Differential activity of regions of transversus abdominis during trunk rotation.

Abstract

The role of the abdominal muscles in trunk rotation is not comprehensively understood. This study investigated the electromyographic (EMG) activity of anatomically distinct regions of the abdominal muscles during trunk rotation in six subjects with no history of spinal pain. Fine-wire electrodes were inserted into the right abdominal wall; upper region of transversus abdominis (TrA), middle region of TrA, obliquus internus abdominis (OI) and obliquus externus abdominis (OE), and lower region of TrA and OI. Surface electrodes were placed over right rectus abdominis (RA). Subjects performed trunk rotation to the left and right in sitting by rotating their pelvis relative to a fixed thorax. EMG activity was recorded in relaxed supine and sitting, and during an isometric hold at end range. TrA was consistently active during trunk rotation, with the recruitment patterns of the upper fascicles opposite to that of the middle and lower fascicles. During left rotation, there was greater activity of the lower and middle regions of contralateral TrA and the lower region of contralateral OI. The upper region of ipsilateral TrA and OE were predominately active during right rotation. In contrast, there was no difference in activity of RA and middle OI between directions (although middle OI was different between directions for all but one subject). This study indicates that TrA is active during trunk rotation, but this activity varies between muscle regions. These normative data will assist in understanding the role of TrA in lumbopelvic control and movement, and the effect of spinal pain on abdominal muscle recruitment.

Full text

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Introduction
There is consistent evidence that the oblique abdominal
muscles are major axial rotators of the trunk. Obliquus
externus abdominis (OE) has been reported to be active
with contralateral trunk rotation (rotation of the thorax
relative to a fixed pelvis), and obliquus internus abdo-
minis (OI) with ipsilateral trunk rotation [1, 6, 7, 15, 25,
32]. However, the role of transversus abdominis (TrA) in
trunk rotation is controversial.
Cresswell and colleagues [7] reported both unilateral
and bilateral recruitment of TrA during trunk rotation,
with greater electromyographic (EMG) activity on the
side to which the trunk was rotated. Juker et al. [24] also
reported bilateral activity of TrA, but found no differ-
ence in activation between sides with movement in dif-
ferent directions. However, inspection of the latter
study’s average data suggests a trend for direction spe-
cific activation. In contrast, De Troyer and co-workers
[10] observed minimal activity of TrA during trunk
Donna M. Urquhart
Paul W. Hodges
Differential activity of regions of transversus
abdominis during trunk rotation
Received: 2 February 2003
Revised: 14 August 2004
Accepted: 27 August 2004
Published online: 30 November 2004
? Springer-Verlag 2004
Abstract The role of the abdominal
muscles in trunk rotation is not
comprehensively understood. This
study investigated the electromyo-
graphic (EMG) activity of anatomi-
cally distinct regions of the
abdominal muscles during trunk
rotation in six subjects with no his-
tory of spinal pain. Fine-wire elec-
trodes were inserted into the right
abdominal wall; upper region of
transversus abdominis (TrA), middle
region of TrA, obliquus internus
abdominis (OI) and obliquus exter-
nus abdominis (OE), and lower re-
gion of TrA and OI. Surface
electrodes were placed over right
rectus abdominis (RA). Subjects
performed trunk rotation to the left
and right in sitting by rotating their
pelvis relative to a fixed thorax.
EMG activity was recorded in re-
laxed supine and sitting, and during
an isometric hold at end range. TrA
was consistently active during trunk
rotation, with the recruitment pat-
terns of the upper fascicles opposite
to that of the middle and lower fas-
cicles. During left rotation, there was
greater activity of the lower and
middle regions of contralateral TrA
and the lower region of contralateral
OI. The upper region of ipsilateral
TrA and OE were predominately
active during right rotation. In con-
trast, there was no difference in
activity of RA and middle OI be-
tween directions (although middle
OI was different between directions
for all but one subject). This study
indicates that TrA is active during
trunk rotation, but this activity var-
ies between muscle regions. These
normative data will assist in under-
standing the role of TrA in lumbo-
pelvic control and movement, and
the effect of spinal pain on abdomi-
nal muscle recruitment.
Keywords Transversus abdominis Æ
Trunk rotation Æ Regional
recruitment Æ Abdominal muscles Æ
Electromyography
Eur Spine J (2005) 14: 393–400
DOI 10.1007/s00586-004-0799-9
ORIGINAL ARTICLE
D. M. Urquhart
Department of Physiotherapy,
The University of Melbourne,
Melbourne, Victoria, Australia
D. M. Urquhart (&)
Department of Epidemiology
and Preventive Medicine,
Monash University,
Melbourne, Victoria, Australia
E-mail:
donna.urquhart@med.monash.edu.au
Tel.: +61-3-99030590
Fax: +61-3-99030556
P. W. Hodges
Prince of Wales Medical Research Institute,
Sydney, NSW, Australia
P. W. Hodges
Department of Physiotherapy,
The University of Queensland,
Brisbane, Queensland, Australia

Page 2

rotation and Misuri et al. [29] reported no significant
changes in muscle thickness with ultrasound imaging.
Furthermore, no difference in recruitment of TrA was
evident with changes in the rotatory forces associated
with rapid limb movement in different directions [19].
To further complicate the issue of the contribution of
TrA to trunk rotation, anatomical studies have high-
lighted variation in the attachments and morphology of
separate regions of the abdominal muscles [2, 43]. For
instance, the upper fibres of TrA that arise from the rib
cage have a horizontal orientation, while the middle and
lower fibres that attach to the thoracolumbar fascia and
the iliac crest respectively are inferomedial. In contrast,
the orientation of OI fascicles ranged from superomedial
in the upper and middle regions to inferomedial in the
lower region [43]. Due to differences in the mechanical
advantage of fascicles in each region, it is possible that
differences in recruitment may exist.
In accordance with these reports, there is preliminary
evidence from EMG studies to indicate differential
activation of regions of TrA and OI. Greater tonic
activity of lower TrA was reported with limb movement
[18] and activity of the lower region of OI predominated
with ipsilateral straight leg raise and pelvic tilting in
standing [6]. A few studies have also investigated
recruitment of compartments of the abdominal muscles
during trunk rotation. Activity of the upper region of
OI has been reported to be greater than that of the
lower region [6] and activity of the lateral fibres of OI
greater than the anterior fibres [8, 28]. However, in the
former study there was only a small difference in
activity between regions and no statistical analyses were
performed, and in the latter study interpretation of
results is difficult as EMG recordings were made with
surface electrodes [8, 28]. The aims of the current
investigation were to compare activity of the abdominal
muscles, and to compare recruitment of regions of TrA
and OI during isometric trunk rotation at the end of
range. This is a fundamental step towards understand-
ing the role of the abdominal muscles in trunk rotation
(a risk factor for low back pain [23]), and the motor
control changes associated with these muscles in people
with spinal pain.
Materials and methods
Subjects
Six subjects (three male, three female) with a mean age,
height, and weight of 30 years (SD, 4 years), 1.74 m
(SD, 0.09 m), and 68 kg (SD, 15 kg) participated in the
study. Subjects were excluded if they had a history of
spinal or leg pain in the preceding 2 years that limited
function or for which they sought medical or allied
health intervention, a history of a neurological, respi-
ratory or gastrointestinal disorder, previous abdominal
surgery, an abdominal or inguinal hernia, observable
spinal deformity or recent pregnancy. The activity level
of the subjects was ‘‘average’’, as determined by a
standard activity questionnaire [3]. All procedures were
approved by the institutional ethics committee and
conducted in accordance with the Declaration of
Helsinki.
Electromyography (EMG)
EMG activity of the abdominal muscles was recorded
with bipolar fine-wire electrodes fabricated from Teflon-
coated stainless steel wire (75 lm) (A-M Systems,
Everett, WA, USA). One millimetre of Teflon was
removed from the ends of the wires and the ends were
bent back 1 mm and 2 mm to form hooks. The wires
were threaded into
(0.70 mm·38 mm) and inserted into the right TrA, OI
and OE under the guidance of ultrasound imaging
(5 MHz curved array transducer) (128XP/4, Acuson,
Mountain View, CA, USA) [7, 10, 16]. Fine-wire elec-
trodes were inserted immediately adjacent to the eighth
costal cartilage in the upper region of TrA, halfway
between the iliac crest and lower border of the rib cage
in the middle region of TrA, OI and OE, and adjacent to
the anterior superior iliac spine (ASIS) in the lower re-
gion of TrA and OI (Fig. 1A). Although the use of
selective intramuscular EMG electrodes results in sam-
pling from a small population of motor units, this
technique was critical to investigate differences between
muscles and between regions of muscles with minimal
cross-talk.
As rectus abdominis (RA) is distant from the other
abdominal muscles near the midline, it was considered
that cross-talk would be less problematic than for OE,
OI and TrA, and surface recordings would be suitable.
Pairs of surface EMG electrodes (Ag/AgCl discs, 1 cm
diameter and 2 cm inter-electrode distance) were placed
over RA, halfway between the umbilicus and the pubic
symphysis. A ground electrode was positioned over the
iliac crest. EMG data were bandpass filtered between
20 Hz and 1 kHz and sampled at 2 kHz using a Pow-
er1401 and Spike2 software (CED, Cambridge, UK).
Analysis was performed using Matlab 6 (release 12;
MathWorks, Natick, MA, USA).
a hypodermicneedle
Trunk rotation
Rotation of the trunk was recorded using a 3-Space
Fastrak motion analysis system (Polhemus, Colchester,
VT, USA). Two motion sensors were attached to the
undersurface of a rotating chair. Data were sampled at
100 Hz and used to identify the end of rotation.
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Procedure
Subjects sat on a rotating seat with their thorax fixed
firmly to an immobile backrest to prevent trunk move-
ments other than rotation occurring (Fig. 1B). Neutral
position of the trunk and pelvis was ensured before each
trial, as pre-rotation of the trunk has been reported to
affect muscle recruitment [35]. With arms across their
chest, subjects performed two repetitions of rotation of
their pelvis and lower limbs in each direction. Rotation
was performed with effort just sufficient to hold end
range. EMG recordings were made during relaxed sit-
ting (with back support) prior to movement and at the
end of range of trunk rotation. To determine the EMG
amplitude of each muscle/region in sitting relative to the
signal with no or minimal activity, recordings were also
made in supine (with arms beside the trunk, and hips
and knees in neutral). In order to normalise the EMG
data, maximum voluntary isometric contractions were
performed in supine with hips and knees flexed to
approximately 45? against manual resistance for 5 s. The
normalisation tasks involved isometric trunk flexion
(RA), and ipsilateral (OI) and contralateral rotation
(OE), and a maximal Valsalva and forced expiratory
manoeuvre (TrA). The peak activity for each muscle
across these tasks was selected for normalisation.
Data processing
Root mean square (RMS) EMG amplitude was calcu-
lated for 2 s of steady state EMG in the neutral sitting
and supine positions, and for 2 s from the time the
subject reached the end range of movement. To
investigate the activity of the abdominal muscles in quiet
sitting prior to movement, the difference in RMS EMG
amplitude between sitting and supine was determined
and normalised to that recorded during maximal vol-
untary manoeuvres. To reduce inter-subject variability
and to ensure there was maximum sensitivity to detect
differences between directions of rotation, the activity in
sitting was subtracted from that recorded at the end of
range of trunk rotation, and the activity in each direc-
tion was expressed as a proportion of the peak activity
recorded during rotation for that muscle in either
direction. Thus, a value of ‘‘1’’ indicated the direction in
which the muscle was most active. The calculation of
absolute scores (normalised to maximum) allowed the
amplitude of the data for each muscle region to be
examined (Table 1), while ratio scores (normalised to
peak activity recorded for either direction of rotation)
enabled activity between movement directions to be
Fig. 1 A EMG electrode sites.
Horizontal lines indicate the
borders of defined regions of
the abdominal wall and shaded
circles represent the electrode
insertion sites; B Experimental
setup. Subjects sat on a rotating
chair with electrodes in situ and
their thorax fixed firmly to an
immobile backrest with a belt
Table 1 Group median (minimum : maximum) change in RMS
EMG amplitude from relaxed sitting to end range of rotation for
each muscle and muscle region expressed as a percentage of RMS
during maximal change. Negative values indicate a decrease in
EMG activity from tonic activity in sitting. (RMS root mean
square, L lower, M middle, U upper, TrA transversus abdominis,
OE obliquus externus abdominis, OI obliquus internus abdominis,
RA rectus abdominis)
Muscle/region *Left rotation
RMS EMG (%)
*Right rotation
RMS EMG (%)
LTrA
MTrA
UTrA
LOI
MOI
OE
RA
24 (0:74)
17 (5:107)
1 (-46:9)
4 (1:8)
14 (0:53)
7 (-1:30)
10 (-2:65)
0 (-2:2)
1 (-23:7)
6 (1:194)
0 (-1:1)
3 (1:15)
9 (5:283)
2 (0:4)
*Left trunk rotation = rotation of the pelvis to the left (relative to
a fixed thorax)
*Right trunk rotation = rotation of the pelvis to the right (relative
to a fixed thorax)
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compared irrespective of the absolute amplitude of the
EMG data.
Statistical analysis
To compare the EMG activity of the abdominal muscles
and regions during quiet sitting (baseline), a one-way
analysis of variance (ANOVA) was performed. A two-
way repeated measures ANOVA was also undertaken
using the ratio scores to compare the activity between
movement directions and between muscle regions. Post
hoc testing was performed using Duncan’s multiple
range test. The alpha level was set at 0.05.
Results
When subjects sat quietly in the experimental apparatus
prior to rotation, activity of the abdominal muscles was
greater than that recorded in supine for the majority of
subjects. Data for individual subjects are shown in
Fig. 2. Although there was no difference in the mean
EMG amplitude between the muscle regions (due to the
large inter-subject variability) (p=0.4), several subjects
had greater tonic activity of the lower region of TrA
compared with the other regions of this muscle and the
other abdominal muscles. The median (minimum /
maximum) tonic activity of lower TrA was 13.7%
(0.2% / 26.1%) of the activity recorded during the
maximal voluntary effort, while activity of the middle
and upper regions was 3.3% (0.5% / 29.3%) and 1.4%
(0.02% / 27.5%) respectively.
For analysis of abdominal muscle recruitment asso-
ciated with trunk rotation, the resting level of tonic
activity in sitting was subtracted from that recorded
during the isometric hold at the end of trunk rotation
range. Therefore, these data provide a sensitive measure
of abdominal muscle activity that relates to rotation.
The results of this analysis indicated that during trunk
rotation, each abdominal muscle (with the exception of
RA) was active to a greater degree in one direction of
rotation (Figs. 3 and 4). OE was more active with right
rotation of the trunk (p<0.01), and the lower region of
OI was more active in the opposite direction of rotation
(p<0.004). The middle fascicles of OI were not different
between directions of rotation (p=0.1), however greater
activity was evident with rotation of the trunk to the left
for all subjects except one. Although there were some-
what higher levels of activity of RA during left rotation,
there was no difference in activity of RA between
movement directions (p=0.4), as greater activity was
evident in half of the subjects for each direction.
All regions of TrA were active during rotation of the
trunk (relative to a fixed thorax). However, there was
greater activity of the upper region of TrA with right
trunk rotation (p<0.02), and greater activity of the
lower and middle regions with left rotation (lower region
(both directions): p<0.009; middle region (both direc-
tions): p<0.003). While activity of the upper region of
TrA was different from the lower and middle regions
(left rotation: p<0.02; right rotation: p<0.01), the lower
and middle regions were similar in their activation in
both movement directions (left rotation: p=1.0; right
rotation: p=0.6). Although the activity of the lower
region of OI was found to be direction specific and the
middle region did not differ between movement direc-
tions, there were no differences in activity between these
regions of OI with rotation to the left (p=0.6) or right
(p=0.2).
Discussion
The results of this study provide evidence that TrA is
active during trunk rotation. A novel finding was that
compartments of TrA differed in their activity between
movement directions, indicating that regions of TrA
have differentialrecruitment
attachments, fibre orientation and recruitment patterns
of TrA, it is hypothesised that this muscle may con-
tribute to torque production and/or stabilisation of the
rib cage (upper), lumbosacral region (middle and lower)
and/or the anterior aponeuroses (all regions) during
rotation. Greater activity of OI and OE in one direction
of rotation, along with some degree of activity in the
opposite direction, may indicate that these muscles serve
several roles during rotatory tasks.
patterns.Giventhe
Fig. 2 Mean RMS EMG amplitude of abdominal muscles and
regions of these muscles in sitting minus that recorded in supine
and expressed as a proportion of that during a maximal voluntary
contraction (MVC). Note there is recruitment of all the abdominal
muscles and regions in the majority of subjects, but a trend for
greater recruitment of lower TrA and minimal or reduced activity
of OE (RMS root mean square, L lower, M middle, U upper, TrA
transversus abdominis, OE obliquus externus abdominis, OI
obliquus internus abdominis, RA rectus abdominis)
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Abdominal muscle recruitment in relaxed sitting
As predicted from the numerous studies of upright trunk
postures [5, 9, 12, 24, 32, 42], tonic activity of the
abdominal muscles was present at rest in sitting. Spe-
cifically, our data are consistent with reports by Snijders
et al. [39] that indicate the oblique abdominals are active
during quiet sitting, and are also in accordance with the
results of Juker et al. [24] that TrA is recruited in this
upright position. In the current study, tonic activity of
the abdominal muscles was quantified as a change in
activity from that recorded in supine. There were no
identifiable motor unit action potentials in the EMG
recordings in supine for the majority of cases, and thus
our data indicate the true increase in activity from the
relaxed state of the muscle. However, in some instances
there may have been small amounts of activity in the
supposedly relaxed muscle [13], indicating a change from
baseline activity rather than an absolute level of
recruitment. Negative values in Fig. 2 indicate that tonic
activity at rest in supine was present for some muscles in
specific subjects. Although there were no significant
differences between muscles/regions (probably due to the
high inter-subject variability), mean scores indicate a
graduated increase in TrA activity from the upper to the
lower regions of this muscle. This may reflect a greater
need to support the lower abdominal contents in
dependent parts of the abdomen in which hydrostatic
pressure is elevated [9] and/or to control stability of the
lumbar spine and pelvis [17, 38, 40].
Abdominal muscle recruitment during trunk rotation
As expected the oblique abdominal muscles were active
with trunk rotation, with ipsilateral OE recruited with
rotation to the right and the lower and middle regions of
contralateral OI active with rotation to the left. These
findings are consistent with previous EMG studies that
Fig. 3 Raw EMG data from a
representative subject in neutral
and at the end range of trunk
rotation. Note greater activity
of the upper region of TrA in
the opposite direction to that of
lower and middle regions.
While the activity of ipsilateral
upper TrA and OE predomi-
nates during right rotation,
contralateral lower and middle
TrA and lower OI are most
active during left rotation, and
recruitment of middle OI and
RA does not vary between
movement directions (L lower,
M middle, U upper, TrA trans-
versus abdominis, OE obliquus
externus abdominis, OI obli-
quus internus abdominis, RA
rectus abdominis)
Fig. 4 Peak EMG activity for right (s) and left (d) trunk rotation
is represented as a proportion of the greatest activity recorded
during rotation in either direction. Note the greater activity of
upper TrA and OE during right rotation, and lower and middle
TrA and lower OI during left rotation. * p<0.05 (L lower, M
middle, U upper, TrA transversus abdominis, OE obliquus externus
abdominis, OI obliquus internus abdominis, RA rectus abdominis)
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have recorded the opposite activity of OI and OE during
rotation of the trunk [6, 15, 27, 32], and biomechanical
models that have calculated muscle forces associated
with rotatory motion [22, 36]. It is important to note
that these similarities were present even though subjects
in the present study had their trunk fixed and rotated
their pelvis (in order to control thoracic movement),
which differs from the task performed in other studies.
The finding that OE and OI were direction specific in
their recruitment is consistent with a role in torque
production. However, it has been argued that EMG
activity cannot be used to predict the axial torque pro-
duced during trunk movement [27, 34]. Previous inves-
tigations have also reported bilateral activity of the
abdominal muscles during rotatory tasks [6, 24, 32, 34],
and it has been hypothesised in biomechanical models
that this activity provides stability to the lumbar spine
[14]. Although in the current study some degree of
bilateral activity of both OI and OE was observed with
rotation in each direction, the activity of the lower fas-
cicles was greater with left rotation and all but one
subject showed greater activity of middle OI in one
direction. These findings may differ from those of the
earlier studies, as EMG recordings were previously
made with surface electrodes, and activity from the
adjacent muscles OE and TrA (which may differ from
OI) is likely to have affected the results.
In agreement with previous studies, activity of RA
did not vary with the direction of trunk rotation [15, 24,
32, 34]. This is consistent with its vertical fascicle ori-
entation [30] and its primary role as a trunk flexor [6,
10]. However, as reported by Pope and colleagues [35]
the EMG amplitude of RA was relatively large. This
may be a result of complex coupling of trunk flexion
with rotation [26, 31] or may indicate that RA contrib-
utes to tensioning of the anterior aponeuroses and the
linea alba to provide a stable base from which the other
abdominal muscles can generate force.
Rotatory function of TrA
This study indicates that TrA is active during trunk
rotation. In agreement with Cresswell and colleagues [7],
greater activity of the lower and middle regions of TrA
was evident with rotation of the pelvis to the contra-
lateral side. This similarity in results was present despite
differences in the task, as Cresswell and colleagues [7]
investigated rotation of the thorax relative to a fixed
pelvis. Although Juker et. al. [24] reported no difference
in activation between sides of TrA with each direction of
trunk rotation, observation of the mean data indicate a
trend towards direction specific activation, which may
not have been detected due to high variability in the data
and insufficient statistical power. If the change from
tonic activity in sitting (also reported in their paper) was
accounted for in the analysis, the sensitivity to detect a
change in TrA activity with rotation may have in-
creased. Other investigations have suggested TrA has no
or a minimal role in trunk rotation [10, 29]. For in-
stance, Misuri et al. [29] reported no overall changes in
thickness of TrA with sonographic imaging and mea-
surement. However, increases in thickness were evident
in three of the six subjects during contralateral trunk
rotation and in four during ipsilateral rotation. Al-
though the relationship between changes in muscle
thickness measured with ultrasound imaging and EMG
activity is non-linear, an association between these two
measures has been demonstrated [21]. Furthermore, al-
though De Troyer et al. [10] qualitatively reported
minimal activity of TrA during rotation, it is difficult to
evaluate the absolute EMG activity recorded without
comparison to standardised manoeuvres.
The contribution of TrA to trunk rotation has also
been investigated during challenges to postural control
[19]. Although unilateral flexion and extension of the
arm produces rotatory trunk moments in opposing
directions, the activity of TrA did not vary between
movement directions [19]. These data are inconsistent
with a rotatory function of TrA, however reactive mo-
ments evoked in multiple directions during the unilateral
movements may have obscured rotatory activity. In
addition, these tasks may not have imposed rotation
moments of sufficient magnitude to evoke contraction of
TrA. Recent data suggest that there may be a threshold
magnitude of rotation required to produce rotatory
activity of TrA. For instance, bilateral activity of TrA
recorded during low load rotatory arm movements be-
comes asymmetrical as postural demand increases
(Hodges, unpublished observations).
Regional recruitment
Regions of TrA differed in their activation with trunk
rotation. The upper region was active in an opposing
direction to that of the ipsilateral lower and middle re-
gions. This is consistent with dissection studies that have
documented distinct anatomical compartments of TrA
that vary in their muscle attachments and fascicle ori-
entation [2, 43]. Furthermore, intramuscular septa have
been identified [11, 33, 45], particularly between the
middle and upper regions [43], that may limit lateral
transmission of forces and allow regions of TrA to act
independently [43].
Differential activation of regions of other muscles,
such as trapezius [37] and gluteus medius [41], has been
reported. Four compartments of trapezius have been
identified according to their varying attachments and
morphology. Distinct differences in recruitment of these
compartments have been observed, with the action of
the upper fibres opposing that of the lower fibres [37].
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The observations of this current study are consistent
with compartments within a muscle displaying differen-
tial activity.
Role of TrA in rotation
Previous studies have argued that the contribution of
TrA to trunk movement is minimal due to its horizontal
fascicle orientation and poor mechanical advantage for
torque generation [36]. However, variation in fascicle
orientation of TrA has been documented, with the fas-
cicles of the lower and middle regions being oriented
inferomedially and those of the upper region horizon-
tally [43]. Although these findings suggest that TrA may
have a minor role in torque production, the horizontal
upper fascicles of TrA were recruited during ipsilateral
rotation, while the inferomedial lower and middle fas-
cicles, with a greater mechanical advantage for this
movement, were relatively silent.
There are several possibilities for the function of
activity of different regions of TrA during rotation. For
instance, the lower and middle fascicles may stabilise the
anterior aponeuroses and linea alba against the superior,
lateral pull of contralateral OE, and the upper fascicles
may oppose the inferolateral pull of contralateral lower
and middle OI. This would provide a stable platform
from which OE and OI can generate rotatory forces.
Alternatively, due to the limited mechanical advantage
of TrA it may be hypothesised that during rotation TrA
controls motion of the rib cage, lumbar spine and/or
sacroiliac joints via tensioning of its musculofascial
attachments [4, 44] and/or generation of intra-abdomi-
nal pressure [20]. However, further investigation of these
hypotheses is required.
Conclusion
This study indicates that TrA is recruited during rota-
tion of the trunk, with activity of the upper region
opposing that of the lower and middle regions. The
results have implications for biomechanical modelling of
the lumbar spine and pelvis, indicating the importance
of using multi-vector methods to represent regions of the
abdominal muscles, and for future studies that investi-
gate the effect of spinal pain on abdominal muscle
recruitment during trunk rotation.
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