Fatigue During Muscular Exercise

1
Fatigue During Muscular Exercise

• Brooks Chapter 33

2
Muscular Fatigue

• Inability to maintain a given exercise intensity.
• Several causes for fatigue.
• Fatigue is task specific.
• Can have impairment within the active muscle.
• peripheral fatigue
• Fatigue can also be due to central factors.
• psychological
• environmental (heat)

3
Muscular Fatigue

• Depends on the training and activity status of
  the individual.
• Can be due to depletion of key metabolites in the
  muscle.
• Can be due to accumulation of metabolites.
• Identifying the cause of fatigue is not simple.

4
Identifying Fatigue

• The inability to maintain a given exercise
  intensity.
• An athlete is rarely completely fatigued.
• they adopt a lower power output
• Often fatigue can be related to a specific cause
  or site. (? glycogen, ? Ca2)
• The causes of fatigue can also be general and
  involve several factors (dehydration).

5

• Compartmentalization in physiological
  organization make it difficult to identify the
  site of fatigue.
• eg. ATP depleted at myosin head, but adequate
  elsewhere?
• The effect of exercise at an absolute, or
  relative, exercise intensity will be more severe
  on an untrained individual.

6

• Heat and humidity will affect endurance
  performance.
• ? sweat, heat gain, dehydration, electrolyte
  shift
• redistribution of CO in the heat
• uncouple oxidation and phos in mitochondria
• less ATP with same VO2
• irritant to CNS, affect psychological perception
  of exercise
• fatigue is cumulative over time
• previous day dehydration will influence current
  performance
• glycogen depletion ? endurance performance

7
Metabolite Depletion

• ATP and CP
• the immediate source of ATP is CP
• CP in muscle is limited
• when CP is depleted, muscle ATP is ?
• must match use with restoration
• otherwise you can not maintain exercise
• the greater the work load, the greater the CP
  depletion
• CP depletion leads to muscle fatigue

8

• CP levels decline in two phases drop rapidly,
  then slowly (Fig 33, 1a).
• both severity of the first drop and extent of the
  final drop are related to work intensity
• Fatigue in maximal cycling coincides with CP
  depletion.
• tension developed related to CP level, therefore
  CP related to fatigue

9
ATP

• ATP is well maintained up to maximum effort.
• due to compartmentalization
• down regulation by muscle cells for protection
• muscle cell shuts off contraction, with ATP
  depletion, in favor of maintaining ion gradients
• free energy of ATP declines 14 in physiological
  pH range
• also depends on ATP/ADP ratio
• consequence less energy available for work with
  given VO2 flux
• fatigue also influences ATP binding in X-bridge
  cycle

10
Glycogen

• Glycogen depletion in skeletal muscle is
  associated with fatigue.
• With moderate activity glycogen is depleted
  uniformly from different fiber types.
• With low resistance activity there is selective
  recruitment and depletion of glycogen from slow
  twitch (type I) fibers.
• With high resistance type II fibers are depleted.
• Thus glycogen can be depleted from specific
  fibers.

11
Blood Glucose

• During short intense exercise bouts, blood
  glucose ? above pre-ex levels as the CNS
  stimulates hepatic glycogenolysis.
• During prolonged exercise glucose production may
  be limited to gluconeogenesis because of hepatic
  glycogen depletion.
• Thus glucose production may fall below that
  required by working muscle and other essential
  tissues.

12
Lactic Acid Accumulation

• During short term high intensity exercise
• lactic acid production exceeds removal
• strong organic acid pH ?
• it is the H rather than the lactate ion that ?
  pH
• H accumulation within muscle
• inhibit PFK and slow glycolysis
• displace Ca2 from troponin (inhibit contraction)
• main stimulate pain receptors

13

• H in blood
• reacts in the brain and causes pain, nausea
• inhibits combination of O2 with Hb in the lung
• reduces hormone sensitive lipase in adipose
  tissue
• limits release of FFA
• It is still uncertain if ? pH stops exercise.

14
Phosphates

• Phosphagen depletion during exercise results in
  phosphate (Pi, or HPO42-) accumulation.
• Phosphate behaves like H and inhibits glycolysis
  (PFK) and interferes with Ca2 binding.
• Phosphate and H produce hydrogen phosphate which
  is a very harmful metabolite that accumulates in
  working muscle.

15
Calcium Ion

• There are several reasons why Ca2 may be
  involved in muscle fatigue
• Ca2 from the SR during EC may be taken up by
  mito
• interferes with mitochondrial function
• reduced ability of SR to release Ca2 during
  twitches
• less forceful contraction
• actin-myosin sensitivity to Ca2 is reduced
• less forceful contraction
• Ca2 re-uptake by SR is slowed
• prolongs contraction, slows relaxation

16
O2 Depletion and Muscle Mitochondrial Density

• The depletion of O2 stores, or inadequate O2
  delivery to muscle, can result in fatigue.
• impaired circulation
• high altitude
• strenuous exercise
• Adequate O2 supply is essential to support
  maximal aerobic work.

17

• Inadequate O2 supply or utilization can be
  represented by
• ? CP levels
• ? lactate production
• both
• Thus inadequate O2 can result in at least 2
  fatigue causing effects.

18

• Skeletal muscles contain a greater mito
  respiratory capacity than can be supplied by the
  circulation.
• ? mito density in response to endurance exercise
  will provide benefits other than VO2.
• ? capacity to oxidize fatty acids as a fuel
• minimize mito damage during exercise
• free radical accumulation
• more mitos, reduced effect of free radicals

19
Disturbances to Homeostasis

• The continuation of exercise depends on the
  integrated functioning of many systems.
• Any factor that upsets this integrated function
  can cause fatigue.
• Some important factors that maintain homeostasis
  include ions (K, Na, Ca2), blood glucose,
  FFA, plasma volume, pH, core temp, hormone levels.

20
Central and Neuromuscular Fatigue

• In the linkage between afferent inputs and the
  performance of a task, several sites require
  adequate functioning.
• A decrement at any site will ? performance
  (fatigue).
• Therefore it is possible to have muscular fatigue
  when the muscle itself is not impaired.

21

• It is very difficult to obtain data on CNS
  function during exercise.
• The relationship between central and peripheral
  functions should not be overlooked.
• Physiological signals can lead to psychological
  inhibition.
• eg. painful inputs affect willingness to continue
  activity

22
Setchenov Phenomenon

• The exhausted muscle of one limb recovers faster
  if the opposite limb is exercised moderately
  during recovery.
• repeated by other researchers
• not due to ?muscle blood flow
• it is attributed to afferent input having a
  facilitatory effect on the brains reticular
  formation and motor centers.

23
Psychological Fatigue

• We have a very limited understanding of how
  afferent input during exercise (pain, breathing,
  nausea, motivation) can influence the physiology
  of the CNS.
• Through training or intrinsic mechanisms, some
  athletes learn to minimize the influence of
  distressing afferents and approach performance
  limits of the musculature.
• Some athletes (altitude) will slow down to reduce
  discomforting inputs to a tolerable level.
• Training at high intensities allows athletes to
  select a proper race intensity.

24
Heart as a Site of Fatigue

• No direct evidence that exercise is limited by
  fatigue of the heart muscle.
• Well oxygenated during exercise.
• Heart gets first choice at CO.
• Can use lactate or FFA as fuel.
• During severe dehydration
• major fluid and electrolyte shift
• K, Na, Ca2can affect e-c coupling
• cardiac arrhythmia is possible

25
VO2max and Endurance

• Relationship between max O2 consumption and upper
  limit for aerobic metabolism.
• 1. VO2max limited by O2 transport
• CO and arterial content of O2
• 2. VO2max limited by the respiratory capacity of
  contracting muscles.
• Currently we can conclude that VO2max is a
  parameter set by maximal O2 transport, while
  endurance is also determined by muscle
  respiratory capacity.

26
Muscle Mass

• Muscle mass influences VO2max.
• Once a critical mass of muscle is utilization VO2
  is independent of muscle mass.
• VO2 when cycling with 1 leg is lt than with 2
• 2 x VO2 of 1 leg is much greater than 2 legs
• VO2max when cycling and arm cranking is not
  greater than just cycling alone.
• VO2max ? as active muscle mass ? to a point
  beyond which O2 delivery is inadequate to supply
  working muscle.

27
Muscle Mitochondria

• Correlation observed between VO2max and mito
  activity - 0.8.
• Mito and VO2max with training and detraining
• muscle mito ? 30, VO2 ? 19
• VO2 persistent longer during detraining than
  muscle respiratory capacity
• illustrates independence of these factors
• The maximal ability of muscle mito to consume O2
  is several times the ability to supply O2.
• hence, VO2max is limited by arterial O2
  transport

28
Arterial O2 Transport

• Arterial O2 transport (TaO2) is equal to the
  product of cardiac output (Q) and arterial O2
  content(CaO2).
• TaO2 Q(CaO2)
• Attempts to raise arterial O2 content by
  breathing ?O2 conc or blood doping raise VO2max.

29
VO2max and Performance

• Maximal capacities of cardiac output, arterial O2
  transport, VO2max and physical performance are
  all interrelated.
• Despite these correlations, VO2max is a poor
  predictor of performance among elite athletes.
• This is due to the importance of peripheral, as
  opposed to central, factors in determining
  endurance.

30
Catastrophy Theory

• Physiological processes are highly controlled and
  often redundant in function.
• Successes and failures in integrated functions
  involve multiple cells, tissues, organs, and
  systems.
• Catastrophy theory the failure of one enzyme
  system, cell, tissue organ or system places a
  burden on related systems, such that they may
  fail simultaneously.

31
Future of Fatigue

• Technology is making available new devices that
  will further investigation of fatigue.
• NMR
• nuclear magnetic resonance spectroscopy
• radio freq signal emitted by a particular atomic
  species
• determine concentrations of ATP, CP, Pi, water,
  fat, metabolites without breaking the skin

32

• PET
• Positron emission tomography.
• Great potential for studying regional blood flow
  and metabolism.
• NIRS
• Near infrared spectroscopy.
• Noninvasively and continuously monitor the state
  of oxygenation of iron containing compounds
  (myoglobin).

33
Fatigue and Physical Training

• To date there is no formal training theory that
  quantitatively and accurately prescribes the
  pattern, duration and intensity of exercise to
  elicit a specific physiological adaptation.
• Without accurate quantification of a training
  dose, the results from training studies to date
  remain qualitative and argumentative.

34

• A training model developed by Banister et al.
  (1975) uses a unit of measure called a training
  impulse (TRIMP) to accurately quantify a training
  dose.
• This training theory proposes that a precisely
  measured quantity of training above that
  currently practiced will improve physical,
  physiological, and biochemical indices of
  adaptation and growth.

35

• An individuals daily training is quantified by
  calculating a training impulse w(t), which
  represents the integrated effect of duration (D)
  and intensity (Y) of exercise.
• Exercise performance may be predicted by
  transforming a daily TRIMP score w(t) into
  separate daily scores of a hypothesized fitness
  g(t) and fatigue h(t).

36

• The time course of the difference between fitness
  and fatigue represents the time course of
  predicted physical performance p(t), due to the
  training. Thus fitness and fatigue grow and decay
  exponentially throughout a period of training.
• During a taper period fatigue decays much faster
  than fitness, and the predicted performance
  increases.

37

• An effective training format is one that has an
  on stimulus of 28 days, in which the exercise
  has the proper intensity and duration to induce a
  positive exponential growth response in
  physiological and biochemical variables.
• A 7 14 day taper at the end of the 28 day
  training program, will then allow fatigue to
  decay faster than fitness.
• The end of the taper period provides a time when
  there is a maximal separation between fitness and
  fatigue, and performance reaches a peak.

38
12 Week Training Program
39
Fitness/Fatigue Graph