Influence of Pelvis Position on the Activation of Abdominal and Hip Flexor Muscles

Abstract

A pelvic position has been sought that optimizes abdominal muscle activation while diminishing hip flexor activation. Thus, the objective of the study was to investigate the effect of pelvic position and the Janda sit-up on trunk muscle activation. Sixteen male volunteers underwent electromyographic (EMG) testing of their abdominal and hip flexor muscles during a supine isometric double straight leg lift (DSLL) with the feet held approximately 5 cm above a board. The second exercise (Janda sit-up) was a sit-up action where participants simultaneously contracted the hamstrings and the abdominal musculature while holding an approximately 45 degrees angle at the knee. Root mean square surface electromyography was calculated for the Janda sit-up and DSLL under 3 pelvic positions: anterior, neutral, and posterior pelvic tilt. The selected muscles were the upper and lower rectus abdominis (URA, LRA), external obliques, lower abdominal stabilizers (LAS), rectus femoris, and biceps femoris. The Janda sit-up position demonstrated the highest URA and LRA activation and the lowest rectus femoris activation. The Janda sit-up and the posterior tilt were significantly greater (p < 0.01 and p < 0.05, respectively) than the anterior tilt for the URA and LRA muscles. Activation levels of the URA and LRA in neutral pelvis were significantly (p < 0.01 and p < 0.05, respectively) less than the Janda sit-up position, but not significantly different from the posterior tilt. No significant differences in EMG activity were found for the external obliques or LAS. No rectus femoris differences were found in the 3 pelvis positions. The results of this study indicate that pelvic position had a significant effect on the activation of selected trunk and hip muscles during isometric exercise, and the activation of the biceps femoris during the Janda sit-up reduced the activation of the rectus femoris while producing high levels of activation of the URA and LRA.

Full text

INFLUENCE OF PELVIS POSITION ON THE ACTIVATION
OF ABDOMINAL AND HIP FLEXOR MUSCLES
JCHAD WORKMAN,
1
DAVID DOCHERTY,
2
KEVIN C. PARFREY,
1
AND DAVID G. BEHM
1
1
School of Human Kinetics and Recreation, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada;
2
School of Physical Education, University of Victoria, Victoria, British Columbia, Canada
ABSTRACT
Workman, JC, Docherty, D, Parfrey, KC, and Behm, DG. Influ-
ence of pelvis position on the activation of abdominal and hip
flexor muscles. J Strength Cond Res 22(5):1563–1569, 2008—A
pelvic position has been sought that optimizes abdominal
muscle activation while diminishing hip flexor activation. Thus,
the objective of the study was to investigate the effect of pelvic
position and the Janda sit-up on trunk muscle activation. Sixteen
male volunteers underwent electromyographic (EMG) testing of
their abdominal and hip flexor muscles during a supine
isometric double straight leg lift (DSLL) with the feet held
approximately 5 cm above a board. The second exercise (Janda
sit-up) was a sit-up action where participants simultaneously
contracted the hamstrings and the abdominal musculature
while holding an approximately 45°angle at the knee. Root
mean square surface electromyography was calculated for the
Janda sit-up and DSLL under 3 pelvic positions: anterior,
neutral, and posterior pelvic tilt. The selected muscles were the
upper and lower rectus abdominis (URA, LRA), external
obliques, lower abdominal stabilizers (LAS), rectus femoris,
and biceps femoris. The Janda sit-up position demonstrated the
highest URA and LRA activation and the lowest rectus femoris
activation. The Janda sit-up and the posterior tilt were
significantly greater (p ,0.01 and p,0.05, respectively)
than the anterior tilt for the URA and LRA muscles. Activation
levels of the URA and LRA in neutral pelvis were significantly (p
,0.01 and p,0.05, respectively) less than the Janda sit-up
position, but not significantly different from the posterior tilt. No
significant differences in EMG activity were found for the
external obliques or LAS. No rectus femoris differences were
found in the 3 pelvis positions. The results of this study indicate
that pelvic position had a significant effect on the activation of
selected trunk and hip muscles during isometric exercise, and
the activation of the biceps femoris during the Janda sit-up
reduced the activation of the rectus femoris while producing
high levels of activation of the URA and LRA.
KEY WORDS isometric exercise, muscle activation, electromy-
ography, rectus abdominis, rectus femoris
INTRODUCTION
Therapists, trainers, and coaches have emphasized
the importance of abdominal exercises for years.
Reasons have included sport performance, injury
prevention and rehabilitation (especially low back
pain), and aesthetics. Much of the interest has centered on the
perceived need to stabilize the ‘‘core,’’ which has generated
a variety of abdominal exercises designed to target specific
muscles. Several studies have examined the interplay between
the hip flexors and the abdominal muscles during a variety of
exercises (2,3,10,20). The general consensus has been that
high levels of hip flexor activity during abdominal strength-
ening exercises are undesirable. Ways in which the
abdominal muscles can be optimally activated while mini-
mizing activation of the hip flexors would seem to have practical
importance.
Abdominal muscle activity has been found to be very
dependent on the position of the pelvis during the
execution of the exercise. In particular, a posterior pelvic
tilthasbeenfoundtohaveamarkedinfluenceonthe
activation of abdominal musculature (9,20,23). Shirado and
colleagues (21) reported that pelvic alignment could
influence the electromyographic (EMG) activity of the
trunk flexors and extensors during isometric trunk
exercises. Full flexion of the lumbar spine has been
reported to be unnecessary for maximum electrical activity
of the abdominal muscles, suggesting that it is the position
of the pelvis that influences the activation of the trunk
muscles (19). Although there are some studies that have
examined the effect of a posterior pelvic tilt on activation
of the trunk musculature, the effect of an anterior tilt or
neutral position of the pelvis has not been clearly
elucidated. Many therapists and exercise specialists
advocate the maintenance of a neutral spine and pelvis
(17,18) during abdominal exercises in order to facilitate
carryover into functional activities. In addition,
Address correspondence to Dr. David G. Behm, dbehm@mun.ca
22(5)/1563–1569
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observation of people performing a variety of abdominal
exercises reveals that most do not prevent moving from
a neutral to an anterior tilt, which potentially changes the
purpose of the exercise and may predispose them to a risk
of low back problems (13). It would seem important to
define more clearly the effect of pelvic position, especially
neutral and anterior tilt positions, on the activation of the
trunk flexors.
The Janda sit-up (Figure 1), devised by Czech physician
Vladimir Janda and also referred to as a heels-press sit-up, has
received popularity in part because it is purported to decrease
hip flexor activity during the sit-up movement through
reciprocal inhibition (10). By actively contracting the ham-
strings muscles, an individual will theoretically deactivate the
hip flexors (10). However, there is little published evidence to
support or refute this theory.
The purpose of this study was to determine the influence of
pelvis position on therelative activity of selected abdominal and
hip musculature. A second purpose of the study was to
compare the relative muscle activity during anisometric hold at
approximately 45°of the Janda sit-up to the relative activity of
the muscle in the 3 pelvic positions. It was hypothesized that
the neutral and posterior pelvic tilt would increase the
activation of the anterior trunk muscles and that anterior
pelvic tilt would increase the activation of the rectus femoris
(reflecting the iliopsoas). In addition, it was hypothesized that
the Janda sit-up would produce high levels of activation in the
anterior trunk muscles while decreasing the activation of rectus
femoris.
METHODS
Experimental Approach to the Problem
Participants assumed a supine position and were fitted
unilaterally with surface EMG electrodes on the upper rectus
abdominis (URA), lower rectus abdominis (LRA), lower
abdominal stabilizers (LAS), external obliques, rectus femoris,
and biceps femoris muscles. Participants were asked to
perform a Janda sit-up (exercise 1) and hold a position during
the sit-up with the trunk at approximately 45°to the bench
while contracting the hamstrings. The second exercise
involved an isometric double straight leg lift (DSLL)
(exercise 2) in each of 3 pelvis positions: anterior tilt, neutral,
and posterior tilt. Each contraction was randomly allocated
and held for 5 seconds. Two trials of each exercise were
performed with a 30-second rest between trials and a
3-minute rest between each different exercise test position.
The EMG activity of each muscle was monitored across
each condition.
Subjects
A convenience sample of 15 subjects was selected to
participate in this study. All participants were male, with
a mean age of 25.9 68.4 years, mean height of 177.4 69.5 cm,
and mean weight of 78.9 611.9 kg. The participants were
instructed on the nature of the study and the equipment and
apparatus involved and were provided with the opportunity
to clarify this information. All subjects were either compet-
itive rugby players (n= 11) or recreational athletes (racquet
sports, running) (n= 4) who presently had no apparent or
known musculoskeletal injuries. All participants had exten-
sive experience with resistance training and performing
a variety of abdominal exercises throughout their training
career. Furthermore, they all scored in the excellent category
in the partial curl-up test of the Canadian Physical Activity,
Fitness, and Lifestyle appraisal. Fourteen participants were
able to complete the exercise movements correctly. One
participant was unable to maintain the pelvic tilt position
during the isometric portion of the leg raise activity. His data
were not included in the statistical analysis. Each subject was
required to read and sign a consent form before participation.
The Human Investigation Committee, Memorial University
of Newfoundland, approved this study.
Surface Electromyography Preparation and Placement
It has been suggested that a valid EMG signal is compromised
when the muscles of interest are performing a dynamic
contraction (4,8). As the joint moves through a range of
motion, the distance between the muscle and the detection
surface changes, which results in a change in the EMG
amplitude. It is recommended that isometric contractions be
used to control for movement during surface EMG testing.
Although this detracts from the ecological validity, it does
increase the validity and reliability of the EMG signal (8). An
effective start for analyzing the effect of pelvis position on
trunk muscle activity would be to use quantified and
controlled EMG procedures.
The electrode placement sites were prepared by shaving,
exfoliating with sandpaper, and wiped with isopropyl alcohol.
Participants were placed in a supine position on a plinth,
Figure 1. A Janda sit-up with resistance provided to the posterior ankle/
heel area so that the participant can contract the hamstrings while
attempting to curl the trunk/abdominal region.
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providing support to the entire length and width of the body.
Electromyographic surface electrodes (Kendall Medi-trace
100 series; Kendall, Chikopee, MA) were placed in parallel
with the muscle fibers with an interelectrode distance of 2 cm.
A ground electrode was placed at the nearest bony
prominence for each pair of active electrodes. The 6 muscle
sites were the URA, LRA, external obliques, LAS (reported to
represent activation of the internal obliques and transversus
abdominis [1,5,6]), biceps femoris, and rectus femoris. The
rectus femoris was used to approximate the activity of the
deep hip flexors, namely, the iliopsoas muscle group (14).
Landmarking for the URA was achieved by measuring 3 cm
lateral to the midline and midway between the xiphoid
process and the umbilicus. The LRA was positioned 3 cm
lateral to the midline and 2 cm inferior to the umbilicus.
Additional electrodes were placed superior to the inguinal
ligament and 1 cm medial to the anterior superior iliac spine
for the lower abdominals. McGill et al. (14) reported that
surface electrodes adequately represent the EMG amplitude
of the deep abdominal muscle within a 15% root mean square
difference. However, Ng et al. (15) indicated that electrodes
placed medial to the anterosuperior iliac spine would receive
competing signals from the external obliques and transverse
abdominis with the internal obliques. Based on these
findings, the EMG signals obtained from this abdominal
location are described in the present study as the LAS, which
would be assumed to include EMG information from both
the transverse abdominis and internal obliques. The external
obliques were positioned superior to the anterior superior
iliac spine at an oblique angle, at the level of the umbilicus.
Biceps femoris electrodes were positioned at the midpoint
of the muscle belly of the biceps femoris. Rectus femoris
electrodes were positioned at the most proximal aspect of the
muscle belly. All muscle sites were measured on the right side
of the body only.
Exercise Instruction
The participants were instructed on proper technique to
complete a maximal anterior pelvic tilt and maximal posterior
pelvic tilt. The anterior pelvic tilt was achieved by asking the
participants to tilt the pelvis forward in order to create as
much space as possible between the plinth and the lower back
area. The posterior pelvic tilt was achieved by asking the
participants to flatten their lower back into the plinth. Manual
guidance was also provided during the instruction and
familiarization period to ensure proper technique and
understanding. The neutral position was described as the
participants’ normal, comfortable resting supine position. One
investigator was positioned by the side of each participant to
ensure proper pelvic positioning during data collection as well
as palpating the anterosuperior iliac spine as a way of
monitoring pelvic position. A second investigator was
positioned at each participant’s feet to ensure proper leg
lifting during the exercise. Participants were instructed to keep
their head resting on the plinth and to rest their hands by their
sides. Participants began in a supine position on the plinth
with their legs straight and their feet placed on a stable bench
15 cm in height. For the anterior pelvic tilt position,
participants were asked to assume the proper position. A
reference mark was placed on the lateral malleolus and the
bench supporting the feet. This mark would be used to ensure
the same starting position for the second trial of the exercise.
The participants were asked to raise their feet off the support 5
cm, hold the position for 5 seconds, and return to the support.
A 30-second rest period was provided before a second trial
was performed. The same procedure was followed for the
neutral and posterior pelvic tilt positions. A 3-minute rest
period was provided between the anterior, neutral, and
posterior tilt trials. The order of exercises was randomized.
If the position was not held properly, then the position and
the data acquisition was terminated and attempted again after
an appropriate rest period.
The Janda sit-up was performed in a supine, crook-lying
position (Figure 1). A padded bar was placed at the back of
the lower leg and held in place manually by one of the
investigators. This bar provided an object against which each
participant was able to contract the hamstring muscles, by
attempting to perform bilateral knee flexion. This bar was
held manually by an investigator in order to ensure that
consistent hamstring contraction occurred throughout the
entire exercise trial. The participants were instructed to
contract the hamstring muscles, perform the sit-up, and hold
for 5 seconds at approximately 45°before returning to the
start position. A 30-second rest period was provided before
the second trial. The order of pelvic position and the Janda
sit-up was randomly assigned.
The isometric BSLL was used in this study to allow us to
maintain a relatively constant torso and leg position, while
changing only the pelvis position. We do acknowledge that
with any change in pelvis rotation there will be changes in the
rest of the kinetic chain, both above and below the pelvis.
However, this exercise would provide the most consistency in
the upper and lower body segments, allowing us to examine
the influence of the pelvis on abdominal muscle activity.
Electromyographic Data Collection
Electromyographic data were collected during the concentric
and isometric contractions of each exercise. The EMG signals
were amplified (MEC 100 amplifier; Biopac Systems Inc.,
Santa Barbara, CA), monitored, and directed through an
analog-digital converter (Biopac MP100) to be stored on the
computer (Sona, St. John’s, Newfoundland, Canada). The
EMG signals were collected over 15 seconds at 2000 Hz and
amplified (3500). The EMG activity was sampled at 2000 Hz
with a Blackman 61-dB band-pass filter between 10 and 500
Hz, amplified (Biopac Systems MEC bipolar differential 100
amplifier, Biopac Systems, Inc.; input impedance = 2 MV
common mode rejection ratio .110 dB minimum (50/60
Hz), noise .5 UV) and analog-to-digitally converted (12 bit)
and stored on personal computer for further analysis. The
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EMG signal was rectified and integrated over the 5-second
static (isometric) contraction period of the movement. An
average of the 2 trials was obtained, and the mean integrated
value used for statistical analysis. Similar to previous
published research from this laboratory (6), absolute rather
than normalized EMG data were analyzed because it was a
repeated-measures design that was completed in a single
experimental session (no change in electrode position). Since
the focus was on changes in activation of individual muscles
and not between muscles or individuals, normalization of the
electromyogram was not considered necessary.
Statistical Analyses
A 1-way, repeated measures analysis of variance (GBStat;
Dynamic Microsystems, Silver Spring, MD) was performed to
detect differences in muscle activation for each muscle,
relative to pelvic position and Janda sit-up exercise. When
statistical significance was found, the Dunn’s (Bonferroni)
post hoc test was used to reveal the differences. Descriptive
statistics include mean 6SD.
RESULTS
Upper Rectus Abdominis
For the URA site, the Janda sit-up demonstrated the highest
EMG activity. Relatively, the anterior pelvic tilt position
showed 70.9% less activity in the URA (p,0.01). The EMG
activity in the neutral position was 52.1% less than that seen
in the Janda sit-up (p,0.01). There was no significant
difference between the Janda sit-up and the posterior pelvic
tilt position. The anterior position demonstrated 57% less
activity than the posterior pelvic tilt position (p,0.05)
(Figure 2).
Lower Rectus Abdominis
For the LRA site, the Janda sit-up elicited the highest EMG
activity. This was significantly different than the anterior
pelvic tilt position (p,0.01), which showed 68.4% less
activity, and the neutral position (p,0.05), which showed
46.3% less activity. There was no significant difference
between the Janda sit-up and the posterior pelvic tilt position
for LRA activity. The anterior pelvic tilt position showed
significantly less (56.6%) activity in the LRA than in the
posterior pelvic tilt position (p,0.05) (Figure 3).
Rectus Femoris
The rectus femoris site demonstrated the highest activity in
the anterior pelvic tilt position. This was significantly different
from the Janda sit-up (p,0.05), which showed 38.1% less
activity. There were no other significant rectus femoris
differences when compared to the other test positions. The
Janda sit-up was not significantly different from the posterior
pelvic tilt or neutral positions (Figure 4).
Biceps Femoris
In the biceps femoris site, the Janda sit-up provided the
highest EMG activity. This was significantly higher than all
other test positions (p,0.01). The neutral, anterior, and
posterior pelvic tilt positions demonstrated 91.1%, 88.6%, and
87.8% less biceps femoris activity, respectively. There were no
other significant differences in biceps femoris activity among
the pelvic tilt positions (Figure 5).
External Obliques
There were no statistically significant differences in external
obliques EMG activity when comparing the 4 test positions
(p= 0.09). The Janda sit-up and the neutral pelvis positions
showed the greatest difference
in EMG activity.
Lower Abdominal Stabilizers
There were no statistically sig-
nificant differences in LAS
EMG activity when comparing
the 4 test positions.
DISCUSSION
The major findings of this study
show that changing the posi-
tion of the pelvis significantly
changes the pattern of activa-
tion of the URA, LRA, and
rectus femoris. This is in agree-
ment with a study by Shields
and Heiss (20) who found that
the double straight leg lowering
exercise, while maintaining
posterior pelvic tilt, achieved
greater abdominal muscle acti-
vation compared to a typical
Figure 2. Upper rectus abdominis (URA) electromyographic activity in each pelvis position and the Janda sit-up.
Bars and accompanying lines represent mean electromyographic activity and SDs, respectively. *Significant
difference at the p,0.05 level; **significance level of p,0.01.
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crunch exercise. Posterior pelvic tilting has also been found to
activate the rectus abdominis to a greater degree than in the
abdominal hollowing exercise (9). Other studies have
identified high levels of rectus abdominis activity during
the posterior pelvic tilt maneuver (24) and leg lifting exercise
(2). This differs from the results of Urquhart et al. (23) who
found the internal oblique mus-
cle more active than the rectus
abdominis during a posterior
pelvic tilt. In the Urquhart et al.
study, participants were asked
to gently and slowly rock their
pelvis backward. Urquhart et al.
(23) describe this as a gentle
effort, corresponding to a 2 on
the Borg scale. The present
methodology differed in that
the posterior pelvic tilt was
accompanied by the isometric
DSLL, a much more demand-
ing task. Our study was in
agreement, however, with the
authors’ conclusion that ab-
dominal muscle activity was
dependent on body position,
including lumbopelvic motion
or position.
There is general agreement
that an individual cannot pref-
erentially activate the URA
versus LRA (7,12) unless highly trained (19). The results of
the present study also found similar activation patterns for
the URA and LRA throughout the exercises. Moreover, it
has been found that no single exercise is able to optimally
recruit all the abdominal musculature simultaneously (3).
Therefore, a comprehensive, individualized program is
required to sufficiently chal-
lenge each of the abdominal
muscles (3) in different planes
of movement.
The anterior pelvic tilt posi-
tion provided the highest EMG
activity in the rectus femoris
and the lowest EMG record-
ings in both the URA and LRA.
The anterior tilt may place the
rectus femoris and underlying
iliopsoas muscle group in
a more optimal length position.
This will change the muscle
length–tension relationship and
produce higher contractile
forces. As the rectus femoris is
in an optimal position, the LRA
and URA will be placed in
a relatively lengthened position.
For the LRA and URA, the
change in length-tension rela-
tionship may place the muscles
in a disadvantageous position
and cause a reduction in
Figure 3. Lower rectus abdominis (LRA) electromyographic activity in each pelvis position and the Janda sit-up.
Bars and accompanying lines represent mean electromyographic activity and SDs, respectively. *Significant
difference at the p,0.05 level; **significance level of p,0.01.
Figure 4. Rectus femoris (RF) electromyographic activity in each pelvis position and the Janda sit-up. Bars and
accompanying lines represent mean electromyographic activity and SDs, respectively. *Significant difference at the
p,0.05 level.
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contractile forces. Furthermore, several authors have cau-
tioned against the use of the BSLL because of the risk of low
back injury caused by increased shear and compressive forces
(3,9). Invariably, individuals may adopt an anterior pelvic tilt
position when performing sit-ups or leg raises that can be
considered contraindicated considering the increased shear
and compressive forces (3,9) placed on the lower back by
stronger hip flexors.
A secondary finding showed that the Janda sit-up produced
relatively high levels of URA and LRA activity and low levels
of rectus femoris activity; however, this was not significantly
different from the posterior pelvic tilt. Our results regarding
the inability of the Janda sit-up to significantly reduce hip
flexor activity in comparison to the posterior pelvic tilt are in
agreement with Juker et al. (10) who found no decrease in
psoas activity using the ‘‘press heels’’ sit-up. The Janda sit-up
is identical to a traditional bent-knee sit-up when considering
the trunk flexion component. The difference is in the
contraction of the hamstring muscles during the exercise. As
this is a sit-up movement, it is typically performed in
a posterior pelvic tilt start position (16). Participants were not
instructed regarding pelvis position before the Janda sit-up
trials. Therefore, pelvis position was not controlled during
this exercise. This may account for some of the similarities
between the Janda sit-up and the results from the posterior
pelvic tilt position. The contraction of the hamstrings during
the Janda sit-up purportedly reduces hip flexor activation
through reciprocal inhibition (10). Our data cannot conclude
whether the low rectus femoris activity can be attributed to
reciprocal inhibition through contraction of the hamstring
musculature. The Janda sit-up did demonstrate the highest
biceps femoris activity as antic-
ipated. However, the rectus
femoris activity was not signif-
icantly different from the pos-
terior pelvic tilt position.
Differences in the posterior
pelvic tilt and Janda sit-up are
seen when we examine their
relationship to the neutral pel-
vis position. For both the URA
and LRA sites, the Janda sit-up
demonstrated significant differ-
ences from the neutral position;
however, the posterior pelvic
tilt position did not. This may
be explained by the investiga-
tors’ definition of neutral pelvis.
The participants were asked to
maintain their normal, comfort-
able supine position. The dis-
crepancy of neutral for each
participant may have influ-
enced the results. In addition,
anatomically, the neutral posi-
tion may be closer in range of available motion to the
posterior tilt than the anterior direction. This may account for
the lack of significant difference in muscle activity when
comparing the neutral position to the posterior pelvic tilt
position.
When we examine the overall trend of muscle site activity,
a pattern emerges. As the participant moves from a posterior
pelvic tilt position through neutral to the anterior pelvic tilt
position, the relative activity of the URA, LRA, and rectus
femoris becomes reversed. During the posterior pelvic tilt and
Janda sit-up, there are high levels of activity in both the URA
and LRA and low activity in the rectus femoris. In the neutral
position, the level of activity of the URA and LRA decreases,
although this was shown to be only significantly different from
the Janda sit-up. The activity of the rectus femoris increased
slightly when mean EMG activity was examined; however, the
change was not significant. In the anterior pelvic tilt position,
the URA and LRA exhibited their lowest activity levels, while
the rectus femoris shows the highest level of activity.
There was no significant difference between the exercises in
the amount of EMG activity in the LAS or external obliques
muscle sites. This differs from Shields and Heiss (20), who
found varying levels of oblique muscle activity during their
isometric double straight leg lowering exercise. The finding
in the present study would suggest that the stabilizing role of
the LAS (24) was similar for all pelvic positions as well as the
Janda sit-up. As the DSLL is not a trunk flexion exercise,
a significant difference in the activity of a trunk flexor such as
the external obliques might not be expected. The Janda sit-
up, however, is a trunk flexion exercise, but it did not show
significant differences in external obliques activity compared
Figure 5. Biceps femoris (BF) electromyographic activity in each pelvis position and the Janda sit-up. Bars and
accompanying lines represent mean electromyographic activity and SDs, respectively. **Significance level of p,
0.05.
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to the 3 different pelvis positions. During these exercises, the
external obliques probably also act as a stabilizer (2).
The biceps femoris muscle site was also unaffected by
a change in pelvis position. This hip extensor muscle may be
expected to have little activation during a hip flexion type of
activity. During the Janda sit-up, there was significantly
greater biceps femoris activity compared to the other test
positions. This is to be expected as the participant is instructed
to actively contract the hamstrings while performing the
Janda sit-up.
PRACTICAL APPLICATIONS
The results of this study will be of value when instructing
persons in correct posture during supine abdominal strength-
ening activities. There is evidence showing that specific
exercise instruction is important for a client to learn and retain
the proper technique and form of an exercise (11). Particular
attention should be given to individuals with increased
lumbar lordosis or very weak abdominal muscles. Several
authors have stressed the potential increase in lumbar
compression and shear force with some abdominal exercises.
The BSLL is not recommended for individuals who have
known lumbar pathologies or very weak abdominal mus-
culature (3,10). These individuals may be at risk of moving
into an anterior pelvic tilt position due to postural habit or
fatigue while exercising (9,22). By changing the rotation of
the pelvis, the focus of the strengthening exercise may shift
from the abdominals to the hip flexors. These results will add
to the existing and emerging scientific literature regarding the
relationship between the pelvis, hip, and lumbar spine and
the interplay of the supporting musculature.
From these data, we can conclude that a change in pelvis
position demonstrates significant differences in URA, LRA
and rectus femoris muscle activity, as measured by surface
electromyography. When considering pelvis position in-
dependently, the highest abdominal muscle activity occurs in
the posterior pelvic tilt position. The Janda sit-up also seems
to be effective in producing significant activation of the rectus
abdominis.
ACKNOWLEDGMENTS
The National Science and Engineering Research Council of
Canada supported this research.
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