Muscle Structure and Function

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Muscle Structure and Function

• Chapter 3

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Learning Objectives

• To describe muscles macro and micro structures
• To explain the sliding-filament action of
  muscular contraction
• To differentiate among types of muscle fibres
• To describe group action of muscles

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Types of Muscle

• 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

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A. Skeletal (Striated) Muscle

• 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

• Located in the blood vessels, the respiratory
  tract, the iris of the eye, the gastro-intestinal
  tract
• The contractions are slow and uniform
• Functions to alter the activity of various body
  parts to meet the needs of the body at that time
• Is fatigue resistant
• Activation is involuntary

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C. Cardiac Muscle

• Has characteristics of both skeletal and smooth
  muscle
• Functions to provide the contractile activity of
  the heart
• Contractile activity can be gradated (like
  skeletal muscle)
• Is very fatigue resistant
• 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

• Cylinder-shaped cells that make up skeletal
  muscle
• Each fibre is made up of a number of myofilaments
• Diameter of fibre (0.05-0.10 mm)
• Length of fibre (appr. 15 cm)
• Surrounded by a connective tissue sheath called
  Sarcolemma
• Many fibres are enclosed by connective tissue
  sheath Perimycium to form bundle of fibres
• Each fibre contains contractile machinery and
  cell organelles
• Activated through impulses via motor end plate
• Group of fibres activated via same nerve motor
  unit
• Each fibre has capillaries that supply nutrients
  and eliminate waste

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Muscle Teamwork

• Agonist (prime mover)
• - the muscle or group of muscles producing a
  desired effect
• Antagonist
• - the muscle or group of muscles opposing the
  action
• Synergist
• - the muscles surrounding the joint being
  moved
• 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 MachinerySarcomeres

• 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

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High microscope magnification of sarcomeres
within a myofibril
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Contractile MachineryCrossbridge formation and
movement

• Cross bridge movement
• - similar to the stroking of the oars and
  movement of rowing shell
• - movement of myosin filaments in relation to
  actin filaments
• - shortening of the sarcomere
• - shortening of each sarcomere is additive

• Cross bridge formation
  
  - a signal comes from the motor nerve
  activating the fibre
  - the heads of the myosin filaments
  temporarily attach themselves to the actin
  filaments

Longitudinal section of myofibril
b) Contraction
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Contractile MachineryOptimal Crossbridge
formation
Longitudinal section of myofibril

• 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
• If the sarcomeres are stretched farther apart
  than optimal distance
• - fewer cross bridges can form ? less force
  produced
• If the sarcomeres are too close together
• - cross bridges interfere with one another
  as they form ? less force produced

c) Powerful stretching
d) Powerful contraction
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Contractile MachineryOptimal muscle length and
optimal joint angle

• The distance between sarcomeres is dependent on
  the stretch of the muscle and the position of the
  joint
• Maximal muscle force occurs at optimal muscle
  length (lo)
• Maximal muscle force occurs at optimal joint
  angle
• 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 MachineryTendons, origin, insertion

• In order for muscles to contract, they must be
  attached to the bones to create movement
• Tendons strong fibrous tissues at the ends of
  each muscle that attach muscle to bone
• Origin the end of the muscle attached to
  the bone that does not move
• Insertion the point of attachment of the
  muscle on the bone that moves

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Muscle Fibre Types
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A. Slow Twitch Fibres

• Suited for repeated contractions during
  activities requiring a force output of lt 20-25
  of max force output
• Examples lower power activities, endurance
  events

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B) Fast Twitch Fibres

• Significantly greater force and speed generating
  capability than slow twitch fibres
• Well suited for activities involving high power
• Examples sprinting, jumping, throwing

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The Muscle Biopsy

• 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

• 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

• Skeletal muscle activation is initiated through
  neural activation
• NS can be divided into central (CNS) and
  peripheral (PNS)
• The NS can be divided in terms of function motor
  and sensory activity
• Sensory collects info from the various sensors
  located throughout the body and transmits the
  info to the brain
• Motor conducts signals to activate muscle
  contraction

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Activation of motor unit and its innervation
systems

• Spinal cord 2. Cytosome 3. Spinal
  nerve
• 4. Motor nerve 5. Sensory nerve 6. Muscle
  with muscle fibres

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Motor Unit

• 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

• Whether or not a motor unit activates upon the
  arrival of an impulse depends upon the so called
  all-or-none principle
• An impulse of a certain magnitude (or strength)
  is required to cause the innervated fibres to
  contract
• Every motor unit has a specific threshold that
  must be reached for such activation to occur

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Intra-muscle Coordination

• 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.

• 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

• 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.

• 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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Muscles Adaptation to Strength Training

• Individuals performance improvements occur
  through a process of biological adaptation, which
  is reflected in the bodys 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 athletes unique biological
  make-up
• Enzymes adapt within hours, cardiovascular
  adaptation within 10 to 14 days