Chapter 6 – The Biomechanics of Skeletal Muscle

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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

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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

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Structural organization of skeletal muscle
From Principles of Human Anatomy (7th edition),
1995 by Gerard J. Tortora, Fig 9.5, p 213
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From Basic Biomechanics by Susan Hall (3rd
edition), Fig 6.6, page 153
6-6
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From Skeletal Muscle Form and Function (2nd ed)
by MacIntosh, Gardiner, and McComas. Fig 1.4, p.
8.
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From Basic Biomechanics by Susan Hall (3rd
edition), Fig 6.5, page 152
6-5
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Structural organization of skeletal muscle
From Principles of Human Anatomy (7th edition),
1995 by Gerard J. Tortora, Fig 9.5, p 213
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From Basic Biomechanics by Susan Hall (3rd
edition), Fig 6.3, page 150
6-3
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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
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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
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From Basic Biomechanics by Susan Hall (3rd
edition), Fig 6.7, page 154
6-7
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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 Type IIa Type IIb
• ST Oxidative FT Oxidative - FT
  Glycolytic
• (S0) Glycolytic (FOG)
  (FG)
• Contraction speed slow fast (2xI)
  fast (4xI)
• Time to peak force slow fast
  fast

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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.)
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• Types of muscle fiber Fast twitch vs Slow Twitch
• Type I Type IIa Type IIb
• ST Oxidative FT Oxidative - FT
  Glycolytic
• (S0) Glycolytic (FOG)
  (FG)
• Contraction speed slow fast (2xI)
  fast (4xI)
• Time to peak force slow fast
  fast
• Fatigue rate slow inter.
  fast
• Fiber diam. small inter.
  large
• Aerobic capacity high inter.
  low
• Mitochondrial conc. high inter.
  low
• Anaerobic capacity low inter.
  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

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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

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Types of muscular contraction

• Concentric fibers shorten
• Eccentric fibers lengthen
• Isometric no length change

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Factors affecting force Production

• 1. Cross-sectional area
• 2. Frequency of stimulation
• 3. Spatial recruitment
• 4. Velocity of shortening
• 5. Muscle length
• 6. Action of the series elastic component
• 7. Muscle architecture
• 8. Electromechanical delay
• 9. Muscle temperature

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Factors affecting force Production
1. Cross sectional area

• Hypertrophy increase in the of myofibrils and
  myofilaments
• Hyperplasia increase in the number of fibers???

• 1. Cross-sectional area

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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
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2. Rate Coding frequency of stimulation
From Basic Biomechanics by Susan Hall (3rd
edition), Fig 6.9, page 155
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• 3. Spatial recruitment
• Increase of active motor units (MUs)
• Order of recruitment
• I ---gt IIa -----gt IIb
• Henneman's size principle MUs are recruited in
  order of their size, from small to large
• 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.

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4. Velocity of shortening Force inversely
related to shortening velocity
The force-velocity relationship for muscle
tissue When resistance (force) is negligible,
muscle contracts with maximal velocity.
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The force-velocity relationship for muscle
tissue As the load increases, concentric
contraction velocity slows to zero at isometric
maximum.
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Force-Velocity Relationship in different muscle
fiber types
Type II fiber
Type I fiber
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Force -Velocity Relationship (Effect of
strength-Training)
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Force/Velocity/Power Relationship
Force/velocity curve
Power/velocity curve
Force
Power
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From Basic Biomechanics by Susan Hall (3rd
edition), Fig 6.25, page 175
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Velocity
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Effect of Muscle Fiber Types on Power-Velocity
Relationship
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Force-velocity Relationship During Eccentric
Muscular Contractions
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From Skeletal muscle structure, function, and
plasticity (2nd Edition) by R.L. Leiber, P 312
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5. Muscle length
From Skeletal muscle structure, function, and
plasticity by R.L. Leiber, P. 55
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From Exercise Physiology Human Bioenergetics and
its applications (2nd edition) by Brooks, Fahey,
and White (1996) P. 306
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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
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6.Action of the series elastic component

• 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.

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7. Muscle Architecture
Fibers are roughly parallel to the longitudinal
axis of the muscle
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
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From Basic Biomechanics by Susan Hall (3rd
edition), Fig 6.13, page 161
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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
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9. Temperature Effect on the Force-Velocity
Relationship (22oC, 25oC, 31Co, and 37oC)
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Two- joint Muscles

• 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.

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Two- joint Muscles

• 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