SKELETAL MUSCLE STRUCTURE

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SKELETAL MUSCLE STRUCTURE
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SKELETAL MUSCLE STRUCTURE
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A MUSCLE FIBER
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ARRANGEMENT OF FILAMENTS
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Events Leading to Muscle Fiber Action
1. A motor neuron releases acetylcholine (ACh).
2. ACh binds to receptors on the sarcolemma.
3. The action potential triggers release of Ca2.
4. The Ca2 binds to troponin on the actin
filament, and the troponin pulls tropomyosin off
the active sites, allowing myosin heads to
attach to the actin filament.
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EVENTS LEADING TO MUSCLE ACTION
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ARRANGEMENT OF FILAMENTS IN A SARCOMERE
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AN ACTIN FILAMENT
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The Sliding Filament Theory
w When myosin cross-bridges are activated, they
bind strongly with actin, resulting in a change
in the cross-bridge.
w The change in the cross-bridge causes the
myosin head to tilt toward the arm of the
cross-bridge and drag the actin and myosin
filaments in opposite directions.
w The tilt of the myosin head is known as a power
stroke.
w The pulling of the actin filament past the
myosin results in muscle shortening and
generation of muscle force.
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CONTRACTING MUSCLE FIBER
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Did You Know?
The difference in force development between FT
and ST motor units is due to the number of muscle
fibers per motor unit, not the force generated by
each fiber.
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What Determines Fiber Type?
w Genetics determine which motor neurons
innervate our individual muscle fibers.
w Muscle fibers become specialized according to
the type of neuron that stimulates them.
w Endurance training and muscular inactivity may
result in small changes in the percentage of FT
and ST fibers.
w Aging may result in changes in the percentage
of FT to ST fibers.
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Slow-Twitch (ST) Muscle Fibers
w High aerobic (oxidative) capacity and fatigue
resistance
w Low anaerobic (glycolytic) capacity and motor
unit strength
w Slow contractile speed (110 ms) and myosin
ATPase
w 10180 fibers per motor neuron
w Low sarcoplasmic reticulum development
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Fast-Twitch (FTa) Muscle Fibers
w Moderate aerobic (oxidative) capacity and
fatigue resistance
w High anaerobic (glycolytic) capacity and motor
unit strength
w Fast contractile speed (50 ms) and myosin ATPase
w 300800 fibers per motor neuron
w High sarcoplasmic reticulum development
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Fast-Twitch (FTb) Muscle Fibers
w Low aerobic (oxidative) capacity and fatigue
resistance
w High anaerobic (glycolytic) capacity and motor
unit strength
w Fast contractile speed (50 ms) and myosin ATPase
w 300800 fibers per motor neuron
w High sarcoplasmic reticulum development
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The All-Or-None-Response
w For a motor unit to be recruited into activity
the motor nerve impulse must meet or exceed the
threshold.
w When this occurs, all muscle fibers in the
motor unit act maximally.
w If the threshold is not met no fibers in that
unit act.
w More force is produced by activating more motor
units.
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Recruitment Order

• How does the musculoskeletal system work in terms
  of the order muscle fibers are recruited?
• What is the name of the Principle that outlines
  this order?
• Why might the body work in this manner?
• What are the benefits/costs?
• CNS load changes?

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RAMPLIKE RECRUITMENT OF FIBERS
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TYPES OF MUSCLE ACTION
How do the control aspects change with joint
angle in two or three joint muscles?
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MUSCLE LENGTH vs FORCE PRODUCTION
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TENSION LENGTH CURVES
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MUSCLE VELOCITY vs FORCE PRODUCTION
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Mechanical Muscle Model
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SEC PEC CE???

• What do these structures contribute to the
  movement
• SEC (series)
• PEC (parallel)
• CE

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MUSCLE ACTION VELOCITY vs FORCE PRODUCTION
Hamil Knutzen 1995
Kreighbaum Barthels 1996
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MUSCLE ACTION VELOCITY vs. FORCE PRODUCTION
Why would less force be produced bilaterally than
the summed unilateral force for the same muscles?
Dickin Too (2006)
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Functional Classification of Muscles
Agonistsprime movers responsible for the
movement
Antagonistsoppose the agonists to prevent
overstretching of them
Synergistsassist the agonists and sometimes
fine-tune the direction of movement
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MUSCLE ACTION DURING ELBOW FLEXION
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Key Points
Use of Muscles
w Muscles involved in movement can be classified
as agonists, antagonists, and synergists.
w Three types of muscle action are concentric,
static, and eccentric.
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Key Points
Use of Muscles
w All joints have an optimal angle at which the
muscles crossing the joint produce maximal force.
w The angle of maximal force depends on the
relative position of the muscle's insertion on
the bone and the load placed on the muscle.
w Speed of action affects the amount of force
produced.
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MUSCLE FIBER ARRANGEMENT

• Force production is highly dependent upon CSA
• So why do pinnated muscles produce more force
• What is the downfall of the pinnation?

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Factors Influencing Force Generation
Rate coding plays an important role in the
magnitude of tension produced by each muscle
fiber.
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Factors Influencing Force Generation
w Number of motor units activated
w Type of motor units activated (FT or ST)
w Muscle size
w Initial muscle length
w Joint angle

• Speed of muscle action
• (shortening or lengthening)

w Angle of the muscle fibers (Pinnation)
w Frequency of neural activation
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MEASURING MUSCLE ACTIVITY
EMG Electromyography
Measures electrical changes within the muscle as
a function of the chemical changes going on
within the muscle
Change is a difference between the electrodes
Why is a ground electrode important in EMG?
Why and what is the process of normalization
doing?
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Bi-phasic Control of Movement

• Rapid movement to a target results in a specific
  EMG trace in the agonists and antagonists, and
  velocity profile

• Why do we see both Biceps and Triceps bursts
  when we move rapidly in an aiming task?

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Tri-phasic Control of Movement

• Why would some movement show a tri-phasic
  pattern?

• Auditory tone presented at 0
• Dashed line Extension trials
• Solid line Flexion trials
• Note the velocity curves and also the triphasic
  pattern for each movement.

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Describing Motion - Kinematics

• Displacement
• Velocity
• Acceleration
• Relative Motion
• Degrees of Freedom
• Planes of movement
• What is missing from this in regards to motion?

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Describing Motion - Kinetics

• Motion is caused by forces
• Newtons Laws of motion
• Body in motion stays in motion unless acted upon
• Fma
• For every action there is an equal and opposite

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Force Models of Movement

• Impulse-Momentum Model
• Momentum will increase with
• Larger force
• Force applied over longer time
• Think about preparing a movement based on this
  principle?
• How do we know when to turn muscle off?
• How would we deal with missing the target?