 10 (age, 22.5 ± 1.4years; height, 173.3 ± 3.6cm; weight, 63.6 ± 6.1kg). Determination of 53° as a
α
C7
Figure1. Measurement of the craniovertebral angle. In upright standing posture, the craniovertebral angle (α)
was calculated as the angle between the horizontal line passing through C7 and a line extending from the tragus
of the ear to C7.
Content courtesy of Springer Nature, terms of use apply. Rights reserved

3
Vol.:(0123456789)
Scientic Reports | (2022) 12:19484 | https://doi.org/10.1038/s41598-022-24095-8
www.nature.com/scientificreports/
reference angle was conducted by study Lee etal.16, Yib etal.17, and Salahzadeh etal. who reported 55° as a nor-
mal range and subjects with FHP had a smaller angle than normal subjects.
Experimental protocols. All subjects were measured for MVC in the neutral position (see below for
details) and then held in the sitting posture for 30min in three dierent head postures (neutral, forward, and
backward) to examine the inuence of head position on muscle activity and fatigue (Fig.2). e sessions were
conducted once each.
In sitting, participants adjusted the seat recline while looking straight ahead and identied the most comfort-
able head and neck position as the “neutral head position” (Fig.2B). Next, the seat was reclined 5° (θ′ = 5°), a
pillow corresponding to the height of x was prepared, and the participant was instructed to place the back of the
head on the pillow and lean back in a neutral position (Fig.2C). en + 3 cm (“Forward”) and − 3cm (“Back-
ward”) from the neutral position were dened (Fig.2D,E). EMG data were then measured in each of the three
head holding positions. e reclining angle (θ′ = 5°) was set to prevent the head from falling forward when in a
forward displaced position (+ 3cm). EMG measurements were taken during the isometric maximal voluntary
isometric contraction (MVC) measurement and the rst minute of posture maintenance and every 10min there-
aer for 1min. e EMG data during these periods were used for analysis. All participants held each position
(e.g., forward, backward, and neutral) a total of 30min. e arms were allowed to droop along the trunk, and
the knees were placed in a comfortable70°–90° exed position. e order of positions was randomized, and the
interval between tasks was at least one day to minimize the eects of fatigue.
We adjusted the resistance pad on the cervical device movement arm so that it was at a level that was just
superior to the external occipital protuberance. e resistance pad was locked in place, and participants were
instructed to perform a series of two MVC attempts against the xed resistance pad. For the MVC measurement,
the participant was instructed to perform head extension as hard as possible for 5s without force in the hip and
shoulders. To prevent the subject from exerting force in the hip and shoulders, the subject was asked to sit deeply
in the seat so that the hip would not li, and the arms were kept relaxed. Each of these eorts was held for a two-
minute rest period between each of these two eorts (Fig.2A). For each participant, we treated the highest EMG
voltage (MVC-Max) observed in these two MVC trials as the maximum voltage that the participant could attain
during an MVC eort. Additionally, we measured the visual analogue scale (VAS) at each posture at 30min as a
subjective fatigue assessment. Subjective fatigue was assessed for fatigue related to the head and neck area. e
VAS was measured on a scale of 0 to 100, with 0 dened as not fatigued and 100 dened as maximally fatigued18.
EMG recording. e 64-electrode grid (1mm, diameter; 4mm, intra-electrode distance, GR04MM1305,
OT Bioelettronica, Turin, Italy) was placed on the trapezius pars descendens muscle of the dominant side
MVCs
2 min 30 min
1 min 1 min 1 min 1 min1 min
A
1 min 10 min 20 min
X
θ
X
θ
θ’
θ
X
Determination of
neutral head position
Neutral position
y
θ
θ’
z
θ
θ’
Forward position Backward position
BCED
Figure2. Study protocol and determination of three head positions. (A) All participants were asked for the
maximal voluntary contraction (MVC) of head extension twice. Aer MVC measurements, participants held
the sitting posture for 30min, and electromyography measurements were taken for 1min at the beginning
of the posture and for 1min every 10min thereaer (gray bar). (B) Looking straight ahead and adjusting the
seat recline, the most comfortable head and neck position is the neutral head position. X indicates the distance
between the back of the head and the seat. (C) e neutral position was dened as the posture with the 5°
recline folded down from the neutral head position (θ′ = 5°). A pillow with a height of x was fabricated for each
participant, and the participant was made to lean against the pillow. (D,E) e position with the pillow height
3cm higher or lower than the neutral position was dened as the + 3 position or − 3 position (y = x + 3cm and
z = x − 3cm).
Content courtesy of Springer Nature, terms of use apply. Rights reserved

4
Vol:.(1234567890)
Scientic Reports | (2022) 12:19484 | https://doi.org/10.1038/s41598-022-24095-8
www.nature.com/scientificreports/
(Fig.3A). e medial side of the electrode grid was placed at the lateral side of C7 and axed on a line connect-
ing C7 and the acromion. Aer cleaning the skin (80% alcohol), an electrode grid was attached to the muscle
surface with a two-adhesive sheet (KIT04MM1305, OT Bioelettronica) with a conductive paste (Elex ZV-181E,
NIHON KOHDEN, Tokyo, Japan)19. e seventh cervical spine was placed with a ground electrode. Monopolar
HD-SEMG signals were recorded using a 16-bit AD converter (Quattrocento, OT Bioelettronica, sampling fre-
quency at 2048Hz), amplied at a 150 gain and ltered at a 10–500Hz o-line bandpass20,21. MATLAB soware
(MATLAB 2021b, Math Works GK, MA, USA) was used to analyze EMG signals.
Data processing. A total of 59 bipolar EMG signals were calculated from adjacent electrodes (12 bipolar
recordings in each row except the upper row, which had 11 electrode pairs, Fig.3A). e root mean square
(RMS) and mean frequency were calculated for each electrode and the mean values were calculated for all elec-
trodes from all of the data at each period (1min, 10min, 20min, and 30min). Furthermore, the RMS of the
MVC was calculated from 1s of data centered on the maximum voltage during MVC. e RMS value was nor-
malized to the MVC value. RMS and mean frequency were computed as follows:
where N is the length of the signal, EMG is the EMG signal, and i is the ith sample.
RMS
=

1
N
N
i
=
1
(EMGi)2
,
4 mm
1 mm
A
Root mean square (mV)
Lateral
Medial
Cranial
Caudal
Lateral
Medial Caudal
Cranial
B
Root mean square (mV)
30 min
1 min 10 min
20 min
C7
Figure3. Electrode placement and color map of the representative high-density surface electromyography
(HD-SEMG) in each period during posture maintenance. (A) e 64-electrode array was placed on the
trapezius pars descendens muscle (electrode diameter; 1mm and interelectrode distance; 4mm). Topographic
map of the root mean square value of the bipolar EMG recorded at the neutral position during posture
maintenance. (B) Illustration of a color map of the representative HD-SEMG in a neutral position for each
period during posture maintenance in a young adult (age 21years).
Content courtesy of Springer Nature, terms of use apply. Rights reserved

5
Vol.:(0123456789)
Scientic Reports | (2022) 12:19484 | https://doi.org/10.1038/s41598-022-24095-8
www.nature.com/scientificreports/
where f is the sampling frequency and P is the power at that frequency.
To characterize the heterogeneity in the spatial distribution of the HD-SEMG potential at each period, we
determined the modied entropy and correlation coecient. e modied entropy was calculated for 59 RMS
values at MVC and each period, as performed in a previous study14.
where p(i)2 is the RMS value square of electrode i, which is normalized by the total of 59 RMS values over a given
period. e correlation coecient was calculated using 59 RMS pairs of the same region for 1min compared to
10min, 20min, and 30min (Fig.3B).
e reduced modied entropy indicates an increase in the heterogeneity of the spatial distribution of the
HD-SEMG potential in the electrode grid. A reduction in the correlation coecients indicates an increase in the
temporal distribution of the HD-SEMG potential. Changes in the spatial and temporal distribution patterns of
the HD-SEMG potential show relative adaptations to muscle activity intensity during contraction and may be
attributed to changes in the peripheral characteristics of the muscle or to the control of the motor unit within
the muscle22.
Statistical analysis. All statistical analyses were conducted using Stata ver. 17 (Stata Corp LLC, Texas,
USA), and GraphPad Prism version 8 (GraphPad Soware Inc, California, USA) was used to generate graphics.
Shapiro–Wilk tests were conducted on all data to ensure normality. Separate unpaired t-tests were used to detect
dierences in age, height, and weight between the FHP and normal groups. e generalized linear mixed-eects
model with random intercepts and random slopes with Bonferroni’s multiple comparison test as a post hoc test
was applied to analyze the normalized RMS, modied entropy, correlation coecients, VAS, and mean fre-
quency. e explanatory variables were group (FHP and normal), period (1min, 10min, 20min, and 30min),
and position (neutral, forward, and backward). e signicance level was p < 0.05.
Results
Age, height, and weight were not dierent between the groups (p = 0.8021, p = 0.3064, and p = 0.2566, respectively).
ere was a signicant interaction eect of group
×
period
×
position for VAS (F = 2.80, p = 0.0100, η2 = 0.141),
modied entropy (F = 5.84, p < 0.0001, η2 = 0.256), and the correlation coecient (F = 2.25, p = 0.0359, η2 = 0.117);
on the other hand, the normalized RMS (F = 0.15, p = 0.9891, η2 = 0.008) and mean frequency (F = 0.235,
p = 0.9650, η2 = 0.013) did not show a signicant interaction eect of group
×
period
×
position.
e modied entropy and correlation coecient values were signicantly lower at 10min, 20min, and 30min
in the normal group in the neutral position than in the FHP group (p < 0.001) (Figs.4A, 5A). Furthermore, the
normal group showed signicantly lower modied entropy at 20min and 30min in the backward position
than the FHP group (p < 0.0001) (Fig.4B). On the other hand, the forward position did not show a signicant
dierence at each period between the groups (Figs.4C, 5C). e normal group showed signicantly decreased
modied entropy and correlation coecients over time in neutral and backward positions compared with 1min
(p < 0.01) (Figs.4A,B, 5A,B), but the forward position did not show a signicant dierence between each period
(Figs.4C, 5C). e normal group showed signicantly higher modied entropy at 10min, 20min, and 30min
in the forward position than in the neutral position (p < 0.0001), and signicantly higher modied entropy at
20min (p = 0.001) and 30min (p < 0.0001) in the forward position than in the backward position (Fig.6A). e
correlation coecients in the normal group were signicantly higher at 20min (p = 0.005) and 30min (p < 0.0001)
in the forward position than in the neutral position. Furthermore, the normal group showed a signicantly lower
correlation coecient at 10min in the neutral position than in the backward (p = 0.049) and forward (p < 0.0001)
Mean frequency
=
∞
0
f·P

f

df /
∞
0
P

f

df
,
E
=−
59
i=1
p(i)2log2p(i)2
,
ABC
Figure4. Comparison of modied entropy between groups in neutral (A), backward (B), and forward (C)
postures. e forward head posture (FHP) group showed signicantly higher values at 10min, 20min, and
30min in the neutral posture, and signicantly higher values at 20min and 30min in the backward posture.
Furthermore, the normal group showed a signicant decrease over time during posture maintenance in the
neutral and backward postures. Data showed median ± 95% CI. #p < 0.05, FHP vs. normal; †p < 0.05, compared
with 1min.
Content courtesy of Springer Nature, terms of use apply. Rights reserved

6
Vol:.(1234567890)
Scientic Reports | (2022) 12:19484 | https://doi.org/10.1038/s41598-022-24095-8
www.nature.com/scientificreports/
A
BC
Figure5. Comparison of correlation coecients between groups in neutral (A), backward (B), and forward
(C) postures. In the neutral posture, the forward head posture (FHP) group showed a signicantly higher
correlation coecient than the normal group (A). e normal group showed a signicant decrease over time
during posture maintenance in the neutral and backward postures (A,B). In the forward posture, each group
did not show a signicant dierence between each period (C). Data showed median ± 95% CI. #p < 0.05, FHP vs.
normal; †p < 0.05, compared with 1min.
AB
Figure6. Comparison of modied entropy between each posture in normal (A) and forward head posture
(FHP) (B) groups. In the forward posture, the normal group showed signicantly higher levels than in the
neutral and backward postures (A). On the other hand, the FHP group did not show a signicant dierence
between each posture. Data showed median ± 95% CI. *p < 0.05, compared with neutral; †p < 0.05, compared
with backward.
AB
Figure7. Comparison of correlation coecients between each posture in the normal (A) and forward head
posture (FHP) (B) groups. e normal group showed signicantly higher correlation coecients at 20min
and 30min in the forward posture than in the neutral posture (A). Furthermore, the neutral posture showed
a signicantly lower correlation coecient at 10min than the backward and forward postures in the normal
group. On the other hand, the FHP group did not show a signicant dierence between each posture (B). Data
showed median ± 95% CI. *p < 0.05, compared with neutral; ‡p < 0.05, compared with backward and forward.
Content courtesy of Springer Nature, terms of use apply. Rights reserved

7
Vol.:(0123456789)
Scientic Reports | (2022) 12:19484 | https://doi.org/10.1038/s41598-022-24095-8
www.nature.com/scientificreports/
positions (Fig.7A). On the other hand, the FHP group did not show signicant dierences in modied entropy
and correlation coecients among postures (Figs.6B, 7B).
e VAS score was signicantly lower at 10min, 20min, and 30min in the normal group at each position
than in the FHP group (p < 0.0001) (Fig.8). e FHP group showed a signicantly increased VAS score over time
compared with 1min in each posture (p < 0.0001,) (Fig.8). e normal group showed a signicantly increased
VAS score over time compared with 1min in the forward posture (p < 0.0001) (Fig.8C). e normal group
showed signicantly higher VAS scores at 10min (p = 0.001), 20min (p < 0.0001), and 30min (p < 0.0001) in
the forward position than in the neutral position and signicantly higher VAS scores at 20min (p = 0.004) and
30min (p < 0.0001) in the forward position than in the backward position (Fig.9A). e FHP group showed a
signicantly higher VAS score at 30min in the forward position than in the neutral position (p = 0.002) (Fig.9B).
e FHP group showed a signicantly higher normalized RMS value than the normal group (p < 0.0001,
η2 = 0.141) (Fig.10A). e normalized RMS value did not show a signicant dierence among the periods (1min
vs. 10min; p = 1.000, 1min vs. 20min; p = 1.000, 1min vs. 30min; p = 0.559, 10min vs. 20min; p = 1.000, 10min
vs. 30min; p = 1.000, 20min vs. 30min; p = 1.000) (Fig.10B). e neutral position showed a signicantly lower
normalized RMS value than the forward head position (p = 0.006), but there was no signicant dierence between
neutral and backward positions (p = 0.307) or backward and forward (p = 0.401) (Fig.10C).
In the mean frequency, there was no signicant dierence between the normal and FHP groups (p = 0.5633,
η2 = 0.03) (Fig.11A). e mean frequency was signicantly higher at 1min than at 20min and 30min (p < 0.0001),
that at 10min was signicantly higher than that at 20min and 30min (p < 0.0001), and that at 20min was signi-
cantly higher than that at 30min (p < 0.0001) (Fig.11B). On the other hand, there was no signicant dierence
between each position (neutral vs. backward; p = 1.000, neutral vs. forward; p = 0.432, backward vs. forward;
p = 0.415) (Fig.11C).
A
BC
Figure8. Comparison of visual analogue scale (VAS) scores between groups in neutral (A), backward (B), and
forward (C) postures. e forward head posture (FHP) group showed signicantly higher VAS scores at each
period in all postures than in the normal group. e FHP and normal groups showed signicantly increased
VAS scores over time during posture maintenance in each posture. Data showed median ± 95% CI. #p < 0.05,
FHP vs. normal; *p < 0.05, compared with 1min; ‡p < 0.05, compared with 10min; †p < 0.05, compared with
20min.
A
B
Figure9. Comparison of visual analogue scale (VAS) scores between each posture in the normal (A) and
forward head posture (B) groups. e normal group showed signicantly higher VAS scores in the forward
posture than in the neutral and backward postures during posture maintenance (A). On the other hand,
the forward head posture (FHP) group showed signicantly higher VAS scores in the forward posture only
at 30min in the forward posture than in the neutral posture (B). Data showed median ± 95% CI. *p < 0.05,
compared with neutral; †p < 0.05, compared with backward.
Content courtesy of Springer Nature, terms of use apply. Rights reserved

8
Vol:.(1234567890)
Scientic Reports | (2022) 12:19484 | https://doi.org/10.1038/s41598-022-24095-8
www.nature.com/scientificreports/
Discussion
is study compared the spatial and temporal distribution patterns of HD-SEMG in the trapezius pars descen-
dens muscle between FHP and normal conditions. e primary novel results were as follows: the FHP group
exhibited a (1) greater RMS amplitude, (2) lower heterogeneity, (3) smaller temporal changes, and (4) greater
quantitative/subjective fatigue (e.g., mean frequency and VAS score) during the long-term sitting position than
the normal group. Of particular importance, we found greater muscle activity in the FHP group even in the
neutral position. Furthermore, the mean frequency analysis used in this study was able to detect muscle fatigue,
while the spatial and temporal distribution analysis of muscle activation was able to identify abnormalities in
muscle activity between dierent postures in each group. ese ndings suggest that, in addition to frequency
analysis, analysis of the distribution of muscle activation patterns can be used to identify more detailed fatigue
in the head and neck region.
In this study, we measured muscle activity during three dierent positions (e.g., neutral, forward, and back-
ward) for both the FHP and normal groups and compared changes in the temporal and spatial distribution
patterns of trapezius pars descendens muscle activity and quantitative/subjective fatigue. Previous studies have
shown that head displacement forward or backward from a neutral position muscle’s EMG activity increased
in trapezius pars descendens muscle23,24. ese previous ndings are consistent with the results of this study
showing that compared to the neutral position, the forward position exhibited a higher RMS value in the normal
group. Interestingly, the FHP group showed signicantly higher normalized RMS values than the normal group.
Previous studies by Hollgren etal. demonstrated that voluntary head retraction and/or protrusion results in a
statistically signicant increase in EMG activity in the posterior rectus capitis muscle, resulting in eccentric
and/or concentric contractions23,24. e deep muscle of the higher spine functions to stabilize the head and
neck, maintain posture, and protect against movements caused by unexpected external forces25. ese ndings
suggest that deviation from the neutral position of the head and neck is associated with increased muscle activ-
ity. Importantly, our results showed that muscle activity increased despite leaning the head and neck against a
headrest, and for the FHP group was found to have highly subjective fatigue even in comfortable head and neck
A
B
C
Figure10. Comparison of normalized root mean square (RMS) values between groups (A), period (B), and
each posture (C). e forward head posture (FHP) group showed a signicantly higher normalized RMS values
by maximal voluntary contraction (MVC) than the normal group (A), but the normalized RMS values did not
show a signicant change (B). e neutral posture showed a signicantly lower normalized RMS value than the
forward posture (C). Data showed median ± 95% CI. *p < 0.05.
ABC
Figure11. Comparison of the mean frequency between groups (A), period (B), and each posture (C). ere
was no signicant dierence in the mean frequency between the forward head posture (FHP) and normal
groups (A). e mean frequency showed a signicant decrease over time (B). ere was no signicant dierence
in the mean frequency of each posture (C). Data showed median ± 95% CI. *p < 0.05, compared with 1min;
†p < 0.05, compared with 10min; #p < 0.005, compared with 20min.
Content courtesy of Springer Nature, terms of use apply. Rights reserved

9
Vol.:(0123456789)
Scientic Reports | (2022) 12:19484 | https://doi.org/10.1038/s41598-022-24095-8
www.nature.com/scientificreports/
positions. ese ndings suggest that deviation from the neutral position could potentially induce headache and
neck disorders if the same posture is held for long periods of time, and with respect to FHP, prolonged postural
holding, including the neutral position, can cause headache and neck health problems.
In neutral and backward postures, the modied entropy and correlation coecient were signicantly lower
in the normal group than in the FHP group. Furthermore, these variables were signicantly decreased over time
during posture maintenance in the normal group. e mean frequency was found to decrease with increasing
postural retention time, but there were no dierences between groups and postures. e modied entropy and
correlation coecient assess the temporal and spatial distribution of muscle activity, and changes in HD-SEMG
potential distribution patterns indicate relative adaptations in the intensity of activity within muscle regions
during contraction and may be attributed to variations in peripheral properties or in the control of motor units
within a muscle26,27. A previous study reported a relationship between the spatial distribution of muscle activity
and endurance time, with a greater spatial distribution of muscle activity being associated with less fatigue14. Con-
sistent with this previous nding, our results showed signicantly lower subjective fatigue in the normal group
among head positions than in the FHP group. ese results indicate that the FHP group has problems with the
adaptive control function of muscle activity in postural retention within a muscle. Previously, it was investigated
whether there is a relationship between head posture and neck pain and whether FHP diers between neck pain
and asymptomatic people. ere was a signicant dierence between asymptomatic and symptomatic adults
with FHP, as determined by the results28. Furthermore, increased FHP can be associated with lower endurance
of the deep neck extensors and exors as well as greater activity of the supercial muscles in adults with neck
pain9,29. ese ndings support the results of this study that people with FHP exhibit greater subjective fatigue
and muscle activity. Importantly, this study found signicant dierences in subjective fatigue and muscle activ-
ity, although only young adults without head and neck pain were included in this study. e long-term eects of
reduced exibility and endurance of neck muscles during adolescence have been suggested to predispose adults
to headache and neck pain7, and our results of the spatial and temporal distribution patterns of muscle activity
analyses used in this study indicate early detection of abnormal muscle activity caused by postural displacement
of the head and neck. Our results also showed that the FHP group was not eective in reducing head and neck
fatigue when changing head position. erefore, people with FHP may need to reduce fatigue in ways other than
adjusting the position of the head and neck position (e.g., using armrests or changing the shape of the pillow).
In the future, when providing therapeutic intervention for people with FHP, it may be necessary to clarify the
comfortable position for people with FHP.
is study has several limitations. First, this study included only young males. Potential confounders that
can inuence neck pain and FHP, such as age and sex, must be controlled, and female participants and a wider
age range should be included to clarify FHP and abnormal muscle activity. Second, this study measured only
the trapezius pars descendens muscle. e muscle activity control mechanisms of not only the extensor muscles
of the head and neck but also the exor muscles play an important role in maintaining the posture of the head
and neck. In the future, including the head and neck exor muscles in the measurement will lead to a greater
understanding of functional abnormalities in FHP.
In conclusion, we compared the spatial muscle distribution and quantitative/subjective fatigue during postural
retention between the FHP and normal groups. Compared with the normal group, the FHP group exhibited
greater subjective fatigue and muscle activity and lower spatial and temporal changes in muscle activation pat-
terns. ese ndings suggest that long-term neck displacement may be a potential factor contributing to neck
pain. is study revealed that head and neck position had little eect on muscle activity and fatigue in the FHP
group, suggesting that other interventions, such as the use of arm rests or adjusting the buttock position, are
important for FHP. In the future, it is necessary to examine methods to reduce head and neck muscle strain in
the FHP group to prevent head and neck disorders.
Data availability
e datasets analyzed in this study are available from the corresponding author on reasonable request aer
approval by institutional authorities.
Received: 14 July 2022; Accepted: 9 November 2022
References
1. Abbott-Ganey, C. & Jacobs, K. Telehealth in school-based practice: Perceived viability to bridge global OT practitioner shortages
prior to COVID-19 global health emergency. Work 67, 29–35 (2020).
2. Romero-Blanco, C. et al. Physical activity and sedentary lifestyle in University students: Changes during connement due to the
COVID-19 pandemic. Int. J. Environ. Res. Public Health 17, 6567 (2020).
3. Hoy, D. et al. e global burden of neck pain: Estimates from the global burden of disease 2010 study. Ann. Rheum. Dis. 73,
1309–1315 (2014).
4. Hallman, D. M., Gupta, N., Mathiassen, S. E. & Holtermann, A. Association between objectively measured sitting time and neck-
shoulder pain among blue-collar workers. Int. Arch. Occup. Environ. Health 88, 1031–1042 (2015).
5. Lin, Y.-H. et al. Time distortion associated with smartphone addiction: Identifying smartphone addiction via a mobile application
(App). J. Psychiatr. Res. 65, 139–145 (2015).
6. Abdel-Aziem, A. A., Abdel-Ghafar, M.A.-F., Ali, O. I. & Abdelraouf, O. R. Eects of smartphone screen viewing duration and body
position on head and neck posture in elementary school children. J. Back Musculoskelet. Rehabil. 35, 185–193 (2022).
7. Mikkelsson, L. O. et al. Adolescent exibility, endurance strength, and physical activity as predictors of adult tension neck, low
back pain, and knee injury: A 25 year follow up study. Br. J. Sports Med. 40, 107–113 (2006).
8. Titcomb, D. A., Melton, B. F., Miyashita, T. & Bland, H. W. Evidence-based corrective exercise intervention for forward head posture
in adolescents and young adults without musculoskeletal pathology: A critically appraised topic. J. Sport Rehabil. 31, 640–644
(2022).
Content courtesy of Springer Nature, terms of use apply. Rights reserved

10
Vol:.(1234567890)
Scientic Reports | (2022) 12:19484 | https://doi.org/10.1038/s41598-022-24095-8
www.nature.com/scientificreports/
9. Shahidi, B., Johnson, C. L., Curran-Everett, D. & Maluf, K. S. Reliability and group dierences in quantitative cervicothoracic
measures among individuals with and without chronic neck pain. BMC Musculoskelet. Disord. 13, 215 (2012).
10. Oliveira, A. C. & Silva, A. G. Neck muscle endurance and head posture: A comparison between adolescents with and without neck
pain. Man. er. 22, 62–67 (2016).
11. Silva, A. G., Punt, T. D., Sharples, P., Vilas-Boas, J. P. & Johnson, M. I. Head posture and neck pain of chronic nontraumatic origin:
A comparison between patients and pain-free persons. Arch. Phys. Med. Rehabil. 90, 669–674 (2009).
12. Merletti, R., Holobar, A. & Farina, D. Analysis of motor units with high-density surface electromyography. J. Electromyogr. Kinesiol.
18, 879–890 (2008).
13. Watanabe, K. & Yoshida, T. Eect of arm position on spatial distribution of upper trapezius muscle activity during simulated car
driving. Int. J. Occup. Saf. Ergon. 28, 1–7 (2021).
14. Farina, D., Leclerc, F., Arendt-Nielsen, L., Buttelli, O. & Madeleine, P. e change in spatial distribution of upper trapezius muscle
activity is correlated to contraction duration. J. Electromyogr. Kinesiol. 18, 16–25 (2008).
15. Shaghayegh Fard, B., Ahmadi, A., Marou, N. & Sarrafzadeh, J. Evaluation of forward head posture in sitting and standing posi-
tions. Eur. Spine J. 25, 3577–3582 (2016).
16. Lee, J.-H. Eects of forward head posture on static and dynamic balance control. J. Phys. erapy Sci. 28, 274–277 (2016).
17. Yip, C. H. T., Chiu, T. T. W. & Poon, A. T. K. e relationship between head posture and severity and disability of patients with
neck pain. Man. er. 13, 148–154 (2008).
18. Lee, K. A., Hicks, G. & Nino-Murcia, G. Validity and reliability of a scale to assess fatigue. Psychiatry Res. 36, 291–298 (1991).
19. Nishikawa, Y. et al. Spatial electromyography distribution pattern of the vastus lateralis muscle during ramp up contractions in
Parkinson’s disease patients. J. Electromyogr. Kinesiol. 37, 125–131 (2017).
20. Nishikawa, Y. et al. e eect of electrical muscle stimulation on quadriceps muscle strength and activation patterns in healthy
young adults. EJSS 21, 1414–1422 (2021).
21. Nishikawa, Y. et al. Identication of the laterality of motor unit behavior in female patients with Parkinson’s disease using high-
density surface electromyography. Eur. J. Neurosci. 53, 1938–1949 (2021).
22. Tucker, K., Falla, D., Graven-Nielsen, T. & Farina, D. Electromyographic mapping of the erector spinae muscle with varying load
and during sustained contraction. J. Electromyogr. Kinesiol. 19, 373–379 (2009).
23. Hallgren, R. C., Pierce, S. J., Prokop, L. L., Rowan, J. J. & Lee, A. S. Electromyographic activity of rectus capitis posterior minor
muscles associated with voluntary retraction of the head. Spine J. 14, 104–112 (2014).
24. Hallgren, R. C., Pierce, S. J., Sharma, D. B. & Rowan, J. J. Forward head posture and activation of rectus capitis posterior muscles.
J. Am. Osteopath. Assoc. 117, 24–31 (2017).
25. Cooper, S. & Daniel, P. M. Muscle spindles in man; their morphology in the lumbricals and the deep muscles of the neck. Brain
86, 563–586 (1963).
26. Holtermann, A., Roeleveld, K. & Karlsson, J. S. Inhomogeneities in muscle activation reveal motor unit recruitment. J. Electromyogr.
Kinesiol. 15, 131–137 (2005).
27. Holtermann, A. & Roeleveld, K. EMG amplitude distribution changes over the upper trapezius muscle are similar in sustained
and ramp contractions. Acta Physiol. 186, 159–168 (2006).
28. Mahmoud, N. F., Hassan, K. A., Abdelmajeed, S. F., Moustafa, I. M. & Silva, A. G. e Relationship between forward head posture
and neck pain: A systematic review and meta-analysis. Curr. Rev. Musculoskelet. Med. 12, 562–577 (2019).
29. Lee, H., Nicholson, L. L. & Adams, R. D. Neck muscle endurance, self-report, and range of motion data from subjects with treated
and untreated neck pain. J. Manipul. Physiol. er. 28, 25–32 (2005).
Acknowledgements
e authors thank all participants who volunteered to participate in this study.
Author contributions
Y.N., T.C., K.K., A.K., and K.I. conceived and designed the study. Y.N. and N.M. analyzed the data. Y.N., K.W.,
and A.H. interpreted the results of the experiments. Y.N. and K.K. prepared gures. Y.N., K.W., and A.H. draed
the manuscript. J.S., T.K, and S.T. edited and revised the manuscript. All authors reviewed the manuscript.
Competing interests
e authors declare no competing interests.
Additional information
Correspondence and requests for materials should be addressed to Y.N.
Reprints and permissions information is available at www.nature.com/reprints.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and
institutional aliations.
Open Access is article is licensed under a Creative Commons Attribution 4.0 International
License, which permits use, sharing, adaptation, distribution and reproduction in any medium or
format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the
Creative Commons licence, and indicate if changes were made. e images or other third party material in this
article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the
material. If material is not included in the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from
the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
© e Author(s) 2022
Content courtesy of Springer Nature, terms of use apply. Rights reserved

1.
2.
3.
4.
5.
6.
Terms and Conditions
Springer Nature journal content, brought to you courtesy of Springer Nature Customer Service Center GmbH (“Springer Nature”).
Springer Nature supports a reasonable amount of sharing of research papers by authors, subscribers and authorised users (“Users”), for small-
scale personal, non-commercial use provided that all copyright, trade and service marks and other proprietary notices are maintained. By
accessing, sharing, receiving or otherwise using the Springer Nature journal content you agree to these terms of use (“Terms”). For these
purposes, Springer Nature considers academic use (by researchers and students) to be non-commercial.
These Terms are supplementary and will apply in addition to any applicable website terms and conditions, a relevant site licence or a personal
subscription. These Terms will prevail over any conflict or ambiguity with regards to the relevant terms, a site licence or a personal subscription
(to the extent of the conflict or ambiguity only). For Creative Commons-licensed articles, the terms of the Creative Commons license used will
apply.
We collect and use personal data to provide access to the Springer Nature journal content. We may also use these personal data internally within
ResearchGate and Springer Nature and as agreed share it, in an anonymised way, for purposes of tracking, analysis and reporting. We will not
otherwise disclose your personal data outside the ResearchGate or the Springer Nature group of companies unless we have your permission as
detailed in the Privacy Policy.
While Users may use the Springer Nature journal content for small scale, personal non-commercial use, it is important to note that Users may
not:
use such content for the purpose of providing other users with access on a regular or large scale basis or as a means to circumvent access
control;
use such content where to do so would be considered a criminal or statutory offence in any jurisdiction, or gives rise to civil liability, or is
otherwise unlawful;
falsely or misleadingly imply or suggest endorsement, approval , sponsorship, or association unless explicitly agreed to by Springer Nature in
writing;
use bots or other automated methods to access the content or redirect messages
override any security feature or exclusionary protocol; or
share the content in order to create substitute for Springer Nature products or services or a systematic database of Springer Nature journal
content.
In line with the restriction against commercial use, Springer Nature does not permit the creation of a product or service that creates revenue,
royalties, rent or income from our content or its inclusion as part of a paid for service or for other commercial gain. Springer Nature journal
content cannot be used for inter-library loans and librarians may not upload Springer Nature journal content on a large scale into their, or any
other, institutional repository.
These terms of use are reviewed regularly and may be amended at any time. Springer Nature is not obligated to publish any information or
content on this website and may remove it or features or functionality at our sole discretion, at any time with or without notice. Springer Nature
may revoke this licence to you at any time and remove access to any copies of the Springer Nature journal content which have been saved.
To the fullest extent permitted by law, Springer Nature makes no warranties, representations or guarantees to Users, either express or implied
with respect to the Springer nature journal content and all parties disclaim and waive any implied warranties or warranties imposed by law,
including merchantability or fitness for any particular purpose.
Please note that these rights do not automatically extend to content, data or other material published by Springer Nature that may be licensed
from third parties.
If you would like to use or distribute our Springer Nature journal content to a wider audience or on a regular basis or in any other manner not
expressly permitted by these Terms, please contact Springer Nature at
onlineservice@springernature.com