Mechanics of Movement II: Muscle Action Across Joints

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Mechanics of Movement II Muscle Action Across
Joints

• Review muscle force generation
• Muscle Physics
• --force versus cross section
• --length versus strain
• Lever mechanics
• Stabilizing the jointisometric and eccentric
• contraction

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Muscle Structure Review
Fig. 10.1

• Muscle fiber muscle cell
• Fibers lined up direction of pull
• Tendon attaches to bone
• Muscle pulls on bone

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Muscle Origin and Insertion

• Origin
• Proximal
• Fixed
• Insertion
• Distal
• Moves
• (usually!!)

Fig. 10.3
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Mechanics of Contraction

• Muscle cell is unit
• Role of actin/myosin
• Action potential or depolarization of membrane
  makes cell contract
• (motor neuron action potential stimulates muscle
  membrane depolarization)

Fig. 10.4
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Visualizing muscle contraction
How actin-myosin complex (sarcomere) shorten
muscle
Fig. 10.7
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Summary of Muscle Organization/Function
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Summary of Muscle Organization/Function
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Summary of Muscle Organization/Function
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Levels of Muscle Organization
Table 10.2
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Muscle Physics Principle I

• Cross sectional area is proportional to Force of
  muscle

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Muscle Physics Principle II

• Length of muscle is proportional to ability to
  shorten (strain)
• Number of sarcomeres in series gives shortening
  ability
• Short, fat muscles
• Lots of force
• Less shortening range
• Long, skinny muscles
• Less force
• More shortening range

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Muscle Physics Principle III

• Force generation depends on current length of
  muscle or overlap in actin/myosin of sarcomeres
• Muscle force strongest between 80-120 of normal
  resting lengthWHY? (dont forget role of
  cross-bridges)
• Most muscles arranged to work in this range

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Types of fascicle arrangements

• Affects length and cross section of muscle
• Thus affects force and shortening properties of
  muscle
• See Muscle Physics Principles I-III if this
  doesnt make sense

Fig. 11.3
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Muscle movement across joints is like lever system
Fig. 11.1
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First-class lever
Fig. 11.2
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Second-class lever
Fig. 11.2
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Third-class lever
Fig. 11.2
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Stabilization and Control Around Joint

• Antagonist often fires or contracts or is
  stimulated simultaneously with agonist to
  stabilize around joint during movement
• NOTE Muscle contraction or stimulus to fire
  does not always result in muscle shortening

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Agonist/Antagonist
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Relation between muscle contraction (or firing)
and shortening

• Concentric contractionmuscle contracts and
  shortens to cause movement across joint
• Isometric contractionmuscle contracts but stays
  same length to hold joint or body in same
  position
• Eccentric contractionmuscle contracts while
  lengthening to stabilize joint during movement
  (most common in antagonist to slow movement
  caused by agonist)