Biomechanics of Biomechanics of Skeletal Muscle Skeletal Muscle Professor Ming-Shaung Ju Professor Ming-Shaung Ju 朱朱朱 朱朱朱 Dept. of Mechanical Engineering Dept. of Mechanical Engineering National Cheng Kung University, Tai National Cheng Kung University, Tai nan, Taiwan nan, Taiwan
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Biomechanics of Skeletal Muscle Professor Ming-Shaung Ju 朱銘祥 Dept. of Mechanical Engineering National Cheng Kung University, Tainan, Taiwan.
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Biomechanics of Skeletal Biomechanics of Skeletal MuscleMuscle
Professor Ming-Shaung Ju Professor Ming-Shaung Ju 朱銘祥朱銘祥
Dept. of Mechanical EngineeringDept. of Mechanical EngineeringNational Cheng Kung University, Tainan, TaiwNational Cheng Kung University, Tainan, Taiwan an
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ContentsContents
I. Composition & structure of skeletal I. Composition & structure of skeletal musclemuscle
II. Mechanics of Muscle ContractionII. Mechanics of Muscle Contraction
III. Force production in muscleIII. Force production in muscle
IV. Muscle remodelingIV. Muscle remodeling
V. SummaryV. Summary
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Muscle types: Muscle types: – cardiac muscle: composes the heartcardiac muscle: composes the heart– smooth muscle: lines hollow internal organssmooth muscle: lines hollow internal organs– skeletal (striated or voluntary) muscle: skeletal (striated or voluntary) muscle:
attached to skeleton via tendon & attached to skeleton via tendon & movementmovement
Skeletal muscle 40-45% of body weightSkeletal muscle 40-45% of body weight– > 430 muscles> 430 muscles– ~ 80 pairs produce vigorous movement~ 80 pairs produce vigorous movement
Dynamic & static workDynamic & static work– Dynamic: locomotion & positioning of Dynamic: locomotion & positioning of
segmentssegments– Static: maintains body postureStatic: maintains body posture
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I. Composition & I. Composition & structure of skeletal structure of skeletal musclemuscle
Structure & organization
• Muscle fiber: long cylindrical multi-nuclei cell 10-100 m
fiber endomysium fascicles perimysium
epimysium (fascia)
• Collagen fibers in perimysium & epimysium are continuous
A bands: thick filaments in cA bands: thick filaments in central of sarcomereentral of sarcomere
Z line: short elements that liZ line: short elements that links thin filamentsnks thin filaments
I bands: thin filaments not oI bands: thin filaments not overlap with thick filamentverlap with thick filamentss
H zone: gap between ends oH zone: gap between ends of thin filaments in center of thin filaments in center of A bandf A band
M line: transverse & longitudM line: transverse & longitudinally oriented linking proinally oriented linking proteins for adjacent thick filteins for adjacent thick filamentsaments
A bandI band
Z ZM
sarcomere
H
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Sarcoplasmic reticulumSarcoplasmic reticulum network of tubules & sacs; network of tubules & sacs; parallel to myofibrilsparallel to myofibrils enlarged & fused at junction enlarged & fused at junction
between A & I bands: transverbetween A & I bands: transverse sacs (terminal cisternae)se sacs (terminal cisternae)
T system: duct for fluids & proT system: duct for fluids & propogation of electrical stimulupogation of electrical stimulus for contraction (action potes for contraction (action potential)ntial)
Sarcoplasmic reticulum store Sarcoplasmic reticulum store calciumcalcium
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Molecular composition Molecular composition of myofibrilof myofibril Myosin composed of indiviMyosin composed of indivi
dual molecules each has a dual molecules each has a globular head and tailglobular head and tail
Cross-bridge: actin & myosCross-bridge: actin & myosin overlap (A band)in overlap (A band)
Actin has double helix; twActin has double helix; two strands of beads spiralino strands of beads spiraling around each otherg around each other
troponin & tropomysin regtroponin & tropomysin regulate making and breakinulate making and breaking contact between actin & g contact between actin & myosinmyosin
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Molecular basis of muscle Molecular basis of muscle contractioncontraction Sliding filament theory: relative movement of aSliding filament theory: relative movement of a
ctin & myosin filaments yields active sarcomerctin & myosin filaments yields active sarcomere shorteninge shortening
Myosin heads or cross-bridges generate contraMyosin heads or cross-bridges generate contraction force ction force
Sliding of actin filaments toward center of sarcSliding of actin filaments toward center of sarcomere: decrease in I band and decrease in H zoomere: decrease in I band and decrease in H zone as Z lines move closerne as Z lines move closer
PEC: parallel elastic componentCC: contractile componentSEC: series elastic component
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II. Mechanics of Muscle II. Mechanics of Muscle ContractionContraction Neural stimulation – impulseNeural stimulation – impulse Mechanical response of a motor unit - twitchMechanical response of a motor unit - twitch
T
t
eT
tFtF
0)(
T: twitch or contraction time, time for tension to reach maximum
F0: constant of a given motor unit
Averaged T valuesTricep brachii 44.5 ms Soleus 74.0 msBiceps brachii 52.0 ms Medial Gastrocnemius 79.0 msTibialis anterior 58.0 ms
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Summation and tetanic contractionSummation and tetanic contraction
(ms)
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Generation of muscle Generation of muscle tetanustetanus
10 Hz
100Hz
Note: muscle is controlled by frequency modulation from neural input very important in functional electrical stimulation
Motor unit recruitmentMotor unit recruitmentAll-or-nothing event2 ways to increase tension:- Stimulation rate- Recruitment of more motor unit
Size principleSmallest m.u. recruited firstLargest m.u. last
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III. Force production in III. Force production in musclemuscle
Force –length characteristicsForce –length characteristics Force – velocity characteristicsForce – velocity characteristics Muscle ModelingMuscle Modeling Neuromuscular system dynamics Neuromuscular system dynamics
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3-1 Force-length curve of 3-1 Force-length curve of contractile componentcontractile component
Resting 2.0-2.25 um Resting 2.0-2.25 um max. no. of cross brimax. no. of cross bridges; max. tensiondges; max. tension
2.25-3.6 um no. of cr2.25-3.6 um no. of cross bridge oss bridge
< 1.65 um overlap of < 1.65 um overlap of actin no. of cross briactin no. of cross bridge dge
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Influence of parallel elastic Influence of parallel elastic componentcomponent
l0
Fc
Fp
Ft
Note: Fc is under voluntary control & Fp is always present
Fp
l0
Fc
Ft
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Overall force-length characteristics of a Overall force-length characteristics of a musclemuscle
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Series Elastic ComponentSeries Elastic Component
Tendon & other series tissueTendon & other series tissue Lengthen slightly in isometric contractionLengthen slightly in isometric contraction Series component can store energy when Series component can store energy when
stretched prior to an explosive shorteningstretched prior to an explosive shortening
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Quick-release for determining Quick-release for determining elastic constant of series elastic constant of series componentcomponent Muscle is stimulated to build Muscle is stimulated to build
tensiontension Release mechanism is Release mechanism is
shortening x while force is shortening x while force is kept constantkept constant
Contractile element length Contractile element length kept constant during quick kept constant during quick releaserelease
F
x
FKSC
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In vivo force-length In vivo force-length measurementmeasurement Human Human in vivoin vivo experiments (MVC) experiments (MVC) Challenges:Challenges:
– Impossible to generate a max. Impossible to generate a max. voluntary contraction for a single voluntary contraction for a single agonist without activating remaining agonist without activating remaining agonistagonist
– Only moment & angle are Only moment & angle are measurable. Moment depends on measurable. Moment depends on muscle force and moment arm.muscle force and moment arm.
– Muscle contracts and shortening, positive Muscle contracts and shortening, positive work was done on external load by musclework was done on external load by muscle
– Tension in a muscle decreases as it Tension in a muscle decreases as it shortensshortens
Eccentric contractionEccentric contraction– Muscle contracts and lengthening, Muscle contracts and lengthening,
external load does work on muscle or external load does work on muscle or negative work done by muscle.negative work done by muscle.
– Tension in a muscle increases as it Tension in a muscle increases as it lengthens by external loadlengthens by external load
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Force-velocity Force-velocity characteristics of skeletal characteristics of skeletal muscle (Hill model)muscle (Hill model)
0
0
0
0
0
0Fwhen velocity max. v
heat shortening oft coefficien a
tensionisometric max. F
)(
F
vab
abv
baFF
eccentric concentricF
v
Increased tensions in eccentric due to:• Cross bridge breaking force > holding force at isometric length• High tendon force to overcome internal damping friction
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Length and velocity versus Length and velocity versus ForceForce
Note: maximum contraction condition; normal contractions are fraction of the maximum force
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Equilibrium of load and muscle Equilibrium of load and muscle forceforce
Muscle force is function Muscle force is function of of lengthlength, , velocityvelocity and and activationactivation
The load determines The load determines activation and length of activation and length of muscle by the muscle by the equilibrium conditionequilibrium condition
Neuromuscular system Neuromuscular system modelingmodeling
ActivationDynamics
Central Command& reflexes
ContractionDynamics
TendonCompliance
TendonCompliance
Force
Muscle-TendonVelocity
Muscle-TendonLength
-
-
+
+
muscle
tendon
tendon
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Muscle fatigueMuscle fatigue
Drop in tension followed prolonged stimulation.Fatigue occurs when the stimulation frequency outstrips rate of replacement of ATP, the twitch force decreases with time
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V. Muscle RemodelingV. Muscle Remodeling Effects of Disuse and ImmobilizationEffects of Disuse and Immobilization
– Immediate or early motion may prevent muscle atroImmediate or early motion may prevent muscle atrophy after injury or surgeryphy after injury or surgery
– Muscle fibers regenerate in more parallel orientatioMuscle fibers regenerate in more parallel orientation, capillariaztion occurred rapidly, tensile strength rn, capillariaztion occurred rapidly, tensile strength returned more quicklyeturned more quickly
– Atrophy of quadriceps developed due to immobilizaAtrophy of quadriceps developed due to immobilization can not be reversed by isometric exercises.tion can not be reversed by isometric exercises.
– Type I fibers atrophy with immobilization; cross-secType I fibers atrophy with immobilization; cross-sectional area decreases & oxidative enzyme activity retional area decreases & oxidative enzyme activity reducedduced
– Tension in muscle afferent impulses from intrafusal Tension in muscle afferent impulses from intrafusal muscle spindle increases & leading to increase stimmuscle spindle increases & leading to increase stimulation of type I fiberulation of type I fiber
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Effects of Physical TrainingEffects of Physical Training– Increase cross-sectional area of muscle fibers, muIncrease cross-sectional area of muscle fibers, mu
scle bulk & strengthscle bulk & strength– Relative percentage of fiber types also changesRelative percentage of fiber types also changes– In endurance athletes % type I, IIA increaseIn endurance athletes % type I, IIA increase– Stretch out of muscle-tendon complex increases Stretch out of muscle-tendon complex increases
elasticity & length of musculo-tendon unit; store elasticity & length of musculo-tendon unit; store more energy in viscoelastic & contractile componmore energy in viscoelastic & contractile componentsents
– Roles of muscle spindle & Golgi tendon organs: inRoles of muscle spindle & Golgi tendon organs: inhibition of spindle effect & enhance Golgi effect to hibition of spindle effect & enhance Golgi effect to relax muscle and promote further lengthening.relax muscle and promote further lengthening.
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V. SummaryV. Summary
Structure unit of muscle: fiberStructure unit of muscle: fiber Myofibrils are composed of actin & myosinMyofibrils are composed of actin & myosin Sliding filament theory & cross-bridgeSliding filament theory & cross-bridge Calcium ion & contractivityCalcium ion & contractivity Motor unit: a single neuron & all muscle fiMotor unit: a single neuron & all muscle fi
bers innervated by itbers innervated by it Force production depends on length, veloForce production depends on length, velo
D.A. Winter, Biomechanics and Motor ContD.A. Winter, Biomechanics and Motor Control of Human Movement, 2nd ed. John Wilrol of Human Movement, 2nd ed. John Wiley & Sons, NY, 1990. ey & Sons, NY, 1990.
M. Nordin & V.H. Frankel, Basic BiomechanM. Nordin & V.H. Frankel, Basic Biomechanics of the Musculoskeletal System, 2ne ed., ics of the Musculoskeletal System, 2ne ed., Lea & Febiger, London, 1989.Lea & Febiger, London, 1989.
Y.C. Fung, Biomechanics: Mechanical PropY.C. Fung, Biomechanics: Mechanical Properties of Living Tissues, 2nd ed., Speinger-erties of Living Tissues, 2nd ed., Speinger-Verlag, NY, 1993.Verlag, NY, 1993.