Original research The effects of eccentric hamstring s - PDF document

OriginalresearchTheeffectsofeccentrichamstringstrengthtrainingondynamicjumpingperformanceandisokineticstrengthparameters:apilotstudyontheimplicationsforthepreventionofhamstringinjuriesRossClark,AdamBryant,John-PaulCulgan,BenHartleyMusculoskeletalResearchUnit,CentralQueenslandUniversity,NorthRockhampton,Qld.,4701Australia :Althoughpreviousresearchshowsthatthehamstringlength–tensionrelationshipduringeccentriccontractionsplaysaroleinhamstringinjury,trainingmethodstopromotebenecialadaptationsarestillunclear.Thepurposeofthispilotstudywastodeterminewhetheraneccentrichamstringspecictrainingprogrammeresultsinfavourableadaptations.:EccentrictrainingconsistingoftheNordichamstringexerciseperformedtwiceaweekforfourweeks.Pre-andpost-training 1.IntroductionOneofthemostcommoninjuriesinnearlyallformsofteamandindividualsportsinvolvingthelowerbodyisthehamstringstrain(Bennell&Cossley,1996;Moseley,1996;Orchard,James,Alcott,Carter,&Farhart,2002;Orchard,Wood,Seward,&Broad,1998).Analysisofepidemiolo-gicalinjurystudiesassessingthesesportsconsistentlyrankshamstringstraininjuriesasoneofthemostprevalentfactorsresultinginmissedplayingtimebyathletes( PhysicalTherapyinSport6(2005)67–73www.elsevier.com/locate/yptsp1466-853X/$-seefrontmatter2005ElsevierLtd.Allrightsreserved.doi:10.1016/j.ptsp.2005.02.003 Correspondingauthor.Tel.:610749309704;fax:610749E-mailaddress:r.clark@cqu.edu.au(R.Clark). duringthedescendinglimbofthemuscle’slength–tensionrelationshipthatresultsinhamstringinjuries(Brockett,Morgan,&Proske,2004).Thisispostulatedtobeduetonon-uniformlengtheningofsarcomeresduetosarcomerelengthinstability,resultinginmicroscopicdamagetothemusclesofthehamstring(Gordon,Huxley,&Julian,1966;Morgan,1990).Ifasportrequiresmultipleeccentriccontractions,thesemicroscopicareasofdamagemayresultina“weaklink”ofthemusculature,fromwhichamajorsofttissuetearmayarise(Brockettetal.,2004Thisleadstoanotherpotentialriskfactorcitedinpreviousresearch,thepositionofkneeextensionatwhichpeakhamstringtorqueisproduced(Armstrong,Warren,&Warren,1991;Brockettetal.,2004).Inregardstothelength–tensionrelationship,itissuggestedthatthegreaterthekneeextensionangleatwhichpeaktorqueisproducedthelowerriskofhamstringinjury(Brockettetal.,2004Therefore,trainingtoincreasethekneeextensionangleatwhichpeakhamstringtorqueisproducedwouldresultinreducedeccentrichamstringloadingoccurringduringthisdescendinglimbofthelength–tensionrelation.Aprevioussinglesessioneccentrictrainingstudyfoundthatjustonetrainingsessionresultedinbenecialadaptationstothelength–tensionrelationshipofthehamstrings(Brockett,Morgan,&Proske,2001).However,whetheramultiplesession,longitudinaltrainingprogrammespecicallydesignedtoincreaseeccentrichamstringstrengthalsoaffectsthekneeextensionangleatwhichpeakhamstringtorqueisproducedisunknown.OnemethodofhamstringtrainingknowntoincreaseeccentricstrengthistheNordichamstringexerciseMjølnes,Arnason,Østhagen,Raastad,&Bahr,2004Thismethodoftrainingwasshowntoincreaseeccentrichamstringstrengthmoreeffectivelythantraditionalhamstringcurls.However,theeffectsofthismethodoftrainingonhamstringpositionofpeaktorqueanddynamicperformanceisunknown.ThispilotstudyattemptstodeterminewhethertheNordichamstringexerciseresultsinfavourableadaptationsinrelationtothelength–tensionrelationshipandhamstringstrengthlevels.Analysisofverticaljumpheightwillalsobeusedtodeterminewhetherthetraininginterventionhasanimpactonlowerbodypoweroutput.2.Methodology2.1.OverviewThetestingprotocolconsistedofpre-andpost-traininginterventionisokineticdynamometertestingofthequadricepsandhamstringsatavelocityof60.Peaktorque,positionofpeaktorqueandverticaljumpwereassessedtodeterminelowerbodystrength,effectsonthelength–tensionrelationshipanddynamicpoweroutput.Thesetestingsessionswereseparatedbyasupervised4weektraininginterventionconsistingoftheNordichamstringexercise.2.2.SubjectsNineamateurAustralianRulesfootballplayers(height6.76cm,bodymass10.33kg)participatedinthisexperiment.Thesubjectsinvolvedinthisstudyparticipatedinsportbuthadlittlestrengthtrainingexperience.Potentialsubjectswhohadparticipatedinregularresistancetrainingprogrammeswereexcluded.ThisparticipationconsistedoftwoAustralianRulesfootballtrainingsessionsandoneamateurAustralianRulesfootballgameperweek.Allsubjectshadnopriorhistoryofmusculoskeletalinjuriesandinparticularhamstringinjuriesthatmayhaveaffectedtheresultsofthestudy.EthicalapprovalwasgrantedbyCentralQueenslandUniversity.AllsubjectshadtocompleteanInformedConsentformandpassaPre-ActivityReadinessQuestionnairebeforecommencementofthestudy.Availabilityofpotentialsubjectsforeverysupervisedtrainingandtestingsessionlimitedthesubjectnumberstonine.2.3.IsokineticdynamometryIsokineticmeasurementofconcentric/concentricham-string/quadricepstorquewasmeasuredusingaBiodexSystem3isokineticdynamometer(BiodexMedicalSystems,Shirley,NewYork,USA)samplingat300Hzpersecond.Thissystemhasbeenpreviouslyshowntoproducevalidandreliablemeasurementsoftorqueandposition(Drouin,Valovich-McLeod,Shultz,Gansneder,&Perrin,2004Angularvelocitywassetat60withverepetitionsperformedforeachleg.Thesesetswereperformedwitha2minrestbetweensets.Thisvelocityandmethodoftestingwerechosenbecausetheycloselyresemblethetestingprotocolperformedinapreviousepidemiologicalstudywhichreportedtheimportanceofpositionofpeaktorqueasariskfactorinhamstringinjuries(Brockettetal.,2004Testingwasprecededbyathreeminutestandardisedwarm-uponastationarycycleergometerat50W.Thedominantandnon-dominantlimbsweretestedwiththeorderchosenbyrandomassignment.SubjectswereseatedontheBiodexwiththeirhipjointatapproximately90exion,theirupperbodiessecuredwithdualcrossoverstrapsandtheirwaistsecuredbyawaiststrap.Therangeofmotionofthekneewassetat90offullextension,withtheupperlegsecuredusingthethighstraptolimitexcessmovementofthekneeandlimb.Fullkneeextensionwasstandardisedbetweenthetestingsessionsbyequalizingkneejointangleswithahandheldgoniometer.Thisensuredanaccurateassessmentofkneejointangleatwhichpeaktorquewasproducedbetweenthetestingsessions.AnalysisofisokineticdatawasperformedusingcustomwrittenanalysissoftwareforLabview(NationalR.Clarketal./PhysicalTherapyinSport6(2005)67–73 Instruments,Austin,TX,USA).Thepositionofpeaktorquedatawasmeasuredindegreesfromthestartoftheconcentriccontraction.Therefore,forthequadricepstheresultwasindegreesfrom90kneeexionandforthehamstringstheresultwasindegreesfromfullkneeextension.Thismeansthatalowervalueindegreesforthehamstringsresultsinagreaterangleofkneeextensionwhereasalowervalueindegreesforthequadricepsresultsinalowerangleofkneeextension.2.4.DynamiclowerbodyperformancePreandposttestingofverticaljumpwasperformedtoassesswhetheratrainingprogrammeemphasisingthehamstringmusclegroupwouldaffectlowerbodydynamicpoweroutput.VerticaljumpwasassessedusingaVertec(SwiftPerformanceEquipment,Lismore,NSW,Australia)onthesamesurfaceforbothpreandposttesting.Thistestingwasperformedafterathreeminute,standardisedstationarycyclewarm-upreplicatingtheoneperformedpriortotheisokinetictesting.SubjectswereinstructedtowearthesameshoesforbothtestingsessionstoreducetheinuenceofshoepropertiesonverticaljumpperformanceStefanyshyn&Nigg,2000).Threetrialswereallowedfortheverticaljump,withthemeanofthethreetestsforeachsessiondeterminingthesubjectsverticaljumpheight.Subjectswereinstructedtoperformtheverticaljumpfromastationarypositionwithfeetshoulderwidthapartandnodgetingforvesecondspriortothecountermovementjump.Priortothejumpthesubjectswereinstructedtoraisetheirrighthandintotheairashighaspossibleandpushthetabsonthevertecsothatabaselinelevelcouldbeattained.Whentheathletejumpedtheywererequiredtolightlytapthehighesttabonthevertecsothatanaccuratemeasureofverticaljumpheightcouldbeattained.Theverticaljumpresultwasrecordedfromsubtractionofthebaselinegurefromthehighesttabtouchedwhileinight.2.5.TraininginterventionTheNordichamstringexercisewaschosenforthisstudybecauseoftheeaseofapplicationandminimaltimerequirementsnecessary.Thisexerciseconsistsoftheathletestartinginakneelingposition,withtheirtorsofromthekneesupwardsheldrigidandstraight.Atrainingpartnerappliedpressuretotheathletes’heelstoensurethefeetstayincontactwiththegroundthroughoutthemovement,isolatingthemusclesofthehamstrings.Theathletebeginstheexercisebyslowlyloweringtheirbodyforwardsagainsttheforceofgravitytowardstheground,usingthehamstringstocontroldescentintotheproneposition.Thiseccentriccontractionofthehamstringswasheldforaslongaspossiblebythesubjectsduringloweringofthebodytoensurethatthehamstringswerecontractingataslongalengthaspossible.Oncetheathletecouldnolongercontroldescentusingtheeccentriccontractionofthehamstrings,theyperformedapush-upjumpfollowedbyconcentriccontractionofthehamstringstoraisethemselvesbackuptothestartingposition.TheexerciseprotocolisshowninFigs.1and2,whichdisplaythestartingpositionandtheupperbodygroundcontactrespectively.ThefourweektrainingprotocolwascarriedoutaccordingtotheguidelinesoutlinedinTable1.AlltrainingsessionsweresupervisedbytheresearchersandtookplaceafterthesubjectsAustralianrulesfootball-trainingsessions.2.6.StatisticalanalysesRepeatedmeasuresANOVAwasusedtocomparethepeakandthepositionofpeaktorqueproducedbythequadricepsandhamstringsofthedominantandnon-dominantlimbspriorto,andfollowingeccentrichamstringstrengthtraining.Therefore,eachANOVAdesignincludedtwowithinfactors(testlimb;dominantandnon-dominantandtestoccasion;preandpost). Fig.1.StartingpositionfortheNordicHamstringStretchexercise.Theathletelowerstheirtorsotowardsthegroundusingthehamstringmusclesastheprimarybrakeagainsttheforceofgravity. Fig.2.UpperbodygroundcontactduringtheNordicHamstringStretchexercise.Theathleteperformsadynamicpush-upjumpandattemptstousethismomentumalongwithconcentriccontractionofthehamstringstoreturntothestartingposition.R.Clarketal./PhysicalTherapyinSport6(2005)67–73 Themainpurposeofthisdesignwastodeterminewhethertherewereanysignicantdifferencesinthedependantvariablesasaconsequenceoftestlimbortestoccasion.IntheeventofasignicantmaineffectorinteractionfollowingANOVAcontrasts,posthoccomparisonsofthemeanswereconductedusingtheleastsignicantdifference(LSD)testtodelineatedifferencesamongsttestlimbsortestoccasion.Pairedsamples-testwereusedtocompareverticaljumpperformancepreandposttraining.Thelevelofsignicancewassetat0.05foralltests.AllanalyseswereperformedusingSPSSversion12.3.ResultsThepre-andpost-traininginterventionresultsfortheisokinetictestingincludingpeaktorqueandpositionofpeaktorqueforthequadricepsandhamstringsofbothlimbsarepresentedinTable2.Statisticalanalysisrevealedasignicantmaineffectoftestoccasionforquadricepspeaktorquewithan11.3%reductioninpeaktorquefromthepretoposttestswhenthedatawerepooledacrosstestlimbs(seeFig.3).Statisticalanalysesalsorevealedasignicantmaineffectoftestlimbforthepositionofhamstringpeaktorquewitha36.4%increaseinkneeexionangletowardsfullextension(0kneeexion)recordedforthedominantlimbcomparedtothenon-dominantlimbwhenthedatawerepooledacrosstestoccasions(seeFig.4Inaddition,statisticalanalysisofthepositionofhamstringpeaktorquedataindicatedasignicantmaineffectoftestoccasionwitha19.4%increaseinkneeexionangletowardsfullextension(0kneeexion)inthepost-trainingtestingsessioncomparetopre-trainingsessionwhenthedatawerepooledacrosstestlimbs(seeFig.5).Finally,asignicantincreaseof6.6%wasfoundforverticaljumpheightbetweenthepreandpost-trainingsessionsFig.64.DiscussionThepurposeofthispilotstudywastoassesstheeffectofapredominantlyeccentrichamstringtrainingprogrammeonisokineticvariablesassociatedwithhamstringinjuriesandlowerbodydynamicpower.Thepre-interventionresultsforpositionofpeakhamstringtorquefortheuntrainedsubjectsparticipatinginthisexperimentweresimilartothosefoundinuninjuredeliteathletesinapreviousstudy(Brockettetal.,2004).Thesubjectsinthepresentstudyrecordedslightlyhigherpre-trainingkneeextensionanglesforproductionofpeaktorqueincomparisontotheeliteathletesinthepreviousstudyforbothright(14.9%)andleft(5.9%)hamstrings.However,thepost-interventionresultsforthesubjectsinthepresentstudywerebothlowerthantheresultsforthepreviouslymentionedeliteathletes,by4.9and38%fortherightandleftlegrespectively.Thissuggeststhatthehamstringtrainingprotocoltheeliteathletesinthepreviousstudyareparticipatinginhasnothadagreateffectonpositionofpeakhamstringtorque.Incontrast,thesubjectsinthepresentstudyshoweda19.4%changeinpositionofpeaktorqueforthehamstringmusclesafterjusta4weektrainingintervention.Thisrapidalterationofthelength–tensionrelationshipissimilartothesignicantshiftinpositionofpeakhamstringtorquereportedinaprevioussinglesessioneccentrictrainingstudy(Brockettetal., Table1FourweektrainingprotocolWeekSessionsperweekSets–RepsTechnicalnotes112/5Thesubjectisencouragedtoresistfallingaslongaspossible222/6Subjecttriestoreduceloweringspeed333/6Subjectscanresistfallingevenlonger,andforanincreasednumberof433/8Loadonthesubjectincreasesbyallowingmorespeedinthestartphase,aswellasanothergradualincreaseinAdaptedfromOsloSportsTraumaResearchCenter(2004) Table2Isokineticperformancemeasuresassessedpreandposta4weekeccentrichamstringtraininginterventionIsokineticperformancePrePostDominantNon-dominantDominantNon-dominantLTLPT204.34204.93180.84182.090.900.01*0.9520.00.029.770.00PosPT69.3262.8266.8465.110.060.950.077.64.850.004.34PT98.6199.0097.30103.640.490.680.3619.30.530.190.93PosPT36.7628.2130.7721.610.01*0.01*0.919.011.7710.30.01L,effectforLimb(dominantandnon-dominant);T,effectfortest(pre-andpost-intervention);LT,interactionbetweenlimbandtest;PT,peaktorque;PosPT,positionofpeaktorque.*Indicatessignicantdifference(R.Clarketal./PhysicalTherapyinSport6(2005)67–73 ).Thendingsofthisstudyshowedthatthepositionofpeakhamstringtorqueshiftedtoamoreextendedkneepositionafterthetrainingintervention,whichmayallowforreducedeccentricmuscledamageofthehamstringsoccurringduringeccentriccontractions.Thismaybeduetoareducedlengthofthedescendinglimbofthelengthtensionrelationship,possiblyresultingindiminishednon-uniformlengtheningofthesarcomeresofthehamstringmuscles.Areduceddescendinglimbofthelength-tensionrelationshiphasbeenpreviouslysuggestedtolowertheriskofhamstringinjury(Brockettetal.,2004).Theseresultssuggestthatatrainingprotocoldesignedtoproduceamorefavourablehamstringlength–tensionrelationshipintermsofhamstringinjurypreventioncanbebenecialafteronlyaminimalamountoftrainingsessions.Itisalsoworthnotingthatthepositionofpeakhamstringtorqueinboththepresentandthepreviouslymentionedstudy(Brockettetal.,2004)variedbetweenthedominantandnon-dominantlimb.However,intheuninjuredeliteathletes(Brockettetal.,2004)therewasonlya7%differencebetweenlimbs,whereasinthepresentstudytherewasa30.3%pre-and42.4%post-interventiondifferenceinthepositionofpeakhamstringtorque.Thisbetweenlimbimbalancesuggeststhattheremaybeaneedforunilateraleccentrichamstringtraininginuntrainedathletestoreducethedifferencebetweenthelegsbeforebilateraleccentricspecictrainingcommences.Theresultsofthepresentstudysuggestthatthetraininginterventioncreatedagreatermagnitudeofdifferencebetweentestlimbs.Itappearsthatthelimbwiththeinitiallyhigherkneeextensionanglebenetsmostfromthismethodoftraining,whichislogicalduetothenatureofthetrainingprotocol.Astheathletelowersthemselvestowardsthegroundusingeccentriccontractionofthehamstrings,thelimbwiththehigherkneeextensionangleofpeakhamstringtorquemayberequiredtotakeovercontrolofthemovementtowardstheendoftherepetition.Thisplacesagreatermagnitudeofstressonthelimbwiththegreaterkneeextensionangleofpeaktorque,becauseitmayberequiredtodominatecontrolofthedescendingtorso.Thisgreaterdegreeofoverloadonthealreadydominanthamstringmayresultinenhancedneuromuscularadaptationinthislimbtothetrainingprotocol,furtherincreasingthemagnitudeofimbalancebetweenthelimbs.Thispotentialdrawbacktobilateraleccentrichamstringtrainingwarrantsfurtherexamination. Non-dominantDominantPositionofPeakTorque( * Fig.4.Hamstringjointanglepositionofpeaktorquefromfullkneeextensionbetweenthedominantandnon-dominantlimbs.*Indicatessignicantdifferencebetweendominantandnon-dominantlimbresults PrePostTestingDayPeakTorque(N.m.) * Fig.3.Quadricepspeaktorquepre-andpost-trainingintervention.*Indicatessignicantdifferencebetweenpre-andpost-testingresults PrePostTestingDayPositionofPeakTorque( * Fig.5.Hamstringjointanglepositionofpeaktorquefromfullkneeextensionpre-andpost-trainingintervention.*Indicatessignicantdifferencebetweenpre-andpost-testingresults( PrePostJumpHeight(cm) * Fig.6.Verticaljumpheightmeasuredpre-andpost-trainingintervention.*Indicatessignicantdifferencebetweenpre-andpost-testingresultsR.Clarketal./PhysicalTherapyinSport6(2005)67–73 Despitethebenecialadaptationsintermsofthelength-tensionrelationship,therewasnoincreaseinhamstringconcentricpeaktorqueasaresultofthetrainingintervention.ThisreplicatesthendingsofMjølnesetal.whofoundalimitedeffectofthismethodoftrainingonconcentrichamstringstrength.Anotherinterestingndingofthisstudywasthesignicant11.3%reductioninpeaktorqueproducedbythequadricepsafterthetrainingintervention.Thismaybeduetoanumberoffactors,suchaschangesintheviscoelasticpropertiesofthemuscularunitinresponsetothetrainingstimulusand/orincreasedantagonisticactivationofthehamstringsduringtheconcentricquadricepsphaseofthetestingprotocol(Solomonow,Baratta,&D’Ambrosia,1989).Achangeinthestiffnesspropertiesofthehamstringsand/orincreasedantagonisticactivationmayadverselyaffecttheforceoutputofthequadricepsbyapplyinginternaloppositiontothequadricepscontractioninadditiontotheexternaloppositionsuppliedbytheisokineticdynamometerSolomonowetal.,1989Althoughadramaticreductioninquadricepspeaktorqueandonlyminorchangesinhamstringpeaktorquewereobservedintheopenkineticchaintesting,asignicant6.6%increaseinverticaljumpheightwasreported.ThisrevealedthattheNordichamstringexercisenotonlyappearstohaveabenecialeffectonthelengthtensionrelationshipofthehamstrings,butresultsinenhancedexplosivepowerperformanceinuntrainedathletes.Althoughtherewasareductioninopen-kineticchainquadricepspeaktorqueinthepost-trainingresults,thismaynotcarryoverintoclosedkineticchainjumpingmovements.Furthermore,theminorchangesinhamstringpeaktorquemaynotbethereasonfortheimprovementinverticaljump.Thechangeinhamstringpositionofpeaktorquetowardsamoreextendedkneeangleislikelytohavecontributedtotheincreaseinverticaljumpheight.Thismaybeduetoincreasedjointstabilityofthekneeduringthenaltakeoffphaseofthejumpingmovement,allowingformoreefcienttransferofforcethroughthejoint(Baratta,Solomonow,Zhou,Letson,Chuinard,&D’Ambrosia,1988Previousstudieshaveshownthatthehamstringsplayakeyroleinpreservingjointstiffnessandstabilityduringthedecelerationphasewhichoccurstowardstheterminalstageofthekneeextensionmovement(Barattaetal.,1988;Hagood,Solomonow,Baratta,Zhou,&D’Ambrosia,1990).Thereforeitwouldbeexpectedthattooptimiseforcetransferthroughthekneejointduringdynamicmovementsitwouldbenecessarytomaintainorincreasehamstringcoactivationduringtheterminalphaseofthemovement.However,Barattaetal.(1988)foundthatinuntrainedathleteswhoparticipatedinsportsrequiringrepetitivejumpingmovements,thisactivationoftheantagonisthamstringmusclesduringthenalphaseofthekneeextensionwasmarkedlyreduced.Thissuggeststhatinnon-weighttrainedathletesthehamstringmusclesreducetheirlevelofactivationduringthenalphaseofkneeextensiontopotentiallydecreaseoppositiontothequadriceps,resultinginincreasedforceoutputofthequadricepsattheexpenseofthejointstabilitysuppliedbythehamstrings.Notonlywouldthisreductioninjointstabilityreducetheefciencyofthemovement,butitmayincreasetheriskofkneejointinjury(Barattaetal.,1988).Incontrast,theathletesinvolvedintheBarattaetal.(1988)studywhowereundertakinghamstringspecicweighttrainingproducedsimilaractivationpatternsofthehamstringstocontrolsubjectswhoweresedentary.Theseweighttrainedathletesrecordedsimilarorincreasedactivationofthehamstringsincomparisonwiththesedentarysubjects,suggestingthatperforminghamstringspecicweighttrainingmaintainedorincreasedtheirkneejointstability.Whilethepreviousstudydidnotlookatdynamicperformancecharacteristicsofthesubjects,theresultsofthepresentstudysuggeststhathamstringspecictrainingcanresultinbenecialadaptationstohamstringactivationintermsofjointstabilityduringthenalphaseofkneeextensionaswellasincreasedynamicperformance.4.1.LimitationsDespitethelimitednumberofsubjectsparticipatinginthisstudy,anumberofsignicantndingswereobserved.However,furtherstudiesincorporatinggreatersubjectnumbers,theinclusionofacontrolgroupandmultipletrialsforassessmentofreliabilitywouldhelptodeterminetheroleoflowerbodyeccentrictrainingonperformanceandinjuryriskfactors.Theresultsofthisstudysuggestthateccentrichamstringtrainingmayresultinadaptationsthatreducetheriskofhamstringstraininjury,howeverfurtherstudiesarerequiredbeforethesebenetscanbedeemedconclusive.5.PracticalapplicationsandclinicalrelevanceThendingsofthisstudysuggestthattheNordichamstringexerciseresultsinfavourableadaptationstothelength–tensionrelationshipinthehamstringmusclegroup.AlthoughtheNordichamstringexercisewasfoundtobebenecialintermsofthehamstringspositionofpeaktorque,ithadlittleeffectonoverallpeaktorquevalues.PossiblyacombinationofNordichamstringexercisetrainingandtraditionalhamstringweightliftingmovementsmayprovidebenecialstrengthandlength-tensionadaptationstopreventsofttissuehamstringinjuries.OveralltheresultsofthisstudysuggestthattheNordicHamstringexercise,becauseofitseaseofimplementation,maybeaneffectivemethodofbothenhancingperformanceandreducinginjuryinsub-eliteathletesperforminginsportsrequiringjumpingmovements.R.Clarketal./PhysicalTherapyinSport6(2005)67–73 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