Muscular Fatigue Mechanisms

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Chapter 6

• Muscular Fatigue Mechanisms

• Text Sources
• Nelson Physical Education VCE Units 34 4th
  Edition Malpeli, Horton, Davey and Telford
  2006.
• 2. Live It Up 2 2nd Edition Smyth, Brown,
  Judge, McCallum and Pritchard 2006.

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What is Fatigue?

• Muscular Fatigue Mechanisms

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What is Fatigue?

• Fatigue occurs when the body is unable to
  function at its optimal level. The muscles are
  unable to exert maximal force levels as a result
  of exercise.
• Fatigue occurs through everyday physical
  activity.
• Exercise increases the physiological effects of
  fatigue.

• Our response to fatigue depends on
• The type, intensity and duration of the activity
• The fitness level and mental state of the
  performer.
• The muscle fibre being used
• Types of muscular contraction occurring
• The amount of metabolic by products being
  produced
• The athletes hydration levels

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Muscle Fibre Type and Fatigue

• Fast Twitch Fibres - Fast reaction time which
  relies on PC stores.
• Unfortunately these stores deplete quickly.
• Fast twitch also have a reduced oxygen supply in
  comparison to the aerobic slow twitch fibres.

• Slow Twitch High stores of glycogen and
  triglycerides helps produce energy for endurance
  activities.
• Eventually fuel reserves are depleted or the
  neuromuscular process breaks down.
• See fig 6.2 and 6.3 p.132-133

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Muscular Contraction Types
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Types of fatigue

• General
• Fatigue in all muscles groups
• Eg. After completing full weights session

• Chronic
• Unhealthy level of fatigue, caused by breakdown
  of bodys defences.
• Eg. Chronic fatigue syndrome

• Local
• Fatigue in specific muscle group
• Eg. Biceps during bicep curls

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Levels of Fatigue
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Causes of Fatigue

• Muscular Fatigue Mechanisms

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Causes of Fatigue

• A number of processes cause muscular fatigue
  including
• Energy Systems used
• Depleted fuel stores (Glycogen / PC and other
  phosphate compounds)
• Metabolic by-products
• Reduced ability to extract energy
• Increased body temperature
• Dehydration
• Changes in blood flow

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Causes of Fatigue
Body Temperature
p.134
Metabolic By-products (LA, H, Pi, Creatine)
Energy pathways
Fuel stores (PC and glycogen)
Hydration levels
Blood flow redistribution
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Depletion of Fuels

• Muscular Fatigue Mechanisms

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Fuel Depletion

• Most commonly exhausted energy stores are PC and
  glycogen.
• Stores of glycogen in the muscle and liver can
  fuel continuous exercise for over 90 mins.
• Muscle glycogen is generally the first fuel
  source used during aerobic exercise then liver
  glycogen and eventually blood-borne and stored
  fat.
• Fat conversion to energy is far less efficient
  than that for glycogen, resulting in a reduced
  intensity.
• As energy stores are continually depleted,
  fatigue occurs and therefore the quality of
  performance decreases

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Aerobic Pathway Fatigue

• Aerobic Exercise
• Less than 20 minutes
• No major fatigue and carbos and fats used as
  energy (Very little lactic acid).
• Extended activities (Greater than an hour) Fats
  used as fuel therefore body must slow down (More
  oxygen required).
• Fatigue caused by depleted fuel stores,
  dehydration, increased body temp, physical and
  mental stress.
• Low Energy Stores
• Athletes should follow strict diets when
  preparing for events so that fatigue is minimised
  or delayed.

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Anaerobic Pathway Fatigue

• Anaerobic Exercise
• Energy supplied by Phosphate Creatine (1-15 sec)
  and anaerobic glycolysis (15sec-2min).
• Oxygen deficit occurs.
• Fatigue caused by lactic acid accumulation, which
  inhibits muscular contractions (Prevents calcium
  ion flow to myosin filament).
• Lactic acid also prevents enzymes breaking down
  glucose stores

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Metabolic By-Products

• Muscular Fatigue Mechanisms

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Metabolic by-products
p.137

• Metabolic by-products are compounds made as a
  result of chemical reactions within the body.
  They are the left-overs as such.
• When making ATP using phosphocreatine, the
  by-product is creatine
• ADP PC ATP creatine
• By-products can prove harmful to the body by
  causing it to function in a less efficient way,
  such as through the effect of lactate and
  hydrogen ions during physical activity.
• These by-products effect
• Contraction of myosin, disruption to the work of
  enzymes, the neuromuscular junction, ionic
  concentrations.
• Summary See fig 6.7 p.136

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By-products - Accumulation of LA

• Lactic acid Produced as a result of pyruvic
  acid reacting in the absence of oxygen.
• As the rate of anaerobic glycolysis increases,
  lactic acid accumulates.
• It effects the ability of energy extracting
  enzymes to work effectively.
• It also lowers the bloods pH levels.
• Lactic acid eventually reaches maximum levels
  within both the muscle fibres and in the blood
  (Lactate threshold).
• Fatigue results as the accumulated lactic acid
  inhibits muscular contractions.

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How LA affects muscular contractions

• Inhibiting the secretion of calcium ions that
  enable the coupling of the actin and myosin
  protein filaments. Protein filaments cannot
  attach to each other. The sliding of filaments is
  not possible.
• Inhibiting the action of the glycolytic enzymes
  resulting in glucose not being broken down.

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By-Product 2 - Accumulation of H ions

• Another by-product of anaerobic glycolysis.
• Accumulation of H ions within the working muscle
  and blood plasma results in the levels of pH of
  the cell decreasing to an extent where muscle
  contraction is no longer possible and fatigue
  occurs.
• The low pH created by the H ions causes the
  glycolytic enzymes to become inoperative. Without
  the glycolytic enzymes, the breakdown of glucose
  cannot take place.
• Inorganic phosphate (Pi) can also have a similar
  fatiguing effect on the body.

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The Redistribution of Blood Flow

• Muscular Fatigue Mechanisms

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Redistribution of Blood Flow
p.138

• During exercise
• Increased demand for oxygen
• Increased waste products
• Increased blood volume to working muscles
• Increased cardiac output
• Less blood flow to vital organs and more to the
  working muscles
• This can cause an increase in core body
  temperature.
• The body therefore needs to monitor its balance
  between cooling and muscle supply
  (Thermoregulation)

• Body Temperature Increase
• Due to blood being brought to the surface
  (vasodilation of veins) of the skin (cooling
  mechanism), less blood is supplied to the working
  muscles. This lessens the ability to produce ATP
  and lactic may be produced.
• See fig 6.2 p.138 and 6.8 p.139

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Dehydration

• Muscular Fatigue Mechanisms

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Dehydration

• Athletes can become dehydrated due to
• Not having access to fluids during competition
• Not tolerating drinking while exercising
• Inability to match intake with loss of fluids
• If dehydrated, athletes are more prone to Cramp,
  heat stress and heat stroke, poor performances
  and increase the risk of injury.

• Sweating causes
• A loss of salt, electrolytes and water.
• This can cause
• Impaired coordination, decision making and
  endurance levels.
• An impact on the cardiovascular system
• A rise in body temperature
• Levels of dehydration are affected by
• Duration and intensity of session
• Environmental conditions and acclimatisation.
• The individual physiological characteristics
• Fluid intake

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Dehydration

• Sweating is the bodys natural process for
  regulating body temperature during exercise.
• As an athletes core temperature increases so
  does the rate of sweat production.
• Sweat contains electrolytes, salts and water, and
  dehydration is the loss of this body fluid.
  Varying rates of sweat, work, fluid intake and
  individual physiological characteristics all
  contribute to the level of dehydration that may
  be experienced.

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Dehydration
p.141

• Signs of dehydration
• Mild to severe thirst
• Rapid loss in weight (1 kg of weight lost 1L of
  sweat lost).
• Dry lips and tongue, confusion
• Decreased urine volume
• Dark urine
• Increased breathing rate
• Light headedness nausea and headache
• Confusion, nausea, headache
• faster breathing rate than normal,

• Fluid loss can be prevented by
• Drinking water prior, during and after events.
• Use sports drinks for extended activities
• Use a fluid replacement routine
• Avoid being dehydrated before sport
• Written Report p.140
• Case Study p.141

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Dehydration continued

• Combination of dehydration and electrolyte loss
  can make the athlete more susceptible to cramp,
  heat stress and heat stroke.
• A loss of 2 of body weight (just 1 kg for a
  50-kg person) causes an increase in perceived
  effort and could reduce performance by 10-20 .
• Loss exceeding 3-5 of body weight reduces
  aerobic exercise performance noticeably and
  impairs reaction time, judgment, concentration,
  cognitive abilities and decision making.
• Complete hydration is vital for achieving optimal
  performance and minimising the negative effects
  of fatigue.

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Guidelines for Reducing Dehydration

• Do not wait until you are thirsty - thirst is a
  poor indicator of hydration levels.
• Drink cool water - absorbed more rapidly
• Use a sports drink if exercise is 1 hour .
• Avoid starting exercise dehydrated.
• 500 mL of water 30-60 mins prior to the game.
• Drink at least 200 mL of water every 15mins
  during
• Weighing yourself before and after sport is a
  good way to assess fluid levels.
• One kilogram of weight lost one litre of fluid
  lost.
• After participating, aim to replace more than the
  fluid lost as sweating and fluid loss continues
  after exercise.

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Web Links Chapter 6

• Nicholas Institute of Sports Medicine and
  Athletic Trauma exercise physiology
  http//www.nismat.org/physcor/index.html
• Information on skeletal muscles in the human
  body http//www.ptcentral.com/muscles
• Sport science (site for sports research)
  http//www.sportsci.org/
• Anaerobic management (training and recovery)
  http//www.anaerobic.net/resources2.html
• Biophysical journal online http//www.biophysj.or
  g
• PubMed (includes links to full text articles and
  other related resources) http//www.ncbi.nih.gov/
  entrez/query
• Article Unravelling the Process of Muscle
  Fatigue http//www.ucsf.edu/cooke/research/intere
  sts/fatigue.htm
• Physiology online magazine (American
  Physiological Society) http//physiologyonline.ph
  ysiology.org
• Science-a-go-go (science news, research and
  discussion) http//www.scienceagogo.com/news
• University of Western Australia, physiology
  department http//www.physiol.biomedchem.uwa.edu.
  au
• Innovations Report Forum for science,
  industry and business http//www.innovations-
  report.com