muscle contraction 09

7/31/2019 Muscle Contraction 09

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Skeletal Muscle BasicsContraction and Basic mechanical

properties

Taken from:Professor Bruce Lynn


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Skeletal Muscle Basics

3 Lectures:

Basic structure of muscle

Muscle activation & relaxation

Basic mechanical properties


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Outline

Sliding filaments & the crossbridge cycle

Force & Power

Antagonistic muscles

Series & Parallel structures

Arrangement of fibres with muscle

Force-velocity relation, also power

For shortening & stretch


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Dependence of isometric force on sarcomere length

Sarcomere length (% of optimum)

Force (%

of max)

Force is proportional to filament overlap:

important evidence for sliding filaments

The Tension Length Curve


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What causes the filament sliding?

Myosin heads bind to actin, then gothrough a cycle of events the cross

bridge cycle

Overall effect is force generation and ATP

hydrolysis

As all myosin molecules are identical, can

reduce problem to considering just a

single myosin head interacting with actin


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Z

ADP Pi

1

Z

ADP Pi

attach

2

Z

ADP

Pi release &

weak to strong

Pi

3

Z

ADP release

& filamentsliding

ADP4ATP

binding

Z

ATP

ATP

5

detach &

ATP

hydrolysis

Does not occur

when [Ca] low


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Force in Isometric contraction: no sliding

Thin filament

Thick filament

Attached crossbridge, no

force (spring not stretched)

Attached crossbridge has

changed shape to stretch

spring, force but no sliding

2

3Direction of isometric force: toward M line


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Cross-bridge Cycle: Key Features

1)ATP is used in each cycle to provide the energyRigor mortis occurs if ATP concentration = 0

2) Direction of filament force and sliding (if sliding

occurs) is one-way (thin filament moves toward M-line

at the centre of the sarcomere)

3) Step size is small: sliding produced by one cycle is

only about 1% of the sarcomere lengthMany cycles occur in succession to cause largemovements (as in running, walking, etc)


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Why so complicated?

Some constraints due

to muscle properties


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Outline


Force & Power






What are they? How are they

different?


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Isometric

Force

Power


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What is isometric contraction?

Muscles are active (=contracting)

producing isometric force

The muscle force resists gravity and

prevents the arm and book falling

Isometric means the muscle length is

constant


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Contraction with shortening (concentric)

Biceps contracts and its shortening

flexes the elbow

Biceps does work lifting the book

POWER is the rate at which work

is done


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Outline


Force & Power






Required due to crossbridge cycle

& sliding filament arrangement


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Tendon

Tendon

Biceps

Tendon

Triceps

Tendon

Example: Rotation around the elbow

Active (contracting) muscle can shorten (pull towards its

center)

Therefore, antagonistic muscles are required

BUT it cannot elongate (push away from its center)


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Rotation around the elbow: Flexion

Biceps contracts & shortens

Triceps is lengthened

(not contracting)


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Rotation around the elbow: Extension

Biceps is lengthened

(not contracting)

Triceps contracts & shortens


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Outline


Force & Power






How the arrangement ofstructures affect force and

length change


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Springs connected in Parallel

Fixed position

Springs connected in Series

Fixed position


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Structures in Series:

Force at A and B are equal.

For structures in series, forces do NOT add up

Fixed position

A

Fixed position

A B


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A

Fixed position

Force = A

Fixed position

A

B

Force = A+B

For structures in parallel, forces add up


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Length changes

Length 2.0 m

Length 1.5 m

= 0.5 m

Length 3.0 m

= 1.0 m

Length 4.0 m

Connect in series

For structures in series, length changes add up


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Length changes

Length 2.0 m

Length 1.5 m

= 0.5 m

Length 2.0 m

Connect in parallel

= 0.5 mLength 1.5 m

For structures in parallel, length changes do NOT add up


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Contractile and Elastic Structures

In series and in parallel


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A muscle-tendon complex (MTC)

Bone

Bone

Muscle fibresor Contractile Component (CC)

Tendon

or Series elastic component (SEC)

Parallel elastic

component


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A muscle-tendon complex (MTC)

Because muscle and tendon are in series:

Both experience the same force at each

moment. An observed length change of MTC could

be due to either component

Tendon can only be stretched when

muscle is active

Muscle cannot move bones without first

stretching tendon

Bone

Bone

Muscle

or CC

Tendon

or SEC


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Elasticity also in parallel

The parallel element:

Can exert force when CC

is relaxed.

Adds its force to that of

muscle when CC is active.

More complicatedconnections can switch

elasticity between series

and parallel.

Bone

Bone

CC

SEC

PEC


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Where and what are the SEC and PEC

relative to the crossbridges?

Tendon (collagen) series

Aponeuroses (collagen) series

Epimysium (collagen) parallel

Filaments (titin) parallel

Filaments (myosin, actin) series


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Outline


Force & Power






How the arrangement ofstructures affect force and

length change


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Arrangement of muscle fibres: some examples

parallel

fusiform

triangular

unipennate

bipennate

multipennate


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Volumes equal and line of muscle force is the same

Each fibre in pennate muscle is half the length

of the fibres in the parallel muscle

and at angle to the line of muscle force;

force along line of muscle (F) = cos * force

along line of fibre (f)

For = 30o, cos = 0.87

But there are twice as many fibres in thepennate muscle as in the parallel muscle

Net effect: pennate muscle produces 2 * 0.87 =

1.74 times more force than the parallel muscle

Arrangement of fibres within muscle:

Pennation increases muscle force

parallel pennate


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Net effect: pennate muscle shortening is only 0.5 * 0.87

= 0.41 times as much as the parallel muscle per unit

time

the cos rule means that muscle shortening

is cos * fibre shortening.

Pennation reduces muscle shortening velocity

also each fibre in the pennate muscle only

shortens half as far as each fibre in the

parallel muscle.

In each unit of time


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Outline


Force & Power







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Contraction with shortening (concentric)

Biceps contracts and its shortening

flexes the elbow

Biceps does work lifting the book

POWER is the rate at which work

is done


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Muscle Force

Muscle length

time(Lever movement)

Before stimulation of the muscle


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start stimulation of the muscle

Isometric phasemuscle force toosmall to lift weight

Muscle Force

Muscle length

time

(Lever movement)

Stim

I i h i


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During stimulation, muscle force

enough to lift weight

Muscle Force

Muscle length

time

(Lever movement)

Stim

Isotonic shortening:constant forceduring shortening


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Muscle Force

Muscle length

time(Lever movement)

Before stimulation of the muscle

Larger weight


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During stimulation of the muscle

time

Muscle Force

Muscle length

(Lever movement)

Stim

Isometric phasemuscle force toosmall to lift weight

I t i h t i


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During stimulation of the muscle

Muscle Force

Muscle length

time

(Lever movement)

Stim

Isotonic shortening:constant forceduring shortening

Larger force &

slower velocity

P k t


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Power = work rate

Velocity

0.0 0.5 1.0 1.5

Power

0.0

0.1

0.2

Velocity

0.0 0.5 1.0 1.5

Force

0.0

0.5

1.0

Inverse relation between

force and velocity of

shortening

The Force Velocity Curve

= (force x length ) / time

= force x (length / time)

= force x velocity


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Contraction with lengthening (eccentric)

Biceps is acting as a brake.

Biceps is producing force, EMG, etc,(=contracting)

The book is lowered in a slow,controlled movement.

The elbow extends as the length of

biceps increases due to the book &

gravity. Work is done on biceps.


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Force during isovelocity stretch of active

muscle

stim

stim

stim

Force-Velocity relation for Stretch

Velocity

stretch shorten


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Stretch of active muscle

Occurs during normal every-day activities

Contracting muscle fibres act as a brake

Large forces can be produced

But not much fuel (ATP) is used

Forces can be large enough to cause

damage

Not covered in many standard textbooks


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Skeletal Muscle BasicsContraction. Basic mechanical properties

Summary

Tension-length curve, max force at max filament overlap

Cross bridge cycle, myosin head binds to actin, ATP splitting,

repetitiveMuscle morphology:

short fat muscles, high force, low speed;

long thin muscles, low force, high speed

Inverse force-velocity relationPower = Force*velocity; max power at ca 1/3 max force or velocity

Eccentric contractions, high force.


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Skeletal Muscle Basics

Contraction. Basic mechanical properties

Good source of information

Jones et al., Skeletal Muscle from Molecules to Movement, 2004,

Churchill Livingstone.

Acknowledgements

Thanks for Nancy Curtin, Imperial College, for use of many of her slides.

muscle contraction 09

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