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EMG analysis of abdominal exercises
1. An Electromyographic Analysis of
Commercial and Common Abdominal
Exercises: Implications for Rehabilitation
and Training
Rafael F. Escamilla, PT, PhD, CSCS1
Michael S.C. McTaggart, MS2
Ethan J. Fricklas, MSE3
Ryan DeWitt, MPT4
Peter Kelleher, MPT4
Marcus K.Taylor, PhD5
Alan Hreljac, PhD6
Claude T. Moorman, III, MD7
Study Design: A repeated-measures, counterbalanced design.
Objectives: To test the effectiveness of 7 commercial abdominal machines (Ab Slide, Ab Twister,
Ab Rocker, Ab Roller, Ab Doer, Torso Track, SAM) and 2 common abdominal exercises (crunch,
bent-knee sit-up) on activating abdominal and extraneous (nonabdominal) musculature.
Background: Numerous abdominal machine exercises are believed to be effective in activating
abdominal musculature and minimizing low back stress, but there are minimal data to substantiate
these claims. Many of these exercises also activate nonabdominal musculature, which may or may
not be beneficial.
Methods and Measures: A convenience sample of 14 subjects performed 5 repetitions for each
exercise. Electromyographic (EMG) data were recorded for upper and lower rectus abdominis,
external and internal oblique, pectoralis major, triceps brachii, latissimus dorsi, lumbar
paraspinals, and rectus femoris, and then normalized by maximum muscle contractions.
Results: Upper and lower rectus abdominis EMG activities were greatest for the Ab Slide, Torso
Track, crunch, and Ab Roller, while external and internal oblique EMG activities were greatest for
the Ab Slide, Torso Track, crunch, and bent-knee sit-up. Pectoralis major, triceps brachii, and
latissimus dorsi EMG activities were greatest for the Ab Slide and Torso Track. Lumbar paraspinal
EMG activities were greatest for the Ab Doer, while rectus femoris EMG activities were greatest for
the bent-knee sit-up, SAM, Ab Twister, Ab Rocker, and Ab Doer.
1
Associate Professor of Physical Therapy, California State University Sacramento, Sacramento, CA.
2
Graduate student (at the time of study), Duke University Medical Center, Durham, NC.
3
Student (at the time of study), Duke University Medical Center, Durham, NC.
4
Student (at the time of study), California State University Sacramento, Sacramento, CA.
5
Lieutenant, Medical Service Corps, US Navy, Naval Health Research Center, San Diego, CA.
6
Associate Professor of Kinesiology and Health Science, California State University Sacromento,
Sacromento, CA.
7
Associate Professor of Orthopaedic Surgery, Duke University Medical Center, Durham, NC.
The protocol used in this study was approved by the Institutional Review Board at Duke University
Medical Center, Durham, NC. The authors of this manuscript affirm we have no financial affiliation
(including research funding) or involvement with any commercial organization that has a direct financial
interest in any matter included in this manuscript.
Address correspondence to Rafael Escamilla, Associate Professor of Physical Therapy, California State
University Sacramento, Department of Physical Therapy, 6000 J Street, Sacramento, CA 95819-6020.
E-mail: rescamil@csus.edu
Conclusions: The Ab Slide and Torso Track
were the most effective exercises in activating
abdominal and upper extremity muscles while
minimizing low back and rectus femoris (hip
flexion) activity. The Ab Doer, Ab Twister, Ab
Rocker, SAM, and bent-knee sit-up may be
problematic for individuals with low back pa-
thologies due to relatively high rectus femoris
activity. J Orthop Sports Phys Ther 2006;36:45-
57.
Key words: EMG, low back pain,
lumbar spine, rectus abdominis,
sit-up
S
trong abdominals are im-
portant for stabilizing
the trunk and helping
unload stress in the lum-
bar spine.3,13
Abdominal
muscles (rectus abdominis, exter-
nal oblique, internal oblique, and
transverse abdominal) are com-
monly strengthened by actively
flexing the trunk with a concentric
muscle action or by resisting trunk
extension (due to an external
force such as gravity) with an iso-
metric or eccentric muscle action.
Journal of Orthopaedic & Sports Physical Therapy 45
RESEARCHREPORT
Journal of Orthopaedic & Sports Physical Therapy
Official Publication of the Orthopaedic and Sports Physical Therapy Sections of the American Physical Therapy Association
2. There are numerous exercises used for abdominal
strengthening. Many of these exercises also activate
extraneous (nonabdominal) muscles, such as the hip
flexors, lumbar paraspinals, or upper extremity mus-
culature, which may or may not be beneficial. For
example, high activation levels from the hip flexors
and lumbar paraspinals tend to generate a force
couple that attempts to anteriorly rotate the pelvis
and increase lumbar lordosis. When coupled with
weak abdominal musculature, activation of these ex-
traneous muscles may increase the risk of low back
pathologies.
Understanding the muscle activation generated by
different abdominal exercises is useful to therapists
and other health care or fitness specialists who
develop specific abdominal exercises for their pa-
tients or clients to facilitate their rehabilitation or
training needs and objectives. For example, abdomi-
nal exercises that actively flex the trunk may be
problematic for individuals with lumbar disk patholo-
gies due to increased intradiscal pressure18
and lum-
bar spine compression,3
and for individuals with
osteoporosis due to the risk of vertebral compression
fractures.21
However, these same individuals may be
asymptomatic during abdominal exercises that main-
tain a relatively neutral spine and pelvis. In contrast,
individuals with facet joint syndrome, spondylolis-
thesis, and vertebral or intervertebral foramen
stenosis may not tolerate exercises such as the Ab
Slide and Torso Track due to the extended spine
position.
There are numerous commercially available ab-
dominal machines that are believed to be effective in
activating abdominal musculature and minimizing
low back stress, but there are little or no scientific
research data to substantiate these beliefs. While
there are numerous studies that examined muscle
activity during more traditional abdominal exercises,
such as the sit-up or crunch exercises,3,14,16,26,27
there
is a scarcity of data related to the use of abdominal
machines. A limited number of studies compared
select abdominal muscle activity while performing
exercises using the Torso Track, Ab Doer, Ab Shaper,
Ab Flex, and Ab Roller,6,7,12,24,26
but there are no
studies that we are aware of that have quantified
abdominal muscle activity while using the Ab Twister,
Ab Rocker, Super Abdominal Machine (SAM), and
Ab Slide. Moreover, when using abdominal machines,
the extent of recruitment of extraneous musculature,
such as low back or upper and lower extremity
musculature, is currently unknown because there
have been no studies that have reported extraneous
muscle activity while performing these exercises. It is
also unknown how abdominal machines compare to
traditional abdominal exercises, such as the sit-up and
crunch, in recruiting the abdominal musculature.
Abdominal machines use various techniques to
target different muscles. For example, some abdomi-
FIGURE 1. Ab Rocker.
FIGURE 2. Ab Roller.
nal machines allow only uniplanar motions, such as
trunk flexion, while others use multiplanar motions,
such as trunk flexion and rotation. It is commonly
believed that performing simultaneous trunk flexion
and rotation recruits the external and internal ob-
lique musculature to a greater extent compared to
trunk flexion only. However, there is currently no
scientific evidence to support this assertion.
46 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
3. The purpose of this study was to test the effective-
ness of 7 popular commercial abdominal machines
and 2 common abdominal strengthening exercises on
activating abdominal and extraneous musculature. It
was hypothesized that significant differences would be
found in the normalized electromyographic (EMG)
data of both abdominal and extraneous muscle activ-
ity among exercises.
METHODS
Subjects
To optimize the EMG signal, this study was limited
to a convenience sample of 14 healthy, young subjects
(7 male and 7 female) who had normal or below
normal amounts of body fat for their age group.
Baseline skinfold calipers (model 68900; Country
Technology, Inc, Gays Mill, WI) and appropriate
regression equations were used to assess percent body
fat, and standards set by the American College of
Sports Medicine were used to determine normal or
below normal amounts of body fat.4
Mean (±SD) age,
mass, height, and percent body fat were 24.1 ± 5.4
years, 58.7 ± 4.9 kg, 166.8 ± 5.9 cm, and 22.7% ±
1.9%, respectively, for females, and 24.0 ± 7.1 years,
78.6 ± 13.9 kg, 179.8 ± 4.1 cm, and 9.7% ± 4.1%,
respectively, for males. All subjects provided written
informed consent in accordance with the Institutional
Review Board at Duke University Medical Center.
FIGURE 3. Ab Doer.
FIGURE 4. Ab Twister.
FIGURE 5. Torso Track.
FIGURE 6. Ab Slide.
Individuals were excluded from the study if they had
a history of abdominal or back pain, or were unable
to perform all exercises pain free and with proper
form and technique for 12 consecutive repetitions.
Exercise Descriptions
The 7 abdominal machine exercises were the Ab
Rocker (Figure 1), Ab Roller (Figure 2), Ab Doer
(Figure 3), Ab Twister (Figure 4), Torso Track
(Figure 5), Ab Slide (Figure 6), and SAM (Figure 7).
The 2 common abdominal exercises tested were the
J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006 47
RESEARCHREPORT
4. bent-knee sit-up (Figure 8) and crunch (Figure 9).
No subject had prior experience in performing the 7
commercial abdominal exercises, but they had mod-
erate experience in performing the crunch and
bent-knee sit-up.
The Ab Rocker, Ab Twister, Ab Doer, and SAM
exercises started and ended in a seated position with
a neutral spine and pelvis. The 2 common move-
ments advertised for the Ab Rocker and Ab Twister
were the crunch (involving sagittal plane trunk flex-
ion) and the oblique crunch (moving obliquely
across the body by simultaneously flexing and rotat-
ing the trunk), and both were tested with rotation
occurring to the left. Three common movements
advertised for the Ab Doer were the body bob
(frontal plane side-to-side motion), body boogie
(moving in a circular motion), and good morning
(involving sagittal plane trunk flexion), and all 3 of
these variations were tested. The movement for the
SAM involved sagittal plane trunk flexion, similar to
how the Ab Rocker (crunch), Ab Twister (crunch),
and Ab Doer (good morning) were performed.
The Ab Roller, crunch, and bent-knee sit-up started
and ended in a supine position. The crunch and Ab
Roller both had 2 variations: a normal crunch involv-
ing sagittal plane trunk flexion and an oblique
crunch involving moving obliquely across the body by
FIGURE 7. SAM.
FIGURE 8. Bent-knee sit-up.
FIGURE 9. Crunch.
simultaneously flexing and rotating the trunk to the
left. The primary differences between the 2 exercises
were that during the Ab Roller the head was sup-
ported by a head pad and the arms were supported
by a supporting bar (Figure 2), while during the
crunch the thumbs were positioned in the ears and
the hands were relaxed against the head (this hand
position was standardized for comfort for both the
crunch and bent-knee sit-up) (Figure 9). Both varia-
tions for the crunch and Ab Roller involved a
curling-up motion (trunk flexion or trunk flexion
with left rotation) until both scapulae were off the
ground. During the bent-knee sit-up the thumbs were
positioned in the ears with the hands relaxed against
the head, the feet were supported and held down,
the knees were flexed approximately 90°, and from
this supine position the subject simultaneously flexed
the trunk and hips until the elbows were even with
the knees (Figure 8).
The Ab Slide and Torso Track started and ended in
the quadruped position (on hands and knees with
hips and shoulders flexed approximately 90°), with a
neutral spine and pelvis. From this position the
subject straightened out the body by rolling forward
in a straight line (Torso Track and Ab Slide straight)
or a curved line to the left (Ab Slide curved), while
maintaining a neutral spine and pelvis (Figures 5 and
6).
48 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
5. Procedures
All subjects became familiar with all abdominal
exercises during a pretest session that took place
approximately 1 week prior to the testing session.
During the pretest session, each subject received
instructions explaining how to correctly perform
each of the abdominal exercises (each abdominal
machine came with written or video instructions for
its use). All exercises were performed with a 3-second
cadence (1 second from start of exercise to end
range, 1-second isometric hold at end range,
1-second return to starting position) and a 1-second
rest between repetitions. The subjects practiced all
exercises under the supervision of trained research
personnel. All of the commercial exercises except the
Ab Roller and Ab Slide had adjustable elastic bands
to make the exercise easier or harder. To better
normalize the intensity of each exercise, resistance
was adjusted according to each subject’s preference
and the manufacturer’s recommendation (eg, using a
resistance that was not too hard but hard enough to
allow the execution of at least 15 repetitions)—
similar to how each subject would adjust the resis-
tance and use the equipment if they purchased it for
home use. The selected resistance during the pretest
session was also used for that subject during the
testing session. For each exercise, each subject used a
resistance that enabled the subject to correctly per-
form at least 15 consecutive repetitions using the
3-second cadence described above. It was not possible
to normalize all exercises with exactly the same
relative intensity because the Ab Roller, Ab Slide,
crunch, and bent-knee sit-up used the body only as
an external resistance, while the remaining exercises
used resistance bands in addition to body as external
resistance. In addition, even when the maximum
resistance possible was used for the Ab Doer, Ab
Twister, and Ab Rocker, all subjects indicated that
they were capable of performing these exercises with
more resistance. A metronome (set at 1 beat per
second) was used to help ensure proper cadence
both during the pretest and testing sessions. Once a
subject was able to correctly perform each exercise
with the proper cadence, a testing session was sched-
uled.
Neuroline (Medicotest Marketing, Inc, Ballwin,
MO) disposable surface electrodes (type 720-00-S)
were used to collect EMG data. These oval-shaped
electrodes (22 mm wide and 30 mm long) were
placed in a bipolar electrode configuration along the
longitudinal axis of each muscle, with a center-to-
center distance of approximately 3 cm between elec-
trodes. Prior to positioning the electrodes over each
muscle, the skin was prepared by shaving, abrading,
and cleaning with isopropyl alcohol wipes to reduce
skin impedance values, which typically were less than
10 k⍀. Electrode pairs were then placed on the
subject’s right side (except for the internal oblique,
which was positioned on the subject’s left side) for
the following muscles in accordance with procedures
previously described5,8,17,20
: (a) upper rectus
abdominis, positioned vertically and centered on the
muscle belly (not on tendinous intersection) near the
midpoint between umbilicus and xiphoid process and
3 cm lateral from midline; (b) lower rectus
abdominis, positioned 8° from vertical in
inferomedial direction and centered on the muscle
belly near the midpoint between umbilicus and pubic
symphysis and 3 cm lateral from midline; (c) external
oblique, positioned obliquely approximately 45° (par-
allel to a line connecting the most inferior point of
the costal margin of the ribs and the contralateral
pubic tubercle) above anterior superior iliac spine
(ASIS) near the level of the umbilicus; (d) internal
oblique, positioned horizontally 2 cm inferomedial to
the ASIS, within a triangle confined by the inguinal
ligament, lateral border of the rectus sheath, and a
line connecting the ASISs (it has been demonstrated
that in this region only the aponeurosis of the
external oblique, and not the external oblique
muscle, covers the internal oblique)20
; (e) sternal
pectoralis major, positioned horizontally 2 cm medial
to the axillary fold; (f) triceps brachii long head,
positioned vertically over the long head muscle belly
near midline of the arm approximately halfway be-
tween the acromion and olecranon; (g) latissimus
dorsi, positioned obliquely (approximately 25° from
horizontal in an inferomedial direction) 4 cm below
inferior angle of the scapula; (h) rectus femoris,
positioned vertically near midline of thigh approxi-
mately halfway between ASIS and proximal patella;
and (i) lumbar paraspinals, positioned vertically 3 cm
lateral to spine and near level of iliac crest between
L3 and L4 vertebrae. A ground (reference) electrode
was positioned over the skin of the right acromion
process. Electrode cables were connected to the
electrodes and taped to skin appropriately to mini-
mize pull on the electrodes and movement of the
cables.
Once the electrodes were positioned, the subject
warmed up and practiced the exercises as needed,
then data collection commenced. EMG data were
collected using a Noraxon 16-channel telemyo EMG
unit (Noraxon USA, Inc, Scottsdale, AZ), and the
amplifier bandwidth frequency was 10 to 500 Hz. The
input impedance of the amplifier was 20 000 k⍀ and
the common-mode rejection ratio was 130 dB. EMG
data were sampled at 1000 Hz, recorded by a
transmitter and amplifier, and broadcast to a receiver
interfaced to a computer. The recorded signals were
processed through an analog-to-digital (A/D) con-
verter by a 16-bit A/D board.
EMG data were collected during 5 repetitions for
each exercise, with all exercises performed in a
randomized order. Each repetition was performed in
J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006 49
RESEARCHREPORT
6. a slow and controlled manner using the 3-second
cadence previously described and 1-second rest be-
tween repetitions. With a relatively low number of
repetitions performed, all subjects acknowledged that
fatigue was minimized. Each testing session took
approximately 45 minutes to complete.
Randomly interspersed within the exercise testing
session, EMG data from each muscle tested were
collected during two 5-second maximum voluntary
isometric contractions (MVICs). After conducting pi-
lot work, we adopted the following protocols for
MVIC testing, which were based on the positions that
elicited the greatest MVIC for each respective muscle
(all MVICs were collected on a plinth with subject in
a prone, supine, or short-sitting position): (a) upper
and lower rectus abdominis, body supine with hips
and knees flexed 90°, feet supported, and trunk
maximally flexed (ie, curl-up position), with resis-
tance at the shoulders in the trunk extension direc-
tion; (b) external and internal oblique, body supine
with hips and knees flexed 90°, feet supported, and
trunk maximally flexed and rotated to the left, with
resistance at the shoulders in the trunk extension
and right rotation directions; (c) sternal pectoralis
major, body supine with right shoulder flexed 90°
and internally rotated, the right forearm supinated,
and the right elbow slightly flexed, with resistance at
the right distal arm and forearm in the horizontal
abduction direction; (d) triceps long head, body
prone with right shoulder abducted 90° and right
elbow flexed 45°, with resistance at the right distal
forearm in the elbow flexion direction; (e) latissimus
dorsi, body prone with right shoulder abducted 0°
and extended maximally, with resistance at the right
distal arm in the direction of shoulder flexion; (f)
lumbar paraspinals, body prone with trunk fully
extended and hands clasped behind head, with resis-
tance at the shoulders in the direction of trunk
flexion; and (g) rectus femoris, body in short-sitting
position with hips and knees flexed 90°, with resis-
tance at the distal leg in the knee flexion direction.
The MVICs were collected to normalize the EMG
data collected during the abdominal exercises. Each
subject was given similar verbal encouragement for
each MVIC to help ensure a maximum effort
throughout the 5-second duration, and the subject
was asked after each MVIC if he/she felt it was a
maximum effort. If not, the MVIC was repeated. An
approximately 1-minute rest was given between each
MVIC and an approximately 2-minute rest was given
between each exercise trial.
Data Processing
Raw EMG signals were full-wave rectified, smoothed
with a 10-millisecond moving average window, and
linear enveloped, then averaged over the entire
duration of each exercise repetition. For each repeti-
tion the EMG data were normalized for each muscle
and expressed as a percentage of a subject’s highest
corresponding MVIC trial, which was determined by
calculating throughout the 5-second MVIC the high-
est average EMG signal over a 1-second time interval.
Normalized EMG data were then averaged over the 5
repetition trials performed for each exercise and
used in statistical analyses.
Data Analysis
A 1-factor repeated-measures analysis of variance
was employed to assess differences in normalized
EMG muscle activity among the different exercise
variations (PϽ.01). Post hoc analyses were performed
using the Bonferroni test to evaluate the significance
of between-exercise pairwise comparisons (PϽ.01).
RESULTS
Normalized EMG data for each muscle and exer-
cise are shown in Table 1. Among all exercises tested,
upper rectus abdominis EMG activities were greatest
for the Ab Slide (straight and curved), Torso Track,
crunch (normal and oblique), and Ab Roller (crunch
and oblique) exercises, and lowest for the Ab Twister
(crunch and oblique), Ab Rocker (crunch and ob-
lique), and Ab Doer (good morning, body boogie,
and body bob) exercises. Lower rectus abdominis
EMG activities were greatest for the Ab Slide (straight
and curved) and Torso Track exercises, and lowest
for the Ab Twister (crunch and oblique), Ab Rocker
(crunch and oblique), and Ab Doer (good morning,
body boogie, and body bob) exercises. Graphical
representations of upper and lower rectus abdominis
activity ranked from highest to lowest among all
exercises are shown in Figures 10 and 11.
The external oblique EMG activity for the crunch
(normal), Ab Roller (crunch), and Ab Doer (good
morning) exercises were significantly lower compared
to the Ab Slide (straight and curved) and bent-knee
sit-up exercises. Internal oblique EMG activities were
greatest for the Ab Slide (straight and curved), Torso
Track, bent-knee sit-up, and crunch (normal and
oblique) exercises, and lowest for the Ab Roller
(oblique), Ab Twister (crunch and oblique), Ab
Rocker (crunch and oblique), and Ab Doer (good
morning) exercises. Graphical representation of ex-
ternal and internal oblique activity ranked from
highest to lowest among all exercises are shown in
Figures 12 and 13.
Sternal pectoralis major EMG activities were great-
est for the Ab Slide (straight and curved), Torso
Track, SAM, and Ab Twister (crunch and oblique)
exercises, and lowest for the Ab Rocker (crunch and
oblique), Ab Doer (good morning, body boogie, and
body bob), Ab Roller (crunch and oblique), and
crunch (normal and oblique) exercises. Triceps
brachii long head EMG activities were significantly
50 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
8. FIGURE 10. Upper rectus abdominis normalized mean (SD) EMG activity among exercises.
FIGURE 11. Lower rectus abdominis normalized mean (SD) EMG activity among exercises.
greater for the Ab Slide (straight and curved) and
Torso Track exercises compared to all other exercises.
Latissimus dorsi EMG activities were greatest for the
Ab Slide (straight and curved), Torso Track, SAM,
crunch (oblique), and Ab Twister (oblique) exercises,
and lowest for the Ab Doer (good morning, body
boogie, and body bob) and Ab Roller (crunch)
exercises.
Lumbar paraspinal EMG activities were significantly
greater for the Ab Doer (good morning, body
boogie, and body bob) exercises compared to all
other exercises. Rectus femoris EMG activities were
greatest for the bent-knee sit-up, SAM, Ab Twister
(crunch and oblique), Ab Rocker (crunch and ob-
lique), and Ab Doer (body boogie) exercises, and
lowest for the Ab Roller (crunch and oblique), Ab
Slide (straight), Torso Track, and crunch (normal
and oblique) exercises. The relative effectiveness of
exercises in muscle recruitment of the trunk, upper
extremity, and hip musculature is shown in Table 2.
0
10
20
30
40
50
60
70
80
90
100Ab
Slide
(straight)Torso
Track
Ab
Slide
(curved)
C
runch
(norm
al)
C
runch
(oblique)
Ab
R
oller(oblique)
Ab
R
oller(crunch)
SAM
Bent-knee
sit-up
Ab
Tw
ister(oblique)
Ab
Tw
ister(crunch)
Ab
R
ocker(crunch)
Ab
R
ocker(oblique)
Ab
D
oer(good
m
orning)
Ab
D
oer(body
boogie)
Ab
D
oer(body
bob)
NormalizedEMG(%MVIC)
0
10
20
30
40
50
60
70
80
90
100
Ab
Slide
(straight)Torso
Track
Ab
Slide
(curved)
C
runch
(norm
al)
SAM
Bent-knee
sit-up
Ab
R
oller(crunch)
C
runch
(oblique)
Ab
R
oller(oblique)
Ab
Tw
ister(oblique)
Ab
Tw
ister(crunch)
Ab
R
ocker(oblique)
Ab
D
oer(good
m
orning)
Ab
R
ocker(crunch)
Ab
D
oer(body
boogie)
Ab
D
oer(body
bob)
NormalizedEMG(%MVIC)
52 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
9. FIGURE 12. External oblique normalized mean (SD) EMG activity among exercises.
FIGURE 13. Internal oblique normalized mean (SD) EMG activity among exercises.
DISCUSSION
Biomechanical Differences Between Flexion and
Extension Exercises
The Ab Slide and Torso Track were the most
effective exercises in activating abdominal muscula-
ture, including the upper and lower rectus abdominis
and the external and internal oblique. While per-
forming these exercises, the abdominal muscles con-
tract in a different manner compared to performing
traditional trunk flexion exercises. During the ‘‘roll-
out’’ portion in performing the Ab Slide and Torso
Track, the abdominal musculature contracts eccentri-
cally or isometrically to resist the attempt of gravity to
extend the trunk and rotate the pelvis. During the
return motion, the abdominal musculature contracts
concentrically or isometrically. If the pelvis and spine
are stabilized and maintained in a neutral position
throughout the roll-out and return movements, then
the abdominal musculature primarily would contract
isometrically. While performing these exercises, a
relatively neutral pelvis and spine were maintained
throughout the movement. In contrast, all other
0
10
20
30
40
50
60
70
Ab
Slide
(curved)
Bent-knee
sit-up
Ab
Slide
(straight)
Ab
Tw
ister(oblique)Torso
Track
C
runch
(oblique)
SAM
Ab
R
ocker(oblique)
Ab
D
oer(body
bob)
Ab
D
oer(body
boogie)
Ab
R
ocker(crunch)
Ab
Tw
ister(crunch)
Ab
R
oller(oblique)
C
runch
(norm
al)
Ab
D
oer(good
m
orning)
Ab
R
oller(crunch)
NormalizedEMG(%MVIC)
0
10
20
30
40
50
60
70
80
Ab
Slide
(curved)
Ab
Slide
(straight)
Torso
Track
Ab
R
oller(oblique)
C
runch
(norm
al)
C
runch
(oblique)
SAM
Ab
D
oer(body
bob)
Ab
R
oller(crunch)
Ab
D
oer(body
boogie)
Ab
Tw
ister(crunch)
Bent-knee
sit-up
Ab
R
ocker(crunch)
Ab
R
ocker(oblique)
Ab
Tw
ister(oblique)
Ab
D
oer(good
m
orning)
NormalizedEMG(%MVIC)
J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006 53
RESEARCHREPORT
10. exercises activated abdominal musculature by actively
flexing the trunk by concentric contractions during
the initial portion of the motion, an isometric con-
traction during the middle portion, and an eccentric
contraction during the final portion of the motion.
Understanding biomechanical differences between
exercises is important because trunk flexion may be
contraindicated in certain populations, such as those
with lumbar disk pathologies or osteoporosis. Main-
taining a neutral pelvis and spine (such as perform-
ing the Ab Slide or Torso Track exercise), rather
than forceful flexion of the lumbar spine (such as
during the bent-knee sit-up), may be desirable for
these individuals. In contrast, an individual with facet
joint pain, spondylolisthesis, and vertebral or
intervertebral foramen stenosis may not benefit from
exercises that maintain the spine and pelvis in a
neutral or extended position, such as when using the
Ab Slide and Torso Track. These exercises may in fact
contribute to the nerve compression. However, trunk
flexion exercises, such as the crunch, bent-knee
sit-up, SAM, Ab Roller, Ab Twister, Ab Rocker, and Ab
Doer may be beneficial.
When the lumbar spine is forcefully flexed, which
may occur when performing many of the commercial
abdominal machines used in the current study, the
anterior fibers of the intervertebral disk are com-
pressed, while the posterior fibers are in tension. In
addition, in extreme lumbar flexion intradiscal pres-
sure may increase several times above the normal
intradiscal pressure from a resting supine posi-
tion.18,19
While these stresses on the disk may not be
problematic for the normal healthy disk, they may be
detrimental to the degenerative disk or pathologic
spine.
There have only been a few studies that have
compared abdominal machine exercises to the tradi-
tional crunch or bent-knee sit-up exercises.6,7,12,24,26
Most of these studies compared the crunch to the Ab
Roller, and like the results of the current study, there
were generally no significant differences in abdomi-
nal muscle activity between these 2 exercises. The
biggest difference between these exercises is that the
Ab Roller provides head support, which may make it
more comfortable to perform compared to the
crunch. The only known study to investigate abdomi-
nal muscle activity between the crunch and the Torso
Track and Ab Doer (good morning) was by Sternlicht
and Rugg,24
and these authors found similar results
as the current study: that abdominal muscle activity
was significantly greater in the crunch and Torso
Track compared to the Ab Doer (good morning).
TABLE 2. Relative muscle recruitment of the trunk, upper extremity, and hip musculature. Note: the Ab Slide (straight and curved) and
Torso Track were the exercises that produced the greatest activation of the abdominal, oblique, and upper extremity musculature, while
only minimally recruiting the hip flexors.
Abdominal and
Oblique Muscles
Upper Extremity
Muscles Low Back Muscles Hip Flexor Muscles
Greatest recruitment • Ab Slide (straight and
curved)
• Torso Track
• Ab Slide (straight and
curved)
• Torso Track
• SAM
• Ab Doer (good morn-
ing and body boogie)
• Bent knee sit-up
• Ab Rocker (crunch)
• Ab Twister (crunch)
• Ab Doer (body boogie)
• Ab Twister (oblique)
Intermediate recruitment • Crunch (normal and
oblique)
• Bent-knee sit-up
• SAM
• Ab Roller (crunch and
oblique)
• Ab Twister (crunch
and oblique)
• Ab Rocker (crunch
and oblique)
• Crunch (oblique)
• Bent-knee sit-up
• Ab Roller (crunch and
oblique)
• Ab Doer (body bob) • Ab Rocker (oblique)
• SAM
• Ab Doer (body bob)
• Ab Doer (good morn-
ing)
Least recruitment • Ab Twister (crunch
and oblique)
• Ab Rocker (crunch
and oblique)
• Ab Doer (good morn-
ing, body boogie, and
body bob)
• Crunch (normal)
• Ab Doer (good morn-
ing, body boogie, and
body bob)
• Ab Twister (crunch
and oblique)
• Ab Rocker (crunch
and oblique)
• Ab Roller (crunch and
oblique)
• Bent-knee sit-up
• Crunch (normal and
oblique)
• SAM
• Ab Slide (straight and
curved)
• Torso Track
• Ab Slide (straight and
curved)
• Torso Track
• Crunch (normal and
oblique)
• Ab roller (crunch and
oblique)
54 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
11. Biomechanical Differences Between the Crunch and
Bent-Knee Sit-up
It should be noted that not all abdominal exercises
involve the same degree of flexion of the lumbar
spine. Halpern and Bleck14
have demonstrated that
lumbar spinal flexion was only 3° during the crunch
but approximately 30° during the bent-knee sit-up. In
addition, the bent-knee sit-up has been shown to
generate greater intradiscal pressure18,19
and lumbar
compression3
compared to exercises similar to the
crunch, largely due to increased lumbar flexion and
hip flexor activity. This implies the crunch may be a
safer exercise to perform than the bent-knee sit-up
for individuals who need to minimize lumbar spinal
flexion or compressive forces due to lumbar pathol-
ogy.
Although the crunch and bent-knee sit-up were
both effective in recruiting abdominal musculature,
there were some differences. Several studies, includ-
ing the current study, have shown that external
oblique activity, and to a lesser extent internal ob-
lique activity, are significantly greater in the bent-
knee sit-up compared to the crunch.1-3,16
However,
upper and lower rectus abdominis activities have
been shown to be greater in the crunch compared to
the bent-knee sit-up.6,14
In addition, like the current
study, hip flexor activity has been shown to be greater
in the bent-knee sit-up compared to the crunch.3,16
The Role of Abdominal Muscles in Trunk Stability
The role of the abdominal muscles, especially the
transverse abdominal and internal oblique, in en-
hancing spinal and pelvic stabilization and increasing
intra-abdominal pressure (IAP) has been well studied,
but still remains controversial.9,10,15,22,25
IAP has been
shown to unload the spine by generating a trunk
extensor moment and tensile loading to the spine.11
By making the trunk a more solid cylinder by the IAP
mechanism, there is a reduction in spinal axial
compression and shear loads. The attachments of the
transverse abdominal and internal oblique into the
thoracolumbar fascia may enhance spinal and pelvic
stabilization, because when these muscles contract
they tense the thoracolumbar fascia. The transverse
abdominal, which is the deepest of the 4 abdominal
muscles, has been shown to exhibit a similar (within
15%) muscle activation pattern and amplitude as the
internal oblique muscle during many of the same
trunk flexion movements (eg, bent-knee sit-up,
crunch) used in the current study.16,17
The highest
EMG activities from the internal oblique were for the
Ab Slide, Torso Track, crunch, and bent-knee sit-up
exercises, which implies that these exercises may offer
more effective stabilization to the spine and pelvis
compared to the other exercises.
Technique Variations
Despite slightly greater external oblique EMG ac-
tivities in oblique and curved techniques, mean
abdominal and oblique EMG activities generally were
not significantly different between technique varia-
tions for exercises such as the Ab Slide, crunch, Ab
Roller, Ab Twister, and Ab Rocker (eg, normal
crunch versus oblique crunch, straight Ab Slide
versus curved Ab Slide). Because simultaneous trunk
flexion and rotation have been shown to increase the
risk of torsional injury to the annulus fibrosis of the
intervertebral disk, as well as generate relatively high
lumbar compressive forces,3
and because abdominal
and oblique EMG activities were generally not differ-
ent between uniplanar and multiplanar trunk move-
ments, the additional risks involved when performing
multiplanar trunk flexion and rotation motions are
not warranted for individuals who have lumbar disk
pathologies.
Exercise Intensity
The Ab Slide and Ab Roller were the only 2
commercial exercises in which resistance could not
be adjusted. This may account for more moderate
amounts of muscle activity in the Ab Roller (because
resistance could not be added to make it harder) and
higher amounts of muscle activity in the Ab Slide
(because there was no way to make it easier).
However, the Torso Track, which is performed in the
same manner as the Ab Slide and did have resistance
bands that could be adjusted to make it easier or
harder, had nearly identical muscle activity compared
to the Ab Slide. The subjects used in the current
study were all relatively young, active individuals who
all used the Torso Track in a more difficult resistance
setting. This more difficult resistance may be appro-
priate for younger more active individuals, but older,
less active, or weaker individuals may not be able to
correctly perform the Ab Slide due to its difficulty
level. In addition, all subjects set the resistance for
the Ab Doer, Ab Rocker, and Ab Twister to the
maximum number of resistance bands that could fit
on each device. Even with maximal resistance, these 3
commercial abdominal devices recorded the lowest
amount of abdominal activity. In contrast, the Ab
Slide, Ab Roller, and Torso Track generated signifi-
cantly greater abdominal and oblique muscle activity
compared to the Ab Doer, Ab Rocker, and Ab Twister.
Extraneous (Nonabdominal) Muscle Activity
There are no studies that we are aware of that have
reported extraneous muscle activity for abdominal
machine exercises. Of the exercises tested, the Ab
Slide and Torso Track produced the greatest activa-
tion of the upper extremity musculature, including
the sternal pectoralis major, triceps brachii, and
J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006 55
RESEARCHREPORT
12. latissimus dorsi. Both the sternal pectoralis major
(lower fibers) and latissimus dorsi contract eccentri-
cally during the initial roll-out phase to control the
rate of shoulder flexion due to gravity, and concentri-
cally in the return phase as the shoulders extend.
Because the elbows typically remain slightly flexed
and at a fixed elbow angle throughout the move-
ment, the triceps brachii primarily contract isometri-
cally throughout the movement. Although the hip
flexors would appear to also contract eccentrically
during the initial roll-out phase to control the rate of
hip extension and concentrically during the return
phase to aid in hip flexion, we unexpectedly found
low rectus femoris activity for both the Ab Slide and
Torso Track exercises. It appears that upper extremity
muscles may have a greater role compared to the hip
flexors in controlling and causing exercise move-
ments during these 2 exercises. Although the activity
of the psoas muscle was not measured in the current
study due to being a deep muscle, it has been
demonstrated that during exercises performed in a
similar position and manner as the Ab Slide and
Torso Track that psoas EMG magnitudes are low and
that psoas EMG magnitudes are typically within ap-
proximately 10% of rectus femoris EMG magni-
tudes.16,17
From these data it can be hypothesized
that psoas activity, like rectus femoris activity, is
relatively low during the Ab Slide and Torso Track.
However, because the Torso Track and Ab Slide are
unique exercises in which psoas activity has not yet
been quantified, additional studies are needed to test
this hypothesis.
Because the Ab Slide and Torso Track exercises
produced the greatest activation of both abdominal
and upper extremity musculature, these exercises may
be beneficial for individuals with limited workout
time and whose goal is to perform exercises that not
only provide an abdominal workout but also an
upper body workout. The greater relative intensity
and number of muscles used during the Ab Slide and
Torso Track exercises implies that these exercises may
also achieve a greater energy expenditure compared
to the other exercises. Moreover, tension in the
latissimus dorsi in addition to the internal oblique
(and presumably the transverse abdominal), which all
tense the thoracolumbar fascia, may enhance trunk
stabilization while performing these exercises. It
should also be emphasized that cocontraction of the
lumbar paraspinal muscles, along with abdominal and
latissimus dorsi musculature, may enhance trunk
stability and spine stiffness. Although excessive activity
from the lumbar paraspinals can cause high compres-
sive and shear (especially at the L5-S1) forces on the
lumbar spine,3,16,23
the relatively low lumbar
paraspinal activity in all the exercises tested is prob-
ably not high enough to by itself cause deleterious
effects to the lumbar spine.
Performing exercises that generate high activity
from the hip flexors and lumbar paraspinals may not
be advantageous for those with weak abdominal
muscles or lumbar instability, because the forces
generated when these muscles act to anteriorly rotate
the pelvis may increase the lordotic curve of the
lumbar spine. Individuals with weak abdominal
muscles or lumbar instability may want to avoid the
bent-knee sit-up, SAM, Ab Twister, Ab Rocker, and Ab
Doer exercises due to the relatively high rectus
femoris activity. In exercises performed similarly to
the exercises in the current study, psoas and iliacus
activities have been shown to be similar in magnitude
and recruitment pattern as rectus femoris activity.1,2,17
The psoas muscle, by its attachments into the lumbar
spine, acts to hyperextend the spine as it flexes the
hip during the bent-knee sit-up and similar types of
hip flexion exercises, which may be detrimental to
individuals with lumbar instability. It has also been
demonstrated that the psoas muscle can generate
compression of the lumbar spine and anterior shear
force at L5-S1,16,23
which may be problematic for
individuals with lumbar disk pathologies. In addition,
the role of gravity in generating L5-S1 shear forces in
some exercises, such as the Torso Track and Ab Slide,
should also be considered when examining injury risk
to the low back. Unfortunately, it is unknown how
much L5-S1 shear force is generated while perform-
ing the Torso Track and Ab Slide, and whether or
not these forces are high enough to be problematic
in some patients with low back pathologies.
Effects of Electrode Placement on EMG Crosstalk
The electrode positions used in the current study
have been shown to minimize EMG crosstalk from
other muscles.5,8,20
This is especially true for the
internal oblique, which was the only muscle tested
that is not a superficial muscle. The internal oblique
normally lies deep to the external oblique, and
therefore is susceptible to considerable EMG crosstalk
from this muscle. However, it has been shown that
the internal oblique is only covered by the
aponeurosis of the external oblique, and not the
external oblique muscle, within the triangle confined
by the inguinal ligament, lateral border of the rectus
sheath, and a line connecting the ASISs.20
Therefore,
surface electrodes are appropriate to use for the
internal oblique when electrode placement is within
this area, especially when clinical questions are being
discussed and if a small percentage of EMG crosstalk
is acceptable. In fact, it has been shown that when
performing trunk flexion exercises similar to those in
the current study, mean internal and external ob-
lique EMG data from surface electrodes (similarly
located as in the current study) were only approxi-
mately 10% different compared to mean internal and
external oblique EMG data from intramuscular elec-
trodes.17
These authors demonstrated that appropri-
56 J Orthop Sports Phys Ther • Volume 36 • Number 2 • February 2006
13. ately placed surface electrodes accurately reflect
(within 10%) the muscle activity within the internal
or external oblique muscles.
CONCLUSIONS
The exercises in the current study activated ab-
dominal muscles by actively flexing the trunk
(crunch, bent-knee sit-up, SAM, Ab Roller, Ab
Twister, Ab Rocker, Ab Doer) or by resisting trunk
extension (Ab Slide and Torso Track). The Ab Slide
and Torso Track exercises produced the highest
activation of the abdominal and upper extremity
muscles while minimizing low back and hip flexion
activity. Both the bent-knee sit-up and crunch exer-
cises demonstrated similar amounts of abdominal
activation, while the Ab Twister, Ab Rocker, SAM, Ab
Doer, and bent-knee sit-up exercises exhibited the
greatest rectus femoris activity. The Ab Doer (good
morning and body boogie) exhibited the greatest
amount of lumbar paraspinal activity.
ACKNOWLEDGEMENTS
We would like to acknowledge Mike Andrawes,
Tracy Lowry, and Mark Adams for all their help in
data collection and analyses in this project.
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