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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Types of fatigue
Fatigue Description Example
Local Fatigue in specific muscle group Biceps during bicep curls Quads in snowboarding
General Fatigue in all muscles groups After completing full weights session Playing a full game of football etc.
Chronic Unhealthy level of fatigue, caused by breakdown of bodys defences. Chronic fatigue syndrome Overtraining Reoccurring illness or injuries
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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.

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Causes of Fatigue

• Muscular Fatigue Mechanisms

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Causes of Fatigue

• Our response to fatigue depends on
• 1. The muscle fibre being used
• 2. Types of muscular contraction occurring
• 3. Depleted fuel stores (Glycogen / PC and other
  phosphate compounds)
• 4. Energy Systems used and the ability to extract
  energy
• 5. The amount of metabolic by products being
  produced
• 6. Changes in blood flow and increased body
  temperature
• 7. The athletes hydration levels

• A number of underlying processes that also
  determine the level of muscular fatigue include
• The type, intensity and duration of the activity
• The fitness level and mental state of the
  performer.

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

• Complete questions 1-4 page 134 of Nelson
  Physical Education VCE Units 3 4.

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1. Muscle Fibre Types

• Muscular Fatigue Mechanisms

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Muscle Fibres

• Muscle fibre types can be broken down into two
  main types
• Slow twitch (Type I) muscle fibres and
• Fast twitch (Type II) muscle fibres Contains
  Type II a and b
• Human muscles contain a genetically determined
  mixture of both slow and fast fibre types.
• On average, we have about 50 slow and 50 fast
  fibres in most of the muscles used for movement.

• Slow Twitch (Type I)The slow muscles contain
  more mitochondria and myoglobin which make them
  more efficient at using oxygen to generate more
  fuel (ATP) for continuous, extended muscle
  contractions over a long time.
• Fast Twitch (Type II) Because fast twitch fibres
  use anaerobic metabolism to create fuel, they are
  much better at generating short bursts of
  strength or speed but fatigue more quickly.
• Fast twitch fibres are able to fire more rapidly
  than slow twitch, hence their name.
• The slow twitch fibres, on the other hand, fire
  more slowly, but can go for a long time before
  they fatigue.

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Muscle Fibre Type Summary
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2. Muscle Contraction Types

• Muscular Fatigue Mechanisms

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Contraction Types
Isotonic
Isometric Isokinetic
Muscle Lengthens No change in
length Contraction through full
range of motion
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3. 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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4. Energy Systems

• Muscular Fatigue Mechanisms

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4. 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 (Several hours of constant
  exercise)
• 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.
• Anaerobic Exercise
• Energy supplied by ATP-PC (1-15 sec) and
  anaerobic glycolysis (15sec-2min).

• Energy Stores
• Athletes should follow strict diets when
  preparing for events so that fatigue is minimised
  or delayed.
• Elite athletes have nutritional programs to aid
  their performance and recovery.

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Summary Table
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5. Metabolic By-Products

• Muscular Fatigue Mechanisms

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Metabolic by-products

• Metabolic by-products are compounds made as a
  result of chemical reactions within the body.
  They are the left-overs as such.
• Eg. 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 hydrogen ions
  during physical activity.
• These by-products effect the functioning of the
  working muscle
• eg. Ability to break down glycogen, ability to
  send signals to the muscle and the changing
  concentration of minerals in and around the
  muscle.
• Summary See fig 6.7 p.136 for ATP production
• during muscular activity.

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Current Investigations

• The role of intracellular metabolites in
  controlling membrane excitability and the release
  of intracellular calcium during fatiguing
  exercise are now hotly being investigated - Pi,
  Ca2, Na and K.
• The hydrogen ion theory as a major fatiguing
  product is hotly debated at present.

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Lactic Acid The Myths Busted

• Lactic acid has a bad reputation. Many people
  blame it for fatigue, sore muscles, and cramps.
• Scientists have discovered that lactic acid plays
  a critical role in generating energy during
  exercise.
• When your body makes lactic acid, it splits into
  lactate ion (lactate) and hydrogen ion.
• The lactate ion provides fuels for many tissues,
  helps use dietary carbohydrates, and serves as
  fuel for liver production of glucose and
  glycogen.

• However, lactic acid does have a dark side.
• The Hydrogen ion is the acid in lactic acid.
• It interferes with electrical signals in your
  muscles and nerves, slows energy reactions, and
  impairs muscle contractions.
• The burn you feel in intense exercise is caused
  by hydrogen ion build-up.
• Inorganic phosphate (Pi) can also have a similar
  fatiguing effect on the body.
• So, when you fatigue, don't blame it on lactic
  acid. Rather, place the blame where it belongs-
  on hydrogen ion.

Lactic Acid
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Lactate Inflection Point (LIP)

• The relationship between blood lactate levels and
  anaerobic metabolism is still unclear. The term
  anaerobic threshold (AT) is therefore no longer
  used in the field exercise physiology and this
  area of study due to its conflicting meaning.
• The preferred term, lactate inflection point
  (LIP), is the point where blood lactate levels
  increase, as a direct result of increasing
  exercise intensity.
• Exercise intensities beyond the LIP causes
  fatigue due to a reliance on anaerobic pathways
  to supply ATP and the build up of the metabolic
  by-products.

• LIP can be measured in the laboratory using blood
  analysis or ventilation measurements. It is a
  good measure of an athletes speed and power of an
  athlete over a prolonged period. It can also
  distinguish between middle and long distance
  runners during a VO2 max test.

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L.I.P
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Checkpoints

• Complete questions 1-5 page 137 of Nelson
  Physical Education VCE Units 3 4.

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6. The Redistribution of Blood Flow

• Muscular Fatigue Mechanisms

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Blood Flow and Body Temperature

• 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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Redistribution of Blood Flow
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Checkpoints

• Complete questions 1-2 page 138 of Nelson
  Physical Education VCE Units 3 4.

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7. Dehydration

• Muscular Fatigue Mechanisms

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Dehydration

• 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

• 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.

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Dehydration

• 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

• 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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Coursework 6.1 and 6.2

• Complete the written report task on page 140 of
  Nelson Physical Education VCE Units 3 4.
• Complete the case study on page 141 of Nelson
  Physical Education VCE Units 3 4.

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Checkpoints

• Complete questions 1-9 page 141 of Nelson
  Physical Education VCE Units 3 4.

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Fatigue and Energy Systems
Predominant energy system Likely causes of fatigue Types of recovery
ATP/PC Fuel Depletion ATP PC Rest recovery
Lactic Acid Accumulation of metabolic by-products H (hydrogen ions) Pi (inorganic phosphates) NB Lactic Acid is no longer thought to contribute to fatigue. In fact, it is being regarded more as a positive performance enhancer rather than a negative Non-dietary Active Recovery Massage Hydro/water based therapies e.g. contrasting via hot/cold baths
Aerobic Fuel Depletion Glycogen stores, then fats Elevated body temperature leading to Dehydration Blood flow away from muscles Dietary High GI foods Rehydration via sports drinks Hypertonic to replace glycogen Hypotonic to replace lost fluids Non-dietary Active Recovery Massage Hydro/water based therapies
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Test Your Knowledge

• Complete the review questions 1-4 page 143-4 of
  Nelson Physical Education VCE Units 3 4.

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Peak Performance

• Complete the chapter questions on page 40-50 of
  Nelson Peak Performance Physical Education VCE
  Units 3 4.

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PHYS ED Notes

• Read the summarised information of pages 54-67 of
  PHYS ED Notes and complete the revision questions.

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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