ManualTherapy12(2007)240248OriginalarticleChangesinposturalactivityofthetrunkmusclesfollowingspinalmanipulativetherapyManuelaL.Ferreiraa,b,c,PauloH.Ferreiraa,b,d,PaulW.Hodgesa,aDivisionofPhysiotherapy,TheUniversityofQueensland,BrisbaneQLD4072,AustraliabSchoolofPhysiotherapy,TheUniversityofSydney,Sydney,AustraliacDepartamentodeFisioterapia,PontifcieUniversidadeCatolicadeMinasGerais,BeloHorizonte,BrazildDepartamentodeFisioterapia,UniversidadeFederaldeMinasGerais,BeloHorizonte,BrazilReceived6May2005;receivedinrevisedform10May2006;accepted27June2006AbstractSpinal manipulative therapy (SMT) is common in the management of low back pain (LBP) and has been associated with changesin muscle activity, but evidence is conicting. This study investigated the effect of SMT on trunk muscle activity in postural tasks inpeoplewithandwithout LBP.In 20 subjects(10withLBP and10controls),EMGrecordingsweremadewith ne-wireelectrodesinserted into transversus (TrA), obliquus internus (OI), and externus (OE) abdominis. Rectus abdominis (RA) and anterior deltoidEMG was recorded with surface electrodes. Standing subjects rapidly exed an arm in response to a light, before and after a smallamplitudeendrangerotational lumbarmobilizationatL4-5. Incontrols, therewasnochangeintrunkmuscleEMGduringthepostural perturbation after SMT. In LBP subjects there was an increase in the postural response of OI and an overall increase in OEEMG. There was no change in TrA or RA EMG. These results indicate that SMT changes the functional activity of trunk muscles inpeople with LBP, but has no effect on control subjects. Importantly, SMTincreased the activityof the oblique abdominalmuscleswithnochangeinthedeeptrunkmuscleTrA,whichisoftenthetargetofexerciseinterventions.r 2006ElsevierLtd.Allrightsreserved.Keywords:Spinalmanipulativetherapy;Motorcontrol;Transversusabdominis;Fine-wireEMG1. IntroductionSpinal manipulative therapy (SMT), dened asmanual loading of the spine using short or long leveragemethods,isoneofthemostcommonapproachesinthemanagement of lowbackpain(LBP). TheefcacyofSMT on clinical outcome measures for people with LBPhas been investigated in several systematic reviews whichconcludethat it leadstoclinicallysignicant improve-ments inpainandfunction(vanTulder et al., 1997;Ferreira et al., 2002; Assendelft et al., 2003). Despite thepositive clinical benet, the physiological mechanismsresponsiblefortheseeffectsarestillunclear.Several possiblemechanisms havebeendiscussedinthe literature. For instance, SMThas beenassociatedwith sympathoexcitatory effects (Vicenzino et al., 1998),changes inpassive andactive spinal range of motion(Nilssonet al., 1996; LehmanandMcGill, 2001), andeffectssuchasenhancedproductionoftumournecrosisfactor andsubstance P(Brennanet al., 1992). Alter-natively, it hasbeenarguedthat SMTchangesmuscleactivity. However, fewstudies have investigated theeffects onparameters of muscleactivationandresultsare contradictory. For instance increased (Herzog et al.,1999) and decreased (Dishman and Bulbulian, 2000;Dishman and Bulbulian, 2001) activity of the paraspinalmuscleshavebeenreportedinresponsetoSMT.Several factorsmayexplaintheinconsistencyof theresults. First, studies have used a variety of manipulativetechniques. ThetermSMTisusedtodescribeabroadARTICLEINPRESSwww.elsevier.com/locate/math1356-689X/$ - seefrontmatter r 2006ElsevierLtd.Allrightsreserved.doi:10.1016/j.math.2006.06.015Correspondingauthor.Tel.:+61 7 3365 2008;fax:+61 7 3365 2775.E-mailaddress:p.hodges@shrs.uq.edu.au(P.W.Hodges).spectrumof techniques that mayor maynot includeaudible cavitation. Second, a range of muscle functions,fromresting activity to reex amplitude have beentested. Finally, most studies have involved peoplewithout LBP. Thus, fromthe present data it is notpossible to determine whether manipulation modiesthe activityof the trunkmuscles inpeople withLBPduringfunctionalactivities.One functional taskthat has beenevaluatedexten-sivelyistheposturalresponseofthetrunkmusclestoarapid limbmovement.In this task trunkmuscleactivityisinitiatedpriortomovementtopreparethespinefortheperturbationfromlimbmovement (Belenkii et al.,1967; Aruin and Latash, 1995; Hodges and Richardson,1997). This task provides an ideal model to evaluate theeffect of SMTas it provides a measure of the pre-plannedstrategyusedbythecentral nervoussystemtocontrol the trunk muscles. Furthermore, people withrecurrentLBPhavebeenfoundtohavechangesinthispre-plannedpostural adjustment. Notably, activity oftransversus abdominis (TrA), the deepest abdominalmuscles, is delayed (Hodges and Richardson, 1996;Hodges and Richardson, 1998). Although impairedactivityof thismuscleisarelativelyconsistent ndinginLBP, activity of supercial trunkmuscles is oftenincreased(Arendt-Nielsenetal., 1996; Radeboldetal.,2000;Hodgesetal.,2003a, b).We hypothesized that manipulative therapy wouldchangetheresponseof thetrunkmuscles, but onthebasis of previous data it was not possible topredictwhether activity would be increased or decreased. Thus,theaims of thepresent studywere, rst, todeterminewhetherthepre-plannedpostural activityof thetrunkmuscles could be modied by SMTand second, toinvestigate whether the effect differedbetweenpeoplewithandwithoutLBP.2. Materialsandmethods2.1. SubjectsTwentysubjects (10withahistoryof LBPand10controls) volunteeredfor this experiment. The subjectdemographics are presentedinTable 1andwere notdifferent between the two groups. Subjects wereexcluded if they had any respiratory or neurologicaldisorder,musculoskeletalpainelsewhereinthespineorlower limbs or if they had been pregnant in the previous2 years. Subjects in the control group were also excludedif they had a history of LBP that had restricted functionorfor whichtheyhadsought medical oralliedhealthintervention.TobeincludedintheLBPgroup, volunteersneededto have a history of at least one episode of LBP that hadlimited function or work in the past 18 monthsand hadanepisodeof LBPwithinthepast 6months. Patientswere excludedif they hadneurological signs, specicspinal pathology (e.g. malignancy, inammatory joint orbone disease), if they had undergone back surgery in thepast 12months,oriftheycouldnottoleratea gradeIVlumbar rotarymobilizationtechnique(nosubject wasexcludedonthebasisof thiscriteria). Themean(SD)painintensity(past 7days) was 3.2(2.3) ona10 cmvisual analogue scale (VAS) and the disability score(Roland Morris) was 3.2 (1.8). All included patientspresentedwithLBPofatleast3monthsduration.ThestudywasapprovedbytheInstitutionalMedicalResearch Ethics Committee and all procedures wereconducted in accordance with the declaration ofHelsinki.2.2. ElectromyographyEMG recordings were made with surface andintramuscularne-wireelectrodes. Fine-wireelectrodeswere fabricated from two strands of Teon-coatedstainless-steel wire (75 mm diameter, A-M systems,USA) threaded into a hypodermic needle(0.6 32 mm) andinsertedwithguidance fromultra-soundimagingintotheright ventro-lateral abdominalwall muscles: TrA, obliquus internus abdominis (OI),and obliquus externus abdominis (OE) half-way be-tweentheiliaccrestanddistalborderoftheribcageintheanterioraxillaryline(Hodgesetal., 1999). Surfaceelectrodes were placedonleft anterior deltoidmuscleapproximately inparallel withthe muscle bres, andover the muscle belly of the right rectus abdominis (RA).EMG data were amplied 2000 times, band-pass lteredbetween20and1 kHz(Neurolong,Digitimer,UK)andsampled at 2 kHz using a Power-1401 and Spike-2software(CambridgeElectronicDesign,UK).2.3. ProcedureSubjects rapidly exed or extended the left upper limbin response to visual command in a choice reaction timetask. Twodirections of movement were performedtolimit the predictability of the response, but only theexion data was analysed. Ten repetitions in eachdirectionwereperformedinrandomorder beforeandARTICLEINPRESSTable1DemographicdetailsControlgroupmean(SD)LBPgroupmean(SD)P-valueaAge(years) 33(11) 28(5) P 0.21Height(cm) 159(38) 171(10) P 0.35Weight(kg) 68(13) 69(13) P 0.95LBP,lowbackpain.aP-valuesreferstot-testforindependentsamples.M.L.Ferreiraetal./ManualTherapy12(2007)240248 241after the spinal manipulative technique (see below). Thesecondsetoftenarmmovementscommencedimmedi-atelyafterthecessationofthemanipulativetechnique.2.4. SpinalmanipulativetechniqueA specic small amplitude rotational end-range(grade IV) oscillatory mobilization technique wasdirected to at L4L5 with subjects in left side-lying(Maitland et al., 2001). This procedure was standardizedfor all patients because of the placement of electrodes ontherightsideofthetrunk. AlthoughitisunlikelythatL4L5 was the symptomatic level in all patients weconsideredthistechniquetobeappropriatetotest theaimsoftheexperimentforanumberofreasons. First,althoughattemptsaremadetofocusthetechniquetoatargetsegment, movementwill occuroveranumberoflevelsandsecond,recentdatasuggestthatthatpatientsreportsignicantdecreasesinpainevenwhenmobiliz-ingnon-painful lumbar segments (Chiradejnant et al.,2003). Thetechniquewasrepeatedthreetimesfor30 s,with oscillations at 1 Hz, with feedback from ametronome.Themobilizationtechniquewasperformedat thesamespinal level (L4L5) for all subjects, eventhoughthis might not have beenthe level of paininsomesubjects. PainintensitywasmeasuredonaVASbeforeandaftertheapplicationofthetechnique.2.5. DataanalysisIn order to investigate both temporal and spatialaspects of EMG, root mean square (RMS) EMGamplitude was calculated during four 25 ms epochsbeforethe onset of deltoid EMG and four 25 ms epochsaftertheonsetofdeltoidEMG.Thisanalysis techniquemeasures the postural response without problemsassociatedwithdetectionof EMGonset of the trunkmuscles. EMGonset detectionwasdifcultduetothehighdegreeofbaselineactivity,particularlyinOEafterthe performance of the manipulative technique. Datawere normalized to the epoch with the greatestamplitude in the pre-manipulation condition. Thisnormalization method provides a high sensitivity tocomparethepre-andpost-manipulationconditionsforeachgroup, but doesnot permit comparisonof EMGamplitude between muscles or between groups, althoughthepatternof changes inpatternof activitycouldbecompared between groups. Data were not normalized toEMG amplitude in a maximum voluntary contraction asthese values are not possible to obtain reliably in peoplewith LBP (Allison et al., 1998) and data were notnormalizedtoasubmaximal taskaspeoplewithLBParelikelytouseanabnormal strategyduringsubmax-imal tasks, making it an invalid reference. Furthermore,recent data suggests that normalization to a submaximaltaskmay increase variability (Urquhart et al., 2005).However,asthisstudywasarepeatedmeasuresdesignwitheachsubjectactingastheirowncontrolsnormal-ization would not affect the results, but normalization topre-manipulation peak amplitudenormalizes all data toasimilarscaleforgraphicalrepresentationofthedata.Statistical analysis involved a three-way repeatedmeasures analysis of variance (ANOVA) with tworepeatedmeasures factors (pre- andpost-mobilizationandepoch) andoneindependent factor(group). Post-hocDuncantestswereusedwhenappropriate. Duetotheinabilitytocomparebetweenmusclesasaresultofthe normalization procedure analyses were run sepa-rately for each muscle (RA, OE, OI and TrA). The alphalevelwassetat0.05.3. Results3.1. TrAWhensubjectsrapidlymovedtheleftarm,aburstofEMGactivity of TrAoccurred in association withdeltoid EMG. There was a trend for the activity to occurin an earlier epoch for TrA in the control groupcomparedtotheLBPgroup. Fig. 1showsanincreasein EMG activity greater than 10% above baseline EMGinepochs3and4inthecontrolgroup.Thisincreaseisnot asevident intheLBPgroupuntil epoch5. Whenarmmovement wasperformedaftertheapplicationofspinal mobilizationtherewasnochangeintheactivityofTrAinthecontrolorLBPsubjectscomparedtothepre-mobilization condition (Fig. 1). Table 2 presents themaineffectsandinteractions.3.2. OISimilartoTrA,aburstofOIEMGactivityoccurredinconjunctionwitharmmovement.InbothgroupstheincreaseinEMGactivitytendedtostart intheepochprior to the onset of deltoid EMG. Following themobilization technique there was no change in thepostural response for the control group (P 0:20).Main effects and interactions are shown in Table 2.Althoughtherewasnoconsistenteffectforthecontrolgroup, OIEMGwasincreasedaftermobilizationin2subjects. Thisincreasedthevariabilityof theresponse(Fig. 2). Incontrast, OI EMGactivitywas increasedfollowingtheonsetofdeltoidEMG(EpochsE5: 025,E6: 2550, E7: 5075, E8: 75100 ms) for subjects in theLBPgroup(all: Po0:001). Notably, mobilizationdidnot increase the background activity prior to armmovement, only the postural adjustment associated withthe limbmovement. Variability was increasedinthisgroup. Three subjects hadanaccentuatedincrease inmuscle activity, and two subjects had a decrease inEMGactivityinepochs13.ARTICLEINPRESSM.L.Ferreiraetal./ManualTherapy12(2007)240248 242ARTICLEINPRESSTable2Maineffectsandinteractionsforcomparisonbetweengroups,betweenepochsandpre-andpost-manipulativetechnique(Manip)Maineffect/interaction TrA OI OE RAF P F P F P F PEpoch 5.27 o0.01* 2.44 0.02* 0.5 0.83 1.03 0.42Group 0.32 0.57 5.07 0.03* 29.9 o0.01* 10.88 o0.01*Epochgoup 0.28 0.96 0.51 0.82 0.07 0.99 0.42 0.89Manip 0.40 0.53 16.81 o0.01* 11.99 o0.01* 2.61 0.11Manipepoch 0.17 0.99 0.45 0.87 0.17 0.99 0.02 1.00Manipgroup 3.33 0.07 4.73 0.03* 16.25 o0.01* 1.17 0.28Manipepochgroup 0.23 0.98 0.59 0.76 0.25 0.97 0.11 1.00*po0.05.-0.50.00.51.01.52.02.53.0*Pre-mobilisationPost-mobilisationLBPE1 E2 E3 E4 E5 E6 E7 E8Prop. pre-mobilisation EMGControl subjectsE1 E2 E3 E4 E5 E6 E7 E8***Fig. 2. Change in OI EMG with SMT. EMG amplitude is shown for four 25-ms epochs prior to onset of deltoid (E1E4) and four after the onset ofdeltoid. The dashed line divides the epochs before and after the onset of deltoid EMG. EMG is presented as a proportion of peak activity recordedacrosstheeightepochsduringthepre-mobilizationtrial.Condenceintervals(95%)areshown.AnincreaseinEMGactivitywasobservedpost-manipulation,aftertheonsetofdeltoidintheLBPgroup.*Po0.001.00.20.40.60.81.01.21.4Pre-mobilisationPost-mobilisationLBPE1 E2 E3 E4 E5 E6 E7 E8Prop. pre-mobilisation EMGControl subjectsE1 E2 E3 E4 E5 E6 E7 E8Fig.1. ChangeinTrAEMGwithSMT.EMGamplitudeisshownforfour25-msepochspriortoonsetofdeltoidEMG(E1E4)andfour25-msepochsaftertheonsetofdeltoid(E5E8).ThedashedlinedividestheepochsbeforeandaftertheonsetofdeltoidEMG.EMGispresentedasaproportionofpeakactivityrecordedacrosstheeightepochsduringthepre-mobilizationtrial.Condenceintervals(95%)areshown.M.L.Ferreiraetal./ManualTherapy12(2007)240248 2433.3. OEWhensubjectswithandwithout LBPexedtheleftupper limb in response to a visual stimulus, there was anincreaseOEEMG.Therewasatendencyforthisbursttoinitiateprior totheonset of deltoidEMGinbothgroups. Afterthe application of SMT, no change inOEactivitywas observedas aconsequenceof armmove-ment for thecontrol subjects (P 0:68). MaineffectsandinteractionsareshowninTable2. Intwosubjectsthere was a decrease in OE EMG in epochs 25 after themobilization, whichexplainstheincreaseinvariabilityin this group. In contrast, when subjects with LBP exedtheirleftarmafterthemobilization,anoverallincreasein muscle activity was observed for all epochs (Po0:01).That is, OEEMGwas increased even prior to armmovement. Likewise, the group with LBP presentedgreatervariabilityafterthemobilization(Fig.3).3.4. RAEMGactivityforRAwasincreasedforbothgroupspriortotheonsetofdeltoidactivity(E4: -25to0). Nochange EMG was observed after the mobilization in thecontrol subjects. Althoughnot signicant, therewasatrendfor RAactivitytoincreaseinpeoplewithLBPafter manipulation(Fig. 4). Maineffects andinterac-tionsareshowninTable2.3.5. PainAverage pain intensity immediately before themanipulation was 1.1 (1.7) and immediatelyafter the procedure was 0.7 (2.2). There was nosignicant change in pain intensity following SMT(P 0:34).ARTICLEINPRESS-1.0-0.50.00.51.01.52.02.53.0Pre-mobilisationPost-mobilisationLBPE1 E2 E3 E4 E5 E6 E7 E8Prop. pre-mobilisation EMGControl subjectsE1 E2 E3 E4 E5 E6 E7 E8** ******Fig. 3. Change in OE EMG with SMT. EMG amplitude is shown for four 25-ms epochs prior to onset of deltoid (E1E4) and four after the onset ofdeltoid. The dashed line divides the epochs before and after the onset of deltoid EMG. EMG is presented as a proportion of peak activity recordedacross the eight epochs during the pre-mobilization trial. Condenceintervals (95%) are shown. An overall increase in EMG activity was observedpost-manipulationonlyintheLBPgroup.*Po0.001.Pre-mobilisationPost-mobilisationLBPE1 E2 E3 E4 E5 E6 E7 E800.250.50.751.01.251.51.75Prop. pre-mobilisation EMGControl subjectsE1 E2 E3 E4 E5 E6 E7 E8Fig. 4. Change in RA EMG with SMT. EMG amplitude is shown for four 25-ms epochs prior to onset of deltoid (E1E4) and four after the onset ofdeltoid. The dashed line divides the epochs before and after the onset of deltoid EMG. EMG is presented as a proportion of peak activity recordedacrosstheeightepochsduringthepre-mobilizationtrial.Condenceintervals(95%)areshown.M.L.Ferreiraetal./ManualTherapy12(2007)240248 2444. DiscussionThe results of this study demonstrate that pre-plannedpostural activity of the trunk muscles can be modied bySMT performed as a small amplitude oscillation withoutcavitation. Thiswasevidencedbytheincreasedampli-tudeof OEandOIEMGasacomponent of posturaladjustment associatedwitharmmovement afterSMT.However,thischangewasonlyobservedinpeoplewithLBP. Incontrast tothesupercial obliqueabdominalmuscles, SMTdidnot affect the postural response ofTrAineithergroup.There are several possible mechanisms for the increaseinEMGactivityfollowingSMTthatarerelatedtotheeffectofmanualtechniquesonmotoneuronexcitabilityvia direct effects of stimulation of joint and muscleafferents, theeffectofmanual techniquesonpain, andother neurophysiological mechanisms. However, therehas been considerable debate and ndings are ofteninconsistent.Oneextensivelyarguedpossibilityisthatexcitabilityof spinal muscle motoneurons is changedby afferentinput from stimulation of mechanoreceptors in the jointcapsules(Indahl et al., 1997), ligaments(Indahl et al.,1997), muscles (Herzog et al., 1999) and cutaneousreceptors(Herzogetal., 1999). However, somestudiesreportincreasedexcitability(Herzogetal.,1999;Kellerand Colloca, 2000), while others report decreasedactivity (Dishman and Bulbulian, 2000; LehmanandMcGill, 2001; Lehman et al., 2001). These discrepanciesmaybeexplainedtosomeextent bythemethodsthathave been used to evaluate muscle activity. For instance,Keller and Colloca (2000) reported increased erectorspinaeactivityduringtrunkextensioninprone,Herzoget al. (1999) reportedshort latency excitatory muscleactivity in response to manipulative thrusts, whereasDishmanandBulbulian(2000)reportedreducedHoff-man-reex (H-reex) amplitude in the gastrocnemiusmuscle in the leg, and Lehman and McGill (2001)reported decreased muscle response to a painfulmechanical stimulus appliedover thespinous process.Thus, fewstudies have measuredthe same parameterand it is difcult to predict from existing data how trunkmuscleactivitywouldchangeduringapostural adjust-mentinassociationwitharmmovement.Short latency responses in paraspinal muscles tomanipulative and mobilization techniques have beenextensivelyreported(Herzoget al., 1999; CollocaandKeller,2001;Shirleyetal.,2002).Theseresponseshavebeenarguedtobemediatedbystimulationof musclespindlesandhaveshortduration(100400 ms)(Herzoget al., 1999). However, it is unclear whether theseresponsesareassociatedwithchangesinongoingEMGactivity, although ongoing reduction in abdominalmuscle activity has been identied in a single case reportby Herzog et al. (1999) following initial excitation.Similarly it is difcult to interpret data regardingamplitudeofH-reexesinlowerlegmuscles. Althoughtheseresponseshavebeenshowntobereduced(Dish-manandBulbulian, 2000), andit is arguedthat themotoneurons that innervate these muscles lie in thelumbosacral spinal cord, it is unlikely that theseresponses would reect the properties of the distinctmotoneurons to the spinal muscles. Furthermore,H-reexamplitudeisinuencedbypre-synapticeffectsandmaynotreectchangesinmotoneuronexcitability(Rudomin, 2002). Finally, the response of the paraspinalmuscles toamechanical stimulus tothesegment mayreect changes in local reex mechanisms and notfunctionalactivityofthetrunkmuscles.Studies that haveinvestigatedtrunkmuscleactivityduring trunk movements also have divergent results.Keller andColloca (2000) reportedincreasedactivityduring back extension. In contrast, Lehman and McGill(2001) foundnoconsistent change inmuscle activityduring trunk movements in standing, but reportedatendency for decreased activity in some muscles in somesubjects. Consistent with the data of Keller and Colloca(2000),weidentiedasignicantincreaseinactivityofOEandOI, andatrendforincreasedRAEMG. Thetask used in the present study involved evaluation of theactivityof the trunkmuscles inassociationwitharmmovement. These responses are consideredtobe pre-programmed by the nervous system as they are initiatedin advance of limb movement (Belenkii et al., 1967;Bouisset andZattara, 1981). As such, theyreect thestrategy used by the nervous system to prepare the bodyfor the perturbation resulting from the movement(Belenkii et al., 1967; Hodges et al., 1999). Notably,temporal and spatial parameters of the anticipatorypostural adjustments are matched to the timing,directionandamplitudeoftheperturbationfromlimbmovement (Bouisset and Zattara, 1981; Aruin andLatash, 1995; Hodges andRichardson, 1997; Hodgeset al., 1999). Thus, changes inEMGamplitudecouldreect either changes in motoneuron excitability (i.e.largerresponseinitiatedforthesamedescendingdrive),or changes indescending drive (i.e. increased outputfrom higher centres). In terms of modication ofdescending drive to the motoneuron pool, if SMTmodies the afferent discharge fromthe peripheralmechanoreceptors this may modify the descendingpostural response if input regardingthe status of thespine waschanged. It has beenshown that excitation of1a afferents by mechanical stimulation such as vibrationmodies the perception of the position of the spine(Brumagneetal., 1999)andleadstomodiedposturalresponses (Kasai et al., 2002). Although the present datadonotallowdifferentiationbetweenspinalorsuprasp-inalmechanisms,thedatadoindicatethattheposturalresponse of the trunk muscles is modied by SMT.Importantly;thedataindicatethatthisisnotageneralARTICLEINPRESSM.L.Ferreiraetal./ManualTherapy12(2007)240248 245response, as not all muscles responded in a similarmanner. Whynochangewasidentiedintheresponseof TrA following SMT is unclear. However, the functionofthismusclehasbeenshowntobeimpairedinpeoplewithLBP(HodgesandRichardson, 1996; HodgesandRichardson,1998),andthismaynotberesolvedbythemechanicalstimulus.Thisstudyevaluatedarotationalspinalmobilizationwithout thrust. This technique is likelytostretchtheabdominal muscles, andthus stimulatemusclespindleafferentsintheobliquemuscles. Itispossiblethatthisstimulus mayhave modiedexcitabilityof abdominalmotoneurons. However, the rotational mobilizationtechniqueappliedinthis studyrotatedthepelvis andspinetotherightwhichwouldshortenthe rightOI andstretch the OE, fromwhich EMGwas recorded. AsactivityofbothOIandOEwasincreasedthechangeinlengthof hemuscles withtheprocedure(oppositeforthese muscles) is unlikely toexplainthe increasedinactivityobservedforbothmuscles.The present data indicate that changes following SMTwereonlypresentinpeoplewithLBP, andnochangeswere identied in healthy control subjects. This isconsistent with previous studies. For instance, decreasedparaspinal muscle activity in response to painfulstimulation over the spinous process following SMThas been shown to occur only at painful segments(LehmanandMcGill, 2001). Althoughthereasonthatresponses were only changed in people LBP is unclear, itmayrelatetothefunctionofthemechanoreceptorsordue to the already abnormal control of the trunkmuscles in people with pain. Several studies havereportedthatproprioceptioninthespineisreducedinpeoplewithLBP(Gill andCallaghan, 1998;Brumagneet al., 2000), thusafferent stimulationfromSMTmayleadtodifferent responses. Interms of control, it isincreasinglyacceptedthat motor control of the trunkmuscles is modiedinpeoplewithLBP. For instanceactivityof thedeepabdominal muscle, TrAisdelayed(Hodges and Richardson, 1996) or reduced (Hodges et al.,2004), reex responses of the paraspinal muscles aremodied(Leinonenet al., 2001), co-contractionof theobliqueabdominal andparaspinal musclesisincreased(Radebold et al., 2000), and paraspinal muscle activity isincreasedduringgait(Arendt-Nielsenetal., 1996)andtrunk exion (Zedka et al., 1999). Thus, changes may beapparentinthispopulation,butnotpain-freecontrols,duetopre-existing changes inmusclecoordination. Forexample, activityof thequadricepshasbeenshowntoincrease as a result of reduced inhibition following SMTto the sacroiliac joint (Suter et al., 1999). Thus, thechangethatwasapparentduetopre-existingdecit intheabilitytodrivethemuscle,andnochangewouldbeexpected if the motoneurons were not inhibited initially.Thus, thepre-existingstatusofthetrunkmusclesmaydeterminewhetheractivityismodiedbySMT.Previousauthorshavearguedthatchangesinmuscleactivity may be explained by reduction in pain. Forinstance cervical mobilizationproduces a hypoalgesiceffect (i.e. increased pressure pain thresholds anddecreased visual analogue scores), which has beenassociated with a sympatho-excitatory effect and de-creasedsupercial neckexormuscleactivity(Sterlinget al., 2001). Those data are consistent with thehypothesis that SMTactivates descending inhibitorypathwaysmediatedthroughthemidbrainperiaqueduc-tal grey region (Vicenzino et al., 1998), which could alsoberesponsiblefor themotor responseassociatedwithSMT. Consistent withthepresent data, animal studiesindicate that activation of the dorsal periaqueductalgreyregioninduces motor facilitation(Lovick, 1992).In the lumbar spine, previous studies have identiedthat muscle responses tomechanical painstimuli arechangedbySMTonlywhenit is appliedtoapainfulspinal segment (LehmanandMcGill, 2001). However,thismechanismisunlikelytoexplaintheresult of ourstudyas painlevels didnot change signicantlyaftertheSMT.A notable nding of the present study was thevariabilitybetweenindividuals. This is consistent withseveral previous studies (Herzoget al., 1999; Lehmanet al., 2001). Without complete understanding of themechanismforSMTtochangepostural responsesitisdifculttospeculateonthefactorscontributingtothevariability. However, theLBPpopulationwas hetero-geneous and differences in pathology, pain intensity andfunctional presentation may be responsible for thedifferences. Furthermore, the SMTtechnique appliedin thisstudy wasaimed at a consistentsegment, L4L5,and this is unlikely to be symptomatic in all individuals.AspreviousstudieshavefoundthatresponsesareonlymodiedbySMTatthepainful segment(LehmanandMcGill, 2001), this may account for some variability. Inaddition, although pain relief from the mobilization wasnot signicant across the group, some individuals didreport a reduction in symptoms and this may havecontributedtoindividualvariation.Was the change in muscle activity observed after SMTpositivefor clinical improvement? AlthoughSMThasbeenshowntoreducepaininpeoplewithacuteLBP(Ferreira et al., 2002), whether the change inmuscleresponseinducedbySMTisbenecial forpeoplewithchronic LBP is unclear. Increased activity of the obliqueabdominals has beenidentiedas acommonstrategyusedbypeoplewithLBPtoincreasethestabilityofthespine (Radeboldet al., 2000; vanDieenet al., 2003).Whether a further increase in activity of these muscles isbenecial is debatable. For instance, augmented activityofthesemusclesincreasesspinalloading(McGilletal.,2003), which may have negative consequences in thelongterm. Infact, some contemporaryexercise inter-ventions, which have been shown to be effective inARTICLEINPRESSM.L.Ferreiraetal./ManualTherapy12(2007)240248 246management of people with acute and chronic LBP(OSullivan et al., 1997; Hides et al., 2001), aimtonormalize (i.e. reduce) activity of the supercial muscles(Richardsonetal., 1999). Furthermore, theseinterven-tions argue that activity of deeper trunk muscles, such asTrA, whichhavebeenshowntobeimpairedinpeoplewithLBP(HodgesandRichardson, 1996; HodgesandRichardson, 1998), andhavebeenshowntocontributeto inter-vertebral control (Hodges et al., 2003a, b),shouldbeaugmented. However, SMTdidnot changethe recruitment of this muscle. These results suggestmanipulativetherapyisunlikelytobeanalternativetospecic exercise interventions to improve the recruit-mentofdeepabdominal muscles, althoughithasbeenshown that SMT can change supercial abdominalmuscle recruitment. Further, biomechanical and clinicalstudiesarerequiredtodeterminewhetherthechangeinmuscle activity is benecial to the health of the spine, orleadstofurthereffects.Three methodological issues require consideration.First, EMG was normalized to the peak activityrecordedforeachmusclebeforeSMT. Asaresultitisnot possible tocompare betweenmuscles or betweensubject groups. Moreover, it does not allow comparisonofongoingbackgroundactivitybetweengroups. How-ever, it is not practical to normalize to maximumcontractions in people with LBP as they do not performtrue maximal efforts (Allison et al., 1998). The objectiveof thestudywastoevaluatetheeffect ofSMTontheactivity ofthe trunkmusclesand normalizationtopeakactivityincreasesthesensitivitytochangeinarepeatedmeasures analysis. Second, treatmenttechnique was notpragmatically performed, since the targetedlevel wasnot always the most symptomatic and patients were notalways positionedonthe non-painful side. This mayhaveinuencedtheresults. However, itisunlikelythatthis would explain the variability in response asmobilizationtoL4L5will affectadjacentsegments. Afollow-up study would clarify this issue. 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