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THEORY AND PRACTICE

Spinal Stabilisation3.-Stabilisation Mechanisms of the Lumbar SpineChristopher 111 Norris

Key WordsLumbar support, muscle. fascia, biomechanics.

SummaryThis paper reviews the active stabilising mechanisms of thelumbar spine. Intra-abdominal pressure s produced by contrac-

t i n of the abdominal musdes when the glottis is closed. Theposterior ligamentous system has been shown to produce25% of the moment of the erector spinae (ES), and the ES willthemselves produce an important passive elastic tension. Inthe thoracolumbar fascia (TLF) mechanism, the horizontalpull of transversus abdominis is changed into a vertical force viathe angled deep and superficial ibres of the TLF. The hydraulicamplifier effect is produced as he ES contract within the fascia1envelope formed from the middle layer of the TLF. This mecha-nism increases the stress generated by the ES by 30%. Multi-fidus is especially important to active stabilisation as it hassegmentally arranged fibres whose lines of force may beresolved into a large vertical and small horizontal component.The ES ines of action show it to be more suitable as a primemover than a stabiliser, and it is the endurance of this musclerather than its strength which is mpoltant to stabilkation. 01 heabdominal muscles, transversus abdominis and internal obliqueact as stabilisers. while the lateral fibres of external oblique andthe rectus -minis act as prime movers.

IntroductionThe human spine devoid of its musculature isinherently unstable. A fresh cadaveric spinewithout muscle can only sustain a load of 4-5 lbbefore it buckles Panjabi et al, 1989). Add tothis the fact that, when standing upright, thecentre of gravity of the upper body lies a t sternallevel Norkin and Levangie, 1992). This combi-

nation of flexibility and weight creates a set ofmechanical circumstances which have beencompared to balancing a weight of approximately7 lb at the end of a 14-inch flexible rod Farfan,1988).

When lifting, the situation is intensified. Thespine may be viewed as a cantilever pivoted onthe hip (fig 1). The weight of the trunk combinedwith the weight of an object lifted forms theresistance, balanced by an effort created by thehip and back extensors. Using thi s model anumber of authors have calculated that the forceimposed on the lower lumbar spine greatlyexceeds the failure load of the lumbar vertebral

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Fig 1: Cantibver model of the wi ne in Ilfting. Acting aa acantilever, he spine i s pivoted at the hip. The welght of thebody and the object Ilfted combined form the resistance,balanced by f orce from the spinal extenso rs and thehip extensors. The reaction force created In this modelexceeds tha failure i d f the lumbar discs

discs unless an additional support mechanismi s p r e sen t (B ra d f o rd and Sp u r l i ng , 1945;

Bartelink, 1957; M o r r i s et al 1961).A number of mechanisms have been suggestedto relieve some of the compression stress on thelumbar discs and help stabilise the spine. Thispaper provides an overview of these mecha-nisms.

Intra-abdominal PressureMechanismThe theoretical basis for the intra-abdominalpressure ( IAP) mechanism is t h a t p r es su rewithin the abdomen acting against the pelvis

and diaphragm provides an additional extensormoment to the spine (fig 2). IAP involves a syn-chronous contraction of the abdominal muscles,the diaphragm and the muscles of the pelvicfloor. The obliquely placed muscles ar e th e mostimportant in this respect as they will produce

Fig 2: Intra-abdomin al pressu re mechanism. Pressurewithin the abdomen acting against the pelvis anddiaphragm provides an eddMoMI extennor moment to thespine (after Troup, 1979)

the largest torques. Contraction of the transver-BUS abdominis, and to a lesser degree the inter-na l an d ex t e r n a l ob l i qu es , wi ll c ause a nincreased I AP when the glottis is closed. Thesemuscles will pull on the rectus sheath and socompress t he v is cera . Compress ion of theabdominal contents forces them upwards on tothe diaphragm an d se parates th e pelvis from the

thoracic cage. The IAP will be greater if thebreath is held following a deep inspiration, asthe diaphragm is lower, and the comparativesize of the abdominal cavity is reduced. I P israised when the muscles contract reflexly todefend the abdominal viscera from a direct blow,and to protect the spine from excessive indirectloads. The muscles act involuntarily to fix therib cage an d to compress the abdominal con-tents.

By making the trunk into a more solid cylinder,axial compression and shear loads are reduced

and transmitted over a wider area through theIAP mechanism (Twomey and Taylor, 1987).IAP is greater when heavy lifts are performed,and when the lift is rapid (Davis and Troup,1964).

IAP is related to strength of the abdominal,pelvic floor, and diaphragm muscles. Strongathl etes can produce very large I AP values (Har-man et al 1988). However, strengthen ing theabdominal muscles with sit-up type movementsdoes not permanently increase I P (Hemborg etal 1983). Exercises of this type do not usuallymimic the co-ordination between the abdominalmuscles which is inherent in the IAP mecha-nism (Oliver and Middleditch, 1991). n astudy looking at the effect of abdominal musclet ra in ing on IAP, Hemborg et al (1985) seda n i sometric t run k cur l and twis t . Musclestrengt hening was clearly demo nstrated by a nincreased recruitmen t of motor units i n th eoblique abdominal muscles. However, EMGactivity of these muscles when lifting was shownto decrease, implying that subjects did notmake functional use of their increased abilityto recruit more motor units.

At the onset of a lift, IAP resists trunk flexionand reduces spinal compression. The glut ealsand ha mstring s rotate t he pelvis backwards andflatten t he lordosis. These muscles are bettersuited than t he erector spinae to initiat e the lift.A 150 l b a t h l e t e w o ul d ne ed t o d evelop amoment of 10.000 d b to lift a 450 lb weight.However, it has been calculated that while thehip extensors are able to generate a moment of15,000 d b , the erector spinae can generate amaximum moment of only 3,000 d b , 20 ofth at required to perform the lift (Farfan, 1988).


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minis contracts and pulls on the lateral raphe,the deep and superficial fibres of the TLF willpull laterally, but some force will be transmittedalong the length of th e TLF, tensing it. Origi-nally this approximating force was calculated tobe 57 of the force applied to the lateral raphe(Mackintosh and Bogduk, 19871, an increase inforce termed the ‘gain’ of the TLF (Gracovetskyet al 1985). However, more detailed anatomicalinvestigation has revealed that the moment cre-ated by contraction of transversus abdominis onto th e TLF is between 3.9 and 5.9 Nm comparedto th at from the back extensors of 250 to 280 Nm(Macintosh et al 1987).

Hydraulic AmplifierA mechanism by which the TLF may exert asubstantially greater stabilising effect is that ofth e hydraulic amplifier. The posterior layer ofthe TLF is retinacular and envelops th e erector

spin ae (fig 4). As the erector spinae contract,their expansion is resisted by the TLF, leadingto a build-up of fascial tension.

Tmsversusmsversus

Fig 4: The hydraulic amplifier mechanism. The posteriorlayer of the TLF envelops the erector spinae. As thesemuscles contract and bulge, fascial tendon build s upincreasing the stress generated by the muscles by 30%(Oliver and Middleditch, 1991)

It has been suggested that tensioning the TLFthrough the hydraulic amplifier effect, rathertha n by the pul l of transversus abdominis,may exert the predominant anti-flexion effect(Macintosh et al 1987). Restriction of the radialexpansion of th e erector spinae by the TLF hasbeen shown to increase the stress generated bythese muscles by a s much as 3Wo (Hukins et al1990a).

Arch Model of the SpineThe traditional model of the spine is of a leversubjected to external loads created by th e weightof th e trunk and any object lifted, and the forcescreated by the various muscles and ligamentssurrounding the spine (fig 1 . This lever modelhas been used to calculate forces imposed on the

spine during lifting especially, and relies onexternal forces for its stability.

An alte rna tiv e representat ion of th e spine isthat of an arch (Aspden, 1987, 1989). The ends(abutments) of the arch are provided caudally bythe sacrum and cranially by a combination ofbodyweight, and muscular and ligamentous

forces. The principal difference between a leverand a n arch is th at the lever is externally sup-ported where a s the arch is intrinsically stable.Any load positioned on the convex surface of thearch will create an internal thrust line whichruns straight to the arch abutments (fig 5) . Forthe arch to remain stable, the thrust line muststay within the depth of the arch ring. Thedeeper within the arch the thr ust line stays, themore stable the arch will be. In the case of thespine, the thrust line is positioned within thevertebral bodies.

Fig 5: Arch modsl of the spine. A load poslUoned on theconvex surface of an arch cmates an Internal titrust line.For the arch to remain stable, the thrust lin e must staywithin the depth of the arch ring (Aspden, 1989)

As we have Been above, analysis of a liftingtechnique using the lever model has shown thatthe load imposed on the spine greatly exceedsthe failure load of the lumbar discs, unless anadditional support mechanism is supplied. Ofthese mechanisms, both I P and TLF have beencriticised a s failing to produce a sufficientlyhigh extensor moment.

Using an arch model, however, th e analysis isdifferent. A 100 kg weight lifted i n a stoopedposition (lordosis lost) creates a t h rus t l i ne

which moves outside the spine (fig 6a overleaf).The arch is therefore unstable. However, intro-ducing IAP creates an additional force vectoracting on the anterior surface of the lumbarregion. This will move the thrust line back intothe spine and increase spinal stability (fig 6b).In addi t ion the spinal muscles , which areintrinsic to the arch, may be used to adjust thelordosis so tha t the thrus t line remains withinthe arch of the spine. Further, th e stiffness ofthe spine (resistance to bending) is increasedthrough the TLF an d hydraulic amplifier mech-anisms.

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Fig 6 Amh modal of the spine in ~ ~ O I Io 8taMlrty. Lilt-

IiM A* which mOvW out~ide th arch Of the spine, k v -ing a heavy wdght in a stooped posWon creates a t h ~ r tIng the splne unstable (a). The combitmi effect of IAP ect-Ing on the an- .u of the @no, and adjustment ofthe krdosls m o w the hrust ilne back Into the vert.bralbodk.(b) (Aspden, 987.1989)

I t should be noted th at the ar ch model of thespine h as been criticised as seriously under-estimating the compressive forces on the spineAdams, 1989).

Trunk Muscle ActionIt has been suggested that the mechanisms forspinal stability can be enhanced by facilitating aco-contraction of th e muscles surrounding thelumbar spine, especially the oblique abdominals,transversus abdominis, eredor spinae, and mul-tifidus Richardson et al 1990).

Spinal Extensor MusclesThe spinal extensors may be broadly categorisedinto superficial muscles (the erector spinae)which travel the length of the lumbar spine andattach to the sacrum and pelvis, and deep mus-cles multifidus, interspinales, and intertrans-

versarii) which s pa n between t he individuallumbar segments.

The intersegmental muscles, being more deeplyplaced, ar e closer to the centre of rotation of thespine, and have a shorter lever arm than thesuperficial muscles. However, by being closer tothe centre of rotation, the change in length ofthe intersegmental muscles will be less for anygiven change in angular position of the spine.The shorter length of the intersegmental mus-cles gives them a faster reaction time, creating asmoother and more efficient stabilising controlsystem Padabi et al 1989). The intersegmentalnature of these muscles also means that they

are able to ‘fine tune’ t he spinal movements byacting on individual lumbar segments rathe rthan th e whole spine kspden, 1992).

The superficial muscles being larger in size andfurt her from the centre of rotation are betterplaced to create g r os s sagittal rotation move-

ments, while the intersegmental muscles are ofgreater importance to spinal stability Panjabiet al 1989).

Deep Intersegmental) MusclesOf the deeply placed intersegmental muscles, itis the multifidus which is of most interest withrespect to lumbar stability. The fibres of multi-f idus a r e arranged segmenta l ly, and eachfascicle of a given vertebra has a separa teinnervation by the medial branch of the dorsalram us of the vertebra below Macintosh andBogduk, 1986). he primary function of each fas-cicle of multifidus is therefore likely to befocused on an individual spinous process, and itmay be able to control t he lordosis at each verte-bral level independently to match any imposedloading Aspden, 1992). The line of action of themultifidus can be resolved into a small horizon-tal and very much larger vertical component(fig 7a), which when viewed from the side acts

Flg 7 : Line ot d o n f lumbar muscles. Lateral vbw showing (a) the line of action of multitldus, with its verticalaiignmemt, and (b) he line of the lumbar llbcostaiis andlumbar longluimus, showing their more oblique orlenta-tion. Note the greater horizontal force vector (H) andamalbr ~ N w lorce vector (V) of he ower fibres ol thewmuscles (from Bogduk and Twomey. 1987)

at 90’ o the spinous processes. This configura-tion enables multifidus to produce posteriorsagi ttal rotation rocking) of the lumbar verte-brae Macintosh and Bogduk, 1986). his actionis used to neutralise flexion of the spine causedas a secondary action of the oblique abdominalsas they produce spinal rotation. Because the lineof action of the long fascicles of multifidus lies


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behind the lumbar spine, the muscle will alsoincrease the lumbar lordosis. Multifidus hasbeen shown to be active through the wholerange of flexion, during rotation in either direc-tion and during extension movements of the hip(Valencia and Munro, 1985).

Marked asymmetry of the multifidus has been

shown using real-t ime ultra sound imaging(Hides et al 1994). Cross-sectional area (CSA) ofthe multifidus was markedly reduced on theipsilate ral side to symptoms, th e s ite corre-sponding to th e level of lumbar les ion a sassessed by manual therapy palpat ion. Inaddition, the muscle showed a rounder shapesuggest ing muscle spasm. The suggestedmechanism for the CSA eduction was by inhibi-tion through perceived pain via a long loopreflex. The level of vertebral pathology may havebeen targeted to protect the damaged tissuesfrom movement. The muscle wasting was rapid

(less than 14 days in 20 of the total 26 patientsstudied ) which the auth ors suggested couldillustrat e a metabolic effect of circulation reduc-tion due to muscle spasm.

In addition to changes in muscle bulk, alterationin fibre type has been shown in the multifiduswith low back pain (LBP) patients (Biedermanne t al 1991). Patients who tended to decreasetheir physical and social activities as a result ofLBP showed a reduced ratio of slow twitch tofast twitch muscle fibres in the multifidus. Thiscould possibly be an adaptive response by themuscle to changes in functional demand placedupon it. In addition, there may have been a shifiin the recruitment patterns of the motor units ofthe paraspinal muscles as a resu lt of injury,with th e fast twitch motor units being recruitedbefore the slow twitch units.

The intersp inale s muscles act synergisticallywith multifidus to produce posterior sagittalrotation. However, the intertransversarii lie soclose to th e axes of both sagittal rotation andlateral flexion, that they are unlikely to con-tribu te significantly to these movements. Theirimport ance seems more likely to be proprio-

ception rather than active movement (see parttwo of this article series).

Superficial MusclesThe lumbar erector spinae consists of two mus-cles, th e iliocostalis and the longissimus. Each ofthese muscles has two components, arising fromboth the thoracic and lumbar spine. Function-ally, therefore, the erector sp inae can be consid-ered in four distinct groups, lumbar longissimus,lumbar iliocostalis, thoracic longissimus andthoracic iliocostalis (Macintosh and Bogduk,1987).

The force produced by the lumbar longissimuscan b e resolved into a large vertical vector and asmaller horizontal vector (fig 7b). However, thefascicle attachments are closer to the axis ofsagittal rotation than those of multifidus and sotheir effect on posterior sagittal rotation is less.The horizontal vectors of lumbar longissimusare directed backward and so the muscle is ableto draw the vertebrae backwards into posteriortranslation and restore the anterior translationwhich occurs with lumbar flexion. The upperlumbar fascicles are better equipped to facilitateposterior sagittal rotation while the lower levelsare better suited o resist anterior translation.

The lumbar iliocostalis has a similar action toth at of the lumbar longissimus. In addition, themuscle will co-operate with multifidus as a neu-tral iser of flexion caused by the abdominal8 asthese muscles rotate the trunk.

The thoracic longissirnus has an indirect effecton the lumbar spine through the aponeurosis ofthe erector spinae to increase the lumbar lordo-sis. It will also laterally flex the thoracic spineand thereby indirectly laterally flex the lumbarspine.

The thoracic iliocostalis attaches not to the lum-bar vertebrae but to the iliac crest. On contrac-tion these fascicles will increase the lordosis andthrough their additional leverage from the ribsthey will indirectly l ateral ly flex the l umbarspine. During contralateral rotation the ribs w i l l

separate, stretching the thoracic iliocostaliswhich can therefore act as a limiting factor tothis movement. On contraction, the thoracic ilio-costalis will derotate t he rib cage and l umbarspine from a position of contralateral rotation.

Rather th an the strength of the erector spinae itis their endurance which may be important toLBP rehabilitation. Endurance ha s been used asa predictor for susceptibility to LBP (Beiring-Sorensen, 1984). n addition, subjects with a his-tory of LBP have been shown to have reducedendurance of the back extensors but similar

strength (Jorgensen and Nicolaisen, 1987). Withincreasing fatigue, subjects with LBP show areduction i n precision and control of t runkmovements. Loss of torque from the trunk mus-cles in these subjects is relatively less than theloss of control and precision (Parnianpour et al19881, ndicating tha t a rehabili tation pro-gramme should include restoration of endurancefor the spinal extensors. Separation (selectiverecruitment) of t he torque producing superficialmuscles from the stabilising deep muscles is alsoseen as im porta nt for rehabilitation of activelumbar stabilisation (Ng and Richardson, 1994).


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Abdominal MusclesThe redus abdominis and lateral fibres of exter-nal oblique may be considered a s the primemovers of tru nk flexion, while th e inte rnalobl ique and t ransver sus abdominis are th emajor stabil iserg, as they are the only twoabdominal muscles which pass from the anteriortrunk to th e lumbar spine (Miller and Medeiros,1987). n term s of spi nal stabilisation, rathertha n th e str ength of these muscles, i t is t hespeed with which they contract in reaction to aforce tending to displace the lumbar spine whichis important (Saal and Saal, 1989).

The rect us abdominis will flex th e tr unk byapproxi mating the pelvis an d ribcage. EMGinvestigation ha s shown the supraumbilical por-tion to be emphasised by trunk flexion, whileactivity in th e infka-umbilical portion is greaterin positions where a posterior pelvic tilt is held

(Lipetz and Gutin, 1970; uimaraes et al, 1991).By assessing EMG ctivity of the trunk musclesit has been shown that the muscles do not sim-ply work as prime movers of the spine, but showantagonistic activity during various movements.The oblique abdominals are more active thanpredicted, to help to stabilise the trunk (Zetter-berg et al 1987). During maximum trunk exten-sion, activity of the abdominal muscles variedfrom 32 to 68% of the activity i n longissimus.In resisted lateral flexion, as would be expected,th e ip s i l a t e ra l musc le s showed max imumactivity, but the contralateral muscles werealso active at about 10% to 20% of these maxi-mum values (Zetterberg et al 1987).

During maximum voluntary isometric trun kextension, transversus abdominis is the only oneof the abdominal muscles to show marked activ-ity. The co-ordinated patterns seen between th eabdominal muscles has been shown to be taskspecific, with t rans versu s abdominis beingthe muscle most consistently related to changesin IAP (Cresswell et al, 1992).

Differences between t he proper t ies of theabdominal muscles have led to them being cate-gorised as movement synergists and stabilitysynergists (Richardson, 1992). Transversusabdominis, inte rnal oblique and external obliquehave been classified as stability synergists whilerectus abdominis has been classified as a move-ment synergist. The stability synergists tend tobe more deeply placed, and have a predomi-nance of type I muscle fibres. The movementsynergis t has a greater number of type TIfibres, and is preferentially recruited duringrapid performance of trunk exercise. More detailof movement synergists and stability synergistsin relation to the muscle imbalance process isgiven in pa rt four of this article series.

Conclusionh e spine devoid of its musculature is unableto support large loads (Panjabi et (22, 1989).Although the precise mechanisms of spinal sta-bility ar e subject to debate, th e importance ofactive mechanisms relying on muscle contrac-tion is increasingly recognised. It seems logicalthat co-ordinated action of the lumbar andabdominal muscles is essential for stability ofthe lumbar spine. Therefore, it is proposedth at any rehabilitation programme designedto enhance lumbar stabilisation must aim atrehears ing a nd improving th e motor ski l l sinhere nt in active lumbar stabilisation, rathe rtha n simply increasing strength of the t runkmusculature.

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Thisseries of articles wilt be compleied n next monfh s issue of Physiotherapy.

Binding the JournalVolume 80 of Physiotherapy (1994) can be bo und in cloth covers with the title, volume numb er and badg eblocked in b lack on the co ver by Streetsprinters. Th e charge for bin ding the 12 issues will be €17 inclusiveof return UK postage. Thos e who w ish to have their 1994 Journals bound should send the com pleteset of 12 issues (including th e in dex) with a cheque or postal order for the abov e direct to Streetsprin ters,Royston Road, B aldock, Hertfordshire SG7 6NW, to arrive not later than Marc h 31, 1995. It is regrettedthat Streetsprinters cannot supply m issing issues. Back nu mbers can b e bound as above for €20 andwithou t the badge or l ettering for €16. Cheques should be m ade payable to Streetsprinters . Please writeyour full n ame and address on the title page of each issue (in p encil), and p lease pack them carefully.Orders cannot be acc epted unless prepaid.

Physiotherapy. February 1996, v d 81, no 2

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1995 Spinal Stabilisation, 3. Stabilisation Mechanisms of the Lumbar Spine

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