The Physiology of Aerobic Exercise

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The Physiology of Aerobic Exercise

• Part 1

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Aerobic Exercise

• Intensity
• Duration
• Frequency
• Type

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Intensity Level
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Intensity Level

• As the intensity level increases, ATP demand
  increases
• This stimulates and increase in the oxidation of
  C, F and P through the aerobic systems.
• What is responsible for the increase in
  oxidation?
• ADP (among others)

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Intensity Level VO2

• This increases the oxidation increases the O2
  demand inside the muscles.
• This amount of oxygen utilized is referred to as
  VO2 or oxygen uptake.
• VO2, therefore, is a measure of exercise
  intensity.

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Intensity Level

• Why does VO2 increase during exercise?
• Increase ATP or energy production

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Intensity Level

• The increase in O2 demand (VO2) requires an
  increase in O2 supply
• O2 supply is accomplished through the cardiac
  output (Q) or the amount of blood pumped by the
  heart per minute
• Cardiac output is the product of heart rate and
  stroke volume

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Intensity Level

• What can we use to monitor the intensity level of
  exercise?
• Heart rate
• RPE
• VO2
• Lactate

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Intensity Level Heart Rate

• Of all the variables we can measure in people,
  why is heart rate the most common indicator of
  exercise intensity?
• Q HR x SV
• VO2 Q x a-v O2 difference
• VO2 HR x SV X a-v O2 difference
• Is HR a good predictor of VO2 (or exercise
  intensity?)

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Intensity Level Heart Rate

• What type of relationship is there between heart
  rate and exercise intensity?

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Intensity Level VO2 Heart Rate
VO2 (ml/kg/min)
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Intensity Level

• Which is more indicative of the actual exercise
  intensity level - Heart Rate or VO2?

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Intensity Level

• Recommendations
• VO2 - 40/50-85 of VO2 reserve
• Heart Rate
• 55/65-90 of HRmax
• 40/50-85 of HR reserve
• Why reserve?
• You dont start at a heart rate or VO2 of zero.
  
• RPE of 12-16

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Intensity Level Maximal Heart Rate

• Can be estimated HRmax 220 age in years
• For 20 year old, HRmax 200
• For 43 year old, HRmax 177
• Other formulas
• 210 (0.65 x age)
• 208 (0.70 x age) D. Seals, et al. J. Am.Coll.
  Card. March, 2001.
• For 20 year old, HRmax 194
• For 43 year old, HRmax 178
• 202 (0.72 x age)

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New York Times (April 24, 2001) 'Maximum' Heart
Rate Theory Is Challenged The formula became
increasingly entrenched, used to make graphs that
are posted on the walls of health clubs and in
cardiology treadmill rooms, prescribed in
information for heart patients and inscribed in
textbooks. But some experts never believed
it. Dr. Fritz Hagerman, an exercise physiologist
at Ohio University, said he had learned from more
than three decades of studying world class rowers
that the whole idea of a formula to predict an
individual's maximum heart rate was ludicrous.
Even sillier, he said, is the common notion that
the heart rate is an indication of fitness. Some
people get blood to their muscles by pushing out
large amounts every time their hearts contract,
he said. Others accomplish the same thing by
contracting their hearts at fast rates. As a
result, Dr. Hagerman said, he has seen Olympic
rowers in their 20's with maximum heart rates of
220. And he has seen others on the same team and
with the same ability, but who get blood to their
tissues by pumping hard, with maximum rates of
just 160. "The heart rate is probably the least
important variable in comparing athletes," Dr.
Hagerman said. Heart rate is an indicator of
heart disease, said Dr. Michael Lauer, a
cardiologist and the director of clinical
research in cardiology at the Cleveland Clinic
Foundation. But, he added, it is not the maximum
that matters it is how quickly the heart rate
falls when exercise is stopped. An average
healthy person's heart rate drops about 20 beats
in a minute and the rates of athletes "nose dive
by 50 beats in a minute," Dr. Lauer said. In
three recent studies, Dr. Lauer and his
colleagues found that people whose rates fell
less than 12 beats within a minute after they
stopped exercising vigorously had a fourfold
increased risk of dying in the next six years
compared with those whose heart rates dropped by
13 or more beats. Dr. Lauer pays no attention to
the standard formula when he gives treadmill
tests. More than 40 percent of patients, he said,
can get their heart rates to more than 100
percent of their predicted maximum. "That tells
you that that wasn't their maximum heart rate,"
Dr. Lauer said. The danger, he said, is that when
doctors use that formula to decide when to end a
treadmill test, they can inadvertently mislead
themselves and their patients. Some patients may
be stopping too soon and others may seem to have
a heart problem because they never can get to
what is supposed to be their maximum rate. "Some
people are being pushed and others are not," Dr.
Lauer said. "In my view, that is
unacceptable." Yet, Dr. Seals said, many doctors
want some sort of guide for estimating maximum
heart rates for treadmill tests. And many people
who want to increase their fitness crave a
general formula. So Dr. Seals and his colleagues
decided to take another stab at finding an
equation. In a study published in the March issue
of The Journal of the American College of
Cardiology, Dr. Seals and his colleagues devised
a new formula maximum heart rate equals 208
minus 0.7 times age. They used published studies
involving 18,712 healthy people and data from 514
healthy people they recruited. Their formula
gives much higher average maximum heart rates for
older people, with the new and old heart rate
curves starting to diverge at age 40. But raising
doubts about the heart rate formula is unlikely
to lead people to abandon it, exercise
physiologists say. What would they do without
it? "I've kind of laughed about it over the
years," Dr. Haskell said. The formula, he said,
"was never supposed to be an absolute guide to
rule people's training." But, he said, "It's so
typical of Americans to take an idea and extend
it beyond what it was originally intended for."
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Intensity Level - Heart Rate

• Why does heart need to increase during exercise?
• Increase supply of oxygen to the muscles.
• Oxygen demand by the muscles is greater during
  exercise.

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Intensity Level Heart Rate

• Why does heart rate level off when the exercise
  intensity levels off?
• O2 supply O2 demand (muscles)
• What type of exercise is this called?
• Steady state (and it is also submaximal)

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Steady State Heart Rate
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Intensity Level Heart Rate

• What may disrupt the positive relationship
  between heart rate and intensity during exercise?
• Hyperthermia Dehydration
• Fatigue
• Medications

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Washington D.C. Marathon, 2002
Fatigue
Heart Rate
Mile
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The Flying Pig Marathon, 2003
Fatigue/Dehydration
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Cardiovascular Drift
w What happens to the amount of plasma as
exercise continues? Increase or decrease?
w What affect would a decrease in plasma volume
have on stroke volume?
w If stroke volume decreases, what would have to
happen to heart rate in order to keep cardiac
output at the same level? (see next slide)
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Cardiovascular Drift
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Intensity Level Heart Rate

• What may disrupt the positive relationship
  between heart rate and intensity during exercise?
• Hyperthermia Dehydration
• Fatigue
• Medications

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Intensity Level Heart Rate

• What is responsible for increasing heart rate
  during exercise?

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Intensity Level Heart Rate

• Parasympathetic Nervous System
• Acetylcholine
• Sympathetic Nervous System
• Norepinephrine
• Calcium channels

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Calcium Channels
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Calcium Channels

• The calcium that enters the cardiac cell is
  involved in the excitation-contraction coupling.
• Catecholamines increase the amount of calcium
  that enters the cardiac cells by binding to
  Beta-adrenergic receptors and enhancing the
  L-type channels.
• Calcium channel blockers also reduce calcium flow
  into the smooth muscle of the arteriols, causing
  vasodilation and a reduced afterload.

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Intensity Level Heart Rate

• Adrenal medulla
• Epinephrine
• Increase HR and SV
• Vasoconstriction (alpha) or Vasodilation (beta)
• Norepinephrine
• Increase HR and SV
• Vasoconstriction

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Beta Receptors

• A site in the autonomic nervous system in which
  inhibitory responses occur when adrenergic
  agents, such as norepinephrine and epinephrine,
  bind to the receptors.
• Activation of beta-receptors cause
• Relaxation of the bronchial muscles
• Increase in the rate and force of cardiac
  contraction
• Various other physiological reactions

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Alpha Receptors

• A site in the autonomic nervous system in which
  excitatory responses occur when adrenergic
  agents, such as norepinephrine and epinephrine,
  bind to the receptors.
• Activation of alpha-receptors
• stimulation of smooth muscles
• the constriction of blood vessels
• various physiological reactions

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Adrenergic Receptors
Blocker
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Chronotropic Agents

• Nitrates
• Calcium channel blockers
• Vasodilators
• Some bronchodilators
• Antidepressants
• Tranquilizers
• Thyroid medications
• Nicotine
• Caffeine
• Diet pills

• Beta blockers
• Calcium channel blockers
• Digitalis

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Intensity Level Heart Rate

• What is responsible for increasing heart rate
  during exercise?
• Decrease parasympathetic stimulation
• Increase sympathetic stimulation
• Increase release of catecholamines (epinephrine
  and norepinephrine)
• Other factors

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Intensity Level

• During exercise, the increase in O2 demand
  requires an increase in O2 supply
• O2 supply is accomplished through the cardiac
  output (Q) or the amount of blood pumped by the
  heart per minute
• Cardiac output is the product of heart rate and
  stroke volume
• The increase in O2 demand is met through an
  increase in Q
• The increase in Q is accomplished by either
  increasing both HR and SV

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Intensity Level Stroke Volume

• Increases until about 25-50 of maximum
• After that it typically plateaus (It may continue
  to increase in highly trained individuals)
• Why does SV plateau?

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Intensity Level Stroke Volume

• What is responsible for the increase in SV?
• Increase preload (EDV) or the amount of blood
  returning to the heart through the veins
• Increase venous return
• Muscle pump, etc.
• Decrease afterload (resistance to pumping blood
  out of the heart)
• Vasodilation
• Local control and sympathetic stimulation
• Increase contractility (ESV)
• Increase sympathetic stimulation
• Catecholamines

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Intensity Level Preload
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Intensity Level Afterload
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Intensity Level Heart

• Increasing O2 supply
• Increase Q
• Increase HR
• Dec Para
• Inc Sym
• Catecholamines
• Increase SV
• Inc Preload
• Dec Afterload
• Catecholamines/Sympathetic stim
• Increase oxygen extraction (later)

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RPE

• ACSM recommendation RPE of 12-16
• Why is the RPE scale from 6-20?

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Intensity and Lactic Acid

• If the intensity level of exercise continues to
  increase, there will come a point where O2 supply
  is less than O2 demand

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Intensity and Lactic Acid

• How is lactic acid produced?

Lactic Acid
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Intensity and Lactic Acid

• Lactic acid
• O2 supply is less than O2 demand
• Hs that dont bind with O2 bind with pyruvate
• Lactic acid can be removed at the liver, slow
  twitch muscle fibers or the heart
• If O2 demand is too great, lactic acid levels
  will increase inside the muscle fiber
• Fatigue

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Intensity and Lactic Acid
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Intensity and Lactic Acid

• What is the lactic acid threshold?

• At what percentage of maximal effort does the
  threshold typically occur?
• 50-85 of max
• Where else is the range 50-85 of max used?

Aerobic
Anaerobic
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Intensity and Lactic Acid
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Intensity and Lactic Acid
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Intensity and Lactic Acid
Intensity level too high
Lactic Acid
Proper intensity level
Intensity level too low
Time
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Intensity and Lactic Acid
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Intensity and Lactic Acid
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Intensity and the Ventilatory Threshold
LT1 LT2

• What is the ventilation threshold and what is the
  cause for the sudden increase in ventilation at
  that point?
• Note the CO2 levels and how they relate to
  changes in ventilation

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Intensity and the Ventilatory Threshold
The point during intense exercise at which
ventilation increases disproportionately to the
oxygen consumption.
Glycolysis increases lactic acid levels
Buffering of lactic acids leads to an increase in
CO2 H HCO3 lt---gt H20 CO2
An increase in CO2 triggers a respiratory
response and increased ventilation.
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Ventilatory Threshold
CO2 from Krebs cycle and buffering lactic acid
stimulate ventilation even faster
CO2 from Krebs cycle stimulates ventilation
Aerobic
Aerobic and Anaerobic
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Ventilatory Threshold

• Talk test
• If you can talk while you exercise, your
  ventilation is BELOW your anaerobic threshold and
  you are aerobic
• If you have a difficult time talking while you
  exercise, your ventilation is ABOVE your
  anaerobic threshold and you are aerobic and
  anaerobic
• If you remain above your AT, lactic acid will
  accumulate inside your muscle leading to fatigue

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Intensity and Fatigue

• Of the causes listed below, which is the most
  likely cause of fatigue when the intensity level
  is too high?
• Depletion of glycogen
• Accumulation of H (lactic acid)
• Depletion of PC
• Lactic acid interferes with cross-bridging

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Intensity and Fatigue

• How does lactic acid cause fatigue?
• Inhibit energy producing enzymes inside the
  muscles
• Inhibit calciums ability to bind to troponin
  which is required for muscle contraction
• FYI. Lactic acid produced during exercise is
  removed within the first few hours after
  exercise, i.e. lactic acid is not responsible for
  delayed onset muscle soreness.

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Intensity and Fuels

• Rest
• 60 fat, 35 carb, 5 protein
• Aerobic Exercise
• Burn more of everything
• A greater percentage comes from carbohydrates
• Active muscle cells burn mostly all carbohydrates
• Other inactive cells burn fat
• Very hard exercise
• Burn less fat due to an increase in lactic acid

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Intensity Summary

• Muscles increase O2 demand
• Recruit more fast twitch fibers
• Cardiovascular increase O2 supply
• Sympathetic and Parasympathetic
• Catecholamines
• Fuels
• Increase carbohydrate metabolism
• Increase lactic acid accumulation