ExplosiveExercise

WrittenbyMikeStone,MegStone,andHughLamont

Introduction

Beforediscussingexplosiveexerciseindetailitisnecessarytodefinetermsand

presentatheoreticalbackgroundforthediscussion.Ofparticularimportancefor

thisdiscussionaretheconceptsofstrength,rateofforcedevelopmentandpower.

Strengthcanbedefinedasanabilitytoproduceforce(Siff2001,Stoneetal2001).

Becauseforceisavectorquantity,strengthwillhaveamagnitudeanddirection.The

magnitudeofstrengthoutputcanrangefrom0to100%andthemusclesinvolved

determinethedirectionofforceapplication.Itisimportanttounderstandthat

strengthcanbe"applied"usingdifferentmuscleactions.

Strengthisexhibitedwhenmusclesacttoproduceforce.Muscleactioncantake

differentfourdifferentforms:

• Isometric‐inwhichthemusclegainstensionbutdoesnotappreciably

changeitslength

• Concentric‐inwhichthemusclegainstensionandshortens

• Eccentric‐inwhichthemusclegainstensionandlengthens

• Plyometric‐inwhichaconcentricactionisimmediatelyprecededbyan

eccentricaction,thustakingadvantageofastretch‐shorteningcycle.

• Muscleactionsaresupportedbyanumberofdifferentphysiologicaland

biomechanicalmechanisms.

ThevariousmechanismsinvolvedinmuscularstrengtharelistedinFigure1.Two

primaryfactors,whichgovernmuscleactivationandthegradationofstrengthare:

1)thenumberofmotor‐unitsrecruitedand2)thefrequencyofmotorunit

activationwhichcanbetermed"ratecoding".Thesetwofactorsnormallywork

togetherinincreasingforceproduction.Theexactdegreetowhichonemechanism

isemphasisedovertheotherduringmuscleactivationdependsupontheamountof

forcerequiredandperhapsthesizeandtypeofmusclebeingactivated.

Figure1:NeuromuscularFactorsInvolvedinStrengthProduction

Thereisdoubtthatanuntrainedmusclecanbefullyactivated(Aagaardetal2000;

SemmlerandEnoka2000).Furthermore,strengthtrainingcanresultinagreater

activationofmuscle,thusinfluencingstrengthproduction.

Anothermechanism,whichcaneffectmuscleforce,isthesynchronisationofmotor

units.Undernormallowintensitymuscleactivationmotor‐unitsfire

asynchronously.However,asthemaximumlevelofstrengthisapproachedsome

motorunitsareactivatedatexactlythesametimeasothermotorunits.

Synchronisationisalsoamajorfactorinballisticmovementsandwillbediscussed

later.

Thereisagreatdealofevidencefortheconceptsofintraandintermusculartask

specificity.Intra‐musculartaskspecificitydealswithspecificpatternsofactivation

formotorunitswhileinter‐musculartaskspecificitydealswiththeinterplayand

patternofactivationamongmusclesduringaspecifictask.Theconceptofintra‐

musculartaskspecificitymayhelpexplainthephenomenaofregionalhypertrophy

(Antonio2000),inwhichaspecificexercisemaycausehypertrophyinoneregionof

amusclebutnotinothers.Bodybuildershaverecognisedthisaspectoftraining

arguingthatinordertomorecompletelydevelopamuscle,manydifferentexercises

forthatmusclemustbeperformed.

Bothintraandinter‐muscularactivationpatternscanchangewithveryslight

alterationsinmovementpattern,eccentricversusconcentricactionsorwith

changesinvelocity(SemmlerandEnoka2000,ZajacandGordon1989).Becauseof

thesealterationsinactivationpatterns,selectionofexercisesforstrength/power

trainingshouldbeviewedasmovementspecificratherthansimplytraininga

muscle(s).Improvementintheefficiencyofintraandespeciallyintermuscular

activationimpliesanenhancedcoordinativeabilityandisanimportantmechanism

contributingtoimprovedstrengthexpression(SemmlerandEnoka2001).

Theuseofreflexesandstretch‐shorteningcycles(SSC)canalsoaltertheproduction

offorce(Bobbertetal1996,Croninetal2000).BasicallyaSSCconsistsofa

plyometricmuscleactioninwhichaneccentricactionimmediatelyprecedesa

concentricaction.Themechanismsinvolvedinconcentricenhancementmay

include:useofelasticenergy,astretchreflex,optimisingmusclelength,optimising

muscleactivationandmuscleactivationpatterns(Bobbertetal.1996,Bobbert

2001).Someevidenceindicatesthatimprovingmaximumstrengthcanaugmentthe

concentricportionoftheSSC(Croninetal2000).Learningtouseastretch‐

shorteningcyclemoreefficientlycanmarkedlyincreaseforceproduction.

Thedegreeofneuralinhibitioncanalsoeffectstrengthcapabilities.Inhibitioncan

taketwodifferentformsconsciousandsomatic‐reflexive.Consciousinhibitiondeals

withaperception(rightorwrong)thatagivenweightmayproduceinjury.For

example,ifyouhaveneverperformedsquatsbeforeandyouareaskedtoperforma

300‐kgfullsquat,chancesare(ifyouareremotelyintelligent)youwillrefuse.

Somatic‐reflexiveneuralinhibition,includesfeedbackformvariousmuscleandjoint

receptors,andhasbeensuggestedtobepartofaprotectivemechanism.This

protectivemechanismcanreducemuscletensionduringmaximumandnear

maximumefforts.Strengthtrainingappearstoreducereceptorsensitivity,diminish

inhibitionandispartiallyresponsibleforthegreaterforcesachieved(Aagaardetal.

2000).

Motorunittypecanalsoinfluencestrength.Severalstudieshaveindicatedthata

largecross‐sectionalareaoftypeIImusclefibresmaybeadvantageousintermsof

dynamicforceproduction(Powelletal1984)evenwhenmusclearchitectureand

othermechanicalfactorsaretakenintoconsideration.Strengthtraining,particularly

explosivestrengthtrainingappearstoincreasetheratiooftypeII:Imusclefibre

cross‐sectionareainamannerfavouringstrengthandpowerproduction.

Biomechanicalandanthropometricfactorssuchasgrossmusclearchitecture,

muscleinsertionpoint,height,limblengthandmomentarmmayalterthe

mechanicaladvantageoftheintactmuscleleversystem.Forexample,weightlifters

possessahighbodymasstoheightratio(Bm/h)comparedtountrainedsubjects

andotherathleticgroups.ThisBm/hisadvantageousbecauseitcanprovidean

increasedforceproduction.Thisadvantageisassociatedwiththestrongpositive

relationshipbetweenamuscle'sphysiologicalcrosssectionalareaandmaximum

muscleforcegeneratingcapabilities(SemmlerandEnoka2000).Iftwoathletesof

differentheightsanddifferentlimblengthshavethesamemusclemassandvolume,

theshorterathletewillhavethegreatestmusclecross‐sectionandthereforea

greatermusclegeneratingforce.

Ofthevariousmechanismsdealingwithabsolutemaximumstrength,themost

importantisthephysiologicalcross‐sectionalareaofamuscle.Fromapractical

standpoint,ifcross‐sectionalareawerenotthemostimportantfactoreffecting

absolutemuscularstrengththentherewouldnotbebodyweightclassesinsports

suchasboxing,judo,wrestlingorweightlifting.Therelationshipbetweenstrength

andthephysiologicalcross‐sectionareastemsfromthenumberofsarcomeresin

parallel.Themoresarcomeresinparallelthegreaterthemaximumstrengthofa

muscle.Theprocessofhypertrophy,resultingfromstrengthtrainingadds

sarcomeresinparallelthusraisingthemuscle'spotentialforceproduction.

ExplosiveExercise

Explosiveexercisecanbedefinedasamovementinwhichmaximumornear

maximumratesofforcedevelopmentareattained.Explosiveexercisescanbeeither

isometricordynamic.Severalfactorscontributedirectlytoexplosiveexercisethese

includemuscleactivationrate,andsynchronisation.

RateofActivation:Animportantfactor,whicheffectstherateofforcedevelopment,

isconcernedwiththerateofmuscleactivation.‐WorkbyViitasaloandKomi(1981)

clearlypointedoutthattheriseinmotorunitactivationasmeasuredbyEMGis

associatedwithariseinmuscleforce.Evidenceofthisrelationshipcanbeobserved

inFigure2NoteinthatintracingAtheinitialrateofactivationandforce

developmentishigherthanintracingB.Thus,therateofforcedevelopmentis

largelyafunctionofthenervoussystem'sabilitytoactivatemuscle.Typically,high

ratesofforcedevelopmentarenecessaryforsuccessin"explosiveandhighpower

activities"suchassprinting,throwingandweightlifting.

Synchronisation:Atlowmuscletensionverylittlesynchronisationisnoted.Motor

unitstypicallyareactivatedasbrief"dynamic"twitches.Figure3depictsthe

asynchronousactivationpatternsofseveralmotorunits.Notethatduring

asynchronousactivationwhenonemotorunitdeactivatesanotherisbeing

activated;thispatterncreatesamuscletensionproduction,whichallowsa

relativelysmoothmovementtooccur.Increasedmuscleactivationthrough

recruitmentorratecodingcanincreasemuscleforce.

Asforceoutputisincreasedgreaterlevelsofsynchronisationcanoccur.The

maximumfrequencyofactivationcanrangefrom30‐50htzforlowthresholdmotor

units,upto100htzforhighthresholdmotorunitsdependinguponthetypeandthe

intensityofthemuscleaction.Furthermore,strengthtrainingcanenhancethe

numberofmotorunitssynchronisingandcanresultinsynchronisationatlower

forceoutputs(SemmlerandNordstrom1998).However,thedegreetowhich

synchronisationeffectsmaximumstrength,especiallywhenmeasuredisometrically

appearstobeminimal(Yaoetal2000).Synchronisationdoesappeartoplayan

importantroleinballisticmovements.

Ballisticmovements:DependenceonSynchronisation:InFigure4notethe

characteristictriphasicmuscleactivationpatternasrecordedbyEMG.Inthefirst

phasethereisasilentperiodinwhichthemotorunitshaveenoughtimeto

completetheirrefractoryperiods.This"pre‐motorsilentperiod"precedes

activationoftheprimemoveroragonist.Thepre‐motorsilentperiodallowsfora

largenumberofmotorunitstosynchronise,whichinturnproducesabrief,butvery

largeforceimpulseduringthe2ndphaseorpre‐programmedperiod.Aftertheburst

ofactivityfromtheprimemovertheagonistisactivatedandactsasabraking

system,whichslowsmovementandreducesinjurypotential.Inthe3rdandfinal

phaseof"proprioception"theprimemoveragainbecomesactiveinorderto

producesubtleadjustmentsinthefinalstagesofmovement.

Thisbasictriphasicresponseisactivatedinallballisticmovementsandcanbe

refinedbyappropriatetrainingprocedures.

TheMeasurementofExplosiveStrength

Toadequatelydescribe"explosivestrength"bothpeakforceandareasonable

measureforcedevelopmentisnecessary.Typicallyaforceplateisused.

Isometricforce‐timecurve:Figure5representsatypicalisometricforcetimecurve

producedfromamid‐thighcleanpull.Theforceproducedinthefirst30

millisecondshasbeenterm"StartingStrength".Startingstrengthisassociatedwith

theabilitytoproducehighvelocity"quick"movementssuchaspunchingorkicking.

Peakforceisthemaximumforceattainedunderthemeasurementconditionsandis

associatedwiththeabilitytoliftheavyobjects.Thepeakrateofforcedevelopment

hasbeentermed"ExplosiveStrength"andisassociatedwiththeabilitytoaccelerate

objects.

Concentricforce‐timecurve:Figure6representsaforce‐timecurveforaconcentric

mid‐thighpull.Notethatatanydynamiceffortatweightslessthanthemaximum

isometriccapability,peakforcewillbecorrespondinglyless.Forexamplethepeak

forcewouldbelowerateachdecreasingpercentageofmaximumfrom90to80to

70andsoon.However,someevidenceindicatesthatthepeakrateofforce

developmentiscorrespondinglyhigherastheloaddecreases,atleasttoapoint.

Thus,ingeneral,peakforceandpeakratesofforcedevelopmentareinversely

related.

Plyometricforcetimecurve:Manyexercisesinvolveaplyometricmovementin

whichthereisastretch‐shorteningcycle.Figure7showsatypicalforce‐timecurve,

whichcanbegeneratedasaresultofajumpingmovement.Duringatypical

counter‐movementjumpthereisanun‐weightingphase,whichinitiatesthestretch

shorteningcycleandproducesaplyometricmovement.Theresultingupwardforce

canbeaugmentedbypreviouslystretchingthemuscle.Aspreviouslynotedthe

mechanism(s)bywhichconcentricforcecanbeaugmentedbyapreviousstretchis

notcompletelyclearbutinvolvesseveralpossibilitiesincluding:a)muscleelastic

properties,b)amyototicreflex,c)returningthemuscletoitsoptimumlengthord)

optimisingthemuscleactivationpattern(Bobbert2001).

Formanysportstheabilitytoproduceforcerapidlymaybemoreimportantthan

maximumforceproduction.Rateofforceproductionisachangeinforce/changein

time.Aspreviouslynoted,therateofforcedevelopmentisprimarilyafunctionof

therateofincreaseinmuscleactivationbythenervoussystem(KomiandViitasalo

1976,ViitasaloandKomi1981).Althoughforceisdirectlyresponsibleforthe

accelerationofanobjectitmaybearguedthatthefasteragivenforceisattained,

themorerapidthecorrespondingaccelerationoccurs.Thusrateofforce

developmentcanbeassociatedwiththeabilitytoaccelerateobjects

(Schmidtbelicher1992).So,attainingahighpeakrateofforcedevelopmentor

explosivestrengthwouldbeassociatedwithhighaccelerationcapabilities.The

importanceofbothpeakforceproductionandhighratesofforcedevelopmentcan

beascertainedbyusingNewton's2ndlawandbyconsideringsprintingasan

example.

F=MA+W

InthisequationrepresentingNewton's2ndlaw,force(F)minustheweight(W)of

anobjectisequaltomass(M)timesacceleration.Rearrangingtheequation,force

(F)isequaltotheweight(W)ofanobjectplusmass(M)timesacceleration(A).

Studieshaveindicatedthatthelimitingforcesduringsprintingareverticalforces,

effectingstridelength,ratherthanhorizontal(Weyandetal.2000).Duringsprinting

elitemalesprintersuseanalternatingpatternofverticalgroundreactionforcesand

thecenterofmassmovesupwardatavelocityof0.49mxs‐1anddownwardat0.49

mxs‐1.Theiraveragefootcontacttimeis0.087sandtheaveragebodymassis

approximately79.5kg.Peakforcetypicallyoccursatakneeangleofapproximately

135‐140°(Mann1996).

Substitutingthesevalues,forelitemalesprinters,intotheforceequation(Newton's

2ndLaw)

VF=79.5(0.98mxs‐1)/0.087s=895.5N+779.1=1674.6N

wefindthatthetypicalelitemalesprinterhastoproduce1675Nor375lbs.of

verticalforce,ononeleg.Thussprintersmustbequitestrong.Furthermoreitis

importanttonotethatthisforceproductionmustoccurinonly0.087s,thustherate

offorceproductionisquitehigh.So,thesesprintersmustbeverystrongand

"explosive"inthatthispeakforcemustbeproducedveryrapidly.

Theimportanceofpowerproduction:Workistheproductofforceanddistance.

Poweristherateofdoingworkandcanbeexpressedastheproductofforceand

velocity.Powercanbecalculatedasanaverageoverarangeofmotionorasan

instantaneousvalueoccurringataparticularinstantduringthedisplacementofan

object.Peakpower(PP)isthehighestinstantaneouspowervaluefoundovera

rangeofmotion.Maximumpower(MP)isthehighestpeakpoweroutputoneis

capableofgeneratingunderagivensetofconditionssuchasthestateoftrainingor

typeofexercise.Muscularactionsthatmaximisepowerincludejumping,throwing

andkicking;indeedactivitiesinwhichamovementsequenceresultsinmaximum

achievablevelocitiesprimarilydependsuponpowerproduction(Young1993).

Furthermore,activitiesrequiringarapiddirectionchangeandacceleration,suchas

displaysof"agility",dependuponburstsofhighpoweroutput.Thus,poweroutput

islikelytobethemostimportantfactorinseparatingsportsperformances;thatis

whowinsandwholooses.Althoughaveragepoweroutputmaybemoreassociated

withperformanceinenduranceevents,forexplosiveactivitiessuchasjumping,

sprintingandweightliftingmovements,PPistypicallystronglyrelatedtosuccess

(Garhammer,1993;Kauhanenetal2000;McBrideetal1999;Thomasetal1994).

PotentialTrainingAdaptations

Trainingadaptationscandependuponanumberoffactorsincluding,training

variablessuchasvolumeandintensity,mechanicalspecificityandthetrainedstate.

Differentmethodsoftrainingcanproducedifferentlong‐termadaptations(Figure

8).Forexampletypicalheavystrengthtrainingwouldbeexpectedtoproduce

increasesinthehighforceendofaforce‐timecurve.Explosivetraining,particularly

dynamicexplosivetrainingwouldlikelyeffecttheinitialriseinforceratherthan

peakforce.Itshouldbenotedthatinordertoeffectreasonablelong‐term

adaptationsthatappropriatevolumeandintensitycharacteristicsshouldbe

consideredintraining.

Thresholdvalueforstrength:For,exampleHakkinenetal.(1987,1988)studied

eliteweightliftersovera1‐yearperiod.Itwasnotedthatmaximumstrengthlevels

dependeduponmaximummuscleactivation.Maximummuscleactivationwas

achievedonlywhenthetrainingintensitywas80%ofthe1RMorgreater.Whenthe

averagetrainingrelativeintensitydroppedbelow80%,maximumstrengthalso

decreased.Thesedataindicatethatamongeliteweightlifters,thethresholdfor

maintainingorincreasingmaximumstrengthisabout80%of1RM.

However,Hakkinenetal.(1987,1988)alsonoticethatiftheweightlifterstrainedat

highintensitiesfortoolong,thenmaximumstrengthandpowerdecreased

regardlessoftheintensityoftraining.MorerecentlyFryetal(1994)haspresented

dataindicatingthatconstanthigh‐intensitytrainingcandiminishmaximum

strengthandexplosivestrengthperformanceinaslittleas2‐3weeks.Thistypeof

"overtraining"hasbeenattributedto"neuralfatigue"andpointsoutthenecessityof

variationintraining.Similarargumentscanbemadeforvolumeconsiderations.

SpecificityofTraining

"Transferoftrainingeffect"dealswiththedegreeofperformanceadaptation,which

canresultfromatrainingexerciseandisstronglyrelatedtotheconceptoftraining

specificity.Mechanicalspecificityreferstothekineticandkinematicassociations

betweenatrainingexerciseandaphysicalperformance.Thusmechanicalspecificity

includesmovementpatterns,peakforce,rateofforcedevelopment,accelerationand

velocityparameters.Themoresimilaratrainingexerciseistotheactualphysical

performancethegreatertheprobabilitiesoftransfer(Behm1995,Sale1992,

Schmidt1991).

Therearevariousstrength/powertrainingmethodswhichcanbeemployed.

However,theeffectsofthesetrainingmethodsonneuromuscularphysiologyand

performancevariablescanbedrasticallydifferent.Fourtypesoftrainingwillbe

discussed;thesetrainingmethodsare:isometric,heavyweighttraining,highpower

orspeedstrengthandintentionallyslowtraining.

Table1comparestherelativeeffectsontheneuromuscularsystemresultingfrom4

differenttypesoftrainingprotocols(Hakkinen1994,Jonesetal1999,Jonesetal

2000,Stoneetal2001):isometrictraining,typicalheavyweighttraining,dynamic

explosivetrainingandintentionallyslowtraining.

Isometrictraining,whichreachedpeakpopularityinthe1960's,hasnotbeen

showntoproduceextensivehypertrophy.Heavyweighttrainingischaracterisedby

loadingthatistypically80%of1RMorhigherandtypicallyuses5‐8repetitions.

Theloadliftedmaymoveslowly,evenifperformedexplosively,becauseitisrelative

closetomaximumvalues.Heavyweighttrainingcanproducemarkedhypertrophy,

exceptduringtheinitialstagesofabeginningtrainingprogramme.Speed‐strength

weighttrainingwithahighpoweroutputtypicallydoesnotproducemarked

hypertrophy,exceptinsedentaryindividuals,butcanresultinprofoundalterations

inthenervoussystem.Intentionallyslowtraininghasbecomepopularamong

healthclubsrecently;basicallyarelativelylightweightismovedinanintentionally

slowmovementpatternbotheccentricallyandconcentrically.Theintentionally

slowmovementcanresultinahighmotorunitfatiguerate,whichisbelievedto

causemoremotorunitstoberecruited.Proponentsofintentionallyslow

movementsbelievethatthetimethatamuscleisundertensionenhancesboth

hypertrophyandstrength,Oftenthistypeoftrainingisperformedforonlyoneset.

Although,currently,thereislittleinformationconcerningintentionallyslow

movement'seffectonhypertrophyafewstudiessuggeststhatwhilesome

hypertrophycanoccuritisnotasextensiveasthatresultingformheavyweight

training(Keeleretal2001).

Differentialeffectshavebeennotedforfibretypeadaptations.TypeIIfibres

typicallydisplayafasterrateofhypertrophythantypeIfibres,althoughthereason

forfasterhypertrophyisnotcompletelyclear.Thusweighttrainingcanproduce

fibrehypertrophysuchthattheII/Icross‐sectionalarearatioincreases;thedegree

ofincreasedependsuponthetypeoftraining.Thereisevidencethathighpower

trainingenhancestheII/Iratioofcross‐sectionalareatoagreaterdegreethanother

typesoftraining.AhighII/Iratioislikelyadvantageousinproducing"explosiveness

andhighpoweroutputs.

Table2comparestrainingmethodsbasedonpotentialperformanceoutcomes.

Althoughanglespecificityisoftenobserved,isometrictrainingcanenhance

measuresofmaximumstrength,especiallywhenmaximumstrengthismeasured

isometrically.Inrelativelyuntrainedsubjectsisometrictrainingmayenhancespeed

ofmovement,providedaconsciousefforttomovefastismade(Behm1995).

However,theeffectsonspeedarerelativelyminorcomparedtospeedstrength

training(Hakkinen1994).Heavyweighttraininghasitsgreatesteffectonmaximum

strengthasmeasuredbya1RM.Amongbeginnersandnovices,relativelylargegains

inpower,rateofforcedevelopmentandspeedcanoccur.Speed‐strengthtraining

hasitsgreatesteffectsonrateofforcedevelopmentandpoweroutput,withlesser

effectsonmeasuresofmaximumstrength.Intentionallyslowtraininghasits

greatesteffectonmeasuresofmaximumstrength,withmuchsmallerandperhaps

negativeeffects,onrateofforcedevelopment,powerandspeed.

Thespecificityeffectsoftrainingareveryapparentinacomparisonbetweenheavy

weighttrainingandspeedstrengthtraining(Figure9)carriedoutinaseriesof

studiesbyHakkinenandKomi(1985a1985a).Onegroupofphysicaleducation

studentsweretrainedinthehalfsquatusingheavyweighttrainingmethods,

anothergroupusedexplosivejumpingwithweightsofapproximately30%oftheir

1RM.Isomericforce‐timecurvesmeasuredpre‐postsindicatedifferentadaptations.

Theheavyweight‐traininggroupshoweda27%improvementinpeakforcebut

verylittlealterationinpeakrateofforcedevelopment.SimultaneousEMGtracings

showalterationscorrespondingtochangesintheforce‐timecurvewithonlya3%

increasedactivationinthepeakforceregionandnochangeinthepeakforceregion.

Thegaininpeakforceshownbytheheavyweight‐traininggroupwasattributedto

musclehypertrophy.Ontheotherhandthespeed‐strengthgroupshowedgainsof

11%inthepeakforceregionoftheforce‐timecurveanda24%improvementinthe

peakrateofforcedevelopmentregion.SimultaneousEMGtracingsindicatedthat

EMGenhancementgenerallycorrespondedtothegainsinpeakforceandforce

development.Thus,thespeed‐strengthgroupshowedthegreatestadaptationsin

thenervoussystemwhiletheheavyweight‐traininggroupshowedgreatergainsin

hypertrophy.

Anotherfactor,whichenhancesthetransferoftrainingtoperformance,dealswith

movementpattern.Movementpatterndealswithapplyingforcesinthemost

efficientmannerandintheappropriatedirections.Movementpatternspecificity

includesbothintraandintermuscularspecificaspects.

Movementpatternspecificity(intra‐muscular):Severalstudieshaveshownthat

thereisahighdegreeofintra‐musculartaskspecificity.Thesestudiesindicatethat

foragivenmovement,therearegroupsofmotorneurones,whichareactivatedina

specificmannerforaspecifictask.Ifthetaskischanged,throughalterationsin

movementpatternorperhapsvelocity,thentheneuronaltaskgroupwillbe

changed.Thistypeofdatalendssupportforthepracticeamongbodybuildersof

usingmanydifferentexercisestomorefullydevelopamuscle(Antonio2000).

Movementpatternspecificity(inter‐muscular):Thepatternofactivationofwhole

muscles,aswellastheefficientuseofreflexesandstretchshorteningcyclesisalso

taskspecific.Inthisrespectthefunctionalroleofmusclesasagonist,antagonistor

stabilisersmustbeclassifiedwithcare.Thesefunctionalrolescanchangefrom

singlejointtomultiplejointmovementsandwithchangesmovementvelocity(Zajac

andGordon1989).Thusinsportsordailylivingsettingsinwhichmultiplejoint

movementsoccur,especiallythoserequiringhighpowerorhighvelocity,transferof

trainingeffectismorelikelyaccomplishedusingcomplexmulti‐jointmovements

whichhavesimilarkineticandkinematiccharacteristics.

Becauseofthehighdegreeoftaskspecificity,gainsinstrengthmaybeeffectedbya

numberoffactorsincludingthenumberofjointsinvolved,velocityofmovement

andposition(RachandMorehouse1957,ZajacandGordon1989,Stoneetal2001).

Forexample,Thorstensson(1977)traineduniversityphysicaleducationstudentsin

thehalfsquatfor8weeks.Pre‐postmeasurementsindicatedapproximatelya75%

improvementinthe1RMhalfsquat(Figure10).However,theimprovementinthe

isometriclegpresswasonlyabout40%andessentiallynoimprovementoccurredin

theseatedlegextension.Althoughthehalfsquattrainingeffectedmusclesusedin

allthreetestsitisclearthatmovementpatterndifferencesalteredtheapparent

strengthgains.Thesedataalsoindicatethatthegreaterthesimilaritiesbetween

trainingexercisesandperformancethegreaterthetransfer.

Speed­StrengthExercises

Manysportsrequirethedevelopmentofspeed.Inordertoenhancespeed

developmentaspecialcategoryofexercisetermed"speedstrength"canbeused.

Speedstrengthexercisesareperformedwithmaximaleffortandarecharacterised

byhavinghighpeakratesofforcedevelopmentandhighpoweroutputs.Typically

theseexercisesareperformedwithsub‐maximalweightsselectedtomaximise

power.Evidenceindicatesthatforsinglejointandsmallmusclemassexercisesthat

powerisatitspeakatabout30%ofpeakisometricforce.Formultiplejoint

exercisesinwhichthebodyweightisinvolved,suchasajumporinweightlifting

movements,itappearsthatpeakpowermayoccursomewherebetween10and

40%ofpeakisometricforcedependinguponthetrainedstate.

Ifperformanceisballisticthenevidenceindicatesthatmuch,ifnotmost,ofthe

trainingshouldalsobeballisticinnature(Newtonetal1996).Ballisticexercisesare

notlimitedbyend‐pointdecelerationasarejointrangelimitedexercisessuchas

typicalbenchpressesortypicalsquats.Ballisticexercisesincludevarioustypesof

throws,jumpsandweightliftingmovements.Itshouldbenotedalsothatballistic

movementscanbeconcentricmovementsorcanhaveaplyometricnature.

Plyometricversusconcentriconlyexercises:Exercisesforthedevelopmentof

powerandspeedcanbedividedindifferentcategoriesbasedontheirspeedof

movementandonwhethertheycontainaplyometricelement.Forexample(Figure

11)jumpingmovementscanbeperformedasheavysquatsorheavyjumpsquatsor

theycanbeperformedasspeed‐strengthexercise‐howeverbothwouldhavea

preliminarycounter‐movement.Insomesportsamovementmaybeinitiated

withoutacounter‐movement,forexampleasprintercomingoutoftheblocks.

Thereforesomeofthetrainingexercisesshouldattempttoduplicatethistypeof

start,soforexample,heavysquatscouldbeperformedbydescending,stoppingfor

severalsecondsbeforeascendingorsquatscouldbeperformedfromapinataset

heightinapowerrack.

SuccessfulTransferofTrainingEffect

Aspreviouslynoted,thereareanumberofcriteriathatanexercisemustmeetfor

successfultransferoftrainingeffect.Thesecriteriaincludemovementpattern,force

productionandvelocityconsiderations.Therealsomustbeanoverloadapplication

forsuccessfulperformanceadaptation.Ifthereisnooverloadthensport

performancewillnotimprovebeyondadaptationtosimplepracticeofthesport.

Movementpatterncharacteristicsinclude(SiffandVerkoshansky1998,Stoneetal.

2001):

1. thetypeofmuscleaction

2. accentuatedregionsofforceproduction

3. thecomplexity,amplitudeanddirectionofmovement

4. ballisticversusnon‐ballisticmovements

Factorstobeoverloadedinclude:

1. forceproduction

2. rateofforceproduction

3. poweroutput

TheTrainedState

Figure12representsaqualitativeexpressionofpotentialchronologicalstrength

adaptationsandunderlyingmechanisms.Theunderlyingmechanismshavebeen

grosslydividedintoneuralandhypertrophicfactors.Initialneuraladaptation

occursquiterapidlycomparedtohypertrophicfactorsandrepresentstheprimary

mechanismofstrengthgainduringthisearlyphaseoftraining.Lateradaptationis

typicallymoredependentuponincreasedmusclecross‐sectionalarea.However,

bothofthesefactorshavegeneticlimitationsthatmakeadditionalstrengthor

powergainsamongadvancedathletesdifficult.

Interestingly,almostanyreasonabletrainingprogrammecanenhancemaximum

strength,powerandspeedamonginitiallyuntrainedsubjectsduetorapidneural

adaptations.However,thetrainingofadvancedandeliteathletesrequires

considerablevariationaswellascreativeapproachesinordertoelicitgainsin

performance.

SpecificityofStrength/PowerTraining­Untrained

Table3liststheexpectedprimaryadaptationofthreedifferentmethodsoftraining

ininitiallyuntrainedsubjects.Basedonthecurrentscientificliterature,aswellas

experience,heavyweighttrainingwouldproducemarkedandsubstantial

alterationsinmaximumstrength,peakrateofforcedevelopmentandpower.Speed‐

strengthtrainingwouldhaveitsgreatesteffectsonpeakrateofforcedevelopment

andpowerandintentionallyslowtrainingwouldshowgainsinstrengthbutmuch

smallereffectsonrateofforcedevelopmentandpower.

However,thetrainingofadvancedandeliteathletesrequiresconsiderablevariation

aswellascreativeapproachesinordertoelicitgainsinperformance.Using

previouslystrengthtrainedmalesWilsonetal.1993studiedtheeffectsofvarious

typesoftrainingonlegandmaximumstrengthandmeasuresof"explosiveness"

(Table4).Fiftyfivetrainedsubjectsweredividedinto4groups.Onegroup

continuedwithheavyweighttraining,butdidnotattempttooverload,simply

trainingwithalreadyestablishedweights,thusservingasacontrolgroup.Asecond

heavyweighttraininggroupcontinuedtheirtrainingroutinesbutdidoverloadby

increasingtheweightsliftedovertheexperimentalperiod.Athirdgroupswitched

todepthjumpsbeginningwithboxesat0.2mandprogressingto0.8m.Aforth

groupswitchedtoexplosivejumpingmovementsusingaresistanceequaltoabout

30%oftherepeakisometricforcemeasureat135°kneeangle.Pre‐post

measurementsincludedcounter‐movementandstaticverticaljumps,andisokinetic

legextensionat400°/sandamodifiedWingatecyclemaximumpowertest.After10

weeksoftrainingthecontrolgroupdidnotchangeonanymeasure.Thetraditional

strengthtraininggroupimprovedonthecounter‐movementandstaticjumpsand

thecyclepowertest.Thedepthjumpgroupimprovedonlyonthecounter‐

movementverticaljump.However,thespeed‐strengthgroupimprovedonall

measures.Furthermorethepercentimprovementonthesemeasureswasasgoodor

betterthananyothergroup.Thesedataindicatethatspeed‐strengthexercisescan

optimise"explosive"performanceanditalsopossiblethatpreviousstrength

trainingmayenhancetheoptimisationprocess.

Supportfortheconceptofstrengthtrainingoptimisingsubsequentspeed‐strength

trainingcanbefoundintheobservationofeliteweightlifterstrainingindifferent

manners.Medvedevetal.(1981)dividedseveralhundredeliteSovietweightlifters

intothreedifferenttraininggroups.Group1trainedheavythroughouttheentire

experimentalperiodlastingseveralmonthsandemphasisedstrengthincreases.

Group2trainedwithrelativelylightweights,between70‐80%of1RM.However,

group3trainedinasequencedmannersuchthatthemonthwasdevotedto

strengthtrainingwithheavyweightsandtheremainderoftheexperimentalperiod

wasusedforspeed‐strengthtraining.Attheendoftheexperimentalperiodgroup3

producedsuperiorimprovementsinweightliftingtotal,primarilythroughan

improvedsnatch.Furthermoregroup3realisedsuperiorimprovementsinother

"explosive"measurementssuchassprintingabilityandmedicineballthrows

comparedtotheothertwogroups.Thesedataindicatethatasequencedtraining

programmeinwhichanemphasisonstrengthtrainingprecedespower‐trainingcan

producedsuperiorresults,particularlyinmeasuresofexplosiveness.

Inordertofurtherinvestigatetheconceptofsequencedtraining,Harrisetal.1999

usedagroupof42Americanfootballplayers.Thestudyconcentratedonlegandhip

maximumstrengthandexplosiveness.For4weeksalloftheplayerstrainedusinga

highvolumestrengthenduranceprogramme.Followingtheinitial4weeksthe

playersweredividedintothreegroupsequalisedonthe1RMsquatandbodymass.

Group1trainedforanadditional9weeksusingexplosiveheavyweighttraining.

Group2usingspeed‐strength‐trainingmethodsusedweightsequivalentto30‐40%

oftheir1RMsquat.Group3usedasequencedcombinationtrainingprogramme;for

thefirst5weeksgroup3trainedinthesamemannerasgroup1exceptheavyand

lightdayswereused.Lightdaysconsistedofthesameliftsexceptatusing20%less

weight.Duringthelast4weeksgroup3usedacombinationofheavyweight

trainingandspeedstrengthexercises.Forexample,inthesquat,afterwarm‐upsets,

oneheavysetof85‐90%of1RMwasperformedandthenfollowedby3setsof

jumpsat30%ofthe1RM.Allliftswereperformedasexplosivelyaspossible.

Pre‐postmeasuresincludedvariousmeasurementsofmaximumstrength,a

counter‐movementverticaljump,verticaljumppower,aMargariastairlimbpower

test,a30msprint,9.1magilitytestandastandinglongjump.Theresultsindicate

thattheheavyweighttraininggroup(Gp1)andthecombinationgroup(Gp3)

producedthebestgainsinmaximumstrengthmeasures.However,inmeasuresof

powerandexplosivenessthespeedstrengthgroup(Gp2)andthecombination

group(Gp3)producedthebestgains.Furthermorethepercentgainsfor

combinationgroup(Gp3)inalltestswereasgoodorbetterthantheothertwo

groups.Thesedataindicatethat1)combinationtrainingcanproducesuperiorgains

acrossawidespectrumofperformancevariablesand2)thatsequencedtraining

consistingofstrength‐endurance,strengthandspeed‐strengthphasescanoptimise

thesetrainingresponses(Table5).

SpecificityofStrength/PowerTraining­PreviouslyTrained

Ofconcerntothecoachiscreatingcontinuedgainsintrainedathletes.Table6lists

thepotentialstrength/poweradaptationsinathletesalreadystrengthtrained.For

examplewewouldexpectedthatcontinuedheavyweight‐trainingwouldresultin

diminishedorlittlegaininmaximumstrength,rateofforcedevelopmentorpower;

intentionallyslowmovementswouldalsoresultindiminishedadaptations.Some

evidenceactuallyindicatesthatbyswitchingtointentionallyslowmovements,

maximumstrengthandespeciallyrateofforcedevelopmentandpowermaybe

reduced.Ontheotherhandswitchingtoaspeedstrengthtypeoftrainingcanelicit

beneficialandquitemarkedalterationsinrateofforcedevelopmentandpower

(Wilsonetal.1993,Harrisetal.1999).

FactorsEffectingExplosiveness

Inadditiontospecifictrainingprotocols,severaldifferentfactorscanhaveamarked

impactuponthedevelopmentofexplosivequalitiesinanathlete.Thesefactors

includemaximumstrength,fatiguelevelsandcross‐training.

Theinteractionofstrengthandpowerisofparamountimportance.Evidence

indicatesthat

1. measuresofmaximumstrengthandpowerhavemoderatetoverystrong

correlations

2. thestrengthoftherelationshipinpartdependsuponthemechanicalsimilarity

ofthemeasures

3. althoughmaximumstrengthinfluencespoweroutputatlightresistancesits

effectonpowerappearstoincreasewithload.

4. sequencedperiodisedtraininganditsvariationscanofferadvantages

Thus,thedevelopmentofpowerandexplosivenesscanbeaugmentedthrough

developmentofstrength.

Whilefactorssuchasmaximumstrengthcanhaveapositiveeffectonexplosiveness,

otherfactorssuchasfatigueandcross‐trainingcanhaveanegativeimpact.Two

factors,whichmustbeconsideredintrainingprogrammes,arethedegreeoffatigue,

whichoccurswithinatrainingsession,andthedegreeofresidualfatigue,whichcan

accumulatebetweentrainingsessions.

Fatigueresultsinreductionsinmaximumstrength,peakrateofforcedevelopment

andpoweroutput.Becauseofthefatigue‐inducedreductioninperformance

capabilityhighfatiguelevelscaninterferewithtechniqueandinterferewith

learningorstabilisingtechnique.Thuslearningtobeexplosive"canbe

compromised.

Evidenceindicatesthatthecombinationoftypicallyaerobictraining,suchas

distancerunning,andresistancetrainingcanresultindecreasedmaximumstrength

andpower.Thus,ifmaximumlevelsofstrengthandespeciallypowerandspeedare

desired,thentypicalaerobictrainingshouldbeminimisedoreliminated.

InjuryPotentialofResistanceTraining

Itiswellknownthattheinjurypotentialofweighttrainingislowcomparedtoother

recreational(Powelletal1998)andsportsactivities(Hamill1994).Althoughitis

commonlybelievedthatfreeweightsproduceahigherinjuryratethenmachines

thereisnoevidenceforthisbelief(Requaetal.1993).Thislaststatementis

particularlyimportanttounderstandbecausefreeweightscanproduceasuperior

transferoftrainingeffect,especiallyforexplosivestrengthcomparedtomachines

(Stoneetal2001).

Itisalsocommonlybelievedthatweightliftingandotherballisticexplosive

exercisesproducehighratesofinjury.Againthereislittledatatosupportthisidea.

Hamill(1994)studiedtheinjuryratesofseveraldifferentsportsintheUnited

KingdomandintheUnitedStates.Basedoninjuryratesper100participationhours

bothgeneralweighttrainingandweightliftingtrainingproducedinjuryratesthat

wereamongthelowestofthesportsstudied.Thus,thereislittleevidencethat

weighttraining,includingexplosiveweighttraining,producesexcessiveinjuries

(Table7).

Summary

Insummarisingvariousaspectsofexplosiveexerciseitshouldbenotedthat:

1. differenttrainingprogrammescanelicitveryspecificlong‐termadaptations

2. differenttrainedstatesaltertrainingadaptations

3. inordertoelicitmaximumresponsesallstrengthtrainingshouldincorporate

maximumeffortsregardlessoftheweightused

4. trainingformaximumexplosivenessrequiresemphasesonbothmaximum

strengthandexplosivetraining

5. strainingadvancedathletesrequirescreativeplanning.Thisplanningshould

incorporateaperiodisedsequencedstructure.

Thuswecanconcludethat"explosiveexercise",whenproperlyintegratedintoa

trainingprogramme,canbeavaluablepartoftraining.

References

Aagaard,P.Simonsen,E.B.,Andersen,J.L.,Magnusson,S.P.,Halkjaer‐Kristensen,J.andDyhre‐Poulsen,P.Neuralinhibitionduringmaximaleccentricandconcentricquadricepscontraction:effectsofresistancetraining.JournalofAppliedPhysiology89:2249‐2257,2000.

Antonio,J.Nonuniformresponseofskeletalmuscletoheavyresistancetraining:canbodybuildersinduceregionalmusclehypertrophy?JournalofStrengthandConditioningResearch14:102‐113,2000.

Behm,D.G.Neuromuscularimplicationsandapplicationsofresistancetraining.JournalofStrengthandConditioningResearch9(4):264‐274,1995.

Bobbert,M.F.,Gerritsen,K.G.,Litjens,M.C.andVanSoestA.J.Whyiscountermovmentjumpheightgreaterthansquatjumpheight?MedicineandScienceinSportsandExercise28:1402‐1412,1996.

Bobbert,M.F.Dependenceofhumansquatjumpperfromanceontheserieselasticcomplianceofthetricepssurae:asimulationstudy.JournalofExperimentalBiology204(pt3):533‐542,2001.

Cronin,J.B.,P.J.McNairandR.N.Marshall.Theroleofmaximalstrengthandloadoninitialpowerproduction.MedicineandScienceinSportandExercise,3:1763‐1769,2000.

Fry,A.C.W.J.Kraemer,F.vanBorselen,J.M.Lynch,J.L.Marsit,E.P.Roy,N.T.TriplettandH.G.Knuttgen.Performancedecrementswithhighintensityresistanceexerciseovertraining.MedicineandScienceinSportsandExercise26:1165‐1173,1994.

Garhammer,J.J.AreviewofthepoweroutputstudiesofOlympicandpowerlifting:Methodology,performancepredictionandevaluationtests.JournalofStrengthandConditioningResearch,7:76‐89,1993.

Hakkinen,K.Neuromuscularadaptationduringstrengthtraining,aging,detrainingandimmobilization.CriticalReviewsinPhysicalandRehabilitationMedicine6(3):161‐198,1994.

Hakkinen,K.andP.V.Komi.Changesinelectricalandmechanicalbehaviouroflegextensormuscleduringheavyresistancestrengthtraining.ScandinavianJournalofSportsScience7:55‐64,1985a

Hakkinen,K.andP.V.Komi.Theeffectofexplosivetypestrengthtrainingonelectromyographicandforceproductioncharacteristicsoflegextensormusclesduringconcentricandvariousstretchshorteningcycleexercises.ScandinavianJournalofSportsScience7:65‐76,1985b.

Hakkinen,K.,P.V.Komi,M.AlenandH.Kauhanen.EMG,musclefibreandforceproductioncharacterisitcsduringa1yeartrainingperiodineliteweightlifters.EuropeanJournalofAppliedPhysiology56:419‐427,1987.

Hakkinen,K.A.Pakarinen,M.alen,H.KauhanenandP.V.Komi.Neuromuscularandhormonaladaptationsinathletestostrengthtrainingintwoyears.JournalofAppliedPhysiology65:2406‐2412,1988.

Hamill,B.P.Relativesafetyofweightliftingandweighttraining.JournalofStrengthandConditioningResearch.8(1):53‐57,1994.

Harris,G.R.,Stone,M.H.,O'Bryant,H.,Proulx,C.M.&Johnson,R.Shorttermperformanceeffectsofhighspeed,highforceandcombinedweighttraining.JournalofStrengthandConditioningResearch13:14‐20,1999.

Jones,K.G.Hunter,G.Fleisig,R.Escamilla,andL.Lemak.Theeffectsofcompensatoryaccelerationonthedevelopmentofstrengthandpower.JournalofStrengthandConditioningResearch13:99‐105,1999.

Jones,K.,P.Bishop,G.HunterandG.Flesig.Theeffectsofvaryingresistance‐trainingloadsonintermediate‐andhigh‐velocity‐specificadaptations.JournalofStrengthandConditioningResearch15:349‐356,2001.

Kauhanen,H.,J.GarhammerandK.Hakkinen.Relationshipsbetweenpoweroutput,bodysizeandsnatchperformanceineliteweightlifters.Proceedingsofthe5thannualCongressoftheEuropeanCollegeofSportsScience,Jyvaskala,Finland(J.Avela,P.V.KomiandJ.Komulainen,eds)pp.383,2000.

Keeler,L.K.,L.H.Finkelstein,WMillerandB.Fernhall.Early‐phaseadaptationsoftraditional‐speedvs.superslowresistancetrainingonstrengthandaerobiccapacityinsedentaryindividuals.JournalofStrengthandConditioningResearch15:309‐314,2001.

Komi,P.V.andJ.H.Vitasalo.signalcharacteristicsofEMGatdifferentlevelsofmuscletension.ActaPhysiologicaScandinavica96:267‐276,1976.

Mann,R.Fundementalsofsprintingandhurdling.PresentationatLevel3trackandfieldclinic,Orlando,Fl,1996.

McBride,J.M.,T.T.Triplett‐McBride,A.Davis,andR.U.Newton.Acomparisonofstrengthandpowercharacteristicsbetweenpowerlifters,Olympicliftersandsprinters.JournalofStrengthandConditioningResearch13:58‐66,1999.

Medvedev,A.S.,Rodionov,VF.Rogozkin,V.Gulyants,A.E.Trainingcontentofweightliftersinpreparationperiod.(translation:M.Yessis)TeoriyaIpraktikaFizcheskoiKultury12:5‐7,1981.

Powell,KE.,G.W.heath,M.J.Kresnow,JJ.SacksandC.M.Branche.Injuryraesfromwalking,gardening,weightlifting,outdoorbicyclingandaerobics.MedicineandScienceinsportsandExercise.30:1246‐1449,1998.

Powell,P.L.,Roy,R.R.,Kanim,P.Bello,M.AandEdgerton,V.R.Predictabilityofskeletalmuscletensionfromarchitecturaldeterminationsinguineapigmuscle.JournalofAppliedPhysiology57:1715‐1721,1984.

Rasch,P.J.&Morehouse,L.E.(1957)Effectofstaticanddynamicexercisesonmuscularstrengthandhypertrophy.JournalofAppliedPhysiology11:29‐34.

Requa,R.K.,DeAvillaL.N.&Garrick,J.G.Injuriesinrecreationaladultfitnessactivities.TheAmericanJournalofSportsMedicine21(3):461‐467,1993.

Sale,D.G.Neuraladaptationtostrengthtraining.InP.V.Komi(ed),StrengthandPowerinSport.pp.249‐265,1992.

Semmler,J.G.andEnoka,RM.Neuralcontributiontochangesinmusclestrength.InV.M.Zatsiorsky(ed)BiomechanicsinSport,BlackwellScienceLtd,London,pp.3‐20,2000.

Semmler,J.G.andNordstrom,Motorunitdischargeandforcetremorinskill‐andstrength‐trainedindividuals.ExperimentalBrainResearch119:27‐38,1998.

Schmidt,R.A.MotorLearningandPerformanceChampaign,Il:HumanKinetics,1991.

Schmidtbleicher,D.Strengthtraining:part2:Structuralanalysisofmotorstrengthqualitiesanditsapplicationtotraining.SciencePeriodicalonResearchandTechnologyinSport5:1‐10,1985.

Schmidtbleicher,D.Trainingforpowerevents.InStrengthandPowerinSports(P.V.KomiEd)London,BlackwellScientificPublicationspp.381‐395,1992.

Siff,M.BiomechanicalfoundationsofStrengthandpowertraining‐In:V.Zatsiorskyed.BiomechanicsinSportLondon,BlackwellScientificLtd.,pp.103‐139,2001.

Siff,M.C.&Verkhoshanski.Supertraining:StrengthTrainingforSportingExcellence(3rdedition).Johannesburg:UniversityoftheWitwatersrand,1998.

Stone,M.H.,Plisk,S.andCollins,D.TrainingPrinciple:evaluationofmodesandmethodsofresistancetraining‐acoachingperspective.SportBiomechanics1(1):(InPress),2001.

Thomas,M.,A.FiataronandR.A.Fielding.Legpowerinyoungwomen:relationshiptobodycomposition,strengthandfunction.MedicineandScienceinSportsandExercise,28:1321‐1326,1996.

Thorstensson,a.Musclestrength,fibretypesandenzymeactivitiesinman.ActaPhysiologicaScandinavicaSuppl:443::1977.

VitasaloJ.T.andP.V.Komi.Interelationshipsbetweenelectromyographic,mechanical,musclestructureandreflextimemeasurementsinman.ActaPhysiologicaScandinavica111:97‐103,1981.

Weyand,P.G.,D.B.Sternlight,M.J.,BelliziandS.Wright.Fastertoprunningspeedsareachievedwithgreatergroundforcesnotmorerapidlegmovements.JournalofAppliedPhysiology,89:1991‐1999,2000.

Wilson,G.J.,Newton,R.U.,Murphy,A.J.&Humphries,B.J.(1993)Theoptimaltrainingloadforthedevelopmentofdynamicathleticperformance.MedicineandScienceinSportandExercise25(11):1279‐1286,1993.

Yao,W.X.,Fuglevand,A.J.,andEnoka,R.M.MotorunitsynchronizationincreasesEMGamplitudeanddecreasesforcesteadinessofsimulatedcontractions.JournalofNeurophysiology83:441‐452,2000.

Young,W.Trainingforspeed/strength:Heavyversuslightloads.NationalStrengthandConditioningAssociationJournal15(5):34‐42,1993.

Zajac,F.E.&Gordon,M.E.Determiningmuscle'sforceandactioninmulti‐atrticularmovement.InK.Pandolph,(ed),ExerciseandSportSciencesReviewsWilliamsandWilkins,Baltimore.