Download Skeletal Muscle Mechanics

PHYSIOLOGY 1
LECTURE 19
SKELETAL MUSCLE
MECHANICS
Skeletal Muscle
Mechanics
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Objectives: The student should learn:
1. The relationship of the skeletal muscle
action potential to Ca++ concentrations and
muscle force development.
2. Submaximal single twitch
3. Temporal summation, Tetany, fatigue,
and recruitment
4. The skeletal muscle motor unit
5. Active, passive, and total tension
curves
Skeletal Muscle
Mechanics
I. Introduction:
 Muscle mechanics has to do with how
skeletal muscle operates in order to
perform their function. In general the
movement of the body is a complex
interaction between the motor cortex of
the brain, alpha motor neurons, and
skeletal muscle.
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Skeletal Muscle
Mechanics
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It involves afferent information from
muscle, bone, and connective tissue
sent to the brain, integration by the
motor cortex and return efferent signals
to the contracting skeletal muscle. That
signal carried by the alpha motor
neurons produces a graded response in
the activated skeletal muscle which is
dependent on the weight of the load to
be lifted.
Skeletal Muscle Mechanics
Muscle Twitch
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A. Single Twitch - a single muscle
twitch is the skeletal muscle contraction
resulting from a single action potential.
– 1. Relationship of a single muscle twitch to
the muscle action potential - cytoplasmic
calcium concentrations - and force
development
– Note - The electrical event (AP) must
precede the mechanical event
Skeletal Muscle Mechanics
Muscle Twitch
Skeletal Muscle Mechanics
Muscle Twitch
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Single twitch is submaximal - Due to
the rapid action of the SR Ca++ATPase
calcium concentration falls before the
skeletal muscle can develop it’s
maximum tension.
Skeletal Muscle Mechanics
Muscle Twitch
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B. Multiple Twitch - Skeletal muscle responds
to several (Multiple) action potentials delivered
at a high frequency by increasing it’s force of
contraction up to the maximum tension
development. Therefore, a graded response
can be developed such that just the force
needed to lift the load is generated. There are
two methods of generating this graded
response. (frequency, recruitment)
Skeletal Muscle Mechanics
Muscle Twitch
1. Temporal summation - A series of action
potentials generated at a high enough
frequency to increase the tension in a skeletal
muscle fiber beyond that produced by a single
action potential.
 2. Tetany - Maximal skeletal muscle
contraction produced by a series of high
frequency action potentials.
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Skeletal Muscle Mechanics
Muscle Twitch
Skeletal Muscle Mechanics
Fatigue
In the human there are two types of fatigue.
 1. Nerve fatigue - This is usually defined as a
depletion of neurotransmitter quanta release to
the point where the release falls short of a full
quanta or becomes completely depleted and
fails to produce a response in the effector
organ. Normally this will never occur as most
nerves store sufficient neurotransmitter for
several thousand full quanta releases.
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Skeletal Muscle Mechanics
Fatigue
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2. Skeletal Muscle Fatigue
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Fatigue in skeletal muscle is not well understood but
the most likely scenario is the following. Skeletal
muscle fatigues at different rates depending on the
type of skeletal muscle fiber involved. Fatigue is fairly
tightly correlated with glycogen store depletion. In
exercise conditions over time aerobic ATP production
declines, therefore, a greater degree of reliance must
be placed on glycolysis to produce sufficient ATP to
maintain exercise. As more and more ATP is produced
from glycolysis lactic acid and [H+] begins to build up
inhibiting PFK.
Skeletal Muscle Mechanics
Strength verses Speed
In general the more sarcomeres a skeletal
muscle fiber has in parallel the stronger the
fiber becomes. (Hypertrophy or Hyperplasia)
 The more sarcomeres a skeletal muscle fiber
has in series the faster its velocity of
contraction becomes.
 Note - Velocity of contraction is limited by the type of
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myosin ATPase expressed by the fiber.
Skeletal Muscle Mechanics
Strength verses Speed
Skeletal Muscle Mechanics
Motor Units
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Def. - The alpha motor neuron plus all
of the skeletal muscle fibers it
innervates form a skeletal muscle motor
unit.
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Motor units can be as small as a nerve
and one muscle fiber or as large as a
nerve and several thousand muscle
fibers.
Skeletal Muscle
Mechanics
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Nerve stimulation
of skeletal muscle
can increase
almost infinitely
but skeletal
muscle response
maximizes (max
tension
development)
Skeletal Muscle Mechanics
Motor Units
1. Alpha motor neuron
 2. Muscle fibers innervated
 3. Interdigitation of motor unit fibers
with other motor unit fibers
 4. Graded response (Recruitment)
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Skeletal Muscle Mechanics
Motor Units
Skeletal Muscle Mechanics
Motor Units
Skeletal Muscle Mechanics
Graded Response (Recruitment)
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A skeletal muscle contraction is a
graded response starting with the
smallest motor units, activated first,
larger and larger motor units are added
on until the load is lifted. This process
is called recruitment.
Skeletal Muscle Mechanics
Graded Response (Recruitment)
Skeletal Muscle Mechanics
Tension Development
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Any skeletal muscle contraction involves
both an active tension development due
to crossbridge cycling and a passive
tension development due to
compression and stretch of elastic
elements such as tendons (stretch) and
endomysium (compression).
Skeletal Muscle Mechanics
Tension Development
Active Tension Curve - Sarcomere
shortening
 Passive Tension Curve - Tendons and
endomysium
 Total Tension Curve - The addition of
the passive tension curve plus the
active tension curve equal the total
tension curve.
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Skeletal Muscle Mechanics
Tension Development
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Active Tension Curve
Generated under
isometric conditions
Resting sarcomere
length in skeletal
muscle occurs at the
peak of the active
tension curve.
Skeletal Muscle Mechanics
Tension Development
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Passive Tension
Curve Generated by pulling
on muscle fiber
Starts at resting
sarcomere length
Skeletal Muscle Mechanics
Tension Development
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Total Tension Curve
Adding the Active
tension curve to the
passive tension
curve gives the Total
tension curve
Skeletal Muscle Types
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Skeletal muscle is composed of three
different fiber types, Fast Glycolytic
(White), Fast Oxidative-Glycolytic (Fast
Red), and Slow Oxidative (Red) fiber
types. The name is derived from the
primary form of energy production
which predominates within the fiber.
Skeletal Muscle Types
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Fast Glycolytic - (White muscle fiber) - predominately
expresses the enzymes involved in glycolysis. These are the
power or strength fibers but lack endurance.
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Fast Oxidative-Glycolytic - (Fast Red fibers) - Express a
balance between glycolytic and Krebs cycle enzymes.
Intermediate fibers strong and fast. Most abundant fibers in
the human.
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Slow Oxidative - (Slow Red fibers) - Express primarily
oxidative (Krebs Cycle) enzymes. These are the endurance
fibers but lack strength.
Skeletal Muscle Types
Skeletal Muscle Types
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All types of skeletal muscle fibers occur
in every skeletal muscle group.
Classification is dependent on the
muscle fiber type in greatest
abundance. Therefore, a muscle group
which predominantly expressed the
slow oxidative fibers would be classified
as an oxidative muscle or sometimes as
an endurance muscle group.
Skeletal Muscle Types
Comparison of Enzymes
ENZYMES
 MyosinATPase
 Glycolytic
 Krebs Cycle
 Mitochondria
 Glycogen
 Myoglobin
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F-G
1a
High
Low
Low
High
Low
F-O-G
1b
High
Med.
Med.
High
Med.
S-O
1c
Low
High
High
Med.
High
Skeletal Muscle Types
Affects of Exercise
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While skeletal muscle types are genetically determined
they can be moved toward other muscle types by
exercise training. For example an individual who
predominantly expresses the fast-oxidative-glycolytic
fiber type if trained as a marathoner will begin to
express more oxidative enzymes in his FOG fibers.
The FOG fibers are much easier to move than the two
extremes, therefore, exercise training focus on the
FOG fibers. In humans the FOG fibers are normally
the most abundant and humans show better training
effects than most other animals.
Skeletal Muscle Types
Affects of Exercise
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EXERCISE Predominant fiber type %
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Marathon
Sprints
Power Lift
Rower
Swimmer
Normal
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SO
42
FG
FG
FOG
FOG
FOG
FG
SO
39
37
63
65
48
22
30
SUMMARY
1. What is the difference between single twitch
and multiple twitch muscle contraction?
 2. What are the two methods a muscle can
use to increase its strength of contraction?
 3. What are the active, passive, and total
tension curves?
 4. In what order are motor units added in
muscle contraction?
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Mechanical Model
5. What are the three skeletal muscle types
and major differences?
 6. Which muscle type would be best for
marathons, high jump, wrestling?
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