Journal of Athletic Training
© by the National Athletic Trainers' Association, Inc
Hip Muscle Activity During 3 Side-Lying Hip-
Strengthening Exercises in Distance Runners
Joseph M. McBeth, MS, ATC*; Jennifer E. Earl-Boehm, PhD, ATCt;
Stephen C. Cobb, PhD, ATCt; Wendy E. Huddleston, PhD, PT:t:
*University of Nevada, Reno; tAthletic
+Human Movement Sciences, University of Wisconsin-Milwaukee
Training Education Program, University of Wisconsin-Milwaukee;
cises is important
duced by the gluteus medius (GMed), tensor fascia latae (TFL),
anterior hip flexors (AHF), and gluteus maximus (GMax) during
3 hip-strengthening exercises: hip abduction
tion with external rotation (ABO-ER), and clamshell
Design: Controlled laboratory study.
Patients or Other Participants:
men, 11 women; age =25.45 ±5.80years,
mass =64.43± 7.75 kg) participated.
Intervention(s): A weight equal to 5% body mass was af-
fixed to the ankle for the ABO and ABO-ER exercises,
equivalent load was affixed for the CLAM exercise. A pressure
biofeedbackunit was placed beneath the trunk to provide po-
Main Outcome Measure(s):
(root mean square normalized
of muscles about the hip during strengthening
To comparethe electromyographic
injuries are associated
Understandinggluteus (GMed) the
(ABO), hip abduc-
Twenty healthy runners (9
height= 1.71 ±0.07 m,
to maximal voluntaryisometric
not different from the AHF. For the ABO-ER exercise, the TFL
(70.9±17.2%) was more active than the AHF (54.3±24.8%),
GMed (53.03±28.4%), and GMax (31.7 ±24.1 %). Forthe CLAM
exercise, the AHF (54.2±25.2%)
(34.4±20.1%)and GMed (32.6±16.9%)
from the GMax (34.2±24.8%).
Conclusions:The ABO exercise is preferred if targeted
tivation of the GMed is a goal. Activation
in the ABO-ER and CLAM exercises
which might indicate the exercises
the primary goal is the GMed activation
Key Words: gluteus medius,
wasrecordedover the GMed, TFL, AHF, and
ABO-ER (F357= 10.458, P<.001),
(70.1 ±29.9%), TFL (54.3±19.1%),
was less active than the GMed and TFL but was
was more active than the TFL
but was not different
of the other muscles
exceededthat of GMed,
are less appropriate
• The side-lying
tensor fascia latae and anterior hip flexors.
The clamshell exercise resulted
medius and gluteus maximus.
the goal of rehabilitation.
ment-intensivenature, and the ability to individualize
running intensity and duration. Although the increase in physi-
cal activity has many health benefits, it also brings the inherent
increased risk of lower extremity injuries. Epidemiologic
dence indicates that 19% to 79% of runners will sustain a lower
extremity injury,l-4 and the knee is the most common site of
injury.2.3The lower leg (25% for male and female runners) and
foot (14% for male and 13% for female runners) are the next
most commonly injured areas.3.4The most frequent injuries af-
fecting runners include patellofemoral
iliotibial band syndrome (ITBS), injuries to the gluteus medius
hip-abduction exercise was the best exercise for activating the gluteus medius with little activationof the
• in the greatest activationof the anterior hip flexors with little activation of the gluteus
• with external rotation exercise might activate and strengthen the tensor fascia latae beyond
oth recreational and competitive running have grown in
popularity in recent years. The growth might be attrib-
uted partly to the known health benefits, the non--equip-
pain syndrome (PFPS),
trainers routinely work to prevent, diagnose, and rehabilitate
running-relatedinjuries, so they must possess knowledge
the current research in which exercises commonly used to treat
these injuries have been investigated.
A contemporary clinical theory that might explain the cause
of PFPS and ITBS is that of proximal muscle weakness lead-
ing to dynamic valgus of the knee joint.5Dynamic valgus has
been described as a malalignment characterized by pelvic drop,
which is inferior movement of the contralateral side of the pel-
vis during single-legged stance; femoral adduction and internal
rotation; genu valgum; tibial internal rotation; and hyperprona-
tion, and it occurs when the hip muscles cannot overcome the
external torque caused by gravity acting on the body's center of
(GMed),and greater trochantericbursitis.3Athletic
Journal of Athletic Training
specifically the hip abductors and external rotators, contributes
to a person assuming a position of dynamic valgus each time
he or she is in single-legged stance.6,7 Evidence that hip muscle
weakness is associated with overuse injuries, such as PFPS8-12
and ITBS,13,14 supports this theory. These concepts of hip weak-
ness leading to dynamic valgus and lower extremity
provide the clinical foundation for why strengthening
abductors is a common and important component of preventing
and rehabilitating these injuries.
Incorporating hip strengthening into rehabilitation programs
for overuse injuries has been associated with positive outcomes,
including reduction of symptoms and correction of positional
malalignment.I5-21 Clinicians often use a variety of strengthen-
ing exercises based on knowledge of anatomical structure and
function of the hip, whereas little empirical evidence might
exist to confirm the activation of particular muscles during a
specific movement. 22The functional anatomy of the hip is com-
plex, and actions of muscles often change depending on the po-
sition of the hip.23,24 Therefore, clinicians need to thoroughly
understand the activity of major muscle groups of the hip dur-
ing common strengthening exercises.
Side-lying, open-chain exercises often are performed early
in the rehabilitation process to produce appropriate neuromus-
cular control and strength, supporting
cises later. Researchersusing electromyography
shown that the GMed is most active during a single-plane, side-
lying hip-abduction (ABD) exercise as compared with a variety
of other exercises (Figure 1).25-27 However, they did not include
the tensor fascia latae (TFL), which is also a primary hip ab-
ductor3;therefore, the contribution
cise is not known. Fredericson and WoIF8 believed that people
with GMed weakness might compensate
a greater extent, leading to hypertonicity
ness in the iliotibial band. Therefore, understanding the relative
believe weakness of the hip musculature,
more functional exer-
of the TFL to this exer-
by using the TFL to
and potential tight-
early in rehabilitation
and external rotators exists (Figure 2). Researchers
combination of abduction and external rotation of the hip leads
to strengthening of the gluteus maximus (GMax) and GMed,
but very low activity of these muscles has been reported.25,29
Given the recognized changes in muscle activity when the hip
is flexed,23,24knowing the activity of the other superficial hip
muscles, such as the TFL and AHF, during this exercise is im-
portant, but this has not been examined.
From our experiences and informal querying at professional
meetings, we have learned that many clinicians have patients
perform the ABD exercise with the hip externally rotated and
the toes pointed towardthe ceiling
The theoretical rationale is that introducing
tion will engage the GMax and also minimize the activity of
the TFL because it is an internal rotator. This theory has little
anatomical basis because the external rotator muscles are not
acting against gravity in this position. Furthermore,
the hip is externally rotated, the anterior hip flexors (AHF) are
more in the line of action to resist gravity and therefore might
be more active during the ABD-ER task. We did not find em-
pirical evidence to support the clinical rationale for using the
ABD-ER exercise, so further examination is necessary.
Although the ABD-ER and CLAM exercises commonly are
used, the anatomical rationale for muscle activation is weak,
and activation of the surrounding
GMed during these exercises is not known. Therefore, the pur-
pose of our study was to compare the EMG activity produced
by the GMed, GMax, TFL, and AHF during 3 common hip-
strengthening exercises: ABD, CLAM, andABD-ER
Based on previous research25-27and anatomical function,22 we
of the TFL and GMed to side-lying exercises is
and often is used very
when great weakness of the abductors
hip external rota-
musculature in addition to
Figure 1. Side-lyinghip-abductionexercise.
Volume 47 • Number 1 • February 2012
Figure 2. Clamshellexercise.
Figure 3. Side-lyinghip abduction-externalrotation exercise.
be most active, followed by the TFL, and that the AHF and
GMax would have low activity. For the CLAM exercise, we
hypothesized that the GMed would be the most active, fol-
lowed by the GMax, TFL, and AHF. For the ABD-ER exercise,
we hypothesized that the GMed would be the most active, fol-
lowed by the TFL, AHF, and GMax.
that during the ABD exercise, the GMed would
We recruited 20 distance runners from the community, local
running clubs, and collegiate track teams. Runners were chosen
because of the high incidence of lower extremity overuse inju-
Journal of Athletic Training
ries in this population and because of the overall lean physique
of most distance runners. For each participant, we collected de-
mographic information (Table 1). People were included if they
were aged 18 to 40 years and ran an average of 25 miles (40
kIn) per week over the 6 weeks before the study. We selected 25
miles per week to more closely match the distances run by rec-
reational runners with the distances run by intercollegiate
ners. A person was excluded from the study if he or she had a
lower extremity injury within the 6 months before the study that
had necessitatedmodification to the regular training regimen
for longer than 7 days, had a lower extremity injury or muscle
soreness at the time of the study, had a history of lower extrem-
ity surgery, was pregnant, was incorporating
exercises into the training regimen, or had a body mass index of
25 or more. A body mass index of 25 or more classifies a person
as overweight and might contribute to increased variability of
the EMG signal because overweight people have higher lev-
els of subcutaneousadipose tissue than average-sized
People with anteverted hips have less EMG activity during the
CLAM exercise than people with normal hips30;therefore, pas-
sive hip internal range of motion also was measured for each
participant as a measure of relative femoral anteversion using
the procedures described by Kozic et alY The average amount
of passive hip internal rotation was 31.27°±9.49°,
participants had values greater than 42°, indicating
Researchers26comparing the EMG activity of several exer-
cises have reported a moderate effect (effect size=0.65).
on these data, at least 17 participants were necessary to achieve
a power of 0.8 with an a level set at .05 for comparing muscles
and exercises. We recruited 20 participants to ensure adequate
power. All participantsprovided
study was approved by the Institutional Review Board of the
University of Wisconsin-Milwaukee.
and only 2
informed consent,and the
Testing took place in a single session in a laboratory setting.
Each participant wore loose-fitting running shorts with a built-
in brief to allow access to the hip muscles. A chronology of the
data collection session is presented in Table 2.
The repetition tempo of the exercises was controlled by an
electronic metronome set to 60 beats per minute and consisted
of a I-beat concentric"up" phase, a I-beat eccentric "down"
phase, and a 4-beat rest phase. We chose this tempo because it
is consistent with the tempo patients use to perform these ex-
ercises in a rehabilitation setting, and it is consistent with tem-
pos used in previous research.26To perform the ABD exercise,
participants lay on the nondominant side with the test limb in
a neutral position and the nondominant
(Figure 1). The amount of abduction was standardized using a
horizontal band that the leg would contact when the participant
reached 35° of hip abduction. Participants
their toes straight ahead throughout the exercise.
To perform the ABD-ERexercise,
in the same side-lying position and were instructed to exter-
nally rotate their hips and point their toes up as far as possible
before initiating the abduction movement (Figure 3). The ref-
erence band remained to indicate when 35° of abduction had
been achieved. Participants were instructed to keep the pelvis
in a neutral position and not to tip it backward; this was moni-
tored visually and with a Stabilizer Pressure Bio-feedback unit
(Chattanooga Group, Inc, Hixson, TN). The unit consists of
an inflatable air bag connected to a pressure gauge (Figure 4).
When it is placed beneath the trunk between the iliac crest and
the distal ribs, changes in body position are reflected in changes
in pressure, which the participant views. This provides addi-
tional feedback for unwanted changes in body position during
exercise and has been shown to decrease substitution from sur-
rounding muscles and to increase activity of the GMed during
the ABD exercise.29The Stabilizer Pressure Bio-feedback
was inflated until the pressure reached 40 mm Hg, and the par-
ticipant and the investigator (I.M.M.) monitored this pressure
during the exercises to ensure that it remained between 35 and
To perform the CLAM exercise, participants
sides with the dominant limb up, with both hips flexed to 45°,
and with the knees flexed to 90° (Figure 2). Keeping their feet
together, they separated their knees and rotated the top leg
upward. The reference band was positioned so the top of the
knee would touch it when the angle between the lower leg and
horizontal was 25°. Participants were instructed to visualize a
clamshell opening for this exercise.
Exercise order was counterbalanced
tigue or learning effect. To minimize any potential learning ef-
fect, all participants were taught how to perform the exercises
by the same researcher (I.M.M.) and performed 4 practice sets
of each exercise before data collection. During all trials, in-
vestigators gave oral feedback to correct errors and to assist
in the maintenance of proper tempo. After instruction, the first
practice set was performed with no weight applied and only
oral feedback given. For the second practice set, the Stabilizer
Pressure Bio-feedback unit was used to provide feedback for
maintaining correct position during the exercise.
For the third practice set, a cuff weight equal to 5% body
mass was applied just above the participant's
3% body mass has been used for side-lying hip-abduction
ercises in other studies,26 we chose 5% for our study to ensure
leg flexed for stability
were cued to point
lay on their
to control for any fa-
Table 1. DemographicCharacteristicsof Participants
Body mass index, kg/m2
run, mi/wk (km/wk)
aDefined as the preferred kicking leg.
Volume 47 • Number 1 • February 2012
22.6± 1.2 (36.4± 1.9)
Table 2. Chronologyof Events During the Data CollectionSession
1. Informed consent
2. Exercise instruction
a. Set 1 included
b. Set 2 included
c. Set 3 included
e. Set 4 included
a. For the gluteus medius, electrodes
the iliac crest and the greater trochanter
b. For the gluteus maximus,
of the femur just superior to the level of the greater trochanters
c. For the tensor fascia latae, electrodes
d. For the anterior hip flexors, electrodes
medial to the palpablemass of the quadriceps
activity of the iliopsoas and is representative
5. Data collection
Participantsperformed 3 5-s repetitions
6. Exercise data collection
a. Participantsperformed 7 repetitions
b. Participants repeated the exercise for side-lying
1-min rest between exercises.
rested for 10 min.
with oral feedback,
with oral feedback,
with oral feedback,
with pressure feedback,
with pressure feedback,
pressure feedback,and without
and with 5% body mass.
with oral feedback,with pressure feedback, and with 5% body mass.
for 5 min on a treadmill.
were placed directly superior to the greater trochanter
of the femur.
electrodes were placed one-half the distance
of the femur one-third of the distance between
between the posterosuperior
of the femur.
iliac spine and the greater trochanters
were placed 2 cm inferior and slightly lateral to the anterosuperior
were placed in the femoral triangle just lateral to the femoral pulse below the inguinal ligament and
femoris. This is described as a quasispecific
of the anterior hip flexors.
site for recording surface electromyographic
of maximal voluntaryisometriccontractions for each muscle.
with oral feedback, with pressure feedback,
and with 5% body mass.
hip abduction-external rotation, and clamshellexercises,with a
adequate muscle activation. Investigators32have suggested that
muscle activation greater than 40% of the maximal voluntary
isometric contraction (MVIC)
gains. For the CLAM exercise, the cuff weight was secured
just proximal to the participant's
at the hip between exercises, the weight was increased to ac-
count for the shorter resistance moment arm associated with
the CLAM exercise. A calculation was made based on an es-
timation of torque during the ABD and ABD-ER
(T=Fr), where F equaled the mass of the cuff weight used and
r equaled the length between the participant's
ter and lateral malleolus. For the CLAM exercise, r was the
distance between the greater trochanter and lateral joint line of
is needed to obtain strength
knee. To create equal torque
Bio-feedback unit (Chattanooga
the knee. To maintain T as constant compared with the other
tasks, a new F was calculated, and this value was used as the
weight for the CLAM exercises. Therefore, the torque applied
to the hip was consistent within participants between exercises.
Although torque values would differ among participants due to
leg-length differences, only within-subjects
made in this study.
After a lO-minute rest, participants performed a fourth prac-
tice set of each exercise. They jogged for 5 minutes on a tread-
mill at a self-selected moderate pace to warm up and increase
skin moisture to enhance EMG signal conductivity before elec-
trodes were applied.
The skin was prepared for surface EMG electrode place-
ment by shaving and vigorously rubbing with an alcohol pad.
Two active silver chloride electrodes
Denmark) were placed parallel with each of the muscles'
bers at an interelectrode distance of 2.6 cm, and a differential
electrode was placed over the fibular head. Electrodes
placed in standardized positions on the GMed, GMax, TFL,
and AHF based on the recommendations
man,33 which is a reference commonly used for similar stud-
ies (Table 2).26,27,29 All EMG electrodes were secured with tape,
and the skin electrode impedance was measured with a digi-
tal multimeter (RadioShackCorporation,
the impedance exceeded100 kQ, we prepared the skin again
and replaced the electrodes until the impedance was less than
100 kQ.33 The EMG data were collected using a 16-channel
EMG system (Run Technologies, Mission Viejo, CA) and were
sampled at 1000 Hz with an amplifier gain of 1000. A twin-axis
was secured to the lateral hip using double-sided
arm on the iliac crest and 1 arm on the greater trochanter
the femur to monitor leg movement.
data were used only to provide a visual representation
movement that could be referenced when we visually examined
the EMG data. Electrogoniometer
nously with EMG data and were sampled at 1000 Hz.
of Cram and Kas-
Fort Worth, TX). If
Ltd, Gwent, United Kingdom)
tape with 1
data were collected synchro-
Journal of Athletic Training
manual muscle test positions34were performed to confirm that
muscle crosstalk was minimal and were used for normalization.
Participants performed 3 5-second MVlCs with a lO-s rest be-
tween contractions and a I-minute rest between muscles tested.
To obtain an MVIC for the GMed, the participants lay on their
sides with the test leg up and the bottom hip and knee flexed
for stabilization. The test leg was abducted to approximately
35°, and the hip was positioned in slight extension and exter-
nal rotation. The investigatorapplied a downward force at the
ankle while stabilizing the hip with the other hand. To obtain
an MVIC for the TFL, the participants lay supine with the hip
flexed and internally rotated maximally with the knee extended.
The investigator applied force at the ankle in the direction of
hip extension. To obtain an MVIC for the AHF, the participants
lay supine with the knees extended. The test leg was positioned
in hip flexion and external rotation. The investigator (I.M.M.)
applied force at the ankle in the direction
To obtain an MVIC for the GMax, the participants
with the knee flexed to at least 90° and the hip maximally ex-
tended. The investigator applied a downward force on the pos-
terior thigh near the knee. Participants also performed 1 MVIC
while seated with the knee extended so we could confirm that
minimal crosstalk occurred for the rectus femoris muscle in the
signals for the TFL and AHF. After MVIC data collection, par-
ticipants rested for 2 minutes before exercise data collection.
The EMG and electrogoniometer
the participantsperformed a final set of 7 repetitions of each
exercise in the same manner in which they were practiced.
Seven repetitions were used to ensure that at least 3 trials were
performed with the correct tempo and form and to ensure opti-
mal signal fidelity. The participants were given a I-minute rest
To confirm our confidence in the placement of the TFL elec-
trodes, we collected post hoc data on MVICs for the TFL and
sartorius. Electrodes were placed on the TFL 2 cm inferior and
slightly lateral to the anterosuperior
on the sartorius 4 cm inferior to the anterosuperior iliac crest on
the anterior surface of the thigh. 33
voluntaryisometriccontractions in standard
of hip extension.
data were recorded while
iliac crest and were placed
The MVICs and EMG data for the exercise trials were band-
pass filtered from 10 to 499 Hz using a Butterworth
Datapac 2K2 software (Run Technologies).
determined by establishing the mean and standard deviation of
a I-second quiet baseline that occurred before the initiation of
each exercise. The muscle was considered on when its ampli-
tude exceeded a threshold of 2 standard deviations above the
baseline for at least 1 second, which we confirmed by com-
paring it with the initiation of movement as indicated by the
electrogoniometer data. The root mean square (RMS) of the
EMG data then was calculated using a 20-millisecond
window for the entire on period for the 3 MVICs and for the ex-
ercise trials. The average RMS over a 500-millisecond
surrounding the peak activity was determined for the MVICs
and exercise trials.
The first and last repetitions of the exercise trials were ex-
cluded from analysis, and the 3 trials with the most consistent
EMG signals based on visual inspection were used. Exercise
EMG amplitude was expressed as a percentage of the average
MVIC for each muscle (%MVIC) because this has been shown
to be the most reliable method of EMG normalization
Muscle onset was
Volume 47 • Number 1 • February 2012
abduction exercises.35The %MVIC values for the 3 repetitions
of each exercise then were averaged within each participant for
A I-way, repeated-measures
GMed, GMax, TFL, and AHF muscle activity was performed
for each of the 3 exercises(ABD, ABD-ER,
SPSS (version 13.0; SPSS Inc, Chicago, IL). The a level was
set a priori at <.05. If a main effect was found, post hoc pair-
wise comparisons were performed with a Bonferroni correction
for multiple comparisons.
analysis of variance comparing
For the ABD
(54.3%± 19.1%), andAHF (28.2%±21.5%)
each other (P range, .001-.004).
was less active than the GMed (P<.OOl) and TFL (P=.004) but
was not different from theAHF (P=.99).
For the ABD-ER exercise, the TFL (70.9%±17.2%)
more active than the AHF (54.3%±24.8%,
(53.03% ± 28.4%,
.001). For the CLAM exercise, the AHF (54.2%±25.2%)
more active than the TFL (34.4%±20.1%,
(32.6%± 16.9%, P=.002) but was not different from the GMax
The post hoc data collected on MVICs for the TFL and sar-
torius clearly showed that, whereas the sartorius was somewhat
active during the TFL MVIC, it was much less active than the
TFL. The sartorius was most active during the "hackey-sack"
position of hip flexion, external rotation, and knee flexion.
among muscleactivity for
(F3,57 = 10.458,
were different from
The GMax (25.3%±24.6%)
P=.05) and GMed
The purpose of our study was to compare the EMG activ-
ity produced by the GMed, GMax, TFL, and AHF during 3
hip-strengtheningexercises: ABD, CLAM, and ABD-ER. All
3 exercises are used commonly during rehabilitation
extremity injuries, and a complete examination
activity had not been performed.
hypothesis that during the ABD exercise, the GMed would be
more active than the TFL, AHF, and GMax. Findings for the
ABD-ER and CLAM exercises were not as we expected; the
GMed was not highly active during these exercises.
of hip muscle
Our results supportedour
The ABD exercise activated the GMed 79.1 %± 29%MVIC.
This amplitude is similar to the amplitude DiStefano
observed using no additional load yet is higher than the am-
plitudes Bolgla and UhI26 (42%±27%MVIC),
(39%± 17%MVIC), and Cynn et al29(25.03%± 1O.25%MVIC)
reported. Activity of the TFL during the ABD exercise was also
high (54.3%± 19.1%) and was consistent with its anatomical
role as a primary hip abductor.22We are the first to include the
TFL in the analysis of EMG activity during hip-strengthening
exercises, so no comparisons with previous data can be made.
Ekstrom et al27
• Gluteus medius• Tensor fascia latae
::::J Anterior hip flexors. Gluteus maximus
Hip abductionHip abductionwith external
Figure 5. Comparison
rotation, and clamshell
medius and tensor fascia latae.
sor fascia latae and gluteus medius.
of muscle activity for side-lying
from each other.
side-lying hip abduction-external
less than gluteus
greater than ten-
CIndicates greater than all other muscles.
The higher activation of the GMed in our study than in pre-
vious studies is attributed to 3 factors. We used a load of 5%
body mass in an attempt to have activation greater than the
40% threshold necessary for strength gains.32,37 We also spe-
running at least 25 miles (40 kIn) per week, whereas other
investigators have used a sample of convenience.26,27
pressure biofeedback unit is known to increase the activation
of the GMed during ABD exercise by limiting muscle substi-
tution from the quadratus lumborum.29
46.06% ± 21.2%MVIC activation of the GMed when they used
this unit with no additional load applied to the lower extrem-
ity and reported 25.03% ± 1O.25%MVIC when the exercise was
performed without the feedback unit. These findings lead us to
recommend including a load of at least 5% body mass and the
Stabilizer Pressure Bio-feedback
tion and thus the strengthening
the ABD exercise.
who were moderately active,
Cynn et al29reported
unit to maximize the activa-
potential of the GMed during
The rationale for turning the toe upward and externally ro-
tating the hip in this exercise has been twofold: to engage the
GMax as a hip external rotator and to minimize the contribu-
tion of the TFL as an abductor. Our data contradicted
these ideas. We found that the TFL (70.9% ± 17.2%) was more
active than all the other muscles during the ABD-ER exercise.
Although both the TFL and GMed contribute to hip abduction,
the TFL is also a secondary hip flexor because its line of action
is more anterior to the hip joint center than that of the GMed.38
Therefore, any force acting on the leg to cause hip extension
will result in TFL activation to prevent hip extension. Despite
the use of the biofeedback pressure cuff and visual and oral
feedback, the participants could have rolled their bodies toward
their backs while performing the ABD-ER exercise. Gravity
acting on the lower limb in this position would pull the hip into
neutral position. Greater activation of the AHF also was seen
during this exercise, supporting this explanation. Even a small
change in body position could have a large effect on muscle
activity because of the long lever ann of the lower extremity.
We are the first to examine hip-strengthening
include activity of the TFL. The relevance of this muscle to
hip motion is great because it acts as a flexor and abductor
throughout the entire range of hip motion in the sagittal plane.23
The placement of the electrodes 2 cm distal to the anterosu-
perior iliac spine and slightly lateral on the TFL was based on
the guidelines of Cram and Kasman.33The anterior aspect of
the hip joint is an area where many muscles cross or converge;
therefore, despite the customary
talk from other muscles, other muscle activity could have con-
tributed to the signal. Of particular concern was the potential
influence that the sartorius might have had on the TFL signal.
The sartorius is also a primary hip flexor and secondary abduc-
of the TFL to maintain the
attempts to minimize cross-
Journal of Athletic Training
tor.23The post hoc data we collected on MVICs for the TFL
and sartorius demonstrated that the placements for the TFL and
sartorius do yield distinct patterns of activity on surface EMG.
Although crosstalk between muscles cannot be eliminated with
surface EMG, we are confident that the procedures
electrode placement, interelectrode
sis of the data to minimize the influence of crosstalk were con-
sistent with accepted practices.
The muscles that are considered the primary hip external ro-
tators are the GMax and the deep external rotators (gemellus
superior, gemellus inferior, obturator internus, obturator exter-
nus, piriformis).22 The activation of the GMax was quite low
during theABD-ER exercise (31.7%±24.1 %) and only slightly
greater than during the ABD exercise. These data indicated that
externally rotating the leg to activate the GMax has little added
benefit. A limitation of our study is that the deep external rota-
tors were not monitored, so no conclusions about their activa-
tion can be made. The low GMax activity combined with the
high activity of the TFL suggests that the ABD-ER exercise is
not superior to the ABD exercise for targeting the GMed and
distance, and visual analy-
The rationale for the CLAM exercise is that it can be per-
formed to strengthen the abductor and external rotator muscles
simultaneously. Our data did not support this claim because the
CLAM exercise showed different patterns of muscle activa-
tion than what was expected. The AHF (54.2%±25.2%)
activated more than the other 3 muscles, and activation of the
GMed was quite low (32.6%± 16.9%). The GMed activation
for the CLAM exercise is in the range of what has been re-
ported.25The low activity of the GMed during the CLAM ex-
ercise can be explained by changes in the moment arms and
actions of the muscles with the hip flexed. The CLAM exercise
was performed in 45° of hip flexion, and authors of cadaver-
based anatomical studies have demonstrated
of hip flexion, the GMed no longer functions as a primary hip
abductor.25In more than 40° of hip flexion, the GMed functions
as an internal rotator, and hip abduction is performed by the
deep external rotators.23,24
Nyland et al30examinedmuscle
GMax, TFL, and vastus medialis during an isometric variation
of the CLAM exercise. They reported nonnormalized
peak EMG amplitude, so direct comparison with our data is im-
possible. Their primary finding was that people with greater hip
anteversion demonstrated less vastus medialis and GMed activ-
ity than those with typical anteversion.30We also measured hip
anteversion using the passive hip internal rotation method and
found that only 2 participants had passive hip internal rotation
angles larger than 42°, which was the criterion used as a cutoff
for increasedanteversion.30Both participants
tion levels that were in the middle of the data set. However, the
degree of anteversion might have influenced the muscle activa-
tion during the various exercises.
The high level of AHF activity (54.2%±25.2%)
exercise is attributed to the need to maintain the hip in a flexed
position while the external rotation movement
No researchers have included examination of AHF activity. In
many cases, the goal of this exercise is to strengthen the hip
abductors and external rotators, and our data did not support
activation of the muscles during this exercise. We conclude
that beyond 40°
activity of the GMed,
Volume 47 • Number 1 • February 2012
that the rationale for performing the CLAM exercise is not sup-
ported by anatomical or EMG data; therefore, we question the
relevance of the exercise.
We are the first researchers to evaluate the muscle activity
of the AHF and TFL in addition to the GMed and GMax during
side-lying hip-strengthening exercises. The Stabilizer Pressure
Bio-feedback unit also has not been used in previous studies.
This tool was valuable for providing
the exercises could be performed with minimal substitution of
other muscles.29Our participants
cises when using the device with little practice. Therefore, us-
ing this device clinically might be beneficial.
Although no recommendations
ulations based on our findings, inferences can be made about
the potential to strengthen the studied muscles with these ex-
ercises. Because achieving more than 40%MVIC is necessary
to produce strength adaptations,32 the ABD exercise is clearly
superior to the other 2 for highly activating the GMed with less
activation of other muscles. The ABD-ER and the CLAM ex-
ercises did not produce high activation of the GMed, and they
activated the TFL and AHF to a greater extent. When strength-
ening the GMed and external rotators is a goal of rehabilitation,
activating the TFL and AHF muscles might not be desirable be-
cause they might be used as compensatory
we conclude that the ABD exercise is optimal for GMed activa-
tion when compared with the ABD-ER and CLAM exercises.
positional feedback so
showed mastery of the exer-
can be made for injured pop-
All participants in our study were distance runners who ran
at least 25 miles (40 km) per week, so our results might not be
generalizable to the general population. Distance runners were
chosen as participantsbecause they commonly
such as PFPS and ITBS that are treated with hip strengthening.
Distance runners also typically have little adipose tissue in the
hip region, facilitating the accurate collection of surface EMG.
Although the descriptive data for height and mass indicated
a homogeneous sample, slight variations in height and body
type would alter the torque applied to the hip during the exer-
cise. This limitation was minimized, however, because all com-
parisons of muscle activation were made within participants,
and no between-subjectscomparisons were made.
Although surface EMG carries inherent limitations, such as
crosstalk, we took all measures possible to maximize the in-
tegrity of the signal. Standard skin preparation was performed,
all electrodes were placed by the same examiner,
trode distance was minimized,
performed to confirm that the recorded activity was consistent
with the action of the muscle of interest. Despite these mea-
sures, some crosstalk might have existed between muscles.
and MVIC contractionswere
The ABD exercise is optimal for activating the GMed with
little activation of the TFL and AHF. The CLAM exercise
caused the greatest activation of the AHF and very little acti-
vation of the GMed and GMax. Similarly, the ABD-ER exer-
cise might induce excess activation and strengthening
TFL beyond what is desired depending
on the goals of reha-
bilitation. This information can be used to make more informed Download full-text
clinical decisions about exercise selection for strengthening the
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to Jennifer E. Earl-Boehm, PhD, ATe, University of Wisconsin-MilwaukeePavilion 350 Athletic Train-
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