Discussion Board 2

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RIGINAL ARTICLE

uantification of Lumbar Stability by Using 2 Different bdominal Activation Strategies

ylvain G. Grenier, PhD, Stuart M. McGill, PhD

i i w c v h u m p e s b b g t i

c d a d r t m t a m e w c k a s

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ABSTRACT. Grenier SG, McGill SM. Quantification of umbar stability by using 2 different abdominal activation trategies. Arch Phys Med Rehabil 2007;88:54-62.

Objective: To determine whether the abdominal hollowing echnique is more effective for lumbar spine stabilization than full abdominal muscle cocontraction. Design: Within-subject, repeated-measures analysis of vari-

nce was used to examine the effect of combining each of 4 oading conditions with either the hollow or brace condition on he dependent variables of stability and compression. A simu- ation was also conducted to assess the outcome of a person ctivating just the transversus abdominis during the hollow.

Setting: Laboratory. Participants: Eight healthy men (age range, 20�33y). Interventions: Electromyography and spine kinematics

ere recorded during an abdominal brace and a hollow while upporting either a bilateral or asymmetric weight in the hands.

Main Outcome Measures: Spine stability index and lumbar ompression were calculated.

Results: In the simulation “ideal case,” the brace technique mproved stability by 32%, with a 15% increase in lumbar ompression. The transversus abdominis contributed .14% of tability to the brace pattern with a less than 0.1% decrease in ompression.

Conclusions: Whatever the benefit underlying low-load ransversus abdominis activation training, it is unlikely to be echanical. There seems to be no mechanical rationale for

sing an abdominal hollow, or the transversus abdominis, to nhance stability. Bracing creates patterns that better enhance tability.

Key Words: Abdominal muscles; Back injuries; Low back ain; Motor skills; Rehabilitation; Spine. © 2007 by the American Congress of Rehabilitation Medi-

ine and the American Academy of Physical Medicine and ehabilitation

ITH A GOAL TO DECREASE low back pain (LBP), stabilizing exercises, along with modifications of other

aily activities, have been shown to be effective in randomized linical trials.1,2 However, the source of this effect is uncertain. his study assessed the influence of different abdominal acti- ation strategies on mechanical stability in the lumbar spine. umbar spine stability is an important issue, especially given

From the Spine Biomechanics Laboratory, Department of Kinesiology, Faculty of pplied Health Sciences, University of Waterloo, ON, Canada. Supported by the Natural Sciences and Engineering Research Council of Canada

grant no. RGPIN36516-98). No commercial party having a direct financial interest in the results of the research

upporting this article has or will confer a benefit upon the author(s) or upon any rganization with which the author(s) is/are associated. Reprint requests to Sylvain G. Grenier, PhD, Biomechanics, Ergonomics and

inesiology Laboratory, School of Human Kinetics, Laurentian University, Sudbury, N P3E 2C6, Canada, e-mail: [email protected].

s 0003-9993/07/8801-10692$32.00/0 doi:10.1016/j.apmr.2006.10.014

rch Phys Med Rehabil Vol 88, January 2007

ts potential link to mechanisms of injury and associated clin- cal efforts directed toward enhancing stability in patients.3 The ay in which patients activate their abdominal muscles is

entral to the stability theme. For example, findings that trans- ersus abdominis is recruited later, in some LBP sufferers, ave led to speculation that it is related to an unhealthy or nstable spine.4 The strategy to recruit the transversus abdo- inis, through the abdominal hollowing technique, has been

roposed as an effective way to increase stability.5 Richardson t al6 investigated the effect of bracing and hollowing on acroiliac joint laxity in a nonfunctional task. They found that oth improved stiffness but concluded that hollowing was etter. However, the muscle resting levels differed between roups. Until now, most of the supporting evidence, for the ransversus abdominis being an important contributor to stabil- ty, has been indirect and qualitative.

Motivation to focus on the transversus abdominis in the linic has been provided by Hodges and Richardson5 who ocumented delays of the transversus abdominis during rapid rm movements in some people who have a history of low back isorders, although this result has not been confirmed in recent eports.7-9 It has also been suggested9 that, although the direc- ion of limb movement does not affect preactivation time, the agnitude of the arm movement perturbation does. Although

his does not provide direct evidence linking the transversus bdominis to stability, it was suggested that this is evidence of otor control deficit in chronic back pain patients. Additional

vidence of other types of motor control deficiencies in those ith LBP include larger delays of onset in several torso mus-

les when the entire torso is moved quickly,10 inhibition of nee extensors,11 and perturbed gluteal patterns while walking nd inability to simultaneously breathe heavily and maintain pine stability.12

It appears that the transversus abdominis may be activated ndependently of other abdominal muscles at very low levels f challenge (1%�2% of maximum voluntary contraction MVC]).6 But at higher levels of activation, when people erform tasks requiring spine stability, the transversus abdo- inis has also been shown to be a synergist of internal

blique.13 Some clinical trials, testing the “stabilization exer- ise,” have shown success in addressing LBP.1,2 However, one of these studies have showed a direct link to the trans- ersus abdominis and the mechanism for efficacy is not known. or example, Vezina and Hubley-Kozey14 have measured ab- ominal wall activation levels during abdominal hollowing xercises and reported that none of their subjects were able to ctivate only the transversus abdominis. Furthermore, David- on and Hubley-Kozey15 showed that, as the demand of an xercise progression increased, the abdominal muscles con- erged, such that all muscles were activating to the same ercentage of maximum at the highest exercise level. A recent tudy16 suggests that hollowing and attempts to specifically ctivate the transversus abdominis reduced the efficacy of the xercise. One could argue that there is no quantitative evidence dentifying the transversus abdominis as an important stabi- izer, although our own clinical efficacy study17 has shown that

tabilization exercises are effective for patients with LBP.

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55ABDOMINAL ACTIVATION AND LUMBAR STABILITY, Grenier

evertheless, it is not possible to make legitimate claims as to he stabilizing role of any specific muscles, only that the eneral approach was effective.

uantification of Stability The definition of stability is a critical issue. Because clini-

ians generally do not have the technical capability to quantify tability, kinematic “indicators” of instability have evolved. In nd of itself, this is not a problem, except that these indicators an be disguised with appropriate muscle recruitment. Stabil- ty, as assessed in this study, is defined as the ability of the pinal column to survive an applied perturbation (known as uler column stability). If the work done (input energy or isturbance) is greater than the potential energy of the column energy stored in disks, ligaments, muscles, tendons), equilib- ium will not be regained. From a clinical perspective, there is subtle, but important, difference between excessive motion

nd instability; excessive motion does not imply instability, nly the potential for instability (for a detailed description, see

ig 1. Stability analysis. This chematic depicts the process eading up to the calculation f stability. Abbreviations: M, distribution moment; dL/ t, derivative of length with espect to time; EMG, electro- yography.

holewicki and McGill18). The model used in this experiment, w

hich quantifies stability in this mechanical sense, is fully escribed elsewhere,19 although an overview is provided here fig 1).

In the context of lumbar stability, the effect of the transver- us abdominis on the spine is still inconclusive. Isolated con- raction of the transversus abdominis beyond very low levels of ctivation has not been measured because all abdominal mus- les become involved at more functional levels of activation 5%�20%).15 An investigation of specific abdominal muscle ctivation strategies and individual muscles’ role in that strat- gy should provide insight to the clinical decision-making rocess. The hollowing technique has been developed as a ransversus abdominis motor pattern retraining technique by ull and Richardson20 to address the motor disturbances. It ppears that others have assumed that it should be used as a echnique to enhance spine stability. This article evaluates 2 bdominal activation strategies, hollowing and bracing, which re techniques used clinically to improve lumbar spine stabil- ty. Given the clinical issue stated previously and our previous

ork21,22 examining the stabilizing role of many torso muscles,

Arch Phys Med Rehabil Vol 88, January 2007

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56 ABDOMINAL ACTIVATION AND LUMBAR STABILITY, Grenier

A

e were motivated to quantify the mechanical impact of the 2 trategies on lumbar stability. It was therefore hypothesized hat the abdominal hollowing strategy would be inferior to that f a bracing strategy for enhancing stability. Spine stiffness is lways stabilizing (summarized in McGill23). The question otivating this study therefore is as follows: is the isolated

ransversus abdominis recruitment associated with the abdom- nal hollowing technique a more effective stabilizer than a full bdominal girdle cocontraction?

METHODS In the present study, the comprehensive lumbar spine model

sed to quantify stability19 was enhanced to include a repre- entation of transversus abdominis. Because pilot work showed hat none of our subjects could perform an ideal “hollow”6 (ie, ctivating only the transversus abdominis and internal oblique), imulations were also conducted to artificially activate the uscles in an “ideal” way. This was done, together with real,

n vivo data collection, with the understanding that subjects ay have had imperfect technique. In this way, we were able

o evaluate “perfect hollowing” and bracing, as well as the mperfect clinical reality.

ata Collection The study proceeded with 2 components in parallel. The first

as the in vivo data collection, and the second was the simu- ation. During the in vivo task, torso muscle electromyographic ctivity and spine kinematics were recorded from each subject. hese data were then input to the biomechanic model to de-

ermine spine stability. In the simulation, a single subject’s data ere modified to reflect the anatomic and electromyographic

hanges associated with an “ideal” hollow and brace. In this ay, several combinations of muscle activation were tested to

ssess their effect on stability. In vivo: subjects. Data were collected from 8 healthy (no

ack pain in the past year) men between the ages of 20 and 33. ll subjects provided informed consent, and the study was

pproved by the university ethics committee. Subjects had a ean height � standard deviation (SD) of 1.82�0.06m (range,

.73�1.88m), a mean weight of 79.8�11.5kg (range, 60.5� 3.6kg), and a mean age of 23.8�4.33 years (range, 20�33y). one had any experience with the transversus abdominis recruit- ent training. In vivo: kinematics. Spine kinematics were recorded with 3Space Isotrak unit,a which measured lumbar flexion and

xtension, lateral bend, and axial twist at a sample rate of 0Hz. The 3Space electromagnetic field source was strapped in lace over the sacrum, and a sensor was worn over the T12 ertebrae, isolating lumbar motion. The 3Space unit returns uler angles. These were adjusted for anatomic relevance to

he spine’s orthopedic axes. Flexion and extension corresponds o rotation about y, lateral bend to rotation about x, and twist o rotation about z.24

In vivo: electromyographic activity. Electromyographic ignals were obtained by using Ag-Ag/Cl Meditrace surface lectrodes,b in a bipolar configuration, spaced 25mm apart, in ine with muscle fiber directions.25 The signals were then mplifiedc (12-bit analog-to-digital conversion; sampling rate, 024Hz; frequency response, 10�1000Hz; common mode re- ection ratio, 115dB at 60Hz; input impedance, �10G�). As er the previously validated electrode positions,19 7 muscles on ach side of the body were recorded for a total of 14 muscles, ncluding the rectus abdominis (2cm lateral to the umbili- us), internal oblique (caudal to the anterior superior iliac

pine and medial to the inguinal ligament), external oblique a

rch Phys Med Rehabil Vol 88, January 2007

15cm lateral to the umbilicus), latissimus dorsi over the mus- le belly (15cm lateral to T9), thoracic erector spinae (5cm ateral to T9 over the muscle belly), lumbar erector spinae (3cm ateral to L3), and the multifidus (2cm lateral to L5, angled lightly with superior electrode more medial). The electromyo- raphic signals were normalized to the amplitudes measured uring the MVC procedure after rectification and low-pass filter- ng at 2.5Hz. In addition to preparing the electromyography for orce estimation, this process also removed electrocardiographic nterference.

MVCs, for normalization, were performed in 2 separate rials.26 First, the abdominal flexors were contracted maximally n a seated position, leaning backward at 45°. Then, the exten- ors were contracted maximally in a Biering-Sorensen posi- ion.27 In both cases, the experimenter provided sufficient re- istance for a maximal isometric contraction. Once this was one, the hollowing and bracing strategies were explained and emonstrated. In vivo: protocol. Subjects were asked to hollow their

bdomen without sucking in their belly. They were told that hey should be able to breathe normally. Assuming that the ransversus abdominis was synergistic in its activation with the nternal oblique,13 the subjects practiced until they were able to asily achieve the required internal oblique activation target of 0% of MVC, as displayed on an oscilloscope. This compares ith a range of approximately 12% of MVC of external oblique

ctivation in hollowing and 32% of MVC of external oblique ctivation in bracing measured by Richardson et al.6 A brace is ifferent from the hollow in several ways; the brace involves ctivation of all abdominal muscles to a level that increases orso stiffness. This does not mean that no motion can occur, nly that the motion is controlled. Furthermore, abdominal racing causes the back extensors to become active, which urther enhances spine stiffness. Initially, the electromyo- raphic patterns were verified with oscilloscope, and observa- ions of initial muscle contraction with ultrasound ensured that he activation patterns reproduced the technique reported in the iterature.6 The ultrasound probe was placed at the rectus order so that the 3 layers of the abdominal wall could be iewed. In the current study, there were considerable variations n the recruitment patterns used to achieve the hollowing. ollowing trials were collected when subjects could show, on

n oscilloscope, a decrease in external oblique and rectus bdominis activity, along with an increase in internal oblique ctivity, although this increase was often minimal. The testing as performed in an anatomically neutral standing posture, ith a 10-kg load in either or both hands, depending on the

ondition. Over a period of 25 seconds, subjects were then sked to relax for 5 seconds, “hollow” the abdomen for 5 econds, relax for 5, “brace” the abdomen for 5 seconds, and elax for the final 5 seconds. These trials were repeated, in andom fashion, 3 times each with (1) no load in the hands bilateral no lift), (2) with 10kg in each hand (bilateral lift), (3) 0kg in the right hand only, and finally with (4) 10kg in the left and only. Different load conditions were used to target the ffect of asymmetric activation, as well as to address the issue f increased intervertebral stiffness with higher compression oads. The kinematic and electromyographic data were then nput to the custom stability model. Note that, for this part of he experiment, because transversus abdominis activity was not easured, it was assumed to be a synergist of the anterior and

audal portions of internal oblique. Thus, the internal oblique ecording was used to drive the transversus abdominis in he model. Two studies have shown this to be a realistic

ssumption.13,28

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57ABDOMINAL ACTIVATION AND LUMBAR STABILITY, Grenier

ata-Collection Procedures Quantification of stability. Spine stability was calculated

y computing the potential energy of the spinal system. For the urpose of summing potential energy, the muscle and tendons ere considered as linear springs, whereas the intervertebral isks were considered as torsional springs. The elastic potential nergy was then summed in each rotational degree of freedom, nd the work done by the external load was then subtracted rom this. The second derivative of the potential energy matrix, f greater than or equal to 0, indicated a stable system.19 This ethod of quantifying stability has been applied to mechanical

tructures and validated repeatedly in civil and mechanical ngineering.29 Its application to the osteoligamentous spine as also been validated.30-32 Specific components of the model re described as follows.

Passive contribution to potential energy. The skeleton of he model consisted of 5 lumbar vertebrae between a rigid elvis and sacrum and a rigid ribcage. The vertebrae were inked by torsional springs representing the vertebral disks, hich generated passive stiffness and allowed rotation but no

ranslation (about 3 orthogonal axes). Three-dimensional lum- ar motion, measured with the 3Space Isotrak was proportion- lly distributed among all vertebrae.33 The stiffnesses, includ- ng stiffness of the disks, ligaments, fascia, skin, and viscera,34

ere also distributed among the vertebrae.19,33

Active contribution of potential energy. In addition to the estorative moment of the passive tissues, the muscles also ontributed a restorative moment to balance the external load fig 2). The muscle force and stiffness estimates were obtained rom a distribution-moment muscle model that used electro- yographic and muscle length as input.35 The muscle forces ere then applied through 118 muscle fascicles to the skeletal

omponents such that the moment they created balanced the oment generated by the external load. Stability calculations

sed all of these variables as input, together with the muscle tiffnesses calculated by the distribution-moment model.36

Modifications to the model. Anatomic improvements were ade to better represent the application of transversus abdo- inis forces on the spine. Specifically, the fascial attachment of

ig 2. A schematic representation of a simplified spine motion egment to show the concept of linear (muscles, ligaments, ten-

fi ons) and torsional (intervertebral disks) springs working to sustain n applied load.

he transversus abdominis on the lumbar vertebrae was repre- ented with 10 fascicles bilaterally on the 5 segments (2 orig- nating on the posterior tip of the lumbar spinous processes and he other 2 originating on the transverse process of the lumbar ertebrae). To capture the line of action of the fascial attach- ents on the posterior spinal processes, the 10 fascicles con-

erged on a point 60cm directly lateral of L5. This arrangement lso closely approximated the experimental findings of Tesh t al37 that the compression cosine of the lateral transversus bdominis force was 39% of its resultant magnitude. Because ransversus abdominis activity was not measured, the stiffness f the transversus abdominis was calculated by using internal blique activity magnitude because these have been shown to e synergistic. The length of the transversus abdominis fibers as calculated based on the real hoop-like architecture of the uscle. This architecture was intended to simply evaluate the

ffects of the muscles recognizing their attachment to the spine. here were no assumptions made regarding the role of intra- bdominal pressure to stiffen the abdominal wall.

Simulations. The objective of the simulation trials was to rtificially adjust abdominal muscle activity levels to imitate ideal” hollowing and bracing strategies because subjects could ot completely isolate, or preferentially activate, any single bdominal muscle to the extent required.

A total of 1 each of 5 types of simulations was performed.

. Simulation of the hollowing strategy: the transversus abdo- minis and internal oblique electromyographic signals were adjusted to an activity level at 20% of MVC, whereas the rectus abdominis and external oblique were adjusted to 2% of MVC. The activity levels in the back extensor muscles were simply those measured.

. Simulation of the bracing strategy: all abdominal electro- myographic signals were replaced by activity at 20% of MVC. The activity levels in the back extensor muscles were simply those measured.

. Simulation of the bracing strategy: the stabilizing effect of the transversus abdominis was evaluated, and isolated, with a sensitivity test in which transversus abdominis activation was zeroed during a bracing simulation (bracing with no transversus abdominis).

. Simulation of the bracing strategy: the procedure in simu- lation 3 was repeated by zeroing each abdominal muscle pair, similar to the muscle “knockout” approach of Cholewicki and VanVliet.38

A. A right and left external oblique set to 0% of MVC. B. A right and left internal oblique set to 0% of MVC. C. A right and left rectus abdominis set to 0% of MVC.

In addition to these trials, across conditions, a sensitivity nalysis was also conducted to assess the effect of the trans- ersus abdominis alone on stability. First, only the transversus bdominis was set to 0% of MVC. This was compared with the ondition in which all abdominal muscles were left untouched. hen, all abdominals except for the transversus abdominis ere set to 0, whereas the transversus abdominis was set to 0% of MVC and then to 100% of MVC so that the transversus bdominis was the only active muscle. In addition to modifying he muscle activity of selected muscles in all simulations, the oment arm of rectus abdominis (and consequently the attach- ents of internal and external oblique) was shortened by 5cm,

or the hollowing simulations, to mimic the “drawing in” of the bdomen (figs 3, 4). These artificially modified data were then nput to the spine stability model as described earlier and in

gure 1.

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58 ABDOMINAL ACTIVATION AND LUMBAR STABILITY, Grenier

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tatistical Analyses The dependent variables from the in vivo trials were spine

ompression and the stability index, which is a general indica- or of the stability of the equilibrium state of the spine.19

In vivo analysis. Using the R statistical package,39,d a ithin-subject, repeated-measures analysis of variance was erformed to examine the effect of combining each of 4 load- ng conditions with the hollow or brace condition on the ependent variables of stability and compression. In this study

ig 3. Because the moment is calculated as the product of force and erpendicular distance, moving the muscle attachment (A) closer to he joint (J) (as occurs during hollowing) shortens the moment arm f rectus abdominis. This has a large effect on the resulting poten- ial energy, and, consequently, stability as well. Stability of a col- mn with guy wires (or a spine with muscles) is a function of eometry (the location of the support attachments), stiffness, the balance” in stiffness in all directions, and column imperfections, uch as curvature. Hollowing reduces the potential energy and tability by narrowing the geometric base.

ig 4. In the hollowing simulation, the moment arm of rectus ab- ominis was reduced by 5cm (“B” in left panel), when compared

a ith the bracing condition (right panel), to account for the change

n anatomic geometry. “A” defines the relaxed condition.

rch Phys Med Rehabil Vol 88, January 2007

hen, the independent variable of “muscle activity pattern” had conditions, hollowing and bracing, whereas the second inde-

endent variable of “load type” had 4 conditions: (1) load in oth hands, (2) no load, (3) right-hand load, and (4) left-hand oad. The Tukey post hoc honestly significant difference test as used to investigate differences. Simulation analysis. Simulation trials were compared to

he reference bracing trial, with the “muscle knockout” model, s well as with the simulated hollowing condition by using a aired t test. The change in (dependent) compression and tability index between bracing and hollowing trials was ex- ressed as a percentage change.

% change � BRC � HLW

BRC � 100 (1)

here BRC is bracing and HLW is hollowing.

RESULTS Bracing stability was always greater than hollowing stabil-

ty, and asymmetric loads always produced greater stability han symmetric loads. In in vivo trials, stability differed sig- ificantly for hollowing and bracing conditions (bracing � ollowing, P�.001) and between the loading conditions P�.009) (table 1). Figure 5 shows a hollow-brace composite or 1 subject showing this difference. Compression values were ot different, either between hollowing and bracing (P�.54) or etween loading conditions (P�.095). One of the subject’s esults were excluded because the abdominal recruitment pat- erns were not consistent with what had been requested for the rials (ie, hollowing of the abdomen was not achieved with a 0% of MVC internal oblique activation). Post hoc testing evealed that spine stability, in the no-load condition, was less han both right- and left-side loads but not less than the bilateral oad condition.

The simulation data supported the in vivo data, showing that he hollowing was not as effective as bracing for increasing tability in the lumbar spine. In fact, bracing improved stability ver hollowing by 32%, with only a 15% increase in compres- ion. Figure 6 shows the difference in stability, whereas figure 7 hows the difference in compression for 1 in vivo subject. In able 2, 1 subject’s data were used for the simulation. The nmodified hollow data were compared, for that subject, with he brace data. The various simulations were also compared ith the full brace trial. When removing only the transversus

Table 1: Mean Stability and Compression Value From In Vivo Data Trials

Trial Mean Stability Index (Nm/rad)

Mean Compression (N) at L4-5

Hollowing: no load 474.6�85.4 1866.5�451.4 Hollowing: 2-hand load 495.6�70.8 1929.2�360.0 Hollowing: left-hand load 517.7�55.8 2003.0�264.6 Hollowing: right-hand load 533.4�57.7 2041.7�286.6 Bracing: no load 511.3�39.5 1911.0�189.0 Bracing: 2-hand load 527.3�59.8 1989.1�247.9 Bracing: left-hand load 555.0�70.1 2060.4�223.1 Bracing: right-hand load 546.1�59.7 2008.6�266.1

OTE. Values are mean � SD. In the case of stability, there were ifferences between muscle activity patterns (bracing � hollowing, �.001) and between the loading conditions (P�.009). The compres- ion values had no differences, either between loads or between ctivation types.

bdominis from the bracing pattern (see table 2, Simulation

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59ABDOMINAL ACTIVATION AND LUMBAR STABILITY, Grenier

racing: no transversus abdominis), stability decreased by 14% with a less than 0.1% decrease in compression. Further-

ore, the importance of the transversus abdominis relative to ther abdominal muscles in affecting stability was very small hen assessed by a “muscle-knockout” approach40 (P�.01)

fig 8).

DISCUSSION Is the abdominal hollowing technique and its specific trans-

ersus abdominis recruitment pattern a more effective stabi- izer than a full abdominal girdle cocontraction? These data uggest that it is not. The bracing strategy provided greater tability than hollowing in both the simulation and in vivo data. urthermore, for our subjects, the ability to activate just the

ransversus abdominis at functional levels was extremely chal- enging, if not impossible, as evidenced by all other abdomi- als’ electromyographic activity not being silent. This sug- ested that the attempt to hollow would, in effect, result in ome degree of bracing. The simulations showed that the ransversus abdominis had virtually no effect on stability. Al- hough Hodges et al41 have shown that the transversus abdo-

inis does contribute stiffness to the spine in a porcine model, ur sensitivity tests (see fig 8) show that its relative contribu-

ig 5. A composite of 2 trials of in vivo subject data (a 2-hand load rial chosen at random) shows that bracing had greater stability han an attempted hollowing. The ideal hollowing was not chieved. Ideal hollowing was tested in the simulations. Note the nitial overactivation when trying to establish the hollow.

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ig 6. Lumbar stability increased by a significant margin (32%) nder the simulated bracing condition. The hollow was conducted ith 20% activation in both the lateral oblique and the transversus

bdominis and 2% activation in both rectus abdominis and external blique. This was considered feasible after observing our subjects.

ost of the stability was obtained through the internal oblique. bbreviations: BRC, bracing; HLW, hollowing; Stb, Stability.

A a

ion is very small when compared with the other abdominal uscles. Assessment of this relative performance was absent

rom their experiment. Performing a hollowing pattern by sing just the transversus abdominis greatly reduced stability, s opposed to all other abdominal muscles being active (fig 9). n addition, the compression-stability ratio favored bracing. he fact that the bracing strategy was more effective at stabi-

izing than the hollowing strategy should not be interpreted as vidence to diminish abdominal hollowing as a tool for train- ng, or retraining the recruitment of the transversus abdominis rom a motor control perspective, because this muscle does orm a component of the abdominal girdle. However, many herapists and coaches, in our experience, recommend “draw- ng in” of the abdomen in an effort to increase stability. Based n our results, this appears to be misdirected. In fact, 2 studies ave recently concluded that focusing on general activity42 and onspecific exercise16 is more beneficial for both pain reduc- ion and functionality.

tudy Limitations Several limitations should be addressed to guide interpreta-

ion of these results. The transversus abdominis in the model

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ig 7. Although the external load component contributes to lumbar ompression equally among conditions, compression due to muscle ctivation differed between the simulation bracing and hollowing ondition by as much as 500N. Note that the hollowing here in- luded both the internal oblique and transversus abdominis as in gure 6. Abbreviation: Cmp, compression.

Table 2: One Subject’s Data Were Taken and Modified for the Simulation

Compared With Bracing Stability %

Change Compression

% Change

In vivo: no load hollowing 7.2 2.3 In vivo: 2-hand load hollowing 6.0 3.0 In vivo: left-hand load hollowing 6.7 2.8 In vivo: right-hand load hollowing 2.3 �1.7 Simulation hollowing 32.5 15.3 Simulation bracing: no TA .14 .00 Simulation bracing: no EO 16.5 11.0 Simulation bracing: no IO 32.5 12.7 Simulation bracing: no RA 12.6 10.6

OTE. The unmodified hollow data were compared, for that subject, o the brace data. The various simulations were also compared with he full brace trial. The percentage increase in stability of bracing ver hollow is clear, especially for the ideal simulation of bracing. ach condition was compared with pure bracing trial. Note that in he subject data, compression increased under right-hand load hol- owing conditions, although only by 1.7%.

bbreviations: EO, external oblique; IO, internal oblique; RA, rectus bdominis; TA, transversus abdominis.

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60 ABDOMINAL ACTIVATION AND LUMBAR STABILITY, Grenier

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as driven by the internal oblique activation profile because it as been shown to share a synergistic activation for neutral tatic postures, such as those tested in this study.13,43 The way n which the transversus abdominis was modeled was an at- empt to capture its force, moment, and stabilizing effects on he spine. The architecture of the transversus abdominis sug- ests a limited capability to influence lumbar stability on its wn. In contrast, the oblique vertical “wrap” of the internal blique, for example, has a much higher potential to directly tabilize and compress the spinal column. Also, although a ransversus abdominis muscle was modeled, the interaction etween the transversus abdominis and intra-abdominal pres- ure was not accounted for. Intra-abdominal pressure is linked o abdominal wall activity, which stiffens and stabilizes the umbar spine. However, at least 1 study44 suggests that a ollowing strategy is unlikely to generate a greater intra-ab- ominal pressure than full bracing. This being the case, if ressure does affect stability, even more stability would result hen bracing as compared with hollowing. The work of Tesh

t al45 contributed some insight as to the modeling of the ransversus abdominis, its link with abdominal pressure, and esistance to lateral bending moments in full flexion. The force enerated by our transversus abdominis equivalent was on the rder of 50N for a 20% MVC. They also report that intra- bdominal pressure may contribute as much as 40% of the estorative moment in lateral bending. Our own work shows hat intra-abdominal pressure contributes significant stiffness, specially in the neutral posture.44 We speculate that there may e a binding of the abdominal muscle layers, when contracted ogether, that creates more stiffness than the sum of the parts. f this is the case, then the transversus abdominis would be an mportant member of the “abdominal team” but no more im-

ig 8. Stability index in simulation trial. A single, 2-hand load trial as processed, with 4 sensitivity tests, to isolate the role of differ-

nt muscles. The bottom trace is actually 2 conditions using the ransversus abdominis as the only active abdominal muscle (at 00% and 20% of MVC). Whether the transversus abdominis is ctive at 20% or 100% of MVC had little effect and those 2 plots are uperimposed. The top trace is actually 2 conditions: (1) with all bdominal muscles active in a brace pattern (at whatever level the ubject had activated them) and (2) with only the transversus ab- ominis removed. Only 1 line is visible because removing the trans- ersus abdominis has almost no effect, and its plot overlays the full ctivation plot. The variability in the stability plot, including what tability is present, results from variable activity in the extensor and other modeled) muscles. Abbreviations: abs, abdominals; TrA, ransversus abdominis.

ortant than any of the other muscles. h H

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In this study, the stability response was measured only with ully anticipated loads in isometric, neutral postures, for sym- etric and asymmetric loads. It is possible that for a sudden

oad, the combination of a pressurization response and trans- ersus abdominis activation might maintain sufficient stability ntil the remainder of the torso muscles are recruited. How- ver, we remain skeptical of this possibility because the re- orted 30-ms delay for transversus abdominis onset is just a raction of the 130-ms electromechanical delay inherent in the orce production for trunk muscle.46 In other words, a loss of tability is not likely to occur within the window between 0ms, the time at which the transversus abdominis turns on, nd 130ms, the time at which it begins to produce force. It is herefore suggested that, although transversus abdominis onset elay5 may be statistically significant, it is not mechanically ignificant. In any case, recent work done with cocontraction of he abdominals and its effect on stability, supports our find- ngs.47 A general cocontraction of the abdominal wall, bal- nced against antagonists and the load, is most effective in ttaining and maintaining lumbar stability. There were consid- rable variations in the recruitment patterns, shown by subjects, sed to achieve a hollowing. This is normal and is an advantage f our modeling approach, which is sensitive to the individual ays people activate muscle. The simulation was able to re- roduce the ideal. Furthermore, this study was performed on oung healthy males with no LBP. Injury, LBP, or population ariability may result in different muscle recruitment patterns nd stability profiles. Again, this possible weakness in the n vivo data was addressed via the simulations in which muscle ctivation was under complete control of the experimenters to lucidate the differences between ideal techniques and in vivo eality.

The simulations were important, for interpretation of this tudy, in 2 ways. First, the difficulty that participants had in chieving a hollowing recruitment pattern necessitated some

ig 9. Stability index with abdominal muscles removed from sim- lation. This histogram shows the stability, for actual subjects, erforming the brace and an attempted hollow (shown at the far ight). Then, the right and left abdominal muscle pairs were re- oved from the analysis, each in turn, and the stability was recal-

ulated. Although the removal of the transversus abdominis made ittle difference to stability, internal oblique was the most important n affecting stability, followed by external oblique and finally by ectus abdominis. Abbreviations: Brc, normal brace; BrEO, brace ith external oblique; BrIO, brace with internal oblique; BrRA, brace ith rectus abdominis; BrTA, brace with transversus abdominis; lw, normal hollow; HwEO, hollow with external oblique; HwIO,

ollow with internal oblique; HwRA, hollow with rectus abdominis; wTA, hollow with transversus abdominis.

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61ABDOMINAL ACTIVATION AND LUMBAR STABILITY, Grenier

ay of testing the “ideal” hollowing pattern. Second, the sim- lations confirmed that, even if an ideal hollowing pattern was chieved, it would still fall short of the stability provided by racing. This is the case regardless of the fact that in this study he transversus abdominis was not measured; the simulations ssumed that it was optimally active. Recent work by Howarth t al48 showed that the magnitude of the stability index is roportional to the level of risk of becoming unstable (of aving the stability index go below 0). Nonetheless, it is still a elative measure so that it is not possible to compare the agnitude of the index between people. It should only be used

omparatively within a subject, and even then, only within a esting session. For this reason, the participants acted as their wn controls. Finally, this work examined Euler column sta- ility; it is possible that the abdominal muscles work differ- ntly to buttress shear or pelvic instability. The mechanical ffect on lumbar stability, of either the transversus abdominis r of hollowing, has not, to our knowledge, been reported in the iterature before this. It must be clearly stated that this is a echanical analysis of the role of the transversus abdominis

nd other abdominal muscles. Therefore, whatever role trans- ersus abdominis isolation training has in rehabilitation, it annot be explained by a mechanically based stability princi- le. If spinal stiffness is the ultimate goal in a training program, hen bracing of the abdominal muscles is a clear winner over ollowing. In reality, a single focus on either strategy may not e optimal (or even possible) for functional tasks that show a reat diversity in load and velocity.

CONCLUSIONS This biomechanically based assessment suggests that brac-

ng of the abdominal muscles provides greater lumbar spine tability than hollowing. According to our simulations, the otential of the transversus abdominis to enhance stability, on ts own, appears to be very limited. The inability to isolate the ransversus abdominis in a functional context may be a moot oint because in healthy men bracing increases spine stability ith minimal increase in spine compression loads. Muscles ther than the transversus abdominis contribute relatively more o avoiding an unstable spine. Whatever the benefit underlying ow-load transversus abdominis activation training, it is un- ikely to be mechanical. There seems to be no mechanical ationale for using stabilization exercises to enhance a hollow or stabilization purposes; rather a brace creates patterns that etter enhance stability.

Acknowledgment: We thank Jay Green for assistance in data ollection.

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Suppliers . Polhemus, 40 Hercules Dr, PO Box 560, Colchester, VT 05446. . Graphic Controls Corp, Medical Products Div, PO Box 1274,

Buffalo, NY 14240. . Bortec Biomedical, 239 Springborough Wy, Calgary, AB T3H

5M8, Canada. . R statistical package. Available at: http://www.R-project.org. Ac-

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