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Journal of Electromyography and Kinesiology xxx (2007) xxx–xxx
www.elsevier.com/locate/jelekin
Activity of deep abdominal muscles increases during submaximal
flexion and extension efforts but antagonist
co-contraction remains unchanged
Donna T. McCook, Bill Vicenzino, Paul W. Hodges
*
NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Science,
University of Queensland, Brisbane, Qld 4072, Australia
Received 25 May 2007; received in revised form 1 November 2007; accepted 2 November 2007
Abstract
Lumbo-pelvic stability relies, amongst other factors, on co-contraction of the lumbo-pelvic muscles. However, during submaximal
trunk flexion and extension efforts, co-contraction of antagonist muscles is limited. It was predicted that activity of the deeper lumbo-pelvic muscles that are often excluded from analysis (transversus abdominis (TrA) and the deep fascicles of multifidus (DM)), would
increase with load in each direction. In eleven healthy subjects, electromyographic activity (EMG) was recorded from eight trunk muscles
using surface and fine-wire electrodes. Subjects performed isometric flexion and extension efforts to submaximal loads of 50, 100, 150 and
200 N and a maximal voluntary contraction (MVC). Loading tasks were then repeated in trials in which subjects knew that the load
would release at an unpredictable time. Compared to the starting position, EMG of all muscles, except DM, increased during MVC
efforts in both directions. During the flexion and extension submaximal tasks, there was no increased co-contraction of antagonist muscles. However, TrA EMG increased in both directions. In the unpredictable trials, EMG of all lumbo-pelvic muscles except TrA was
decreased. These findings provide further support for a contribution of TrA to lumbo-pelvic stability. In submaximal tasks, TrA activation may enhance stability as a strategy to improve trunk stiffness without requiring a concurrent increase in activity of the larger torque producing trunk muscles.
2007 Elsevier Ltd. All rights reserved.
Keywords: Co-contraction; Trunk muscles; Lumbar spine; Stability
1. Introduction
Co-contraction of the lumbo-pelvic muscles is required
even in neutral upright postures because the passive structures of the spine are inadequate to maintain stability of
the lumbar spine (Cholewicki and McGill, 1996; Crisco
and Panjabi, 1991; Gardner-Morse et al., 1995; Granata
and Marras, 1995). Modelling studies predict that the level
of co-contraction depends on the demands for spinal control, which are influenced by factors such as the nature
and magnitude of applied loads and the stability of the task
(Granata et al., 2005). However, co-contraction of antago*
Corresponding author. Tel./fax: +61 7 3365 4567.
E-mail address: [email protected] (P.W. Hodges).
nist muscles are not generally observed in tasks that involve
submaximal isometric or isotonic flexion or extension
efforts (Bartelink, 1957; Cresswell and Thorstensson,
1989; Granata and Marras, 1995; Granata et al., 2005; Silfies et al., 2005). It is unclear whether this suggests that no
increase in co-contraction is required during isometric flexion or extension efforts, or whether the data are explained
by lack of consideration of the redundancy in the motor
system (i.e. many muscles are capable of contributing to
spinal stability (Cholewicki and Van Vliet, 2002)) including
the possible contribution of deeper trunk muscles. These
muscles are often excluded from analysis of trunk muscle
function.
It is possible, if not likely, that the strategy for increased
muscle activity with increased demand for stability may
1050-6411/$ - see front matter 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jelekin.2007.11.002
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differ between tasks and individuals. Consistent with this
proposal, recent data suggest that the stability of the spine
is increased in individuals with low back pain (Van Dieen
et al., 2003), but with considerable variation in muscle activation strategy (Hodges et al., 2006). Furthermore, despite
evidence that the deeper muscles of the lumbo-pelvic region
(e.g. transversus abdominis, lumbar multifidus) provide a
contribution to the control of the spine (Barker et al.,
2006; Hodges et al., 2003; Kaigle et al., 1995; Wilke
et al., 1995) and pelvis (Richardson et al., 2002), few studies have considered the contribution of these muscles in
progressive spinal loading (Cresswell, 1993). Activity of
these deeper muscles with increased load may preclude or
compliment increased co-contraction of the more superficial trunk muscles.
This study aimed to investigate the activity of the deep
and superficial trunk muscles during submaximal and maximal flexion and extension efforts. The influence of reduced
task predictability on the activation of deep and superficial
trunk muscles was also examined.
2. Methods
2.1. Subjects
Eleven males with a mean (SD) age, height and weight of 27 (7)
years, 1.8 (0.9) m, 75.5 (10.5) kg, respectively, participated in the
study. Subjects were excluded if they had any prior history of low
back pain that was sufficient to require medical intervention or
absence from their full time activities (work or study). Additional
exclusion criteria included any major orthopaedic, neurological or
cardiorespiratory disorders. Subjects were recruited via local
advertising on notice boards and university web-based notices.
Previous studies have reported differences in electromyographic
activity (EMG) of the trunk muscles between males and females.
This has been attributed to physical variations in body weight,
centre of mass, muscle stiffness, muscle mass and skin impedance
(Granata and Orishimo, 2001). For the purposes of this study male
subjects were nominated for subject homogeneity; however, a
subsequent study may be warranted to determine if there are any
differences between genders for this specific task. The study was
approved by the Institutional Medical Research Ethics Committee
and was conducted in accordance with the Declaration of Helsinki.
2.2. Electromyography
EMG recordings were made from eight trunk muscles using a
combination of surface and fine-wire electrodes. Disposable Ag/
AgCl surface electrodes were placed on the right side over the
thoracic erector spinae (TES: 5 cm lateral to T9 spinous process);
the lumbar erector spinae (LES: 3 cm lateral to L3 spinous process); rectus abdominis (RA: 3 cm lateral to the umbilicus); lower
abdominal wall, recording from obliquus internus and TrA (LA:
midway between the anterior superior iliac spine and symphysis
pubis, above the inguinal ligament); and upper abdominal wall
measuring contributions from all of the abdominal muscle layers
(UA: lateral abdominal wall over the 10th rib and immediately
inferior to this) (Ng et al., 1998).
Recordings of EMG of transversus abdominis (TrA) and the
superficial and deep fibres of the multifidus muscle (SM and DM),
respectively, which have been shown to be differentially active in
response to predictable and unpredictable perturbations (Moseley
et al., 2002, 2003) were made using bipolar fine-wire intramuscular electrodes fabricated from two strands of Teflon-coated
stainless steel wire (75 lm, A-Systems, USA). Wires were threaded through a hypodermic needle (0.7 · 32 mm, 0.7 · 38 mm or
0.7 · 50 mm), 1 mm of Teflon was removed from the end (to limit
cross-talk) and the tips were bent back at 1 and 3 mm to form
hooks. Electrodes were inserted into the right TrA (midway
between the ribcage and the iliac crest) (Hodges et al., 1999), and
right multifidus (Haig et al., 1991) to record from the DM and SM
independently with electrodes inserted under the guidance of
ultrasound imaging (Moseley et al., 2002). The ground electrode
was placed over the lateral aspect of the right iliac crest.
EMG data were amplified 2000 times, band-pass filtered
between 20 Hz and 1 kHz, and sampled at 2 kHz using Spike2
software and Power 1401 Data acquisition system (Cambridge
Electronic Design, Cambridge, UK). Data were exported for
analysis using Matlab 6.5 (The Mathworks, USA).
2.3. Procedure
Subjects were positioned in a semi-seated position on a tilted
seat in a custom-built frame (Cholewicki et al., 2000) (Fig. 1). The
position allowed subjects to maintain a natural lordosis while
reducing the contribution of the hip and lower limb muscles to
force generation. The lumbo-pelvic control system has considerable redundancy. That is, there are multiple ways to generate
trunk force in standing with contributions possible from both the
lower limb and trunk muscles. One advantage of the semi-seated
position with the pelvis fixed was that it limits one of the sources
of variation. A second advantage of this task was that it maintained the spine in its mid position (i.e., gentle lumbar lordosis)
reducing the contribution of passive elements to spinal control.
Although not necessarily functional, it is argued that this task
allows a focused evaluation of neural strategies to control the
spine, with less inherent variability in the redundant lumbo-pelvic
system.
A harness attached to an adjustable light rigid frame was
placed over the subject’s shoulders and tensioned to comfort. The
frame was positioned such that cables for application of load were
attached to the harness at the T9 level. The cable was adjusted to
run horizontally to a pulley and then vertically via a load cell to
an electromagnet at the base of the frame. Subjects either isometrically flexed (posterior load) or extended (anterior load) the
trunk against the resistance of the cable to four different force
levels (50, 100, 150 and 200 N), consistent with forces used by
other authors (Cholewicki et al., 2000, 2005; Reeves et al., 2005;
Stokes et al., 2006; Vera-Garcia et al., 2006). Force output and
target forces were shown to the subject on an oscilloscope. In the
first set of trials, the electromagnet was energised throughout the
trial so that the cable was not released (predictable trials). Subjects were instructed to steadily increase the force to the target
level, maintain the force for 4–5 s and then release slowly. In the
second set of trials, the magnet was released at an unexpected time
during the force matching task (non predictable trials) after the
force had been maintained for 3–8 s. Although subjects anticipated that the load would be released, they could not predict its
timing. Data was obtained up to the point of release and not
following. For both the predictable and non-predictable flexion
and extension trials, three repetitions of each of the four target
loads were presented in a randomised order. During each set of
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Fig. 1. Experimental frame and subject position. The custom made loading device stabilized the pelvis and legs during the load applications. The load was
attached to the trunk by a cable extending from the electromagnet, via the load cell, to the shoulder harness. Directions of force could be alternated by
varying the electromagnet attachment between the front and back of the loading frame.
repetitions (n = 12), subjects rested for 5–10 s between each repetition. Subjects also rested for 5 min between the four trials.
Baseline EMG was recorded prior to the commencement of each
task. Following completion of the submaximal loading trials three
isometric MVC efforts were performed into flexion and extension
with verbal encouragement to optimise contraction potential. The
average difference from the mean for the MVC efforts was 6% for
isometric flexion and 5% for isometric extension efforts.
2.4. Data analysis
Root mean square (RMS) EMG amplitude was calculated for
2 or 3 s during the isometric contractions for all loading conditions. Baseline EMG amplitudes were also calculated for 3 s prior
to onset of force increase. EMG data were normalised as a percentage of the greatest activity recorded across the trials (generally during the MVC). Any data that deviated by more than 2 SD
from the mean data were excluded as outlying data. This was
generally due to the presence of a movement artefact. This was
necessary in less than 5% of trials.
EMG data were compared between conditions (predictable
versus non-predictable), directions (posterior load versus anterior
load) and load levels (0, 50, 100, 150, 200 N and MVC) using
repeated measures ANOVA. Post hoc analysis was undertaken
with Duncan’s Multiple Range test. Significance was set at
P < 0.05.
In order to take into account the individual variations in
strategy between subjects during the submaximal loading tasks,
the net amount of the normalised EMG amplitude of the trunk
flexor and extensor muscles (excluding the deep muscles that have
minimal torque generation capacity) was calculated. This analysis
results in a single value for flexor and extensor activity for each
subject that could be compared to the baseline level of activity,
thus accounting for individual differences in strategy. Although
this analysis did not take into account different muscle mass and
moment arms, it allowed the estimation of net changes in agonist
and antagonist muscle activity despite any variation in individual
muscles between subjects. For this analysis the net change in
EMG amplitude from the baseline taken in the upright starting
position was compared to zero (i.e. no change) with t-tests for
single samples (P < 0.05). Net EMG amplitude of the flexor and
extensor muscles was compared between the predictable and
unpredictable trials with paired t-tests.
3. Results
3.1. Predictable isometric trunk flexion and extension tasks
When subjects isometrically flexed the trunk to the target flexion forces, EMG of all abdominal muscles (RA,
UA, LA and TrA) increased progressively from the baseline EMG amplitude that was recorded in the upright starting position (Force · Direction, P < 0.0001). UA was the
only muscle with increased EMG at the 50 N interval
(P = 0.03). Activity of other muscles was increased at the
100, 150 and 200 N increments (all: P < 0.03) (Fig. 2).
There was no change in EMG from the baseline amplitude
for any of the individual antagonist paraspinal muscles
(SM, DM, LES, TES) at any of the four loads for the isometric flexion tasks (all: P > 0.15) (Fig. 3), and despite individual variability in responses, there was no significant
change in the net EMG of the paraspinal muscles (DM
excluded) at any of the load intervals (50 N, P = 0.059;
all other increments P > 0.074) (Fig. 4). In contrast to the
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Fig. 2. Mean EMG amplitude for the abdominal and paraspinal muscles is shown at all force levels during isometric extension and isometric flexion tasks
in predictable (circle) and non predictable (square) loading trials. Filled symbols indicate force intervals at which activity was increased above or decreased
below the baseline (P < 0.05). Data were normalised to the MVC effort. Mean and standard error of the mean are shown. * – P < 0.05 for the difference
between the level of activity in the predictable and unpredictable trials for the same load.
submaximal tasks, during MVC flexion efforts, EMG of
the antagonist paraspinal muscles SM, LES and TES
increased (all: P < 0.04). However, similar to the low load
tasks, there was no change in DM EMG (P = 0.225).
During isometric extension efforts, EMG of DM, SM,
and LES increased progressively at all 4 force increments:
50, 100, 150 and 200 N (all: P < 0.006) (Figs. 2 and 3).
TES EMG increased with loads of 100 N and above
(P < 0.034) but there was no change at 50 N (P = 0.51).
EMG of the antagonist flexor muscles (RA, UA and LA)
did not increase above baseline at any of the loading increments (all: P > 0.53) (Fig. 3). There was also no change in
the net EMG of the abdominal muscles (TrA excluded)
(200 N, P = 0.054; all other increments P > 0.15) (Fig. 4).
However, during maximal extension efforts, EMG of all
flexor muscles increased above baseline to co-contract with
the extensor muscles (all: P < 0.02) (Fig. 2).
TrA EMG behaved differently to the other abdominal
muscles during extension efforts with increased EMG during trunk extension efforts of 150 and 200 N (both:
P < 0.028) (Figs. 2 and 3). Thus, TrA EMG was increased
in both directions of load. Although the amplitude of TrA
EMG was greater during isometric flexion than extension
at the 200 and 50 N intervals (P < 0.033) it did not differ
between directions at the 100 and 150 N loads (P > 0.25)
or during the MVC efforts (P > 0.53) (Figs. 2 and 3).
Consistent with the limited co-contraction observed in
the submaximal loading task, EMG of UA, LA, SM,
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DM and LES differed between directions at all loads
(P < 0.011). RA and TES EMG differed between directions
at loads of 100 N or more (P < 0.011) but not at 50 N (RA:
P = 0.153, TES: P = 0.22). During the maximal efforts,
EMG of all muscles, except TrA, differed between directions (all: P < 0.03) (Fig. 2).
3.2. Comparison between predictable and non-predictable
trials
Fig. 3. Raw EMG of all muscles for a representative subject for the
isometric flexion (top) and isometric extension (bottom) trials for all
submaximal loads. Note that for this participant there is also a progressive
activation of TES in the flexion trials as the load increments increased.
This was not typical of the group. EMG calibration 250 lV.
Fig. 4. Net EMG of the abdominal muscles (sum of RA, UA and LA
normalised EMG) and the paraspinal muscles (sum of LES, SM, TES
normalised EMG) is shown at all force levels for the isometric extension
(left) and isometric flexion (right) loads for the predictable loading trials.
The mean and standard error of the mean (SEM) are shown.
When subjects anticipated that the load would be
released at an unpredictable time during sustained isometric flexion (i.e. non-predictable trials), EMG of the abdominal muscles was altered compared to that recorded when
subjects matched an identical load during the predictable
trials without release of the load (Force · Direction · Prediction · Muscle: P = 0.05). When the release of the posterior load of 200 N was anticipated (i.e. the non-predictable
isometric flexion trials), EMG of UA, RA and LA was less
than that identified in the predictable trials at the same load
(P < 0.0001 for all muscles). UA EMG was also less in the
unpredictable condition at the 100 N load (P = 0.048). TrA
EMG did not differ between the predictable and non-predictable trials. However, there was a tendency for TrA
EMG to be greater when there was reduced predictability,
particularly at the 150 N (P = 0.069) and 200 N increments
(P = 0.058) (Fig. 2). In the same unpredictable isometric
flexion tasks, activity of the antagonist paraspinal muscles
TES and SM was decreased at the 50 N load (TES by
2.9%, SM by 2.5% (both: P < 0.045)). There was no significant change in EMG for any of the individual paraspinal
muscles at any of the other loads (P > 0.109). There was
a net reduction in activity of the paraspinal muscles (DM
excluded) at the 50 and 100 N loads (P < 0.023) but no
change was found at the higher load tasks (P > 0.20).
Although this reduction differed from the predictable trials
during which there was no reduction in net paraspinal
EMG, there was no difference in net amplitude between
these conditions (all: P > 0.28).
In trials in which the load was released from the front (i.e.,
the non-predictable isometric extension trials), compared to
the predictable isometric extension trials, SM EMG was
reduced at all force increments (P > 0.0011), DM EMG
was less at 100 and 200 N (P < 0.034), LES EMG was less
at 200 N (P < 0.037), but TES EMG was unchanged
(P > 0.155) (Fig. 2). For the same non-predictable extension
task, EMG of TrA and the individual antagonist muscles
(RA, UA and LA) was not different to that measured in
the predictable trials at any of the submaximal load intervals
(all: P > 0.34). There was also no change in the net EMG of
the antagonist abdominal muscles (TrA excluded) between
the predictable and non predictable trials (all: P > 0.07).
4. Discussion
Results of this study demonstrate that although co-contraction of antagonist muscles is increased during maximal
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flexion and extension efforts, antagonist co-contraction
does not increase (above that recorded in the upright sitting
position) during submaximal efforts of the magnitude used
here or when predictability of the task is reduced. In contrast, in both task directions, the EMG of TrA increased
at the higher load increments and did not decrease when
task predictability was reduced. These data are consistent
with the proposal that TrA contributes to stabilisation of
the spine.
4.1. Effect of load level on co-contraction
Consistent with previous data, low level tonic EMG of
the flexor and extensor muscles was evident to maintain
the mechanical stability of the spine in the upright neutral
posture (Cholewicki et al., 1997). During maximal efforts,
co-contraction was enhanced with increased EMG of all
muscles during both isometric flexion and extension tasks,
except for DM which did not increase during flexion tasks.
This increased co-contraction during strong efforts has
been shown to be necessary to maintain intervertebral stability (Cholewicki et al., 1997; Granata and Marras, 1995,
2000; Stokes, 2005).
Despite the challenge to stability of the spine imposed by
the progressive increase in loading (Bergmark, 1989; Cholewicki and McGill, 1996; Cholewicki et al., 1997; GardnerMorse et al., 1995; Thelen et al., 1995), EMG of muscles
that were antagonists to those generating the force (i.e.
the abdominal muscles during the trunk extension task,
and paraspinal muscles during the flexion tasks) was not
increased during the submaximal flexion and extension
efforts in this task. Comparison of the net EMG response
suggests that there was no increase in antagonist co-contraction that may have been obscured from the analysis
of individual muscles by differences in strategy between
subjects. In contrast, analysis of the net EMG amplitude
of flexor and extensor muscles showed that the activity of
muscles that were antagonist to the force direction could
even be decreased relative to that recorded in the initial
upright starting posture in some individuals.
Other studies that have investigated submaximal lifting
(Daggfeldt and Thorstensson, 1997; El-Rich et al., 2004;
Granata and Marras, 1995; Ross et al., 1993), dynamic
flexion and extension (Silfies et al., 2005) or isometric tasks
(El-Rich et al., 2004) have also reported no or minimal
change in co-contraction despite increases in load. In contrast, co-contraction has been shown to increase when load
is elevated by increased movement velocity (Bartelink,
1957; Floyd and Silver, 1950, 1955; Granata and Marras,
1995; Hemborg et al., 1983; Thelen et al., 1995), increased
asymmetry of the load (Huang et al., 2001; McGill, 1991;
Pope et al., 1986), positioning the load away from the body
(Granata and Orishimo, 2001) and during vertical loading
of the trunk (Cholewicki et al., 1997). These findings suggest that increases in co-contraction may be related to the
task complexity, the loading site and loading velocity, but
is not necessarily required for all tasks.
Unlike the activity of the lumbo-pelvic flexor and extensor muscles, activity of TrA was increased when submaximal force was generated in both directions. Taken
together these data suggest that in tasks involving submaximal flexion or extension torque, either: the demands for
stability are not increased; the amount of activity present
in the lumbo-pelvic muscles during upright sitting is sufficient to meet the demands of lumbo-pelvic stability during
submaximal efforts; or the increase in TrA EMG may be
sufficient to maintain stability and there may be some benefit to the nervous system for selection of this strategy. As
TrA does not have a significant moment arm for flexion or
extension (McGill et al., 1988; Urquhart and Hodges,
2005) activity of this muscle may provide an optimal strategy to enhance stability during flexion and extension efforts
(Hodges et al., 2003, 2005).
4.2. Effect of reduced predictability
Consistent with previous studies, it was anticipated that
co-contraction would increase when the predictability of
the task decreased (Cholewicki and McGill, 1996; Granata
and Orishimo, 2001). When a perturbation is applied to the
trunk by unexpectedly adding a load there is increased cocontraction prior to the time of loading in order to optimise the control of the impending event (Andersen et al.,
2004; Chiang and Potvin, 2001; Cresswell et al., 1994; Granata and Orishimo, 2001; Krajcarski et al., 1999; Lavender
and Marras, 1995; Lavender et al., 1989). Increased co-contraction stiffens the spine and reduces the displacement
induced by the perturbation (Janevic et al., 1991; Lavender
et al., 1989; Marras et al., 1987). Furthermore, the amplitude of any additional muscle activity in response to the
perturbation is decreased (Vera-Garcia et al., 2006).
However, a different strategy was observed. When the
perturbation of the trunk was generated by a sudden reduction of the load (i.e., unloading of muscle contraction) at
an unpredictable time, there was no increase in co-contraction in advance of the perturbation. Instead there was
reduced activity of the individual lumbo-pelvic agonist
muscles (muscles generating force against the load prior
to perturbation) and, in some cases, also reduced activity
of the antagonist muscles.
Although agonist activity has been shown to be reduced
immediately prior to predictable unloading of the trunk
(Aruin et al., 1998; Brown et al., 2003) and limbs (Hugon
et al., 1982), little change in agonist muscle activity has
been observed in other studies (Brown et al., 2003) with
unpredictable unloading. However, Brown et al. (2003)
used lighter loads (less than 100 N) and found no change
in LES or TES EMG. In the current study, the activity
of LES was significantly decreased at higher loads
(200 N). Furthermore, activity of DM and SM was reduced
during the extension tasks at several loads and these
muscles were not recorded by Brown et al. (2003).
Although increased co-contraction may be predicted to
be an ideal strategy to prepare the spine for the impending
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perturbation to a sudden loading task, there are a number
of reasons why this may not occur in preparation for a
rapid unloading of the trunk muscles.
In unloading tasks, regardless of the amplitude of cocontraction that preceded the load release, the position of
the trunk to the upright state cannot be restored until the
activity of the agonist muscles is reduced and/or the activity
of the antagonist muscles are rapidly increased. This contrasts with the response to a suddenly applied load, which
requires a rapid increase in activity of the muscles that
oppose the newly applied load (Cresswell et al., 1994; Stokes
et al., 2000; Thomas et al., 1998). Preliminary data comparing a load release with the trunk stiffened by co-contraction
and to a load release with the trunk muscles relaxed prior to
unloading suggest that the acceleration, displacement and
rate of recovery profiles are distinctly different for the two
states. The functional ramifications of either strategy are
currently unclear and warrant further investigation.
4.3. Activation of the deep lumbo-pelvic muscles during
flexion and extension tasks
Activity of TrA responded to changes in load direction
and magnitude in a manner that was different to other lumbo-pelvic muscles. TrA EMG increased in both directions
of loading during submaximal and MVC efforts. This is
consistent with previous studies (Cresswell et al., 1992) in
which subjects generated flexion and extension torques in
side lying. Similarly, TrA activity is not affected by force
direction during arm movements in different directions
(Hodges et al., 1999; Hodges and Richardson, 1996,
1997). These data suggest that TrA may provide a contribution to spinal control when challenged by forces over a
range of directions.
In the present study, TrA was also the only lumbo-pelvic
muscle with no reduction of EMG amplitude during tasks
with unpredictable unloading. As this muscle has minimal
ability to generate torque (Hodges et al., 2005; McGill
et al., 1988; Urquhart and Hodges, 2005) but potential to
contribute to spinal stability (see below), this may provide
an ideal strategy to optimise control in this task. These
findings support the suggestion that TrA provides a contribution to stabilisation of the spine in a manner that differs
from the other lumbo-pelvic muscles (Hodges et al., 2003).
TrA has the capacity to increase spinal stability via its contribution to IAP (Cresswell et al., 1994; Cresswell and
Thorstensson, 1994; Daggfeldt and Thorstensson, 1997;
Hodges, 1999), and via modulation of tension in the thoraco-lumbar fascia (Barker et al., 2006; Hodges et al., 2003,
2005; Tesh et al., 1987). Increased IAP, to which TrA is
closely related, decreases intervertebral motion (Hodges
et al., 2003) and increases stiffness of the spine (Hodges
et al., 2005). Increased tension in the thoraco-lumbar fascia
contributes to the control of intervertebral motion (Hodges
et al., 2003), counteracts flexion moments (Barker et al.,
2006) and controls shear in both directions (Guggenheimer
et al., 2006, unpublished data). Via these mechanisms,
7
increased TrA activity may contribute to the maintenance
of stability during both isometric flexion and extension
efforts. As TrA makes only a minor contribution to torque
production, activation of this muscle may provide a simple
solution to enhance spine stability without the need to control associated torques generated by the muscle.
DM activity is also argued to contribute to intervertebral stability (Kaigle et al., 1995; Moseley et al., 2003; Wilke et al., 1995) and EMG of this muscle has been reported
to be independent of the direction of force during rapid
arm movements (Moseley et al., 2002). However, contrary
to our prediction, DM EMG only increased during isometric extension tasks. In the flexion task, consistent with the
other posterior lumbo-pelvic muscles, there was negligible
change in activity. This finding suggests that either DM
EMG recorded in upright sitting was sufficient to maintain
stability, or activity of this muscle was not required to
maintain control during the isometric flexion efforts.
It is interesting to note that TES differed little between
directions and was not affected by predictability. With
the harness attached at T9, the task may have been more
of a lumbar extension task requiring less contribution from
the TES during the tasks – irrespective of predictability.
However, the possibility that the activity of TES was more
related to the maintenance of stability and the upright posture than generating torque also cannot be excluded.
5. Conclusion
This study shows that healthy individuals do not
increase co-contraction of antagonist muscles during submaximal flexion and extension efforts beyond that which
is required in the upright posture. Instead, activity of the
deep muscle TrA increased with torque generation in both
directions, potentially providing an alternative strategy to
simple co-contraction of antagonist muscles to increase
spine stability during this task.
Acknowledgements
The authors thank David MacDonald, Michel Coppieters and Henry Tsao for assistance with data collection.
Financial support was provided by the University of
Queensland and Paul Hodges was supported by the
National Health and Medical Research Council of
Australia.
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Donna T. McCook MHSci, PGD (Manipulative
Physio), GD (Clinically Applied Exercise Science) is a lecturer in the Physiotherapy Division
at the University of Queensland where she
teaches undergraduate and postgraduate courses in musculoskeletal physiotherapy and exercise
prescription. She
completed
her
undergraduate degree in Physiotherapy (1990),
and her Post-Graduate Diploma in Manipulative Physiotherapy and Masters in Health Science (Physiotherapy) (2000) at Auckland
University of Technology (New Zealand). In
2003, she completed a Post-Graduate Diploma in Clinically Applied
9
Exercise Science (Human Movement Studies) at the University of
Queensland (2003). Currently, she is a PhD student at the University of
Queensland with the Centre for Clinical Research Excellence in Spinal
Pain, Injury and Health. Her current doctorate research is investigating
the muscular contribution to spinal stability and motor control in people
with and without low back pain and the neurophysiological effect that
spinal manipulation may have on the motor system.
Bill Vicenzino is a Professor in Sports Physiotherapy and Head of Division of Physiotherapy
in the School of Health and Rehabilitation
Sciences at The University of Queensland,
Australia. Initially graduating with a Bachelor
of Physiotherapy (1980) from the University of
Queensland, he subsequently completed a
Graduate Diploma in Sports Physiotherapy
and Masters in Science from Curtin University
before completing his PhD (2000). He leads the
Musculoskeletal Pain and Injury Research Unit
and his research focus is on the clinical efficacy
and underlying mechanisms of a range of physical therapies, such as,
manipulation, exercise, taping and foot orthoses.
Paul W. Hodges PhD, MedDr, BPhty (Hons) is
a Principal Research Fellow (National Health
and Medical Research Council) at the University of Queensland where he heads the Centre
for Clinical Research Excellence in Spinal Pain,
Injury and Health. He has doctorates in both
physiotherapy and neuroscience and his work
blends neurophysiological and biomechanical
methods to study the control of movement and
stability of the spine and limbs and how this
changes in pain. He was awarded the 2006
ISSLS Prize for spinal research from the
International Society for the Study of the Lumbar Spine. His primary
research interests include investigation the relationship between pain and
motor control; the coordination of the multiple functions of the trunk
muscles; pain rehabilitation; and the biomechanical mechanisms for control of the spine.
Please cite this article in press as: McCook DT et al., Activity of deep abdominal muscles increases during submaximal ..., J Electromyogr Kinesiol (2007), doi:10.1016/j.jelekin.2007.11.002