Download Muscle Fatigue

PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
9
Muscles and
Muscle Tissue:
Part A
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Review Principles of Muscle Mechanics
1. Same principles apply to contraction of a single fiber
and a whole muscle
2. Contraction produces tension, the force exerted on the
load or object to be moved
3. Contraction does not always shorten a muscle:
•
Isometric contraction: no shortening; muscle
tension increases but does not exceed the load
•
Isotonic contraction: muscle shortens because
muscle tension exceeds the load
4. Force and duration of contraction vary in response to
stimuli of different frequencies and intensities
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Motor Unit: The Nerve-Muscle Functional Unit
• Motor unit = a motor neuron and all (four to several
hundred) muscle fibers it supplies
• Small motor units in muscles that control fine
movements (fingers, eyes)
• Large motor units in large weight-bearing muscles
(thighs, hips)
• Muscle fibers from a motor unit are spread throughout
the muscle so that a single motor unit causes weak
contraction of entire muscle
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Muscle Twitch
• Response of a muscle to a single, brief threshold
stimulus
• Three phases of a twitch:
• Latent period: events of excitation-contraction
coupling
• Period of contraction: cross bridge formation;
tension increases
• Period of relaxation: Ca2+ reentry into the SR;
tension declines to zero
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Muscle Twitch Comparisons
Different strength and duration of twitches are due to variations
in metabolic properties and enzymes between muscles
Latent period
Extraocular muscle (lateral rectus)
Gastrocnemius
Soleus
Single
stimulus
(b) Comparison of the relative duration of twitch responses of
three muscles
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Graded Muscle Responses
• Responses are graded by:
1. Changing the frequency of stimulation
2. Changing the strength of the stimulus
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Response to Change in Stimulus Frequency
• A single stimulus results in a single contractile
response — a muscle twitch
Single stimulussingle twitch
Contraction
Relaxation
Stimulus
A single stimulus is delivered. The muscle
contracts and relaxes
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Response to Change in Stimulus Frequency
• Increase frequency of stimulus (muscle does not have
time to completely relax between stimuli)
• Ca2+ release stimulates further contraction  temporal
(wave) summation
• Further increase in stimulus frequency  unfused
(incomplete) tetanus
• If stimuli are given quickly enough, fused (complete)
tetany results
• Rarely happens in the body under normal
conditions – mostly in lab conditions
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Response to Change in Stimulus Strength
• Threshold stimulus: stimulus strength at which the
first observable muscle contraction occurs
• Muscle contracts stronger as stimulus strength is
increased above threshold
• Contraction force is precisely controlled by
recruitment (multiple motor unit summation), which
brings more and more muscle fibers into action
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Response to Change in Stimulus Strength
• Size principle: motor units with larger and larger fibers are
recruited as stimulus intensity increases
Motor
unit 1
Recruited
(small
fibers)
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Motor
unit 2
recruited
(medium
fibers)
Motor
unit 3
recruited
(large
fibers)
Isotonic Contractions
• Muscle changes in length and moves the load
• Isotonic contractions are either concentric or eccentric:
• Concentric contractions — the muscle shortens and does
work
• For example, concentric contraction is used to lift a glass
from a table
• Eccentric contractions — the muscle contracts as it
lengthens
• Example – someone pulls your arm straight while at the
same time you try to keep the arm locked in one position
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Isometric Contractions
• The load is greater than the tension the muscle is able to
develop
• Tension increases to the muscle’s capacity, but the muscle
neither shortens nor lengthens
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Muscle Metabolism: Energy for Contraction
• ATP is the only source used directly for contractile
activities
• Available stores of ATP are used in 4–6 seconds
• ATP is regenerated by:
• Direct phosphorylation
phosphate (CP)
of
• Anaerobic pathway (glycolysis)
• Aerobic respiration
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ADP
by
creatine
Anaerobic Pathway
• At 70% of maximum contractile activity:
• Bulging muscles compress blood vessels
• Oxygen delivery is impaired
• Pyruvic acid is converted into lactic acid
• Lactic acid:
• Diffuses into the bloodstream
• Used as fuel by the liver, kidneys, and heart
• Converted back into pyruvic acid by the liver
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Aerobic Pathway
• Produces 95% of ATP during rest and light to
moderate exercise
• Fuels: stored glycogen, then bloodborne glucose,
pyruvic acid from glycolysis, and free fatty acids
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Muscle Fatigue
• Physiological inability to contract or sustain the expected
power output
• It is a reversible condition
• Fatigue can potentially occur at any of the point involves
in muscle contraction – from the brain to the muscle fibers
or in any of the systems that are responsible to supply
oxygen to the muscles
• Total lack of ATP occurs rarely during states of continuous
contraction
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Muscle Fatigue
• Central fatigue mechanisms – arise from the CNS
• Includes subjective feeling of tiredness and desire to cease
activity
• Suggested reasons include low pH, failure to produce
enough ACh
• Peripheral fatigue mechanisms – anywhere between the
neuromuscular junction and the muscle
• Lack of glycogen
• Ionic imbalances (K+, Ca2+, Pi) interfere with E-C
coupling
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Muscle Fatigue
• An endurance exercise training can delay onset of
fatigue by increasing oxidative capacity:
• Increased number of mitochondria
• Increased level of oxidative enzymes
• Increased number of capillary beds to muscle
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Force of Muscle Contraction
• The force of contraction is affected by:
• Number of muscle fibers stimulated (recruitment)
• Relative size of the fibers — hypertrophy of cells
increases strength
• Frequency of stimulation —  frequency allows time
for more effective transfer of tension to noncontractile
components
• Length-tension relationship — muscles contract most
strongly when muscle fibers are 80–120% of their
normal resting length
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Length-tension relationship
• The force of muscle contraction depends on the length of the
sarcomeres before the contraction begins
• On the molecular level, the length reflects the overlapping
between thin and thick filaments
• The tension a muscle fiber can generate is directly
proportional to the number of crossbridges formed between
the filament
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Velocity and Duration of Contraction
Influenced by:
1. Muscle fiber type
2. Load
3. Recruitment
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Muscle Fiber Type: Functional Characteristics
• Speed of contraction – determined by speed in which ATPases
split ATP
• The two types of fibers are slow and fast
• ATP-forming pathways
• Oxidative fibers – use aerobic pathways
• Glycolytic fibers – use anaerobic glycolysis
• These two criteria define three categories
• Slow oxidative fibers contract slowly, have slow acting
myosin ATPases, and are fatigue resistant
• Fast oxidative fibers contract quickly, have fast myosin
ATPases, and have moderate resistance to fatigue
• Fast glycolytic fibers contract quickly, have fast myosin
ATPases, and are easily fatigued
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Remodeling muscles to match function
• Muscles are continually being remodeled to match
functions required
• The muscle can alter:
• The diameter
• Length
• Strength
• Vascular supply
• Types of fibers (slightly)
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Muscle diameter changes
• Hypertrophy – increased total mass of muscle
• Result of increased number of the filaments in each
fiber
• The enzyme systems also increase (mainly enzymes
for glycolysis)
• If the muscles are not used for many weeks, the rate of
decay of contractile units is more rapid results in
atrophy (decrease in mass)
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Adjustment of muscle length
• It is a type of hypertrophy in which muscles are
stretched to greater than normal length
• That causes the addition of new sarcomeres at the end
of the fibers
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Effects of Exercise - Aerobic exercise
• Aerobic exercise is physical exercise that intends to improve
the oxygen system
• As a result, aerobic (endurance) exercise:
• Leads to increased:
• Muscle capillaries
• Number of mitochondria
• Myoglobin synthesis
• Results in greater endurance, strength, and resistance to fatigue
• May convert fast glycolytic fibers into fast oxidative fibers
• Examples: running, swimming, cycling
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Effects of Exercise - Aerobic exercise
• Among the recognized benefits of doing regular aerobic
exercise are:
• Strengthening the muscles involved in respiration, to
facilitate the flow of air in and out of the lungs
• Strengthening and enlarging the heart muscle, to
improve its pumping efficiency and reduce the
resting heart rate, known as aerobic conditioning
• Strengthening muscles throughout the body
• Improving circulation efficiency and reducing blood
pressure
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Effects of Resistance Exercise (anaerobic)
• The use of resistance to muscular contraction to build the strength and
size of skeletal muscles.
• Involves contractions of fast fibers
• Exercise that uses glycolysis and the reserve of ATP as a source of
energy (no oxygen use). It depends on:
• The amount of ATP available
• The amount of glycogen
• The ability of the muscle to tolerate the lactic acid (decreased pH
interferes with enzyme activity)
• The energy during the first 10-20 seconds is from ATP reserves
• Typically the onset of muscle fatigue occurs within 2 minutes of the start
of maximal activity
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Effects of Resistance Exercise (anaerobic)
• Resistance exercise results in:
• Muscle hypertrophy (due to increase in fiber size)
• Increased mitochondria, myofilaments, glycogen
stores, and connective tissue
• Examples – weight lifting, Machines that offer
resistance
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