Neuromusculoskeletal Models Provide Insights into the Mechanisms ...

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www.acsm-essr.org ..... Sports 37: 1931Y1938, 2005. 14. Thelen, D.G., E.S. Chumanov, D.M. Hoerth, T.M. Best, S.C. Swanson,. L. Li, M. Young, and B.C. ...

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ARTICLE

NeuromusculoskeletalModelsProvideInsights
intotheMechanismsandRehabilitationof
HamstringStrains
DarrylG.Thelen,
1
ElizabethS.Chumanov,
1
MarcA.Sherry,
2
andBryanC.Heiderscheit
1
1
UniversityofWisconsin-Madison,Madison,WI;and
2
UniversityofWisconsinHealthSportsMedicineCenter,
Madison,WI

THELEN,D.G.,E.S.CHUMANOV,M.A.SHERRY,andB.C.HEIDERSCHEIT.Neuromusculoskeletalmodelsprovide
insightsintothemechanismsandrehabilitationofhamstringstrains.
Exerc.SportSci.Rev.,
Vol.34,No.3,pp.135
Y
141,2006.
Neuromusculoskeletalmodelsareusedtoinvestigatehamstringmechanicsduringsprinting.Weshowthatpeakhamstringstretchoccurs
duringlateswingphaseandisinvariantwithspeed,butdoesdependontendoncomplianceandtheactionofothermusclesinthe
lumbopelvicregion.Theinsightsgainedarerelevantforimprovingthescientificbasisofhamstringstraininjurypreventionand
rehabilitationprograms.
KeyWords:
muscleinjury,tendoncompliance,stretchshorteningcycle,forwarddynamics,computer
simulation,biomechanics,motionanalysis,sprinting

INTRODUCTION
Acutehamstringstrainsareacommoninjuryinsports
involvingsprinting.Straininjuriesarecharacterizedby
observabledisruptionofthemusculotendonjunction(7),
withpostinjuryremodelinginvolvingbothscartissue
formationandmuscleregeneration(6).Theinjurycan
causeanathletetomissafewdaystoafewweeksofsport.
Moreproblematicisthehighrecurrencerate,withapprox-
imatelyoneofthreeathletesreinjuringwithinayearof
returningtosport(9).Theseobservationshighlightthe
prevalenceofhamstringstraininjuriesandthechallengein
preventingtheinitialinjuryandsubsequentreinjury.
Theresidualeffectsofapriorhamstringstrainmaybe
identifiable,andtheassociatedriskforreinjuryreducibleusing
newapproaches.Forexample,Proske
etal.
(10)identifieda
shiftintheisokinetickneestrengthprofileinpreviously
injuredlimbs,aneffectthatmaybeamendablevialength-
eningcontractiontraining.SherryandBest(11)haveshown
thatarehabilitationprogramfocusedonearlymovementand

Addressforcorrespondence:
DarrylG.Thelen,Ph.D.,DepartmentofMechanical
Engineering,UniversityofWisconsin-Madison,1513UniversityAve.,Madison,WI
53706(E-mail:[e-mail]).
Acceptedforpublication:
April20,2006.
AssociateEditor:ChrisMcGibbon,Ph.D.
0091-6331/3403/135
Y
141
ExerciseandSportSciencesReviews
Copyright
*
2006bytheAmericanCollegeofSportsMedicine

oCypirhg t @0260b yht emArecinaC loelego fpSrostM dei531

icen .neuromuscularcontroldramaticallyreducedhamstringreinjury
ratescomparedwithatraditionalstretchingandstrengthening
approach.Althoughthesearepromisingobservations,the
underlyingmechanismsarenotcompletelyunderstood.
Wehaveusedaneuromusculoskeletalmodelofsprinting
toanalyzepotentialhamstringinjurymechanisms.The
modeldescribestherelationshipamongmuscleexcitations,
activationdynamics,musculotendoncontractionmecha-
nics,andsegmentalaccelerations(13,15).Thus,themodel
hasallowedustorelatemechanicsatthemusclelevelto
themovementproducedatthewholebodylevel.Inthis
article,wereviewtheuseofaneuromusculoskeletalmodel
toinvestigatetheeffectsofsprintingspeed,musculotendon
properties,andcoordinationonhamstringmechanics
duringsprinting.Theinsightsgainedarerelevantfor
improvingthescientificbasisofinjurypreventionand
rehabilitationprograms.

HAMSTRINGINJURYREHABILITATION

Hamstringstraininjuriesmostcommonlyoccurinthe
bicepsfemorislongheadandexhibitastrongtendencyto
recur.Forexample,imaginganalysisof170recentlyinjured
athletesfoundthatapproximately80%ofhamstringstrain
injuriesinvolvedthebicepsfemoris(7).Furthermore,a
reviewof858hamstringstrainsinAustralianfootballers
showedtherateofrecurrencewas12.6%duringthefirstweek
ofreturntosportand8.1%forthesecondweek.Thecumulative

nUuahtrozider peorudtcoi nfot ih sraitlc esip orihibet.d
riskforreinjuryforthe22-wkseasonwas30.6%(9).Thehigh
reinjuryratemaybeduetotheuseofinappropriatecriteriafor
determiningsuitabilityforreturntosportor,alternatively,that
traditionalrehabilitationmethodsareinsufficientforreducing
riskforreinjury.
Rehabilitationprotocolsforacutemusclestrainshave
traditionallyemphasizedhamstringstretchingandstrength-
eningexercises.SherryandBest(11)prospectivelycom-
paredsuchanapproachwithaprogressiveagilityandtrunk
stabilization(PATS)rehabilitationprogram.PATS
includedexercisesthatemphasizedearlymovementand
coordinationofthepelvisandtrunkmuscles(Fig.1).Inthe
first2wkafterreturningtosport,noneofthe13athletes
participatinginthePATSprogramexperiencedareinjury,
comparedwith6(54.5%)of11athletesthatperformed
isolatedhamstringstretchingandstrengtheningexercises.A
significantreductioninreinjuryratewasstillevidentin
PATSparticipantsevenafter1yrofreturningtosport.
However,thestudywasunabletorelatereinjuryriskto
commonclinicalmeasuressuchasstrength,flexibility,
speed,andverticaljumpheight.
AlthoughSherryandBest(11)haveshownapromising
clinicaloutcome,itremainsunclearwhichneuromuscular
factorsareresponsibleforthereducedreinjuryriskinthe
PATSgroup.Onehypothesisisthatimprovedneuro-
muscularcontrolofthelumbopelvicregionallowsthe
hamstringstofunctionatsafelengthsandloadsduring
athleticmovement,therebyreducinginjuryrisk(11).An
alternativeexplanationisthattheuseofearlysubmaximal
loadinglimitstheresidualadverseeffectsofscartissue
formedearlyintheremodelingprocess.
RecentobservationsbyProske
etal.
(10)suggestresidual
effectsmayindeedbepresentinapreviouslyinjuredmuscle.

j1Theystudiedisokinetic(60
-I
s)kneeflexionexercises
performedbynineathleteswhohadexperiencedmultiple
unilateralhamstringstrainswithinthepast5yrs.Atthe
timeoftesting,allathleteswereinjury-freeforatleast1
monthandparticipatinginsport.Interestingly,therewere
nostrengthdeficitsinthepreviouslyinjuredlimbs.How-
ever,peakisokinetictorquewasgeneratedatanaverage12
-
greaterkneeflexionangleinthepreviouslyinjuredlimb
comparedwiththeuninjuredlimb.Theauthorsattributed
thedifferencetoashorteroptimalmusculotendonlengthfor
activetensioninthepreviouslyinjuredmuscle.Suchashift
canbeatrainingeffect,forexample,repeatedperformance
ofconcentricstrengtheningexercisesinrehabilitation.
Alternatively,theshiftcanreflectthepresenceofresidual
scartissueatthemusculotendonjunction(6).Scartissueis
stifferthanthecontractiletissueitreplacesand,thus,may
alterthemechanicalenvironmentseenbythemusclefibers.
Specifically,adecreaseinseriescompliancewouldshift
peakforcedevelopmenttoshortermusculotendonlengths
asobserved.Proske
etal.
(10)havealsoshown,atleastin
healthycontrolsubjects,thattheperformanceofcontrolled
lengtheningcontractionexercisescanfacilitateashiftin
peakforcedevelopmenttolongermusculotendonlengths.
Theirinitialdatasuggestthattheincorporationofsuch
exercisesintotrainingmayreducehamstringinjuryrates.
Althoughtherearepromisingnewapproachesfor
improvingthepreventionandrehabilitationofhamstring
strains,questionsremainregardingtheunderlyingmecha-
nisms.Fundamentally,astraininjuryistheresultof
exceedingthelocalmechanicallimitsofthemuscletissue.
Thus,itisrelevanttoinvestigatethemechanicalbehavior
ofthehamstringsduringpotentiallyinjurioustasks.Inthis
article,wereviewtheuseofaneuromusculoskeletalmodel

Figure1.
SherryandBest(11)comparedtheeffectivenessoftworehabilitationprogramsinreducingreinjuryratesinathleteswhosustainedanacute
hamstringstrain.ASSgroup(
N
=11)performedstaticstretching,isolatedprogressivehamstringstrengthening,andgraduatedreturntoactivity.APATS
group(
N
=13)performedagilityexercisesbeginningwithmovementsprimarilyinthefrontalandtransverseplanes,thenprogressingtomovementsin
thesagittalplane.Exercisesrequiringmuscleactivitytomaintainthespineandpelvisinadesiredposture(bridges)werealsoperformed.Foreach
rehabilitationprogram,athletesprogressedfromphase1tophase2whentheycouldwalkwithanormalgaitpatternanddoahigh-kneemarchinplace
withoutpain.ComparedwiththeSSgroup,therewasastatisticallysignificantreductionininjuryrecurrenceinthePATSgroupat2wkandat1yrafter
returntosport.PATSindicatesprogressiveagilityandtrunkstabilization;SS,stretchingandstrengthening.

136
ExerciseandSportSciencesReviews

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hobitide.
toestimatethestretch,loading,andworkdonebymuscles
duringsprinting.Theseanalysesareusedtoaddressthe
followingspecificquestions:Whenduringthesprintinggait
cyclearethehamstringssusceptibletoinjury?Howdoes
speedaffecthamstringmechanicsandpotentiallycontribute
toinjuryrisk?Howcanchangesinmusculotendonproper-
tiesaffectinjurypotential?Canthecoordinationof
individualtrunkmusclesinfluencehamstringmechanics?
HAMSTRINGINJURYMECHANISMS
Despitethefrequencyofhamstringstrainsduringsprint-
ing,itremainsuncertainwheninthegaitcyclethemuscle
isinjured.Ithasbeensuggestedthatinjuriesmayoccur
duringlateswing,whenthehipisflexedandthekneeis
extended.However,othershavespeculatedthatthe
potentiallylargeloadsassociatedwithgroundcontactmay
causeinjury.Animalmodelsclearlyshowthatmuscleinjury
isassociatedwithexcessivefiberstretchduringalength-
eningcontraction(8).Thus,wehaveusedamodelof
sprintingtoquantifythestretchofthebiarticularham-
stringsthroughoutthegaitcycle(14).Thisinf