1. Original Research
Difference in Selective Muscle Activity of Thoracic Erector Spinae
During Prone Trunk Extension Exercise in Subjects With
Slouched Thoracic Posture
Kyung-hee Park, PhD, PT, Jae-seop Oh, PhD, PT, Duk-hyun An, PhD, PT,
Won-gyu Yoo, PhD, PT, Jong-man Kim, PhD, PT, Tae-ho Kim, PhD, PT,
Min-hyeok Kang, MSc, PT
Abstract
Background: The prone trunk extension (PTE) exercise is often used to strengthen the back extensors. Although altered trunk
posture is associated with movement impairment, the influences of a slouched thoracic posture on muscle activity of the thoracic
erector spinae and thoracic movement during the PTE exercise were overlooked in previous studies.
Objectives: To compare the muscle activity of the erector spinae muscles and the relative ratio of the thoracic and lumbar
erector spinae muscles during a PTE exercise in subjects with and without slouched thoracic posture.
Design: Cross-sectional.
Setting: University motion analysis laboratory.
Participants: The study included 22 subjects with slouched thoracic posture (defined as !40
) and 22 age- and gender-matched
healthy subjects.
Methods: All participants performed the PTE exercise.
Main outcome measures: Bilateral surface electromyographic signals of the longissimus thoracis, iliocostalis lumborum pars
thoracis, and pars lumborum muscles were measured during PTE exercises. Thoracic kyphosis (the angle of T1 minus T12) and
lumbar lordosis (absolute value of the angle of L5 minus T12) were recorded using inclinometers during the PTE exercise.
Results: The results showed no difference in muscle activity of the erector spinae in subjects with slouched thoracic posture
versus those without during the PTE exercise. However, selective recruitment of the erector spinae pars thoracis was decreased
significantly, and the thoracic kyphotic angle and lumbar lordotic curve were increased, during the PTE exercise in subjects with a
slouched posture.
Conclusions: Although the PTE exercise has historically been a key component of correction of hyperkyphosis, the increased
spinal curvature inhibits muscle activation of the erector spinae pars thoracis in these individuals, thus limiting effective strength
gains. Therefore, modified methods to maintain a neutral posture of the spine and facilitate muscle activation of the erector
spinae pars thoracis are needed in these individuals.
Introduction
A slouched posture is commonly involved in daily
sitting activities and is defined as a relaxed sitting
posture with a flexed thoracic and lumbar spine [1,2].
An increased or prolonged slouched posture may cause
not only low-back pain (LBP) and movement-related
disorders in the lumbar spine, but it may also result in
thoracic spine pain or movement impairment syndrome
such as thoracic flexion syndrome [3], osteoporotic
compression fractures of the spine [4], or impairment of
shoulder flexion due to disturbances in scapular move-
ment [5,6]. It is likely beneficial to strengthen the
thoracic spine extensors and to correct excessive
thoracic kyphosis to reduce or prevent painful spinal
disorders and other complications [3,7]; however, few
research studies have examined the thoracic versus the
lumbar spine.
PM R XXX (2014) 1-6
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2. The prone trunk extension (PTE) exercise is a familiar
technique used to strengthen the erector spinae in the
treatment of weak and fatigue-sensitive back muscula-
ture; this exercise is typically recommended to prevent
the natural progression of kyphosis [7]. However, it is
questionable whether the PTE exercise is always
effective in individuals with a slouched thoracic posture
[8-11]. A prolonged slouched posture has a tendency to
induce excessive thoracic kyphosis according to the
directional susceptibility of movement [8]. Moreover, a
prolonged slouched posture may lengthen or stretch the
erector spinae, which may decrease the position sense
[1,9,10]. The movements used in an attempt to
decrease the thoracic curve may cause pain or difficulty
and may produce compensatory changes in the more
mobile lumbar region [3,11]. Therefore, lumbar exten-
sion may be performed to a greater degree than
thoracic extension in these individuals during PTE
exercises, and lumbar hyperextension exercises
accompanied by inordinate use of the lumbar erector
spinae musculature seem to be related to LBP due to
abnormal compressive and shear forces [12-16]. There-
fore, very careful observation and posture correction
are crucial to prevent hyperextension of the lumbar
spine and to facilitate the thoracic erector spinae
muscles in such patients during PTE exercises.
Although synergistic activity of the erector spinae
pars thoracis and lumborum muscles is considered the
main mechanism of trunk extension, these muscles do
not comprise a homogeneous muscle mass, but have
anatomical and functional differences [17-20]. Knowl-
edge of the activity of the erector spinae in individuals
with slouched thoracic posture during PTE exercises is
insufficient. The purpose of our research was to
compare the muscle activity of the erector spinae pars
thoracis and lumborum muscles and the relative ratio of
the thoracic and lumbar erector spinae muscles in sub-
jects with a slouched thoracic posture. Because muscle
activity of the erector spine influences trunk posture, a
secondary purpose was to compare thoracic kyphosis
and lumbar lordosis in subjects with and without
slouched thoracic posture during the PTE exercise.
Methods
Study Participants
In total, 22 subjects (10 male and 12 female) with
thoracic slouched posture and 22 healthy subjects (10
male and 12 female) were selected from among 250
young persons engaged in desk work and computer use
for more than 5 hours per day. Participants with meta-
bolic, neuromuscular, or musculoskeletal disorders or a
history of spinal surgery were excluded. In the 22 sub-
jects with slouched thoracic posture, the thoracic spine
alignment tended to demonstrate excessive thoracic
kyphosis in a self-selected, relaxed standing position.
These subjects were selected from among 250 young
persons at 3 universities in South Korea. The criteria
used to place the subjects into the slouched thoracic
posture and control groups were based on data taken
from 250 young persons whose mean kyphotic angle in a
relaxed standing posture was 30.2
(standard deviation
[SD], Æ4.83
). The slouched thoracic posture group was
defined as those subjects with a kyphotic angle !40
,
which represented the group’s mean plus 2 SDs (30.2
þ
[2 Â 4.83
]) [21]. A total of 22 age- and gender-matched
participants with a kyphotic angle within the range of
mean Æ 1 SD were selected as the control group. These
participants reported no instance of LBP or thoracic pain
within the last year, no musculoskeletal disorders that
would limit normal thoracic kyphosis, and no pain during
the test procedure.
This study was approved by the human subjects
committee of the University of Inje. Informed consent
was obtained from all subjects.
Instrumentation
The angles of thoracic kyphosis and lumbar lordosis
during the PTE exercise were measured using 2 gravity-
dependent inclinometers (Zebris Medical GmbH, Isny,
Germany). The spinous processes of the first thoracic
vertebra (T1), twelfth thoracic vertebra (T12), and fifth
lumbar vertebra (L5) were used as landmarks for posi-
tioning the inclinometer sensors [6] (Figure 1). These
spinal levels were marked by palpation; the L5 spinous
process was identified above the sacrum, the T12
spinous process was identified superiorly from the L5
point, and the T1 spinous process was identified inferi-
orly from the seventh cervical vertebra (designated as
the most prominent spinal process) [6]. During PTE
exercise, the angle between T1 and T12 and between L5
and T12 were measured to assess thoracic kyphosis and
lumbar lordosis, respectively, using the inclinometers.
Surface electromyographic (EMG) signals were recor-
ded for each subject using 8 preamplified (gain: 1000)
active surface electrodes (model DE-2.3; Delsys, Inc.,
Wellesley, MA). EMG signals from the recording sites
were band-pass filtered between 20 and 450 Hz, analog-
to-digital converted at a sampling rate of 2048 Hz, and
stored on a computer hard disk for later analysis.
The electrodes were positioned bilaterally on the
iliocostalis lumborum pars lumborum (right ICL and left
ICL) at the L3 level, midway between the lateral-most
Figure 1. Placements of the inclinometers.
2 Muscle Activity of Thoracic Erector Spinae

3. palpable border of the erector spinae and a vertical line
through the posterosuperior iliac spine [17,19,22]; on the
longissimus thoracis (right LT and left LT) at the T9 level,
midway between a line through the spinous process and a
vertical line through the posterosuperior iliac spine,
located approximately 5 cm laterally [19,22]; and on the
iliocostalis lumborum pars thoracis (right ICTand left ICT)
at the T10 level, midway between the lateral-most
palpable border of the erector spinae and a vertical line
through the posterosuperior iliac spine [17,23-25].
Skin impedance was reduced by shaving excess body
hair if necessary, by gently abrading the skin with fine-
grade sandpaper, and wiping the skin with alcohol swabs.
Procedures
The subjects were asked to perform a body
weightedependent isometric back extension exercise in
the prone position. The PTE exercise was performed
with the iliac crests aligned with the table edge and the
subjects’ arms crossed at the chest and lower limbs
fixed by nonelastic straps at the hip, knees, and ankles.
While looking downward at a visual fixation point, the
subjects were instructed to raise their trunk to hori-
zontal (parallel to the ground) and maintain this posi-
tion for 5 seconds [26] (Figure 2). The exercises were
taught to each subject before data collection; 2 prac-
tice sessions were allowed to achieve proper perfor-
mance. A bar indicator was positioned approximately at
the T6 level for feedback about the horizontal position.
The procedure was repeated 3 times with a 3-minute
recovery period between trials.
The maximum voluntary isometric contraction (MVIC)
of the erector spinae was used for normalization. To
measure the MVIC of the ES pars thoracis and lumborum,
the subjects, lying in a prone position, placed their
hands on their head with their legs strapped to the
table. Back extension was performed with maximum
isometric effort against resistance by the experimenter
on the angular inferior aspect of both scapulae [18,25].
This was repeated 3 times, with a 30-second rest period
between sessions.
A root mean square (RMS) processing method was
executed on 250-millisecond (512 points) successive
time windows; EMG signals from the 3 middle seconds of
the 5-second isometric contraction during the PTE ex-
ercise and MVIC testing were used. The data obtained
were normalized (% MVIC) by the mean RMS value during
MVIC testing. The normalized LT:ICL and ICT:ICL ratios
were calculated to measure the selective recruitment of
the thoracic erector spinae.
Inclinometer markers to compare the angles of
thoracic kyphosis and lumbar lordosis were placed over
T1, T12, and L5 and were simultaneously measured in
the isometric PTE position with a surface EMG signal. In
a previous study, intrarater and interrater reliability
were concurrently established during PTE exercises in
15 participants and were found to be highly correlated
(intraclass correlation coefficient [ICC] [1,2] ¼ 0.97, ICC
[2,1] ¼ 0.91).
Clockwise rotation of the indicator (toward the
extension direction) represented positive values, and the
opposite rotation represented negative values. The angle
of T1 minus T12 was the value of thoracic kyphosis, and
the absolute value of the angle of T12 minus T1 was the
angle of thoracic extension. The angle of lumbar lordosis
was the absolute value of the angle of L5 minus T12.
Statistical Analysis
The KolmogoroveSmirnov test was used to assess
homogeneity of variance of the % MVIC of each muscle
and the LT:ICL and ICT:ICL ratios. An independent t-test
was performed to evaluate the differences between the
right and left erector spinae EMG data. Because no
significant differences were found, EMG data of the
right and left erector spinae were averaged and are
reported. Independent t-tests were then performed to
investigate the effect of slouched thoracic posture on
the normalized EMG activity of the erector spinae
(% MVIC), the selective recruitment of the thoracic
erector spinae (LT:ICL and ICT:ICL ratios), the angle of
thoracic kyphosis in a standing posture, and the angle of
thoracic kyphosis and the angle of lumbar lordosis
during the PTE exercise. All statistical analyses were
performed with the statistical software package SPSS
version 18.0 (SPSS Inc., Chicago, IL), and the level of
statistical significance was set at P .05.
Results
The subjects in the slouched thoracic posture group
were a mean (Æ SD) age of 27.54 Æ 4.29 years; their
mean height was 169.37 Æ 8.57 cm, body weight 63.25 Æ
8.79 kg, and thoracic kyphotic angle 44.25
Æ 4.14
while standing (Table 1). The subjects in the control
Figure 2. Prone trunk extension exercise.
Table 1
Thoracic kyphotic angles in subjects with and without slouched
thoracic posture in a standing posture
With Slouched
Posture (n ¼ 22)
Without Slouched
Posture (n ¼ 22) P Value
44.25 Æ 4.14 30.05 Æ 4.72 .001
Data for angles are given in degrees (
).
3K. Park et al. / PM R XXX (2014) 1-6

4. group were a mean (Æ SD) age of 26.15 Æ 5.34 years;
their mean height was 168.25 Æ 7.46 cm, body weight
61.92 Æ 9.17 kg, and thoracic kyphotic angle 30.05
Æ
4.72
while standing (Table 1).
No difference in the muscle activity of the erector
spinae (% MVIC) was found between the 2 groups, but
significant differences in both LT:ICL and ICT:ICL ratios
between the groups were found during PTE exercise.
The LT:ICL and ICT:ICL ratios were significantly lower in
the slouched thoracic posture group (0.72 Æ 0.25 and
0.78 Æ 0.20, P ¼ .040 and .024, respectively) than in the
control group (0.82 Æ 0.37 and 0.88 Æ 0.31, respec-
tively) (Table 2).
In the slouched thoracic posture group, the upper
thoracic spine (T1) was significantly flexed 3.83
Æ 8.03
(P .001), and the lower thoracic spine (T12) was
significantly extended À12.17
Æ 7.70
(P ¼.026), but the
extension movement of L5 during the PTE exercise was
not significantly different between groups (P ¼ .666)
during the PTE exercise. Kyphosis of the thoracic spine
and lordosis of the lumbar spine were significantly higher
at 16.00
Æ 10.36
and 25.13
Æ 6.30
, respectively, in the
slouched thoracic posture group than in the control group
during PTE exercise (7.21
Æ 9.90
and 22.05
Æ 7.20
;
P .001 and .045, respectively) (Table 3).
Discussion
The present study was designed to investigate the
influence of slouched thoracic posture on the level of
activity (%MVIC) of the erector spinae muscles (pars
thoracis and lumborum), the balance between these
erector spinae muscles (LT:ICL and ICT:ICL ratios), and
thoracic and lumbar spine movements during PTE ex-
ercise. Although there was no difference in the muscle
activity of the erector spinae muscles in subjects with
and without a slouched thoracic posture, a significant
decrease in the relative ratio of the thoracic to lumbar
erector spinae muscles was found in the slouched
posture group versus the control group. In addition, the
thoracic kyphotic angle and lumbar lordotic curve were
greater during the PTE exercise in these subjects than in
the control group.
An increased kyphoticelordotic curve of the spine
could be explained by the decreased activity of the
erector spinae pars thoracis and increased activity of
the erector spinae pars lumborum. Although the erector
spinae pars thoracis and lumborum produce forces syn-
ergistic with the trunk extension force, the lumbar
extensors are more naturally suited to spinal stability;
the thoracic extensors, located more superficially, are
designed for higher loads [13,18,19]. Our results showed
no significant difference in muscle activity itself, but
the LT:ICL and ICT:ICL ratios were decreased in the
slouched posture group. To the best of our knowledge,
few studies have investigated the activities of the LT
and ICT during extension exercises or their activity
relative to that of the ICL. The level of erector spinae
pars thoracis muscle activity (37%e41% MVIC) in the
current study was lower than that in previous studies
(41%e74% MVIC) [12,18,22]. The muscle activity of the
erector spinae pars lumborum in the current study
was 52%e56% MVIC, which is similar to some previous
results (55%e57% MVIC) [12,22] and lower than others
Table 2
Differences in normalized EMG activity of each segment in the erector spinae muscles and selective recruitment of the longissimus thoracis and
iliocostalis pars thoracis to iliocostalis pars lumborum during the PTE exercise in participants with and without a thoracic slouched posture
Variable Muscles With Slouched Posture Without Slouched Posture P Value
Muscle activity (% MVIC) Both LT 37.80 Æ 14.52 37.84 Æ 16.51 .933
Both ICT 40.93 Æ 18.38 39.48 Æ 20.91 .701
Both ICL 56.28 Æ 21.90 52.36 Æ 22.75 .151
Ratio to ICL Both LT 0.72 Æ 0.25 0.82 Æ 0.37 .040*
Both ICT 0.78 Æ 0.20 0.88 Æ 0.31 .024*
EMG ¼ electromyography; PTE ¼ prone trunk extension; LT ¼ longissimus thoracis; MVIC ¼ maximum voluntary isometric contraction;
ICT ¼ iliocostalis lumborum pars thoracis; ICL ¼ iliocostalis lumborum pars lumborum.
*
P .05.
Table 3
Differences in thoracic and lumbar spine movement during the PTE exercise in participants with and without habitual slouched posture
Variable
Degree of Trunk Extension (
)
P ValueWith Slouched Posture Without Slouched Posture
T1 3.83 Æ 8.03 À2.98 Æ 9.95 .001*
T12 À12.17 Æ 7.70 À10.20 Æ 3.85 .026*
L5 11.95 Æ 6.40 11.85 Æ 7.01 .666
Kyphosis of thoracic spine 16.00 Æ 10.36 7.21 Æ 9.90 .001*
Lordosis of lumbar spine 25.13 Æ 6.30 22.05 Æ 7.20 .045*
PTE ¼ prone trunk extension.
*
P .05.
4 Muscle Activity of Thoracic Erector Spinae

5. (70% MVIC) [18]. The main reason for this is that the PTE
exercise was performed in a different way in the pre-
vious studies, with both arms raised to the head instead
of folded at the chest or with participants instructed to
achieve an intensity of 60% of 1 repetition maximum
(RM). Notably, the muscle activity of the erector spinae
pars thoracis in the slouched thoracic posture group was
lowest, and decreased activation was not enough to
straighten the thoracic spine.
The PTE exercise is beneficial in strengthening
the erector spinae for the treatment of weak and
fatigue-sensitive back musculature [7]. However, an
appropriate spinal curve should be considered when
performing the PTE exercise, because maintenance of a
stable neutral zone in the spine is assumed to be safe
and to diminish the stress on the spine [12,22,27]. That
is, the PTE exercise with increased thoracic kyphosis
and lumbar lordosis may be less effective and may
produce pain because of greater disk loading,
compression force, and shearing force caused by an
increased spinal curve in the sagittal plane [28,29]. In
the present study, the slouched thoracic posture group
showed increases in the lumbar lordotic and thoracic
kyphotic curves during the PTE exercise. Considering
directional susceptibility to movement, namely,
compensatory movement in a specific direction or a
stress applied in a specific direction [8], persons with
slouched thoracic posture are accustomed to a thoracic
flexed posture not only during standing but also during
other positions, and even during the PTE exercise. Also,
thoracic kyphosis in a relaxed standing posture was
greater in the slouched thoracic posture group than in
the control group in this study. It is inferred that soft
tissues allow greater flexibility, together with less
stiffness, going into thoracic flexion in the slouched
thoracic posture group than in the control group, which
may lead to a greater thoracic kyphotic angle in the
slouched thoracic posture group, even with no signifi-
cant difference in muscle activity of the erector spinae
between the groups during the PTE exercise. Individuals
with a slouched thoracic posture have decreased
thoracic extension range of motion [11,30] and may
compensate for this by increasing their lumbar lordotic
curve instead of increasing thoracic extension during
the PTE task. Considering the greater thoracic kyphotic
angle in the slouched thoracic posture group than in the
control group, individuals with a slouched thoracic
posture may have a mechanical disadvantage in
recruiting the thoracic extensor muscles. Thus, insuffi-
cient thoracic extensor strength and abnormal neuro-
muscular control could be important issues in persons
with a slouched thoracic posture, although these were
not addressed in this study. In addition, a habitually
flexed posture lengthens the intrafusal fibers of the
paraspinal muscles and intervertebral ligaments,
thereby stretching the g-motor neuron and mechano-
receptors, causing diminished position sense in the
spine [1,9]. Thus, these individuals may also have diffi-
culty recognizing their abnormal movement patterns
and correcting their increased thoracic kyphosis and
lumbar lordosis angles to achieve a neutral curve [30].
The first limitation of the current study is that we
examined a young healthy population; thus, the results
cannot be generalized to elderly individuals or to pa-
tients with LBP. Second, only a single exercise (PTE) was
used. Third, although the main function of the erector
spinae is to maintain an erect posture, the endurance of
the erector spinae muscles was not tested. Future
research should use various exercises and resistances
and compare the muscle endurance of the erector spi-
nae muscles in individuals with and without slouched
thoracic posture. In addition, differences in the diam-
eter and contractility of erector spinae muscles be-
tween persons with and without a slouched thoracic
posture should be measured using real-time ultrasound
in a future study. Finally, we measured only the thoracic
kyphotic angle in this study. To fully understand
thoracic mobility and the influence of thoracic kyphosis
on adjacent joints, the mobility of the rib cage and
thoracic facet joints and the alignment of the cranio-
cervical junction should also be assessed in future
studies.
Conclusion
In this study the muscle activity relative ratio of the
thoracic to lumbar erector spinae muscles decreased
significantly, whereas the angle of kyphosis and lumbar
lordosis increased significantly, during the PTE exercise
in the slouched thoracic posture group compared with
the control group. The increased spinal curve in the
sagittal plane and decreased selective activation of the
erector spinae pars thoracis induced during this exercise
could have a negative impact on the spine. Thus, careful
observation and modified methods (eg, support of the
lower trunk to prevent compensatory lumbar lordosis) to
maintain a neutral posture of the spine and to facilitate
muscle activation of the erector spine pars thoracis are
needed in these individuals.
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Disclosure
K.P. Department of Physical therapy, Masan University, Changwon, South Korea
Disclosure: nothing to disclose
J.O. Department of Physical Therapy, College of Biomedical Science and Engi-
neering, INJE University 607 Obang-dong, Gimhae-si, Gyeongsangnam-do, South
Korea, 621-749. Address correspondence to: J.-s.O.; e-mail: ysrehab@inje.ac.kr
Disclosure: nothing to disclose
D.A. Department of Physical Therapy, INJE University, Gimhae, South Korea
Disclosure: nothing to disclose
W.Y. Department of Physical Therapy, INJE University, Gimhae, South Korea
Disclosure: nothing to disclose
J.K. Department of Physical Therapy, Jeonju Univerisy, Jeonju, South Korea
Disclosure: nothing to disclose
T.K. Department of Physical Therapy, Daegu University, Daegu, South Korea
Disclosure: nothing to disclose
M.K. Department of Physical Therapy, Graduate School, INJE University,
Gimhae, South Korea
Disclosure: nothing to disclose
Submitted for publication March 17, 2014; accepted October 5, 2014.
6 Muscle Activity of Thoracic Erector Spinae