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SPINE Volume 29, Number 11, pp 125412652004, Lippincott Williams & Wilkins, Inc.

Determining the Stabilizing Role of Individual TorsoMuscles During Rehabilitation Exercises

Natasa Kavcic, MSc, Sylvain Grenier, PhD, and Stuart M. McGill, PhD

Study Design. A systematic biomechanical analysis

involving an artificial perturbation applied to individual

lumbar muscles in order to assess their potential stabiliz-

ing role.

Objectives. To identify which torso muscles stabilize

the spine during different loading conditions and to iden-

tify possible mechanisms of function.

Summary of Background Data. Stabilization exercises

are thought to train muscle patterns that ensure spine

stability; however, little quantification and no consensus

exists as to which muscles contribute to stability.

Methods. Spine kinematics, external forces, and 14

channels of torso electromyography were recorded for

seven stabilization exercises in order to capture the indi-

vidual motor control strategies adopted by different peo-

ple. Data were input into a detailed model of the lumbar

spine to quantify spine joint forces and stability. The EMG

signal for a particular muscle was replaced either unilat-

erally or bilaterally by a sinusoid, and the resultant

change in the stability index was quantified.

Results. A direction-dependent-stabilizing role was

noticed in the larger, multisegmental muscles, whereas a

specific subtle efficiency to generate stability was ob-

served for the smaller, intersegmental spinal muscles.

Conclusions. No single muscle dominated in the en-

hancement of spine stability, and their individual roles

were continuously changing across tasks. Clinically, if the

goal is to train for stability, enhancing motor patterns that

incorporate many muscles rather than targeting just a fewis justifiable. [Key words: lumbar spine, spine stability,

modeling, muscles] Spine 2004;29:12541265

While muscles function to create torques, which supportpostures and facilitate movement, they are also criticalfor ensuring spine stability.1 Clinically, the question ofhow to train lumbar spine stability requires knowledgeof how the various muscles contribute to ensuring stabil-ity. A common functional distinction used to classify therole of the different muscles is that intersegmental orlocal muscles are hypothesized to function primarily

as stabilizers and multisegmental or global muscles arehypothesized to function primarily as moment produc-

ers.24

This distinction, formalized by Professor AndersBergmark,2 focused the early discussions on stability;however, debate continues over which muscles are im-portant stabilizers and how to best train the neuromus-cular control system to ensure sufficient stability.

Researchers have used various techniques to investi-gate the question of which muscles stabilize the lumbarspine. Electromyographic analysis of torso muscle onsettimes to various perturbations has suggested that themore internal muscles, particularly the transverse abdo-minis and internal obliques, behave in an anticipatorymanner, irrespective of loading condition, suggesting a

proactive control of spine stability.5,6 Others have ob-served a wasting of the multifidus muscle on the side ofthe reported low back pain with MRI techniques,7 sug-gesting that in order to ensure a stable spine this musclerequires specific training to return the cross-sectionalarea of multifidus to normal levels.4 While these studiesdid not quantify stability but rather relied on qualitativeintuition, other approaches have attempted to quantifystability. For example, in vitro approaches have repre-sented muscle forces with wire cables acting on cadavericlumbar spines. Through investigation of predominantlysmall, local muscles, several researchers have found

that these muscles successfully increase the stiffnesswithin the spinal structure, critical for stability.810

While many believe that the local muscles are cru-cial for spine stability, others hypothesize that theglobal, larger muscles play a role. Panjabi et al8 sug-gested that the role global muscles have in stabilizing thelumbar spine comes from their efficient ability to impactthe stiffness of the entire spinal column, opposed to localmuscles that can only act on a few joints. Cholewicki andMcGill11 and Cholewicki and Van Vliet12 suggest fromthe results of their biomechanical analyses, that no singlemuscle, local or global, possesses a dominant responsi-

bility for lumbar spine stability and therefore concludedthat training efforts should not focus on any singlemuscle.

Contrasting results and descriptions for the neuro-muscular control of spine stability have led to the devel-opment of various training theories. Patterns such as an-tagonist cocontraction as a method of increasing spinestiffness has been confirmed through numerous stud-ies1316; however, some argue that enhancing this re-sponse for therapeutic purposes to train spine stabilitycan lead to very high compressive load penalties.4 Someadvocate training isolated groups of muscles, primarily

local, with the goal to minimize global muscle activa-tion and compressive loads. Identifying muscle impor-

From the University of Waterloo, Faculty of Applied Health Sciences,Waterloo, Ontario, Canada.Acknowledgment date: May 21, 2003. First revision date: June 26,2003. Acceptance date: August 6, 2003.Supported by the National Science and Engineering Research Councilof Canada.The manuscript submitteddoes not contain information about medicaldevice(s)/drug(s).Federal funds were received in support of this work. No benefits in anyform have been or will be received from a commercial party relateddirectly or indirectly to the subject of this manuscript.

Address correspondence to Stuart M. McGill, PhD, Faculty of AppliedHealth Science, University of Waterloo, Waterloo, Ontario, CanadaN2L 3G1; E-mail: [email protected]

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tance during different loading conditions is necessary toproperly critique existing clinical practices for training

and restoring a healthy lumbar spine.The purpose of this study is to use a highly sophisti-

cated and detailed torso model, driven with biologic sig-nals measured directly from each study participant, tocompute spine loads and stability. Each muscle was sys-tematically adjusted to assess the impact on the stabilityof the spine, thereby quantifying their contribution at aspecific instant in time. Furthermore, an attempt to iden-tify the different mechanisms as to how the various mus-cles contribute to lumbar spine stability was performed.

Materials and Methods

Ten male study participants performed a series of eight differ-ent exercises (Figure 1) while electromyography, three-

dimensional lumbar motion, and external forces were mea-

sured. These data were input into a series of biomechanical

models in order to calculate a measure of lumbar joint forcesand spine stability. These methods are extremely detailed and

have already been published. While the interested reader can

refer to the manuscripts for details,11,17,18 the essential details

are documented here. A schematic of the protocol is shown in

Figure 2. All procedures were approved by the University Office

for Research Ethics.

Subjects.Ten male university students with an average age of21 years (SD 3 years), height of 177.8 cm (SD 6.2 cm), and

weight of 80.2 kg (SD 12.1 kg) volunteered to participate in

this study. Subjects had no history of low back pain. Before

testing, study participantsheight, weight, and breadth dimen-

sions at the feet, ankles, knees, hips, hands, wrists, elbows, andshoulders were obtained while standing in anatomic position.

Figure 1. Pictures of different stabilization exercises.A, Abdominal curl.B, Right side bridge.C, Sitting on a stool.D, Sitting on a gym ball.E,Four-point kneeling with contralateral arm and leg extension. F,Four-point kneeling with single leg extension.G,Back bridge with singleleg extension. H, Back bridge.

1255Torso Muscles and Rehabilitation Exercises Kavcic et al


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Data Collection

Exercises. Each study participant performed a series of eightexercises presented in random order. The exercises (shown inFigure 1) include the abdominal curl (A), right side bridge (B),sitting on a gym ball (D), four-point kneeling with a left arm

and right leg extension (E), four-point kneeling with right leg

extension (F), back bridging with right leg extension (G), and

back bridging (H). To act as a control trial for the gym ball

condition and allow for assessment of unstable support sur-

faces, study participants performed trials sitting on a stool (C).

Figure 2. Flow chart of the various models used in the stability analysis.

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The stabilization exercises were chosen for ease of analysis aswell as to ensure moments to the spine in all three axes ofrotation (Table 1). Each exercise was performed with a neutrallumbar spine position and controlled limb positioning. Limband/or pelvis position was controlled through the use of anexternal frame with metal bars that was placed alongside bodysegments to act as targets.

Each exercise was held isometrically for 2 seconds with anisometric contraction of the abdominal muscles (termed ab-dominal brace).1 A brace is an isometric contraction of all themuscles of the abdominal wall without any change in the po-sition of the muscles. This is in contrast to the abdominalhollow,described by Richardson et al,4 which is intended tofocus on the recruitment of the transverse abdominis whileminimizing activation of the rectus abdominis and the ob-liques. Bracing has been shown to be superior to hollowing for

enhancing lumbar stability.23 Subjects were shown the tech-nique for performing both the hollowand braceabdom-inal contraction, and the instruction was to perform thebraceto the same perceived intensity as ahollow.Conse-quently, the intensity of the contraction was fairly low, however,since the intensity was chosen subjectively, thestability demandofsome postures may have required that study participants bracemore intensively than originally instructed. Subjects were given anunlimited number of practice trials and once comfortable with thetechnique of performing each exercise with an abdominal brace,three successive trials were measured.

Instrumentation

Electromyography. Fourteen channels of EMG were col-lected from the following muscles bilaterally: rectus abdominis,internal oblique, external oblique, latissimus dorsi, thoracicerector spinae (longissimus thoracis and iliocostalis at T9),lumbar erector spinae (longissimus and iliocostalis at L3), andmultifidus (1cm lateral to L5). We acknowledge the difficultyin capturing multifidus with surface electrodes19 and thereforeassign validity of the EMG signal to the landmarked locationrather than to themultifidusmuscle itself. Ag-AgCl surfaceelectrodes were positioned with an interelectrode distance ofabout 3 cm. The EMG signals were amplified and then A/Dconverted with a 12-bit, 16-channel A/D converter at 1,024Hz. Each study participant was required to perform a maximalcontraction of each measured muscle for normalization of eachchannel. For the abdominal muscles each study participant,

while in a sit-up position and manually braced by a researchassistant, produced a maximal isometric flexor moment fol-lowed sequentially by a right and left lateral bend moment andthen a right and left twist moment; little motion took place. Forthe extensor muscles, a resisted maximum extension in theBiering-Srensen position was performed with focus on quasi-static motion throughout neutral lordosis, which was found tocreate larger neural drive. The EMG signal was normalized to

these maximal contractions, full wave rectified and low-passfiltered with a second-order Butterworth filter. A cutoff fre-quency of 2.5 Hz was used to mimic the frequency response ofthe torso muscles.11

Three-Dimensional Kinematic Positioning of the LumbarSpine.Lumbar spine kinematics was measured about three or-thogonal axes using a 3 Space IsoTRAK, electromagnetictracking instrument (Polhemus Inc., Colchester, VT). This in-strument consists of a single transmitter that was strapped tothe pelvis over the sacrum and a receiver strapped across theribcage, over the T12 spinous process. Thus, the position of theribcage relative to the sacrum was measured, isolating lumbar

motion. Overall rotation of the lumbar spine was normalizedrelative to each study participants standing neutral spine pos-ture. In this way, individual variance in the passive tissue con-tributions as a function of maximum range of motion wasrepresented. However, in this experiment, there was minimalcontribution of the passive tissue restorative moment becauseof the neutral spine posture characteristic of the stabilizationexercises chosen.

External Force Measures.For exercises requiring an inversedynamic load application, namely, the four-point kneeling ex-ercises, back bridging exercises, and the side bridge, externalforce measures were recorded using an AMTI force plate. Thesignals were amplified to produce a peak to peak range of 20 V

( 10V) and then A/D converted with a 12-bit A/D converter at1,024 Hz. Forces and moments were measured about threeaxes and were used to calculate the external force center ofpressure values in the x, y, and z direction. For each exercise,the study participant was instructed to position the contactingsegment on the force plate around the 0, 0, 0-reference pointlocated at the center of the force plate. Reaction forces weremeasured at different parts of the upper body depending on theexercise being performed (Figure 1). Force plate measures werenot recorded for the abdominal curl or ball sitting and chairsitting exercises. The process of using whole body linked seg-ment dynamics and measured external forces has been ex-plained previously.11

Kinematic Limb Positions.Kinematic marker data for eachexercise were measured from a single study participant, notpart of the group of 10 mentioned above. This study partici-panthada heightof 178 cmanda weightof 79kg.Theexternalsegment kinematics were recorded for each exercise posturewith a single digital video image and guided by a space framejig. The isometric position of each exercise was used to analyzethe segment kinematics in the sagittal plane. The joints digi-tized for the kinematic analysis were the metatarsal, ankle, hip,shoulder, elbow, wrist and hand bilaterally, as well as L4 L5and C7T1. The kinematic posture obtained for each exercisewas controlled in the other 10 study participants with the ex-ternal jig, and the marker data were scaled to the height of eachindividual study participant. The joint locations about the z-axis, or in the frontal plane, were scaled to the breadth mea-

Table 1. Summary of the support moments created at theL4L5 joint in order to perform the different exercises

Average L4L5 moment (Nm)

Bend ( 1 SD) Twist ( 1 SD) Flex ( 1 SD)

Abdcurl 1.30 (1.9) 0.72 (0.99) 56.71 (7.0)Chair 0.54 (0.5) 0.10 (0.3) 1.47 (0.5)

Ball 0.72 (1.0) 0.18 (0.5) 1.28 (0.5)Bridge 0.15 (3.9) 2.64 (7.6) 73.81 (32.7)Bridge leg 8.42 (5.0) 15.74 (7.6) 65.94 (33.3)Fpn_leg 4.84 (2.9) 15.62 (8.1) 6.14 (25.3)Fpn_arm/leg 0.05 (5.1) 57.05 (14.6) 32.84 (23.2)Side bridge 69.18 (21.9) 12.80 (3.9) 2.87 (3.4)

Average and standard deviations are listed. In the sagittal plane, flexion isnegative and extension is positive. In the frontal plane, right lateral bend ispositive and left lateral bend is negative. In the transverse plane, right axialtwist is negative and left axial twist is positive.



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sures taken from each study participant. Since no exercise re-quired deviations of the limbs from anatomic position in thefrontal plane, breadth measures were assumed to be constantacross exercises.

Data Analysis

Calculating a Stability Index. The analysis of stability wasperformed using a method documented by Cholewicki andMcGill11 and involved three cascading and interdependentmodels. For the interested reader, these models are described indetail by Cholewicki and McGill,11 McGill and Norman,17

and McGill18; however, a brief description is provided here(Figure 2, flow chart showing the modeling process for thestability analysis).

The first model is an 8-segment link segment model that usesexternal force measures recorded from the forceplate, studyparticipant kinematics, and anthropometrics of height andweight to calculate reaction forces and moments acting at eachof the 6 lumbar intervertebral joints through a top-down, in-verse dynamics approach. The L4 L5 moments calculated

from this linked-segment model are used to ultimately drive theEMG-assisted optimization routine that determines the muscleforce profiles; however, this will be described in more detaillater in this section.20 The reaction forces from the link segmentmodel calculations are used to determine the shear and com-pression forces at the L4 L5 joint.

The second model is thelumbar spine model,which con-sists of an anatomically detailed, three-dimensional ribcage,pelvis/sacrum, and five intervening vertebrae. More than 100laminae of muscle and the passive tissues, which are repre-sented as a lumped parameter of torsional stiffness, are mod-eled about each axis. This model uses the measured three-dimensional relative spine motion data from the 3-space

IsoTRAK system and assigns the appropriate rotation to eachof the lumbar vertebral segments based on findings from Whiteand Panjabi.21 Muscle lengths and velocities are determined fromtheir motions and attachment points on the dynamic skeleton ofwhich the motion is driven from the directly measured lumbarkinematics obtained from the study participant. As well, the ori-entation of the vertebral segments along with stress/strain rela-tionshipsof thepassivetissues wasused to calculate therestorativemoment created by the spinal ligaments and discs.

The third model, termed the distribution-moment mod-el,22 is used to calculate the muscle force and stiffness profilesfor each of the muscles. The model uses the normalized EMGprofile of each muscle along with the calculated values of mus-

cle length and velocity of contraction to calculate the activemuscle force and any passive contribution from the parallelelastic components. When input to the spine model, these mus-cle forces are used to calculate a moment for each of the 18 dfof the six intervertebral joints. The objective function for theEMG-assisted optimization routine is to match the momentswith a minimal amount of change to the EMG driven forceprofiles. In this way, biologic validity of using EMG is pre-served while mathematical validity is addressed with achievingbalanced moments. The adjusted muscle force and stiffnessprofiles are then used in the calculations of L4L5 compressionand shear, as well as in calculating spine stability. The mostrecent updates to the model, specifically regarding the muchimproved representation of the transverse abdominis, are doc-umented by Grenier and McGill.23

The value for stability, or stability index, was obtained by

calculating a level of potential energy in the spinal structure foreach of the 18 df(three rotational axes at six lumbar joints)resulting from the combined potential energy existing in boththe active and passive spinal structures, minus any work donefrom external loads. The 18 values of potential energy wereformed into an 18 18 Hessian matrix and diagonalized. The

determinant of this matrix represented an index of spine stability.For a more detailed description of the mathematical procedures,refer to Cholewicki and McGill11 and for sensitivity testing andmathematical validity of the approach see Howarth et al.24

Before inputting data into the link-segment model, certainmodifications were made to both the data and the model so toenable accurate calculations of spine load and stability for cer-tain exercise postures. They are noted as follows:

Abdominal curl. When performing this exercise, study par-ticipants were directed to perform a curl-up such that rotationof the upper body occurred about the base of the rib cage.Consequently, the weight supported consisted of the head andneck, thorax, and arms. Calculating moments about the L4L5joint would consider the entire torso mass and result in anoverestimation of theflexor moment required by the muscles.To consider the true axis of rotation, the L4 L5 marker wasshifted up along the long axis of the spine to accurately repre-sent a rotation of the thorax opposed to the trunk. A thoraxdistance of 0.4 m, which is characteristic of a 75 percentile malewas used. The mass proportion assigned to the thorax was0.216 of body mass.25 For this exercise only, the abdomensegment was considered a rigid segment and the thorax mo-ment was then translated to the L4 L5 joint, recognizing thatthe rectus abdominis carries equal loading along its length.

Bridging with single leg extension.In this exercise, the inter-

nal oblique activation profile did not accurately represent thatof the psoas muscle because of the extended leg. To account forthe extra force necessary to support the extended leg, the psoasforce in the lifted leg was calculated as a proportion of themoment supporting the leg, which was assumed to be primarilygenerated from combined action of the rectus femoris, iliacus,and psoas. The moment arms and peak isometric muscle forcesused to calculate the proportions for the three listed muscleswere obtained from the literature (Table 2).26,27 Then, for eachstudy participant, the support moment required to maintainthe posture of the lifted leg was calculated. This moment wasthen multiplied by a proportionality constant for psoas anddivided by its moment arm. The resulting force value was inputinto the 18 df lumbar spine model (Figure 2) by adding itdirectly to the compressive force acting on the spine, consistentwith the psoas line of action.28

Table 2. Parameters used to calculate the contributionof the psoas muscle to the support moment of theextended leg for the back bridge with single legextension

MusclePeak isometric

muscle force (N)*Moment

arm (cm)

Relative proportion oftotal hip-flexion

moment

Psoas 370 2.9 0.19Iliacus 430 3.0 0.23Rectus Femoris 780 4.2 0.58

* From Delp et al.26

Moment arms are measured at the hip during the mid stance phase of gait.Arnold et al.27



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Determining a Muscles Impact on Spine Stability. The

contribution of each individual muscle to spine stability wasevaluated in the following way. A value of external force andmuscle activation for each muscle was taken at a point in timecorresponding to the 1-second point of each 2-second trial.This profile was then frozen and extended for the entireduration of the 2-second trial. The activation profile for allfascicles of a single muscle of interest, or target muscle, wasthen replaced by a sinusoid wave that varied from 0% to 100%MVC (Figure 3A), and the analysis was run with the new mus-cle profile. A sinusoid wave was originally chosen as an inputbecause it was thought that some muscles would demonstratemore control over stability than others. Control of spine sta-bility would be reflected by a strong correlation betweenchanges in stability with changes in muscle activation. In thosemuscles that had little control over spine stability, the sinusoi-dal pattern would be less evident in the stability output. A

sinusoid was chosen as a very specific input that could be iden-

tified in the output. Through a pilot analysis, however, thereappeared to be no significant difference in how closely stability

followed muscle activation across the various muscles tested;

therefore, this analysis was not performed.

The specific target muscles assessed were the rectus abdomi-

nis, external oblique, internal oblique, pars lumborum fibers of

longissimus thoracis and iliocostalis lumborum, thoracicfibers

of iliocostalis lumborum, longissimus thoracis, quadratus lum-

borum, latissimus dorsi, multifidus, and transverse abdominis.

This analysis was systematically repeated for each muscle, one

at a time, both unilaterally and bilaterally. To isolate the effect

of each target single muscle at this level of analysis, the EMG-

assisted optimization routine (Figure 2), used to balance the

moments, was not used. This prevented the force and stiffnessprofiles of the other muscles from changing. In effect, this pro-

Figure 3. Sinusoidal muscle acti-vation profile from 0 to 100%MVC (A) and the associatedchange in the stability indexwhen manipulating each muscleEMG profile (B). RL correspondsto the muscle on both the rightand left side. Rect rectus ab-dominis; Ext external oblique;Int internal oblique; Pars pars lumborumfibers of longissi-mus thoracis and iliocostalislumborum; Ilio thoracic fibersof iliocostalis lumborum; Long thoracic fibers of longissimusthoracis; Quad quadratus lum-borum; Lat latissimus dorsi;Mult multifidus; Trans transversus abdominis.



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cedure allowed each muscle to berattledand the subsequent

effect of this perturbation on the spine assessed.The artificial sinusoidal activation profile impacted manyvariables within the analysis; however, the effect was onlyquantified in certain variables of interest: namely, muscle force,muscle stiffness, spine loads, and the stability index. The max-imum increase and decrease in these variables, resulting fromthe sinusoidal manipulation, were computed and compared toa nonmanipulated control trial.

In an attempt to better understand the different mechanicaladvantages for each of the muscles to stabilize, the RMS differ-ence was calculated across each stability index curve resultingfrom the manipulated muscle activation profile, as well asacross the corresponding muscle force curve. The RMS differ-ence was used to quantify of the magnitude of fluctuationwithin the particular curve. The force RMS difference was thendivided into the stability index RMS difference. In this sense, an

efficiency ratio was created to describe the coupling between

the fluctuations in the force of a particular muscle and thecorrespondingfluctuations in spine stability.

Results

The effect of the sinusoidal EMG activation profileonthecalculated stability index is shown for each muscle inFigure 3B The stability index for each manipulated mus-cle is superimposed on the same graph.

Assessing the Absolute Impact of a Single Muscle onLumbar Spine Stability

The effect of increasing each muscle activation profile to100% MVC on increasing the stability index is shown in

Figure 4A, whereas the effect from decreasing muscleactivation to 0% MVC is shown in Figure 4B. A major

Figure 4. A, Increase in stabilityindex resulting from activating amuscle bilaterally to 100% MVC.B,Decrease in stability index re-sulting from turning a muscle off

bilaterally to 0% MVC. Acrosstasks there is no consistent pat-tern in the ability of the differentmuscles to affect stability. How-ever, i t appears as though,across the larger muscles, in-creased activation of the mo-ment antagonist enhances stabil-ity and decreased activation ofthe mom ent ago nist red uce sstability.



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finding in the analysis is that, between the different tasks,there is no consistent pattern across muscles in their abil-ity to affect stability. This is particularly evident withsome of the larger muscles, such as the rectus abdominisand the lumbar and thoracic extensors. In contrast, thequadratus lumborum, latissimus dorsi, multifidus, andtransverse abdominis demonstrated only small changes

in their relative patterns in terms of both increasing anddecreasing stability.

Quantification of each muscles absolute impact onthe stability index shows that, compared with the rectusabdominis, obliques, and lumbar and thoracic extensors,the quadratus lumborum, latissimus dorsi, multifidus,and transverse abdominis each created minimal changes.In contrast, both the internal and external obliques con-sistently demonstrated a large impact on both increasingand decreasing stability irrespective of the task condi-tion. Between the two muscles, a more dramatic effectwas produced from the internal obliques. One important

note is that in thestabilizationexercises assessed, noindividual muscle, either unilaterally or bilaterally, whenartificially reduced in activation, created an unstablesituation.

An interesting result is that certain muscles demon-strated a direction-dependent effect on lumbar spine sta-bility. Specifically, coactivation of what would be con-sidered an antagonist, in a torque context, enhancesstability. Compare the rank order of the predominantflexor: rectus abdominisversusthe major lumbar exten-sors, pars lumborum, iliocostalis lumborum, and longis-simus thoracis. In the abdominal curl, which is a flexion

dominant task (Table 1), the three extensor musclesdemonstrate a greater effect on increasing the stabilityindex compared with the rectus abdominis. However,this pattern is reversed when quantifying each musclesability to reduce spine stability. In contrast, during theextension dominant tasks (Table 1), the rectus abdomi-nis creates a greater increase in stability over the parslumborum and longissimus thoracis. For the iliocostalis,lumborum, this pattern is not so evident; however, care-ful examination shows that across the extension domi-nant tasks, as the required support moment increases,the relative difference between the effects of the rectus

abdominis and the iliocostalis lumborum decreases. Aswith the abdominal curl, when the activation levels arereduced to 0% MVC, the pattern between the flexorsand extensors reverses.

The same association observed between the flexor andextensor muscle groups is observed between certain rightversus left muscle groups during asymmetric tasks such asthe four-point kneeling tasks and side bridge (Figure 5).

It should be noted that the above results refer to groupmeans calculated from the 10 study participants. Acrossthe individual study participants, the pattern of muscleimpact on stability was not consistent for any given task;

however, the direction-dependent effect observed amongthe group means exists at an individual level as well.

Assessment of a Potential Mechanical Stabilizing

Mechanism for Each of the Different Muscles

The cost of certain muscles to stabilize is demonstrated inFigure 6. These figures show that the larger muscles, suchas the rectus abdominis, obliques, and upper and lowererectors, impose larger changes in L4L5 load comparedwith the other muscles tested. Given this, thefinal anal-

ysis assesses theefficiencywith which each muscle cantranslate their respective generated force to spine stabil-ity (Figure 7). Those muscles with higher values of theefficiency ratio have a greater normalized contributionto spine stability for a given change in muscle force. Incontrast to the absolute impact of the various muscles tospine stability, large efficiency ratios were observed in themultifidus, quadratus lumborum and transverse abdomi-nis, internal and external oblique, and iliocostalis lum-borum produced. Relatively smaller values were ob-served in the rectus abdominis, pars lumborum,longissimus thoracis, and latissimus dorsi.

Discussion

Clearly, there is no single muscle that is superior at en-hancing spine stability. In addition, the muscle manipu-lation method described here has provided insight intothe potential neuromuscular control of lumbar spine sta-bility. Results of this analysis indicate that muscles in thetrunk play several roles at once and that their roles de-pend on the instantaneous demand placed on the spinalcolumn. Generally, those muscles that were antagonist tothe dominant moment of the task were most effective at

increasing stability. This finding supports the direction-dependent cocontraction pattern that has been reportedin the more global muscles during different tasks.14,29,30

The greatest reductions in stability were observed whenmuscles that opposed the dominant destabilizing forceswere inactivated. For example, in a lateral bending tasksuch as the right side bridge, the dominant external forceat the spine is the ground reaction force acting at theforearm that forces the L4L5 joint in a left lateral bend.The right abdominal muscles are activated not only tooppose the left lateral bend moment to support the total-body posture, but at the level of a single lumbar joint,

they potentially also protect against an instantaneousinstability resulting from an excessive rotation in lateralbend. During an in vivo study by Cholewicki andMcGill,31 the authors observed a temporary excessivevertebralflexion in a powerlifter who incurred an injurywhile lifting. The authors hypothesized that a motor con-trol error in a crucial back muscle may have been respon-sible for the excessive flexion instability. These tempo-rary reductions may prevent those muscles, whose jobduring a particular task is to oppose crucial destabilizingforces and rotational instabilities, from controlling ver-tebral motion. As the destabilizing forces on the spine

change through different postures, so do the muscles thatare able to oppose these forces.



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Across the various torso muscles, the mechanical ad-vantage to provide stability to the lumbar spine variesdepending on the muscle. It appears as though, on aver-age, the larger, moreglobalmuscles are better able toalter spine stability than the smaller, intersegmental mus-cles. This is most likely because of the larger force-generating potential in these muscles and their ability togenerate higher levels of L4L5 compression, translatingto higher levels of spine stiffness. As well, their largermoment arms enhance their ability to act as guy wires.Interestingly, the increase in compressive loads on thespine that result from muscular cocontraction has beenestimated to increase stability at a higher rate than the

additional compression. Specifically, Granata and Mar-ras32 have estimated that stability is enhanced threefold

for a given increase in compression, whereas Grenier andMcGill23 have computed the enhancement to be at leasttwofold. It would appear that the qualitative assumptionthat activating muscles that impose low compressiveloads as prime stabilizers is problematic when evaluatinga quantitative stability analysis. Among the more localmuscles, it is interesting to observe the minor ability ofthe transverse abdominis to alter spine stability whenmanipulated through its entire force- and stiffness-generating abilities.

The mechanical advantage for the smaller, interseg-mental muscles, particularly the multifidus, and quadra-tus lumborum, appears to come from their efficient trans-

lation of generated force to spine stiffness and stability.These results can potentially be explained by a phenom-

Figure 5. Increase in stability in-dex resulting from activating amuscle unilaterally to 100% MVC.A, Results are shown for twoasymmetrical tasks.B, Decrease

in stability index resulting fromturning a muscle off bilaterally to0% MVC. Results are shown fortwo asy mme tri cal tas ks. Thesame pattern observed betweenagonist and antagonist musclesnoted in Figure 4 is observed be-tween right and left muscles dur-ing asymmetrical tasks. R corre-sponds to the muscle on the rightside; L corresponds to the mus-cle on the left side.



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enon known as the follower load, described by Pat-wardhan et al.33,34 According to this theory, those mus-cles that insert onto the spinal segments are better able totranslate their generated force along the compressive axisof the spine or tangent to the curve of the lumbar spine.It is important to note that previous work by Crisco andPanjabi10 reported the opposite, in that the more multi-segmental muscles are more efficient at creating a criticallevel of lumbar spine stiffness over the intersegmentalmuscles. The discrepancy infindings, however, is consis-tent with the different models used. In their study, thelumbar spine was modeled as a straight elastic columnwith motion restrained to the frontal plane. With such a

linear model, a given level of activation in the multiseg-mental muscles would impact many joints, whereas the

intersegmental muscles may only affect one or two joints,as they described. In our analysis, the spine was modeledwith a natural lordotic curvature and motion existed in atotal of 18 dfThe ability of the intersegmental muscles tofollow the curvature of the spine and direct a large com-ponent of force along the compressive axis is how theefficiency of these muscles dominated over the more mul-tisegmental muscles.

When attempting to apply the results of this study toclinical practice, consideration of the physiologic rele-vance of this technique is necessary. In a human neuro-muscular system, muscle synergies exist, where changesin a given muscle activation level rarely occur in isolation

but rather are associated with changes in that of othermuscles. For example, Richardson and Jull35 reported

Figure 6. A,Increase in L4 L5 compression resulting from activating a muscle bilaterally to 100% MVC. B,Decrease in L4 L5 compressionresulting from turning a muscle off bilaterally to 0% MVC. The larger, multisegmental muscles stabilize through their ability to generatehigh levels of L4 L5 compression, which is associated with increased levels of spine stiffness, together with their action as guy wiresenhancing the systems potential energy.



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that activation of the multifidus is linked to that of thetransverse abdominis. In order to accurately assess thestabilizing role of a given muscle and represent physio-logic reality, synergistic patterns need to be considered.Since this study examined the effect of changing a singlemuscle, this could be considered a limitation in terms ofassessing synergies. Assessing the consequence of syner-gies is much more difficult given the many roles each

component muscle plays, but it is our objective to exam-ine this in the future. The benefit of this analysis, how-ever, is to address the clinical misconception that at anygiven moment a single muscle can provide the necessarystability to the lumbar spine. One finding observed in thisanalysis was that no single muscle, when manipulatedfrom 0% to 100% MVC, created an unstable spine. Itdoes not seem reasonable then that any one muscle inisolation has the capabilities to dramatically impactspine stability, at least in the stabilityexercises testedhere, although we have found some low challenge taskswhere this is not the case. This study showed that asloads are applied to the spine there is an integration ofthe many different muscles in order to balance the stabil-ity and moment demands, and these patterns change asthe spine loading patterns change.

One feature of this technique was that it was successfulat identifying the total contribution that each muscle canmake to stability, relative to the other torso muscles tested.In this light, it is noticed that the smaller muscles have astabilizing role through their efficient generation of force,however, as loads increase the need for the stronger globalmuscles is required. One important note is that only thecontributions of the muscles force profiles to stability wereassessed. Other potential roles that influence stability such

as proprioceptive integration or passive-elastic link withintra-abdominal pressure were not assessed.

Another benefit of manipulating a muscle in isolationis that the changes created in stability can be associatedwith only the manipulated muscle. Allowing an optimi-zation routine to balance the muscle force moments tothe external moments would have caused changes in theforce profiles of various muscles, and associating thesechanges to that observed in the stability index would bea difficult task.

Many assumptions were made in this study as a resultof the biomechanical modeling procedure used. Assump-tions made in the biomechanical models used in thisanalysis have been documented previously,11 and whilegreat attempts were made to achieve biofidelity, thishighly complex analysis could not be performed withoutthem. Lastly, the conclusions of this work are limited tothe contrivedstabilityexercises tested. One importantnote is that in the abdominal curl exercise, the actualtorque is generated at the level of the midthoracic spine;however, in this analysis, in order to assess the stabilizingability of the different muscles in the lumbar spine duringa flexion task, the moment created at the thoracic levelwas translated to the lumbar spine. Given that the studyparticipants were fully supported laying supine on thefloor, the individual muscle contributions to lumbarspine stability during the type of abdominal curl per-formed here remain unknown.

In terms of the practical application of the findings inthis study pertaining to prevention and rehabilitation,the clinical practice of isolated training of a specific mus-cle or group of muscles in attempts to reduce the com-pressive costs must be questioned. According to the re-sults of this study, it appears justifiable to train motorpatterns that involve the contribution of many of the

potentially important lumbar spine stabilizers. This seemsto be the case since although some of the highly regarded

Figure 7. RMS of stability curve normalized to RMS of muscle force curves. One mechanism explaining how the smaller, intersegmentalmuscles stabilize could result from their ability to efficiently translate their respective generated force to spine stability.



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local muscles can create stability very efficiently, their abso-lute contribution is not dominating and may not be suffi-cient during functional tasks. Focusing on a single muscle,or only a few, appears to be misdirected clinical effort if thegoal is to ensure a stable spine.

Key Points Using various assumptions and variations to abiomechanical model, assessment of the stabilizingrole of different muscles was quantified for differentloading scenarios. The role of each individual lumbar musclechanges as the loads placed on the spine changes. Consideration should be given to each potentialstabilizer when designing exercise programs in-tended to increase spine stability.

References

1. McGill S.Low Back Disorders: Evidence-Based Prevention and Rehabilita-

tion. Champaign, IL: Human Kinetics, 2002:143.

2. Panjabi MM, Abumi K, Duranceau J, et al. Spinal stability and intersegmen-

tal muscle forces: a biomechanical model.Spine. 1989;14:194 200.

3. Bergmark A. Stabilityof thelumbar spine:a study in mechanical engineering.

Acta Orthop Scand Suppl. 1989;60:154.

4. Richardson C, Jull G, Hodges P, et al. Therapeutic exercise for spinal seg-

mental stabilization. In:Lower Back Pain. London: Harcourt Brace, 1999.

5. Hodges PW, Richardson CA. Feedforward contraction of transverse abdo-

minis is not influenced by the direction of arm movement. Exp Brain Res.

1997;114:362370.

6. Hodges PW, Richardson CA. Delayed postural contraction of transverse

abdominis in low back pain associated with movement of the lower limb.

J Spinal Disord. 1998;1:46 56.

7. Hides JA, Stokes MJ, Saide M, et al. Evidence of lumbar multifidus muscle

wasting ipsilateral to symptoms in patients with acute/subacute low back

pain.Spine1994;19:165172.

8. Panjabi M, Abumi K, Duranceau J, et al. Spine stability and intersegmental

muscle forces: a biomechanical model.Spine. 1989;14:194 200.

9. Wilke HJ, Wolf S, Claes LE, et al. Stability increase of the lumbar spine with

different muscle groups: a biomechanical In vitro study. Spine. 1995;20:192

198.

10. Crisco JJ, Panjabi M, The intersegmental and multisegmental muscles of the

lumbarspine: a biomechanical model comparing lateral stabilizingpotential.

Spine1991;16:793799.

11. Cholewicki J, McGill S. Mechanical stability of the In vivo lumbar spine:

implications for injury and chronic low back pain. Clin Biomech. 1996;11:

115.

12. Cholewicki J, VanVliet J IV. Relative contribution of trunk muscles to the

stabilityof thelumbarspineduring isometricexertions. Clin Biomech. 2002;

17:99 105.

13. Cholewicki J, Simons A, Radebold A. Effects of external trunk loads on

lumbar spine stability.J Biomech. 2000;33:13771385.

14. Gardner-Morse M, Stokes I. Trunk stiffness increases with steady-state ef-

fort.J Biomech. 2001;34:457 463.

15. Gardner-Mores M, StokesI. Theeffects of abdominalmuscle coactivationon

lumbar spine stability.Spine. 1998;23:86 92.

16. Granata K, Orishimo K. Response of trunk musclecoactivation to changes in

spinal stability.J Biomech. 2001;34:11171123.

17. McGill S, Norman R. Partitioning of the L4 L5 dynamic moment into disc,

ligamentous and muscular components during lifting.Spine. 1986;11:666 677.

18. McGill S. A myoelectrically based dynamic three-dimensional model to pre-

dict loads on lumbar spine tissues during lateral bending.J Biomech. 1992;

25:395 414.

19. Stokes I, Henry S, SingleR. Surface EMGelectrodes do notaccurately record

from lumbar multifidus muscles.Clin Biomech. 2003;18:9 13.

20. Cholewicki J, McGill S. Relationship between muscle force and stiffness in

the wholemammalian muscle: a simulationstudy.J Biomech Eng. 1995;117:

339 342.

21. White A, Panjabi M.Clinical Biomechanics of the Spine. Philadelphia: Lip-

pincott, 1978:79.

22. Ma SP, Zahalak GI. A distribution-moment model of energetics in skeletal

muscle.J Biomech. 1991;24:2135.

23. GrenierS, McGill S. Lumbarspine stabilityfrom hollowing vs. bracing:

the transverse abdominis is no more important than any other muscle to

ensure lumbar stability. Submitted.24. Howarth S, Allison A, Grenier S, et al.On theimplications of interpretingthe

stability index: a spine example.J Biomed. In press.

25. WinterDA. Biomechanicsand MotorControl of HumanMovement,2nded.

Toronto: John Wiley and Sons, 1990:56 57.

26. Delp SL, Loan JP,Hoy MG,et al.An interactive graphics-based model of the

lower extremity to study orthopaedic surgical procedures. IEEE Trans

Biomed Eng1990;37:757759.

27. Arnold A, Salinas S, Asajawa D, et al. Accuracy of muscle moment arms

estimated from MRI-based musculoskeletal models of the lower extremity.

Comput Aided Surg2000;5:108 119.

28. Santaguida PL, McGill SM. The psoas major muscle: a three-dimensional

geometric study.J Biomech. 1995;28:339 345.

29. Thomas JS, Lavender SA, Corcos DM, et al. Trunk kinematics and trunk

muscleactivity duringa rapidly applied load.J Electromyogr Kinesiol. 1998;

8:215225.

30. McGill S. Electromyographic activity of the abdominal and low back mus-

culature during generation of isometric and dynamic axial trunk torque:

implications for lumbar mechanics.J Orthop Res. 1991;9:91103.

31. Cholewicki J, McGill S. Lumbar posterior ligament involvement during ex-

tremely heavy lifts estimated fromfluoroscopic measurements. J Biomech.

1992;25:1728.

32. Granata KP, Marras WS. Cost-benefit of muscle cocontraction in protecting

against spinal instability.Spine. 2000;25:1398 1404.

33. Patwardhan AG,Harvey R, Ghanayem A, et al.A follower load increasesthe

load-carrying capacity of the lumbar spine in compression.Spine. 1999;24:

10031009.

34. Patwardhan AG, Meade KP, Lee B. A frontal plane model of the lumbar

spine subjected to a follower load: implications for the role of muscles.

J Biomech Eng. 2001;123:212217.

35. Richardson C, Jull G. Muscle control-pain control: what exercises would

you prescribe?Man Ther. 1995;1:210.


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