muscle mechanics

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Muscle Mechanics

Related to Chapter 11 in the text

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/

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peroneus longusperoneus brevis

flexor halucis longus

flexor digiturum longus

triceps surea

tibialis anterior

tibialis posterior

ext. hallucis longus

ext. digitorum longus

PreparationHintermann

Muscles crossing the ankle joint complex

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musclemuscle

fasciclefascicle

muscle fibre (cell)muscle fibre (cell)

myofibrilmyofibril

endomysiumendomysium

sacrolemmasacrolemma

perimysiumperimysium

epimysiumepimysium

fasciafascia

Muscle Schematic IllustrationMuscle Schematic Illustration

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I - Band

M Line

A - Band

MyofibrilMyofibril

Z Line

Huxley and Huxley, 1954

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filamentfilament Z-lineZ-line

sarcomeresarcomere

A-bandA-band I-bandI-band

MyofibrilMyofibril

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Current paradigm to describe muscle Current paradigm to describe muscle contractioncontraction

Hugh Huxley and Andrew Huxley published in Hugh Huxley and Andrew Huxley published in 1954 two independent papers (which were 1954 two independent papers (which were basically identical) to describe the sliding of the basically identical) to describe the sliding of the thick and thin filaments past one another.thick and thin filaments past one another.

sliding filament theorysliding filament theory

Refinished in 1957 by A. HuxleyRefinished in 1957 by A. Huxley cross bridge theory cross bridge theory

Cross bridge theoryCross bridge theory

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thick filamentthick filamentthin filamentthin filament

I-BandI-Band A-BandA-Band

Cross bridge theoryCross bridge theory

Z-lineZ-line Z-lineZ-line

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thickthickmyofilamentmyofilament

thinthinmyofilamentmyofilament

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I - Band

M Line

A - Band

thick filaments

Sliding filament model

thin filaments

1m

Z A I M

Huxley and Huxley, 1954

Z Line

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globular headglobular head

tail tail portionportion myosin myosin

moleculemolecule

thickthickmyofilamentmyofilament

Cross bridgesCross bridges

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6060oo

14.3 nm14.3 nm

43 nm43 nm

Cross bridgesCross bridges

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contractioncontraction

restrest cross-bridgecross-bridgethin filamentthin filament

thick filamentthick filament

slidingsliding

Cross bridge theoryCross bridge theory

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Muscle Force Depends on Four Factors

• Sarcomere (muscle) length• Velocity of muscle contraction• Activation level• Previous contraction history

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Fact:Fact:Muscles at very long and very Muscles at very long and very short lengths can not produce short lengths can not produce high forceshigh forces

Fact:Fact:Maximal force production of a Maximal force production of a muscle depends on its lengthmuscle depends on its length

Force-Length Relationship

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100100

7575

5050

2525

00

ascending limbascending limb

plateau regionplateau region

descending limbdescending limb

ForceForce[%][%]

Sarcomere lengthSarcomere length

Force-Length RelationshipForce-Length Relationship

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Force-Length RelationshipForce-Length Relationship

1.60 1.60 mm

0.10 0.10 mm 0.95 0.95 mm

Sarcomere = 1 z-line + 2 thin filament + 1 thick filament - overlapsSarcomere = 1 z-line + 2 thin filament + 1 thick filament - overlaps

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Force-Length Relationship

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aa

bb

cc1.51.5 2.02.0 2.52.5 3.03.03.53.5

aa bb

cc

100100

7575

5050

2525

00

sarcomere length sarcomere length [[m]m]

tens

ion

gene

rate

dte

nsio

n ge

nera

ted

Force-Length Relationship

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100100

7575

5050

2525

0000 1.271.27

3.603.60

11

2233 44

55

1.701.702.002.00

2.172.17

ascending limbascending limb

plateauplateau

descending limbdescending limb

ForceForce[%][%]

[%][%]

Force-Length Relationship

Sarcomere LengthSarcomere Length

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Ascending limb:Ascending limb:

Point 2: Point 2: Thin filaments overlap partially.Thin filaments overlap partially.A reduced number of cross-bridges A reduced number of cross-bridges can attach.can attach.

Point 1:Point 1: Complete overlap of thin filaments.Complete overlap of thin filaments.No cross-bridges can attach.No cross-bridges can attach.

General:General:descending limbdescending limb easy to understandeasy to understandascending limbascending limb more difficult to understandmore difficult to understand

Force-Length Relationship

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Starting position in sprintStarting position in sprint

Knee angle in weight liftingKnee angle in weight lifting

Design of weight lifting Design of weight lifting equipmentequipment

Design of bicyclesDesign of bicycles

Application of F-L Relationship

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Velocity of Muscle Contraction

mus

cle

forc

e

velocity of muscle contraction

eccentric concentric

isometric

– +

• Why less force for faster concentric contractions?• Why more force for eccentric contractions?

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STST FTFT

ForceForce

VelocityVelocity

PowerPower

STST

FTFT

VelocityVelocity

Force/Power - Velocity

ST = slow twitchST = slow twitch

FT = fast twitchFT = fast twitch

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Activation Level• It takes time for muscle to develop tension

1) Electrical signals must be sent from the brain (or spine) to activate muscles. The dynamics of muscle contraction once the signal reaches the muscle also takes time

• Even after activation is initiated, there is a delay in the force applied to the bones2) At the start of a contraction, the sarcomeres will

shorten but will not be able to generate their maximum force. The sarcomeres shorten because the tendons (and other elastic components of muscle) are stretched. The elastic components of muscles and tendons must be sufficiently stretched before the muscular force is transmitted to bone (Springs).

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Previous Contraction History• If a muscle is initially contracting isometrically and

is then stretched….….the muscle will produce a greater isometric force at it’s new length.

• Also, a concentric contraction immediately following an eccentric contraction will be more forceful.

• This is known as the “force enhancement” phenomenon and has been repeated in hundreds of experiments.

• There are several theories behind this behaviour but none are globally excepted.

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• In 1994, two men attempted to set a world bungee jumping record by performing the highest double bungee jump in history off of Royal George Bridge. The bridge was located in Colorado and was suspended 300 m above the Arkansas River. John (69.2 kg) and Rory (90.1 kg) used a bungee cord (linear spring) that was 50 m long. John was physically tied to the bungee while Rory simply held onto John. The duo had meticulously planned their jump so that they would come to a stop just as they touched the water. Rory would let go of John allowing him to make his way back to the top and reel John back to safety.

– Knowing that the 50 m long bungee cord had a stiffness (k) of 15, was their jump successful? In other words, did the pair come to rest just at the surface of the Arkansas River? (3)

– The top of the bungee was fixed to the middle of the underside of a huge metal I-beam. Assuming that 250 KJ of the strain energy was lost as heat (not converted back into kinetic energy) and that the pair dropped in a perfectly vertical path, what happened to John after Rory was dropped into the water? Make sure to include John’s velocity at 300 m above the surface of the river. (3)

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NEXT CLASSREAD CHAPTER 5

AND

Construct a flow chart depicting what the torque developed about a

joint depends on.

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musclemuscle

faciclefacicle

muscle fibre (cell)muscle fibre (cell)

myrofibrilmyrofibril

endomysiumendomysiumsacrolemmasacrolemma

perimysiumperimysium

epimysiumepimysium

fasciafascia

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Z LineZ Line M LineM Line

A BandA Band

thick filamentsthick filaments thin filamentsthin filaments

titintitin

I BandI Band

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globular headglobular head

tail portiontail portion myosin moleculemyosin molecule

thick myofilament thick myofilament

centre of filamentcentre of filament

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6060°°

14.3 nm14.3 nm

42.9 nm42.9 nm

cross bridges cross bridges on thick on thick myofilament myofilament

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actin chainsactin chainstropomyosintropomyosin

actin globuleactin globule troponintroponin

38.5 mm38.5 mm thin myofilamentthin myofilament

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1 µm1 µm

ZZ AA II MMTitinTitin

Sliding filament model:Sliding filament model:

Cross-section area of thick Cross-section area of thick filaments and thick-thin filaments and thick-thin myofilaments overlap myofilaments overlap

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1.01.0

0.50.5

0000 0.20.2 0.40.4 0.60.6 0.80.8

1.01.0

Force Force / / PowerPower[normalized][normalized]

ForceForcePowerPower

VelocityVelocity[normalized[normalized]]

Force/Power - Velocity

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Sarcomere Length

• Maximum overlap of myosin and actin allows for a maximum amount of cross-bridge connection and hence force.

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First experiments:First experiments:

•• Fenn and Marsh, 1935Fenn and Marsh, 1935

•• Hill, 1938Hill, 1938Found (“stumbled” onto) the Found (“stumbled” onto) the Force-velocity relationship while Force-velocity relationship while working on heat production of working on heat production of isolated frog skeletal muscle.isolated frog skeletal muscle.

Force-Velocity Relationship

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6060oo

6060oo

14.3 nm14.3 nm43 nm43 nm

14.3 nm14.3 nm

43 nm43 nm

model Imodel I

model IImodel II

model Imodel I

model IImodel II

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myosin filament

actin filament

AA

A1

B1

B2

M4

A4

MM11

Huxley 1969; Huxley and Simmons, 1971

Cross bridge theoryCross bridge theory

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Cross bridge theoryCross bridge theory

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Knee Extension

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0.00.0 0.50.5 1.01.0 1.51.5 2.02.0 2.5 2.5 [cm][cm]

00

2020

4040

6060

LengthLength

Force Force

passive structurespassive structures

Force-LengthForce-Length

AccumulatedAccumulatedForce-LengthForce-Length

Active and passive structures

[N][N]

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100100

7575

5050

2525

0000

3.603.60

11

2233 44

55

ascending limbascending limb

plateau regionplateau region

descending limbdescending limb

ForceForce[%][%]

Sarcomere lengthSarcomere length

1 z-line1 z-line 0.10 0.10 mm2 thin filament2 thin filament 1.90 1.90 mm1 thick filament1 thick filament 1.60 1.60 mmtotal length total length 3.60 3.60 mm

no cross bridges can attachno cross bridges can attach

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100100

7575

5050

2525

0000

3.603.60

11

2233 44

55

2.172.17

ascending limbascending limb descending limbdescending limb

ForceForce[%][%]

Sarcomere lengthSarcomere length

1 z-line1 z-line 0.10 0.10 mm2 length thin filament2 length thin filament 1.90 1.90 mm1 thick 1 thick filamentfilament no overlap no overlap 0.17 0.17 mmtotal length sarcomertotal length sarcomer 2.17 2.17 mm

all cross bridges can attachall cross bridges can attach

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100100

7575

5050

2525

0000

3.603.60

11

2233 44

55

2.002.002.172.17

ascending limbascending limb descending limbdescending limb

ForceForce[%][%]

Sarcomere lengthSarcomere length

1 z-line1 z-line 0.10 0.10 mm2 thin filament2 thin filament 1.90 1.90 mm1 thick filament no overlap1 thick filament no overlap 0.00 0.00 mmtotal length sarcomertotal length sarcomer 2.00 2.00 mm

all cross bridges can attachall cross bridges can attach