Skeletal Muscle Basics

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Skeletal Muscle Basics 3. Contraction. Basic
mechanical properties Lecture 5 Exercise
Physiology For modules in Cardiac and Pulmonary
Rehab Professor Bruce Lynn MSc School of Human
Health and Performance
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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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Force is proportional to filament overlap
important evidence for sliding filaments
Dependence of isometric force on sarcomere length
The Tension Length Curve
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What causes the filament sliding? Myosin heads
bind to actin, then go through 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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Does not occur when Ca low
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Force in Isometric contraction no sliding
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Cross-bridge Cycle Key Features
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)
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Why so complicated?
Some constraints due to muscle 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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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
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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Active (contracting) muscle can shorten (pull
towards its center)
BUT it cannot elongate (push away from its center)
Therefore, antagonistic muscles are required
Example Rotation around the elbow
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Rotation around the elbow Flexion
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Rotation around the elbow Extension
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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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Fixed position
Structures in Series Force at A and B are
equal. For structures in series, forces do NOT
add up
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Fixed position
For structures in parallel, forces add up
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Length changes
For structures in series, length changes add up
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Length changes
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)
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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

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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 complicated connections can switch
  elasticity between series and parallel.

Bone
CC
PEC
SEC
Bone
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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
Sliding filaments the crossbridge cycle
Force Power
Antagonistic muscles
How the arrangement of structures affect force
and length change
Series Parallel structures
Arrangement of fibres with muscle
Force-velocity relation, also power
For shortening stretch
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Arrangement of muscle fibres some examples
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Arrangement of fibres within muscle Pennation
increases muscle force
parallel
pennate
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 the
pennate muscle as in the parallel muscle
Net effect pennate muscle produces 2 0.87
1.74 times more force than the parallel muscle
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Pennation reduces muscle shortening velocity
In each unit of time
the cos rule means that muscle shortening is cos
fibre shortening.
also each fibre in the pennate muscle only
shortens half as far as each fibre in the
parallel muscle.
Net effect pennate muscle shortening is only
0.5 0.87 0.41 times as much as the parallel
muscle per unit time
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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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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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Before stimulation of the muscle
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Isometric phase muscle force too small to lift
weight
start stimulation of the muscle
Muscle Force
Muscle length
(Lever movement)
Stim
time
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Isotonic shortening constant force during
shortening
During stimulation, muscle force enough to lift
weight
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Larger weight
Before stimulation of the muscle
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Isometric phase muscle force too small to lift
weight
During stimulation of the muscle
time
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Isotonic shortening constant force during
shortening
During stimulation of the muscle
Larger force slower velocity
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Contraction with lengthening (eccentric)
The book is lowered in a slow, controlled
movement.
Biceps is acting as a brake.
Biceps is producing force, EMG, etc,
(contracting)
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
Force-Velocity relation for Stretch
stretch
shorten
Velocity
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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 Basics
• Contraction. Basic mechanical properties
• Summary
• Tension-length curve, max force at max filament
  overlap
• Cross bridge cycle, myosin head binds to actin,
  ATP splitting, repetitive
• Muscle morphology
• short fat muscles, high force, low speed
• long thin muscles, low force, high speed
• Inverse force-velocity relation
• Power Forcevelocity 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.