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ExercisePhysiology:TheoryandApplicationtoFitnessandPerformance ByScottPowers&EdwardHowley Chapter8SkeletalMuscle:StructureandFunction SummaryCreatedbyDanHechler ClassLectureExercisePhysiologyIDr.LaurieMilliken,UMassBoston,Fall2015 SkeletalMuscle 1.HumanBodycontainsover400skeletalmuscles -40-50%oftotalbodyweight -75%water -20%protein -5%salts,phosphates,ions,macronutrients 2.Epimysium -Surroundsallthebundlestoformentiremuscles 3.Perimysium -Srroundsseveralfibersandformsbundlescalledfasciculi 4.Endomysium -Wrapseachfiber 5.Tendons:tissueconnectingmuscletoperiosteumofbone 6.Sacrolemma -Musclecellmembrane 7.Satellitecells: -Playroleinmusclegrowthandrepair(increasenumberofnuclei) -Morenucleiallowforgreaterproteinsynthesis -Importantforstrengthtrainingadaptations 1.Myofibrils -Containcontractileproteins • Actin(thinfilament) • Myosin(thickfilament) 2.Sacromere -IncludesZline,Mline,Hzone,Aband,Iband 3.Sacroplasmicreticulum -Storagesitesforcalcium NeuromuscularJunction 1.Junctionbetweenmotorneuronandmusclefiber -Motorunit • Motorneuronandallfibersitinnervates 2.MotorEndPlate -Pocketformedaroundmotorneuronbysarcolemma 3.Neuromuscularcleft -Shortgapbetweenneuronandmusclefiber 4.Acetylcholinereleasedfrommotorneuron -Causesend-platepotential(EPP) • Depolarizationofmusclefiber • AcHhydrolysisbycholinesterase SlidingFilamentModel 1.Muscleshorteningoccursduetomovementofactinfilamentovermyosinfilament 2.Formationofcrossbridges(PowerStroke) 3.ReductionindistancebetweenZlinesofthesarcomere 1.Tropomyosin -Liesalongactininthegrooveformedbythedoublehelix • Inhibitsactin-myosininteraction 2.Troponin -Embeddedatregularintervalsalongactin • InteractswithCa+2 • Movestropomyosinuncoveringactiveactinsites EnergyForContraction 1.ATPrequiredformusclecontraction • MyosinATpasebreaksdownATPfibercontract • ATPàADP+Pi • ATPaseislocatedonmyosinhead • RelaxationaftermusclecontractionalsorequiredATP • CalciumpumpsrequireATPtopumpcalciumbackintosarcoplasmicreticulum 2.Relaxation • Ca+2activelypumpedinSacroplasmicReticulum • Troponinallowstropomyosintointerferewithactin-myosininteraction Excitation-ContractionCoupling 1.Excitation • Actionpotentialinmotorneuroncausesreleaseofacetylcholineintosynapticcleft. • Acetylcholinebindstoreceptorsonmotorendplatewhichleadstodepolarizationthat isconducteddowntransversetubules,whichcausesreleaseofCa+2fromSR 2.Contraction • Atrest,myosincross-bridgesinweakbindingstate • Ca+2bindstotroponin,causesshiftintropomyosintouncoveractivesites,andcrossbridgeformsstrongbindingstate. • Pireleasedfrommyosin,cross-bridgemovementoccurs • ADPreleasedfrommyosin • ATPattachestomyosin,breakscross-bridgeandformingweakbindingstate.ATPthen bindstomyosin,brokendowntoADP+Pi,whichenergizesmyosin. • ThiscontinuesaslongasCa+andATParepresent CharacteristicsofMuscleFiberTypes BiochemicalProperties -OxidativeCapacity • Numberofcapillaries,mitochondria,andamountofmyoglobin -TypeofmyosinATPase • SpeedofATPdegradation -IsoformofLDH(LactateDehydrogenase) ContractileProperties -Maximalforceproduction • Forceperunitofcross-sectionalarea -Speedofcontraction(Vmax) • MyosinATpaseactivity -Musclefiberefficiency WhatDeterminesFiberType 1.Geneticsdeterminewhichtypeofmotorneuronsinnervatesindividualmusclefibers. 2.Musclefibersbecomespecializedaccordingtotypeofneuronthatstimulatesthem 3.Enduranceandstrengthtraining,andmuscularinactivitymayresultinsmallchanges(less than10%)in%ofFastTwitchandSlowTwitchFibers. 4.Endurancetrainingreduce%ofFTxfiberswhileincreasingthe%ofFTafibers. 5.Agingchangesin%ofFTandSTfibers Slow-TwitchFibers:TypeI 1.Highaerobic(oxidative)capacityandfatigueresistance 2.Lowanaerobic(glycolytic)capacityandmotorunitstrength • Slowcontractilespeed(110mstoreachpeaktension)andmyosinATPase • LowSRdevelopment • SlowerCa2+releaseanduptakebySR • Largeandnumerousmitochondria • Approx50%ofallfibers • 10-180fiberspermotorneuron(hasasmallcellbody) Fast-TwitchFibers:TypeII 1.HighCapacitytotransmitActionPotential 2.HighmyosinATPaseactivity 3.RapidCa2+releaseanduptakebySR 4.Highrateofcrossbridgeturnover 5.Capableofhighforcegeneration 6.Relyonanaerobicmetabolism Fast-TwitchIIaFibers 1.Fastshorteningspeed 2.Moderateaerobic(oxidative)capacityandfatigueresistance 3.Highanaerobic(glycolytic)capacityandmotorunitstrength 4.Fastcontractilespeed(50mstoreachpeaktension)andmyosinATPase 5.300-800fiberspermotorneuron • Motorneuronhaslargecellbody • LargerfibersizecomparedtoSTfibers 6.HighSRdevelopment Fast-TwitchIIxfibers 1.MostRAPIDshorteningvelocity 2.Lowaerobic(oxidative)capacityandfatigueresistance 3.Highanaerobic(glycolytic)capacityandmotorunitstrength 4.Fastcontractilespeed(50mstoreachpeaktension)andmyosinATPase 5.300-800fiberspermotorneuron • MotorNeuronhasalargecellbody • LargerfibersizecomparedtoSTandFTIIa 6.HighSRdevelopment FiberTypeAndPerformance MuscleAtrophyDueToInactivity 1.Lossofmusclemassandstrength -Immobilizedduetoinjury 2.Initialatrophy(2days) -Duetodecreasedproteinsynthesis 3.Furtheratrophy -Duetoincreasemuscleproteinbreakdown 4.Atrophyisnotpermanent -Canbereversedviastrengthtraining TypesofMuscleAction 1.Isometric -Muscleexertsforcewithoutchanginglength -Pullingagainstimmovableobject -Posturalmuscles 2.Isotonic(dynamic) -Concentric • Muscleshortensduringforceproduction -Eccentric • Muscleproducesforcebutlengthincreases FactorsThatRegulateProductionOfForceDuringMuscleContraction 1.Typesandnumberofmotorunitsrecruited • Moremotorunits=GreaterForce:FastMotorunits=GreaterForce 2.Initialmusclelength • “Ideal”lengthforforcegeneration • Increasedcross-bridgeformation 3.Natureofneuralstimulationofmotorunits -Frequencyofstimulation(SimpleTwitch,Summation,Tetanus) WhatCausesMuscleCramps? 1.ElectrolyteDepletion 2.AlteredNeuromuscularControlTheory • AbnormalactivityofbothmusclespindleandGolgitendonorgan • IncreasedmusclespindleactivityanddecreasedGolgitendonactivity WhatCausesMuscleFatigue? HighIntensityActivity • AccumulationofLactate • HydrogenIons • ADP • InorganicPhosphate • FreeRadicalswithinActiveMuscleFibers EnduranceFatigue • AccumulationofFreeRadicals • Muscle/Extracellularelectrolyteimbalances • DepletionofMuscleGlycogen • CentralFatigue(CentralGoverningTheory) SizePrincipalOfMuscleFiberRecruitment -MotorUnitsareRecruitedSmallesttoLargest(SnowballEffect) LengthTensionRelationship 1. Lessthanoptimallengthwillresultinfewercrossbridgeinteractionsandproducelow force 2. Optimallengthwillresultinmaximalcrossbridgeinteractionsandproducingmaximal force 3. GreaterthanoptimallengthwillresultinNOcross-bridgeinteractionandnoforce MuscleForce-VelocityRelationships 1. Absoluteforcethespeedofmovementisgreaterinfasttwitchfibers 2. Maximumvelocityofshorteningisgreatestatthelowestforce • Trueforbothslow-andfasttwitchfibers MuscleForce-PowerRelationships 1. Anygivenvelocityofmovementpowerisgeneratedgreaterinmusclewithhigher percentoffast-twitchfibers 2. PowerdecreasesbeyondPeakPowerbecauseforcedecreaseswithincreasing movementspeed DiseasesThatNegativelyImpactMuscleFunction 1.Agingandmuscleloss -Sarcopenia -10%ofmusclemasslostbetween25-50Years -Additional40%lostbetween50-80Years -Lossoffastfibersandgaininslowfibers -Alsoduetoreducedphysicalactivity -Resistancetrainingcandelayage-relatedatrophy 2.Diabetes -Associatedwithprogressivelossofmusclemass -Combinedwithagerelatedloss -Aerobic/Resistanceareprotective 3.Cancer -50%ofcancerpatientssufferrapidmusclemassloss(Cachexia) -Accountsfor20%ofdeathsincancerpatients 4.Musculardystrophy -Hereditarydefectsmuscleprotein -Resultsinlossofmusclefibers/weakness