Transcript Fatigue During Muscular Exercise

Fatigue During Muscular
Exercise
Brooks Chapter 33
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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)
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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.
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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).
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• 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.
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• 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
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Metabolite Depletion
ATP and CP
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•
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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
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• 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
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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
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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.
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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.
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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
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• 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.
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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.
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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
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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.
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• 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.
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• 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
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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.
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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.
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• 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
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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 brain’s reticular
formation and motor centers.
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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.
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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+, Ca2+can affect e-c coupling
• cardiac arrhythmia is possible
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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.
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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 < 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.
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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
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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.
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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.
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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.
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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
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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).
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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.
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 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.
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 An individual’s 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)].
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 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.
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 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.
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12 Week Training Program
TRIMPS
4 Weeks of Training
(T2)
Week
Taper
4 Weeks of Training
(T1)
Taper
75%
T2
75%
T1
50%
T2
50%
T1
1
2
3
4
5
6
7
8
9
10
11
12
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Fitness/Fatigue Graph
Fitness
9000
Fatigue
Performance
8000
7000
6000
5000
4000
3000
2000
1000
0
-1000
-2000
1
8
15
22
29
36
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Day
50
57
64
71
78
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