Transcript Chapter 6 – The Biomechanics of Skeletal Muscle

Chapter 6 – The Biomechanics of Skeletal
Muscle
1. Principal characteristics of skeletal muscle
2. Structural organization of skeletal muscle
3. Fast versus slow twitch motor units
4. Roles assumed by muscles
5. Types of muscular contraction
6. Factors affecting force production
Chapter 6 – The Biomechanics of Skeletal Muscle

Four principal characteristics:
• Excitability – ability to receive and respond to
a stimulus
• Contractilty (irritability) – ability of a muscle to
contract and produce a force
• Extensibility – ability of a muscle to be
stretched without tissue damage
• Elasticity – ability of a muscle to return to its
original shape after shortening or extension
Structural organization of skeletal muscle
From Principles of
Human Anatomy (7th
edition), 1995 by
Gerard J. Tortora, Fig
9.5, p 213
From Basic
Biomechanics
by Susan Hall
(3rd edition),
Fig 6.6, page
153
6-6
From Skeletal Muscle:
Form and Function (2nd
ed) by MacIntosh,
Gardiner, and McComas.
Fig 1.4, p. 8.
From Basic Biomechanics by Susan
Hall (3rd edition), Fig 6.5, page 152
6-5
Structural organization of skeletal muscle
From Principles of
Human Anatomy (7th
edition), 1995 by
Gerard J. Tortora, Fig
9.5, p 213
From Basic Biomechanics by
Susan Hall (3rd edition), Fig
6.3, page 150
6-3
From Exercise
Physiology: Theory and
Application to Fitness
and Performance (6th
Edition) by Scott K.
Powers and Edward T.
Howley. Fig 8.6 P. 147
A motor unit: single motor neuron and all
the muscle fibers it innervates
From Basic Biomechanics
Instructors manual by Susan
Hall (2nd edition, 1995), Fig TM
31
From Basic Biomechanics by Susan
Hall (3rd edition), Fig 6.7, page
154
6-7
From Basic Biomechanics by Susan Hall
(3rd edition), Fig 6.8, page 154
6-8
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Types of muscle fiber: Fast twitch vs Slow Twitch
Type I
ST Oxidative
(S0)
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Contraction speed
Time to peak force
slow
slow
Type IIa
FT Oxidative Glycolytic (FOG)
fast (2xI)
fast
Type IIb
FT Glycolytic
(FG)
fast (4xI)
fast
FT
ST
Twitch Tension
Fast twitch (FT)
fibers both reach
peak tension and
relax more quickly
than slow twitch
(ST) fibers. (Peak
tension is typically
greater for FT than
for ST fibers.)
Time

Types of muscle fiber: Fast twitch vs Slow Twitch
Type I
ST Oxidative
(S0)
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Contraction speed
Time to peak force
Fatigue rate
Fiber diam.
Aerobic capacity
Mitochondrial conc.
Anaerobic capacity
slow
slow
slow
small
high
high
low
Type IIa
FT Oxidative -
Type IIb
FT Glycolytic
Glycolytic (FOG)
fast (2xI)
fast
inter.
inter.
inter.
inter.
inter.
(FG)
fast (4xI)
fast
fast
large
low
low
High
Sedentary people – 50% slow/50% fast, whereas elite
athletes may differ
e.g., cross country skiers – 75% slow 25% fast
sprinters
- 40% slow 60% fast
Roles assumed by muscles
•
Agonist: acts to cause a movement
•
Antagonist: acts to slow or stop a
movement
•
Stabilizer: acts to stabilize a body part
against some other force
•
Neutralizer: acts to eliminate an
unwanted action produced by an agonist
•
Synergist: acts to perform the same action
as another muscle
Types of muscular
contraction
• Concentric: fibers shorten
• Eccentric: fibers lengthen
• Isometric: no length change
Factors affecting force Production
1.
2.
3.
4.
5.
6.
7.
8.
9.
Cross-sectional area
Frequency of stimulation
Spatial recruitment
Velocity of shortening
Muscle length
Action of the series elastic component
Muscle architecture
Electromechanical delay
Muscle temperature
Factors affecting force Production
1. Cross sectional area
1. Cross-sectional
area
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Hypertrophy: increase
in the # of myofibrils
and myofilaments
Hyperplasia: increase
in the number of
fibers???
Parallel vs serially arranged sarcomeres
Optimal for force
production
Optimal for velocity of shortening and
range of motion
In series
In parallel
From Exercise Physiology: Human Bioenergetics and its
applications (2nd edition) by Brooks, Fahey, and White (1996) Fig
17-20, P. 318
2. Rate Coding – frequency of stimulation
From Basic Biomechanics by Susan Hall
(3rd edition), Fig 6.9, page 155
3. Spatial recruitment
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Increase # of active motor units (MUs)
Order of recruitment
I ---> IIa -----> IIb
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Henneman's size principle: MUs are recruited in
order of their size, from small to large
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Relative contributions of rate coding and spatial
recruitment.
• Small muscles - all MUs recruited at approximately 50%
max. force; thereafter, rate coding is responsible for
force increase up to max
• Large muscles - all MUs recruited at approximately 80%
max. force.
The force-velocity
relationship for
muscle tissue:
When resistance
(force) is negligible,
muscle contracts
with maximal
velocity.
Force
4. Velocity of shortening: Force
inversely related to shortening velocity
(Low resistance,
high contraction
velocity)
Velocity
isometric
maximum
Force
The force-velocity
relationship for
muscle tissue: As
the load increases,
concentric
contraction velocity
slows to zero at
isometric maximum.
Velocity
Force-Velocity Relationship in different muscle fiber types
Type II fiber
Type I fiber
Force -Velocity Relationship (Effect of strength-Training)
Force/Velocity/Power Relationship
Force/velocity curve
Power/velocity
curve
Force
Power
30%
From Basic Biomechanics
by Susan Hall (3rd
edition), Fig 6.25, page
175
30%
Velocity
Effect of Muscle Fiber Types on Power-Velocity Relationship
Force-velocity Relationship During Eccentric
Muscular Contractions
From Skeletal muscle structure, function, and plasticity (2nd Edition) by R.L.
Leiber, P 312
5. Muscle length
From Skeletal muscle structure, function, and plasticity by R.L. Leiber, P. 55
From Exercise Physiology: Human Bioenergetics and its applications (2nd edition) by
Brooks, Fahey, and White (1996) P. 306
The length-tension
relationship: Tension
present in a stretched
muscle is the sum of the
active tension provided
by the muscle fibers and
the passive tension
provided by the tendons
fascia, and titin
6.Action of the series elastic component
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The stretch-shortening
phenomenon
The effectiveness and
efficiency of human
movement may be
enhanced if the muscles
primarily responsible for
the movement are
actively stretched prior to
contracting
concentrically.
Mechanism: storage and
release of elastic strain
energy.
7. Muscle Architecture
Parallel fiber arrangements
Fibers are roughly parallel to
the longitudinal axis of the
muscle
Pennate fiber arrangements
Short fibers attach at an angle
to one or more tendons within
the muscle
From Basic Biomechanics by Susan Hall
(3rd edition), Fig 6.11, page 159
From Basic Biomechanics by
Susan Hall (3rd edition), Fig
6.13, page 161
8. Electromechanical delay
20-100 ms
Time between arrival of a neural stimulus
and tension development by the muscle
From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.20, page 171
9. Temperature: Effect on the Force-Velocity
Relationship (22oC, 25oC, 31Co, and 37oC)
Two- joint Muscles
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Advantages
• Actions at two joints for the price of one
muscle. Possible metabolic saving if
coordinated optimally
• Shortening velocity of a two-joint muscle is
less than that of its single-joint synergists
Results in a more favorable position on the
force velocity curve.
• Act to redistribute muscle torque and joint
power throughout a limb.
Two- joint Muscles
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Disadvantages:
• Active insufficiency: unable to actively
shorten sufficiently to produce a full
range of motion at each joint crossed
simultaneously
• Passive insufficiency: unable to
passively lengthen sufficiently to
produce a full range of motion at each
joint crossed simultaneously