Transcript Muscle Structure and Function

Chapter 3
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Learning Objectives
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To describe muscle’s macro and micro structures
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To explain the sliding-filament action of muscular
contraction
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To differentiate among types of muscle fibres
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To describe group action of muscles
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Types of Muscle
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The human body is comprised of 324 muscles
Muscle makes up 30-35% (in women) and 42-47% (in men) of
body mass.
Three types of muscle:
Skeletal muscle
Cardiac muscle
Smooth muscle
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A. Skeletal (Striated) Muscle
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Connects the various parts of the skeleton through one or more
connective tissue tendons
During muscle contraction, skeletal muscle shortens and moves
various parts of the skeleton
Through graded activation of the muscles, the speed and smoothness
of the movement can be gradated
Activated through signals carried to the muscles via nerves (voluntary
control)
Repeated activation of a skeletal muscle can lead to fatigue
Biomechanics: assessment of movement and the sequential pattern of
muscle activation that move body segments
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B. Smooth Muscle
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Located in the blood vessels, the respiratory
tract, the iris of the eye, the gastro-intestinal
tract
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The contractions are slow and uniform
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Functions to alter the activity of various
body parts to meet the needs of the body at
that time
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Is fatigue resistant
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Activation is involuntary
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C. Cardiac Muscle
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Has characteristics of both skeletal and
smooth muscle
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Functions to provide the contractile
activity of the heart
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Contractile activity can be gradated
(like skeletal muscle)
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Is very fatigue resistant
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Activation of cardiac muscle is
involuntary (like smooth muscle)
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Components of skeletal muscle
d) myofibril
c) muscle fibre
b) muscle fibre bundle a) Muscle belly
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Muscle Fibres
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Cylinder-shaped cells that make up skeletal muscle
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Each fibre is made up of a number of myofilaments
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Diameter of fibre (0.05-0.10 mm)
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Length of fibre (appr. 15 cm)
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Surrounded by a connective tissue sheath called Sarcolemma
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Many fibres are enclosed by connective tissue sheath Perimycium to
form bundle of fibres
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Each fibre contains contractile machinery and cell organelles
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Activated through impulses via motor end plate
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Group of fibres activated via same nerve: motor unit
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Each fibre has capillaries that supply nutrients and eliminate waste
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Muscle Teamwork
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Agonist (prime mover):
- the muscle or group of muscles producing a desired effect
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Antagonist:
- the muscle or group of muscles opposing the action
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Synergist:
- the muscles surrounding the joint being moved
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Fixators:
- the muscle or group of muscles that steady joints closer to the body axis so
that the desired action can occur
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Bending or straightening of elbow requires the coordinated
interplay of the biceps and triceps muscles
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Contractile Machinery:
Sarcomeres
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Contractile units
Organized in series ( attached
end to end)
Two types of protein
myofilaments:
- Actin:
thin filament
- Myosin: thick filament
Each myosin is surrounded by
six actin filaments
Projecting from each myosin
are tiny contractile myosin
bridges
Longitudinal section of myofibril
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(a) At rest
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High microscope magnification of sarcomeres
within a myofibril
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Contractile Machinery:
Crossbridge formation and movement
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Cross bridge movement:
- similar to the stroking of the oars and
 Cross bridge formation:
movement of rowing shell
- a signal comes from the motor
- movement of myosin filaments in relation
nerve activating the fibre
- the heads of the myosin filaments to actin filaments
- shortening of the sarcomere
temporarily attach themselves to
- shortening of each sarcomere is additive
the actin filaments
Longitudinal section of myofibril
b) Contraction
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Contractile Machinery:
Optimal Crossbridge formation
Longitudinal section of myofibril
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Sarcomeres should be optimal
distance apart
For muscle contraction: optimal
distance is (0.0019-0.0022 mm)
At this distance an optimal number
of cross bridges is formed
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If the sarcomeres are stretched
farther apart than optimal distance:
- fewer cross bridges can form 
less force produced
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If the sarcomeres are too close
together:
- cross bridges interfere with one
another as they form  less force
produced
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c) Powerful stretching
d) Powerful contraction
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Contractile Machinery:
Optimal muscle length and optimal joint angle
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The distance between sarcomeres is dependent on the stretch of
the muscle and the position of the joint
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Maximal muscle force occurs at optimal muscle length (lo)
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Maximal muscle force occurs at optimal joint angle
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Optimal joint angle occurs at optimal muscle length
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Muscle tension during elbow flexion at constant speed
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Contractile Machinery:
Tendons, origin, insertion
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In order for muscles to contract, they must be
attached to the bones to create movement
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Tendons: strong fibrous tissues at the ends of
each muscle that attach muscle to bone
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Origin:
the end of the muscle attached to the
bone that does not move
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Insertion: the point of attachment of the muscle
on the bone that moves
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Muscle Fibre Types
Slow twitch fibres:
Slow Oxidative (Type I)
Fast twitch fibres:
Fast Glycolytic (Type IIb)
Fast Oxidative Glyc. (Type IIb)
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A. Slow Twitch Fibres
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Suited for repeated contractions during activities requiring a
force output of < 20-25% of max force output
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Examples: lower power activities, endurance events
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B) Fast Twitch Fibres
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Significantly greater force and speed generating capability than
slow twitch fibres
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Well suited for activities involving high power
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Examples: sprinting, jumping, throwing
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The Muscle Biopsy
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Used to determine muscle fibre type
1. Injection of local anesthetic into the muscle being sampled
2. Incision of approximately 5-7mm is made in the skin and fascia
of the muscle
3. The piece of tissue (250-300mg) removed via the biopsy needle
is imbedded in OCT compound
4. The sample is frozen in isopentane cooled to –180C
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Muscle Biopsy
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The Histochemistry
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The biopsy samples are first sectioned (8-10 μm thickness)
Sections are processed for myosin ATPase:
Fast twitch fibres – rich in myosin ATPase (alkaline labile)
Slow twitch fibres – low in myosin ATPase (acid labile)
Sections are processed for other metabolic characteristics
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Nerve-Muscle Interaction
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Skeletal muscle activation is initiated through neural activation
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NS can be divided into central (CNS) and peripheral (PNS)
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The NS can be divided in terms of function: motor and sensory
activity
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Sensory: collects info from the various sensors located
throughout the body and transmits the info to the brain
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Motor: conducts signals to activate muscle contraction
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Activation of motor unit and its innervation systems
1. Spinal cord 2. Cytosome
3. Spinal nerve
4. Motor nerve 5. Sensory nerve 6. Muscle with muscle fibres
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Motor Unit
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Motor nerves extend from the spinal cord to the muscle fibres
Each fibre is activated through impulses delivered via motor end plate
Motor unit: a group of fibres activated via the same nerve
All muscle fibres of one particular motor unit are always of the same
fibre type
Muscles needed to perform precise movements generally consist of a
large number of motor units and few muscle fibres
Less precise movements are carried out by muscles composed of fewer
motor units with many fibres per unit
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All-or-none Principle
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Whether or not a motor unit activates upon the
arrival of an impulse depends upon the so called
all-or-none principle
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An impulse of a certain magnitude (or strength) is
required to cause the innervated fibres to contract
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Every motor unit has a specific threshold that must
be reached for such activation to occur
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Intra-muscle Coordination
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The capacity to apply motor units simultaneously
is known as intra-muscle coordination
 Many highly trained power athletes, such as
weightlifters, wrestlers, and shot putters, are able
to activate up to 85% of their available muscle
fibres simultaneously (untrained: 60%)
 Force deficit: the difference between assisted and
voluntarily generated maximal force (trained:
10%, untrained: 20-35%)
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Intra-muscle Coordination cont.
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Trained athletes have not only a larger muscle mass than
untrained individuals, but can also exploit a larger number
of muscle fibres
Athletes are more restricted in further developing strength
by improving intra-muscular coordination
Trained individuals can further increase strength only by
increasing muscle diameter
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Inter-muscle Coordination
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The interplay between muscles that generate movement
through contraction (agonists) and muscles responsible for
opposing movement (antagonists) is called inter-muscle
coordination
 The greater the participation of muscles and muscle
groups, the higher the importance of inter-muscle
coordination
 To benefit from strength training the individual muscle
groups can be trained in relative isolation
 Difficulties may occur if the athlete fails to develop all the
relevant muscles in a balanced manner
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Inter-muscle Coordination cont.
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High-level inter-muscle coordination greatly improves
strength performance and also enhances the flow, rhythm,
and precision of movement
Trained athlete is able to translate strength potential to
enhance inter-muscle coordination
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Muscle’s Adaptation to Strength Training
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Individual’s performance improvements occur through a
process of biological adaptation, which is reflected in the
body’s increased strength
Adaptation process proceeds at different time rates for
different functional systems and physiological processes
Adaptation depends on intensity levels used in training and
on athlete’s unique biological make-up
Enzymes adapt within hours, cardiovascular adaptation
within 10 to 14 days
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