ORIGINAL RESEARCH
Electromyographic Examination of Selected Muscle
Activation During Isometric Core Exercises
Gretchen D. Oliver, PhD, ATC, Audrey J. Stone, ATC, CSCS, and Hillary Plummer, BS
Objective: The purpose of the current study was to quantitatively
examine the muscle activations of 3 common isometric core exercises
(abdominal bridge, single-leg abdominal bridge, and superman)
along with a newly introduced isometric exercise (flying squirrel) and
determine if muscle activations differed among the exercises.
Design: The design was a comparison study.
Setting: An athletic training classroom laboratory was where all data
collections occurred.
Participants: Thirty healthy collegiate graduate students (age, 23.4 6
1.4 year; height, 171.3 6 10.3 cm; mass, 73.3 6 16.2 kg), regardless
of sex, consented to participate.
Independent Variable: The independent variable was the muscle
selected.
Main Outcome Measures: The main outcome measures or
dependent variables were the muscle activation reported as percent of
maximum voluntary isometric contraction during each exercise.
Results: Results revealed that the multifidi produced the greatest
muscle activity in all exercises, and the single-leg abdominal bridge
exercise produced greater muscle activation than the general
abdominal bridge exercise (P , 0.025).
Conclusions: The findings of this study demonstrate that any of
these exercises may be a part of a core stability program. In addition,
these findings may be incorporated into an isometric core exercise
program to supplement a currently implemented isometric core exercise program.
Key Words: abdominal muscles, EMG, isometric exercises
(Clin J Sport Med 2010;20:452–457)
INTRODUCTION
The concepts of torso stability, core control, and core
strength have been vastly explored in the realm of athletic
training and strength and conditioning over the past decade.1–11
Submitted for publication April 5, 2010; accepted July 26, 2010.
From the Department of Health Science, Kinesiology, Recreation, and Dance,
University of Arkansas, Fayetteville, Arkansas.
The authors have no financial disclosures or conflicts of interest.
Reprints: Gretchen D. Oliver, PhD, ATC, 309 HPER, 1-University of
Arkansas, Fayetteville, AR 72701 (e-mail: [email protected]).
Copyright Ó 2010 by Lippincott Williams & Wilkins
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There are numerous terms and definitions that surface when
trying to define core stability. However, it is important that
within the sports medicine or strength and conditioning arenas,
we have some operational definitions. The core has been
described as the lumbopelvic–hip complex that incorporates
anterior, posterior, medial, and lateral musculature.7 Trunk,
torso, and core stabilities describe the ability of the body to
maintain or resume a position of the torso after both static and
dynamic muscular contractions11 and are the products of motor
control of the lumbopelvic–hip complex,7 often referred to as
postural control.
Coordinated movement occurs through sequential activation of muscle contractions through the kinetic chain.12 The
kinetic chain is considered a network of individual links (body
segments) that are dependent on each other, allowing for
coordinated and efficient movements. The kinetic chain model
is based on the sequencing of muscle activity patterns in
a proximal to distal nature. The proximal links are composed of
the lower extremity and trunk muscles, which is where the
muscle energy is initiated and then transferred to the distal
extremities. The proximal segments allow for the transfer of
momentum to more distal segments, such as the shoulder to
propel the wrist and hand in throwing activities.6,12,13 An
example would be the anticipatory patterns of proximal segment
activations that are associated with distal joint movements,
which result in effectively moving the center of gravity forward
and up toward the side of unilateral shoulder flexion.10 It is the
proximal to distal muscle synergies that allow for postural or
trunk corrections to counteract disturbances in equilibrium
caused by distal movements.4,5,10
Thus, solely focusing on the kinetic chain model, it is
evident that the functionality of the core is vital throughout all
forms of segmented movement.1,7,12,13 In addition to the core
being a vital contributor to efficient and effective segmental
movement, it has also been documented as a key component of
injury prevention.3,5,7,9 The core musculature innately prevents
postural collapse.8,9 Postural collapse then biomechanically
leads to inefficiencies in movement. In an attempt to perform
with optimal force production, it is vital to be in a biomechanically correct posture.8,9 Lack of torso stability results
in decreased force production and compensatory segmental
movements.
Emphasis has previously been placed on performing
various types of abdominal crunches in an attempt to target
and strengthen the anterior abdominals.1 Improved understanding that the core is composed of the entire lumbopelvic–
hip complex has resulted in an increase in popularity of core
stability routines focused on recruiting the anterior, medial,
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Clin J Sport Med Volume 20, Number 6, November 2010
lateral, and postural musculature. Efforts to target the core in
its entirety have led to the inclusion of core exercises that result
in isometric contractions. Studies have shown that in as few as
4 weeks of implementing an isometric core stability program,
fourth graders were able to improve their core musculature
endurance.14 Thus, with the increased popularity of these
isometric core exercises, it was the purpose of our study to
quantitatively examine the muscle activations of 3 common
isometric core stability exercises (abdominal bridge, single-leg
abdominal bridge, and superman) along with a newly
introduced core exercise (flying squirrel) and determine if
muscle activations differed among the exercises. We hypothesized that all exercises would activate the core musculature
and that there would be significant differences in muscle
activations among the different exercises.
METHODS
Study Design
A single-group repeated measures design was used to
collect muscle activation data for bilateral gluteus maximus,
gluteus medius, external oblique, and multifidus muscle
groups. Electromyographic data were collected for the
following core exercises: superman, flying squirrel, abdominal
bridge, and the single-leg abdominal bridge. Participants were
given instructions on how to properly perform each exercise,
and data were recorded for 3 trials of each exercise. All
participants were instructed by the same certified athletic
trainer (ATC) throughout all data collection. Participants were
instructed on the proper technique (by the same ATC), and
then they practiced the technique. The ATC explained the
verbal cues that would be given and how the participant should
adjust their posture. Instruction was standardized in that the
same ATC gave all instructions so that there was no variability
between those giving instruction and verbal cues.
Participants
Thirty healthy collegiate graduate students (age, 23.4 6
1.4 year; height, 171.3 6 10.3 cm; mass: 73.3 6 16.2 kg),
regardless of sex, consented to participate in the current study.
Healthy collegiate graduate students were defined as having no
history of any type of injury in the past 6 months and currently
participating in physical activity for at least 30 minutes,
4 times a week. In addition, participants were excluded if they
reported a previous history of low back pain. Data collection
sessions were conducted indoors at the Health, Physical
Education, and Recreation building of the University of
Arkansas. All testing protocols used in the current study were
approved by the University of Arkansas Institutional Review
Board, and before participation, the approved procedures,
risks, and benefits were explained to all participants. Informed
consent was obtained from the participants, and the rights of
the participants were protected according to the guidelines of
the University’s Institutional Review Board.
Procedure
Participants reported for testing before engaging in
any vigorous activity on testing day. Location of the core
musculature, bilateral gluteus maximus, gluteus medius,
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sEMG of Common Core Exercises
external oblique, and multifidus, were identified through
palpation. Before testing, the identified locations for surface
electrode placement were shaved, abraded, and cleaned using
standard medical alcohol swabs. Subsequent to surface
preparation, adhesive 3M Red-Dot bipolar surface electrodes
(3M, Saint Paul, Minnesota) were attached over the muscle
bellies and positioned parallel to muscle fibers using
techniques described by Basmajian and Deluca.15 In the
current study, the selected interelectrode distance was
25 mm.16 The use of surface electrodes was chosen because
they have been deemed to be a noninvasive technique that is
able to reliably detect the surface muscle activity.15–17
Electrodes were secured before conducting 2 manual muscle
tests (MMTs) for each muscle, and the protocols were
explained to ensure the participant’s full understanding. These
MMTs were used to identify the approximate maximum
voluntary isometric contraction (MVIC) for each muscle and
were performed following the techniques described by Kendall
et al.16 All MMTs consisted of a 5-second isometric
contraction for each muscle, with MVIC data for the first
and last second of each contraction removed to obtain steadystate results. Each MMT was conducted to establish baseline
readings for each participant’s maximum muscle activity to
which all surface electromyographic (sEMG) data could be
compared.
Electromyographic data were collected via a Noraxon
Myopac 1400L 8-channel amplifier (Noraxon USA, Inc,
Scottsdale, Arizona). The signal was full-wave rectified and
root mean squared at 100 milliseconds. Throughout all testing,
sEMG data were sampled at a rate equal to 1000 Hz. Filtering
of all sEMG data was completed using standard band-pass
filtering techniques with band-pass filters set at cutoffs of
20 Hz and 350 Hz, respectively. In addition, all sEMG data
were notch filtered at frequencies of 59.5 and 60.5 Hz,
respectively.18
After electrode placement, investigators instructed the
participants on proper technique for each of the core exercises.
Participants were randomly assigned the order in which core
exercises would be completed. Each participant performed
each exercise as a warm-up trial before any data were
collected. During the trials, participants were instructed on
proper posture through verbal cures.
The exercises that the participants performed were the
following.
Superman
Participants performed the superman lying prone with
arms straight out over the head and legs fully extended. They
were then instructed to raise arms and legs off the floor and to
hold that position for 10 seconds (Figure 1).
Flying Squirrel
Participants performed the flying squirrel lying prone
with knees flexed, hips internally rotated, and legs lifted off the
floor. Participants’ arms were flexed at the elbows, shoulders
externally rotated, and lifted off the floor. This position was
held for 10 seconds (Figure 2).
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FIGURE 1. Superman exercise. Arms and legs raised off the
floor and held for 10 seconds.
Abdominal Bridge
Participants performed the abdominal bridge lying
supine with knees flexed and feet on the floor. Arms were
crossed over the chest, and the torso was lifted off the floor,
positioning the torso and pelvis in a neutral alignment. The
position was held for 10 seconds (Figure 3).
Abdominal Bridge With One-Leg Support
Participants performed the abdominal bridge with
single-leg support similar to the abdominal bridge. Participants performed the abdominal bridge and then lifted one foot
off the floor by extending the knee. While the participant was
in single-leg support, the torso and pelvis maintained neutral
alignment. This exercise was performed on each leg and held
for 10 seconds each (Figure 4).
Statistical Analysis
Data from each muscle were normalized by being
expressed as a percent contribution of the MVIC. Statistical
analyses were performed using SAS version 9.2 (SAS
Institute, Inc, Cary, North Carolina). Repeated measures
multivariate analysis of variance was conducted to determine
a global difference in muscle activation among all 5 exercises.
Tukey’s Honestly Significant Different (HSD) test was used to
identify all possible pairwise comparisons in muscle activations within each exercise. An overall a level was set a priori
at P # 0.05. In an effort to control for type I error due to
multiple analyses, the a level was adjusted to P # 0.025 using
Bonferroni correction.
FIGURE 2. Flying squirrel. Arms flexed at elbows, shoulders
externally rotated, and lifted off the floor. Knees flexed, hips
internally rotated, and legs lifted off the floor. The position was
held for 10 seconds.
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FIGURE 3. Abdominal bridge. With arms crossed over the
chest, the torso was lifted off the floor and position was held for
10 seconds.
RESULTS
The mean EMG activity of each muscle was expressed
as a percent of MVIC (%MVIC). Figures 5–8 show %MVIC
for each muscle within each exercise. Figure 5 is a graphic
representation of muscle activations (mean 6 SD) in the
multifidi for both left and right sides. The squirrel and
superman exercises produced significantly greater %MVIC in
the multifidi than the bridge exercises (squirrel, 57 6 29 in the
left and 56 6 22 in the right; superman, 61 6 28 in the left and
62 6 22 in the right; bridge, 32 6 18 in the left and 33 6 18 in
the right; P , 0.025). Figure 6 displays the muscle activations
(mean 6 SD) in the gluteus maximus for both left and right
sides. The bridge exercise produced significantly lower
%MVIC in the gluteus maximus for both the left and right
sides (left, 20 6 14 and right, 16 6 11; P , 0.025), and the
bridge with leg lift produced significantly lower %MVIC of
the gluteus maximus ipsilateral of the lifted leg (right bridge,
31 6 19 in the left and 7 6 5 in the right; left bridge, 8 6 5 in
the left and 25 6 18 in the right; P , 0.025). Figure 7 displays
the muscle activations (mean 6 SD) in the gluteus medius for
both left and right sides. Similarly, the bridge exercise
produced significantly lower %MVIC in the gluteus medius
for both the left and right sides (left, 17 6 9 and right, 17 6 11;
P , 0.025), and the bridge with leg lift produced significantly
lower %MVIC of the gluteus medius ipsilateral of the lifted leg
(right bridge, 35 6 17 in the left and 14 6 14 in the right; left
FIGURE 4. Abdominal bridge with one-leg support. Perform
abdominal bridge and then lift one leg off floor by extending at
knee. The position was held for 10 seconds.
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sEMG of Common Core Exercises
lowest power to be 0.94 when examining differences in
%MVIC between multifidi and other muscles during any of the
5 exercises.
DISCUSSION
FIGURE 5. Muscle activation of the multifidi. Percent
maximum involuntary isometric contraction (%MVIC) recorded for multifidi activation are presented per each exercise.
*Significantly lower than squirrel and superman %MVIC (P ,
0.025).
bridge, 10 6 13 in the left and 33 6 16 in the right; P ,
0.025). Figure 8 shows the muscle activations (mean 6 SD) in
the obliques for both the left and right sides. The bridge
exercise produced significantly lower %MVIC in the obliques
for both left and right sides (left, 9 6 8; right, 8 6 8; P ,
0.025) than the bride with leg lift exercise (right bridge, 15 6
11 in the left and 14 6 10 in the right; left bridge, 17 6 15 in
the left and 14 6 11 in the right; P , 0.025). Overall, the
multifidi produced the greatest %MVIC in all exercises, and
the bridge with leg lift exercise produced greater muscle
activation than the general bridge exercise. A post hoc power
analysis was conducted using paired means and found the
FIGURE 6. Muscle activation of the gluteus maximus. Percent
maximum involuntary isometric contraction (%MVIC) recorded for gluteus maximus activation are presented per each
exercise. *Significantly lower than squirrel, superman, and
bridge with right leg lift %MVIC (P , 0.025). †Significantly
lower than squirrel, superman, and bridge with left leg lift
%MVIC (P , 0.025).
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The purpose of this study was to quantitatively examine
the muscle activations of 3 common isometric core stability
exercises (abdominal bridge, abdominal bridge with leg lift,
and superman) and 1 newly introduced core exercise (flying
squirrel). The data presented were a representation of %MVIC
of each muscle acting on the lumbopelvic–hip complex.
Although this study did not examine all the muscles acting
on the lumbopelvic–hip complex, all muscles examined are
a part of the lumbopelvic–hip complex. Typically, the most
commonly mentioned muscles of the lumbopelvic–hip
complex are the transverse abdominis and multifidi. However,
due to the deep location of the transverse abdominis, the
investigators did not feel that sEMG would adequately
describe its muscle activation. Just as data regarding the
muscles controlling the pelvic floor would be advantageous in
displaying the efficiency of the exercises proposed, the
methodology used limited the ability to examine the deep
musculature of the lumbopelvic–hip complex. It is believed
that those muscles examined were a reliable representation of
lumbopelvic muscle activation. The data from the current
study can be applied to core strengthening and rehabilitation
regimens. The current study is in agreement with the previous
work examining the abdominal bridge and the abdominal
bridge with the leg lift.19 However, the previous studies have
not examined the superman or the flying squirrel exercises.
It has been shown that the core musculature, specifically
the transverse abdominis, is the first to fire during unexpected
trunk loading or trunk self-loading and during any upper or
lower extremity movement, regardless of the direction of
FIGURE 7. Muscle activation of the gluteus medius. Percent
maximum involuntary isometric contraction (%MVIC) recorded for gluteus medius activation are presented per each
exercise. *Significantly lower than squirrel, superman, and
bridge with right leg lift %MVIC (P , 0.025). †Significantly
lower than squirrel, superman, and bridge with left leg lift
%MVIC (P , 0.025).
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Oliver et al
FIGURE 8. Muscle activation of the external obliques. Percent
maximum involuntary isometric contraction (%MVIC) recorded for oblique activation are presented per each exercise.
*Significantly lower than bridge with leg lift %MVIC (P ,
0.025).
movement.20 After the transverse abdominis, it is believed that
the multifidi are next to fire.21 In a study examining the
multifidi between healthy patients and patients with low back
pain, the patients exhibiting back pain had a reduced capacity
to activate their multifidi.22 The results presented display that
the multifidi were activated greater than 50% MVIC on both
the right and left sides when performing the superman and
flying squirrel exercise. Clinically, it has been demonstrated
that small increases in activation of the multifidi are required
for spinal stability.2 Ultimately, it has been discussed that the
decreased multifidi activation in patients with low back pain
could be a result of the back pain. MacDonald et al23 also
recently reported that those individuals who had a history of
low back pain had delayed activation of the multifidus than
those without back pain, thus alluding to the discussion that
those with back pain may exhibit altered motor strategies in an
attempt to avoid pain vexation.24 Therefore, it should be noted
that all participants in the current study were pain free and had
no history of low back pain.
It has been proposed that the multifidi and the transverse
abdominis co-contract during abdominal hollowing,21 suggesting that the co-contraction allows for spinal stability or
postural control. Thus, with the evidence that supports the
multifidi as a key component of spinal stability, it would seem
fundamental that the multifidi should be the focus of any core
strengthening and/or rehabilitation program. It should be noted
that the multifidus muscle group is complex, in that there
are superficial and deep fibers. However, it is difficult to
differentiate them both anatomically and biomechanically,25,26
although it has been assumed that the superficial multifidi
allow for more efficient control of lumbar extension and spinal
orientation control.27
The gluteal muscle group also acts to stabilize the pelvis.
Particularly, the gluteus maximus acts to externally rotate the
hip and extend the trunk, whereas the gluteus medius abducts
the hip, stabilizes the pelvis, and assists in external hip
rotation. Grelsamer and McConnel28 describe the effects of
a weak gluteus maximus resulting in a contralateral drop of the
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pelvis and internal rotation at the hip, thus increasing one’s
functional quadriceps angle. It has been reported that weak hip
abductors (gluteal muscle group) have been associated with
anterior knee pain.29,30 In addition, weak hip external rotation
has been correlated with greater incidence of knee injury.7
Recently, it was reported that individuals with patellofemoral
pain had a significant alteration in the gluteal and trunk
musculature.31 The present study demonstrated greatest
gluteus maximus and gluteus medius activations when
performing the squirrel, superman, and single-leg bridge.
The oblique muscle group is activated in directionspecific patterns in an effort to support the pelvis before limb
movement.4,5,10 The obliques in the current study were activated
the least compared with other muscles examined with the
single-leg support bridges, and the squirrel displayed the
greatest activation. The obliques investigated were the external
obliques, which are activated during rotation. Typically, when
rotating to the right, the contralateral or left external oblique is
activated. The decrease in oblique activation was expected
because none of the exercises performed were rotational
in nature.
With the strong relationship of the muscles of the
lumbopelvic–hip complex playing a role in injury prevention,3,6,7,9,11,29,31 it would be ideal if these exercises were
incorporated not only in the rehabilitation process but also in
the basic maintenance conditioning regimens. Based on the
findings of the current study, by incorporating these isometric
core exercises into rehabilitation or maintenance conditioning,
there is a greater chance for patients and athletes to maintain
pelvic neutral throughout their activities and ultimately
attempt to decrease the incidence of injury.
Limitations
It should be noted that all participants in the study were
healthy and free of any injury for the past 6 months and had
no history of back injury or low back pain. Often, isometric
core exercises are encouraged for those individuals who are
suffering from low back pain. If individuals with low back
pain were examined, they may display different trends in
muscle activation. A second limitation was the use of sEMG.
Although electrode placement was in accordance to Kendall
et al,16 sEMG does have a limitation of cross talk between
electrodes. The nature of sEMG often allows for cross talk
among muscles that are close in proximity. In addition, it has
been noted that when trying to record sEMG of the multifidus,
the longissimus thoracis muscle activity is often recorded
instead. When keeping with the sEMG protocol, it is
recommended that if it is the multifidi that are of interest,
then further investigation of the longissimus thoracis should
also be included in attempt to differentiate the 2 muscle
activations.
CONCLUSIONS
The data revealed that the flying squirrel and superman
exercises produced the greatest overall muscle activations of
the core musculature investigated. In addition, the abdominal
bridge with the leg lift displayed increased muscle activation of
the gluteal group, especially the gluteus medius of the support
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leg (opposite the leg lifted). The flying squirrel, abdominal
bridge, abdominal bridge with leg lift, and superman exercises
provided muscle activations in what we previously defined as
the primary musculature of the core. The findings of this study
demonstrate that any of these exercises may be incorporated
into a core stability program. In addition, these findings may
be used as adjuncts to supplement a currently implemented
core stability program. Based on the results of this study,
further research is warranted. Future research should focus
on not only healthy populations but also individuals with
low back pain. An intervention study would allow for these
exercises to be examined on both healthy populations and
individuals with back pain. After performing the exercises for
a period, it would be interesting to examine the muscle
activation in the individuals with back pain after the pain had
subsided.
ACKNOWLEDGMENTS
The authors acknowledge the assistance of Katelyn
Bishop and Jaime Hall.
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