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Scott K. Powers • Edward T. Howley
Theory and Application to Fitness and Performance
SEVENTH EDITION
Chapter
Exercise Metabolism
Presentation prepared by:
Brian B. Parr, Ph.D.
University of South Carolina Aiken
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Chapter 4
Objectives
1. Discuss the relationship between exercise
intensity/duration and the bioenergetic pathways that
are most responsible for the production of ATP during
various types of exercise.
2. Define the term oxygen deficit.
3. Define the term lactate threshold.
4. Discuss several possible mechanisms for the sudden
rise in blood-lactate concentration during incremental
exercise.
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Chapter 4
Objectives
5. List the factors that regulate fuel selection
during different types of exercise.
6. Explain why fat metabolism is dependent on
carbohydrate metabolism.
7. Define the term oxygen debt.
8. Give the physiological explanation for the
observation that the O2 debt is greater following
intense exercise when compared to the O2 debt
following light exercise.
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Chapter 4
Outline
 Energy
Requirements at
Rest
 Rest-to-Exercise
Transitions
 Recovery from
Exercise:
Metabolic
Responses
 Metabolic
Responses to
Exercise:
Influence of
Duration and
Intensity
Short-Term, Intense
Exercise
Prolonged Exercise
Incremental Exercise
 Factors
Governing Fuel
Selection
Exercise Intensity and
Fuel Selection
Exercise Duration and
Fuel Selection
Interaction of Fat/
Carbohydrate
Metabolism
Body Fuel Sources
 Estimation of Fuel
Utilization During
Exercise
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Chapter 4
Energy Requirements at Rest
Energy Requirements at Rest
• Almost 100% of ATP produced by aerobic
metabolism
• Blood lactate levels are low (<1.0 mmol/L)
• Resting O2 consumption:
– 0.25 L/min
– 3.5 ml/kg/min
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Chapter 4
Rest-to-Exercise Transitions
Rest-to-Exercise Transitions
• ATP production increases immediately
• Oxygen uptake increases rapidly
– Reaches steady state within 1–4 minutes
– After steady state is reached, ATP
requirement is met through aerobic ATP
production
• Initial ATP production through anaerobic
pathways
– ATP-PC system
– Glycolysis
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Chapter 4
Rest-to-Exercise Transitions
The Oxygen Deficit
Figure 4.1
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Chapter 4
Rest-to-Exercise Transitions
Comparison of Trained and
Untrained Subjects
• Trained subjects have a lower oxygen
deficit
– Better-developed aerobic bioenergetic
capacity
– Due to cardiovascular or muscular
adaptations
• Results in less production of lactic acid
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Chapter 4
Rest-to-Exercise Transitions
Differences in VO2 Between
Trained and Untrained Subjects
Figure 4.2
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Chapter 4
Rest-to-Exercise Transitions
In Summary
 In the transition from rest to light or moderate
exercise, oxygen uptake increases rapidly, generally
reaching a steady state within one to four minutes.
 The term oxygen deficit applies to the lag in oxygen
uptake in the beginning of exercise.
 The failure of oxygen uptake to increase instantly at
the beginning of exercise suggests that anaerobic
pathways contribute to the overall production on
ATP early in exercise. After a steady state is reached,
the body’s ATP requirement is met via aerobic
metabolism.
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Chapter 4
Recovery From Exercise: Metabolic Responses
Recovery From Exercise
• Oxygen uptake remains elevated above rest into
recovery
• Oxygen debt
– Term used by A.V. Hill
• Repayment for O2 deficit at onset of exercise
• Excess post-exercise oxygen consumption (EPOC)
– Terminology reflects that only ~20% elevated O2
consumption used to “repay” O2 deficit
• Many scientists use these terms interchangeably
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Chapter 4
Recovery From Exercise: Metabolic Responses
Oxygen Debt
• “Rapid” portion of O2 debt
– Resynthesis of stored PC
– Replenishing muscle and blood O2 stores
• “Slow” portion of O2 debt
– Elevated heart rate and breathing =  energy need
– Elevated body temperature =  metabolic rate
– Elevated epinephrine and norepinephrine = 
metabolic rate
– Conversion of lactic acid to glucose
(gluconeogenesis)
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Chapter 4
Recovery From Exercise: Metabolic Responses
EPOC is Greater Following
Higher Intensity Exercise
•
•
•
•
Higher body temperature
Greater depletion of PC
Greater blood concentrations of lactic acid
Higher levels of blood epinephrine and
norepinephrine
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Oxygen Deficit
and
During
Recovery
FromDebt
Exercise: Metabolic
Responses
Light/Moderate and Heavy
Exercise
Chapter 4
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Figure 4.3
Chapter 4
Recovery From Exercise: Metabolic Responses
A Closer Look 4.1
Removal of Lactic Acid Following
Exercise
• Classical theory
– Majority of lactic acid converted to glucose in liver
• Recent evidence
– 70% of lactic acid is oxidized
• Used as a substrate by heart and skeletal muscle
– 20% converted to glucose
– 10% converted to amino acids
• Lactic acid is removed more rapidly with light exercise in
recovery
– Optimal intensity is ~30–40% VO2 max
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Chapter 4
Recovery From Exercise: Metabolic Responses
Blood Lactate Removal
Following Strenuous Exercise
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Figure 4.4
Chapter 4
Recovery From Exercise: Metabolic Responses
Factors Contributing to EPOC
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Figure 4.5
Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Metabolic Responses to ShortTerm, Intense Exercise
• First 1–5 seconds of exercise
– ATP through ATP-PC system
• Intense exercise longer than 5 seconds
– Shift to ATP production via glycolysis
• Events lasting longer than 45 seconds
– ATP production through ATP-PC, glycolysis, and
aerobic systems
– 70% anaerobic/30% aerobic at 60 seconds
– 50% anaerobic/50% aerobic at 2 minutes
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Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
In Summary
 During high-intensity, short-term exercise (i.e., two to
twenty seconds), the muscle’s ATP production is
dominated by the ATP-PC system.
 Intense exercise lasting more than twenty seconds
relies more on anaerobic glycolysis to produce
much of the needed ATP.
 Finally, high-intensity events lasting longer than
forty-five seconds use a combination of the ATP-PC
system, glycolysis, and the aerobic system to
produce the needed ATP for muscular contraction.
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Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Metabolic Responses to
Prolonged Exercise
• Prolonged exercise (>10 minutes)
– ATP production primarily from aerobic metabolism
– Steady-state oxygen uptake can generally be
maintained during submaximal exercise
• Prolonged exercise in a hot/humid environment or at
high intensity
– Upward drift in oxygen uptake over time
– Due to body temperature and rising epinephrine and
norepinephrine
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Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Upward Drift in Oxygen Uptake
During Prolonged Exercise
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Figure 4.6
Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Metabolic Responses to
Incremental Exercise
• Oxygen uptake increases linearly until maximal oxygen
uptake (VO2 max) is reached
– No further increase in VO2 with increasing work rate
• VO2 max
– “Physiological ceiling” for delivery of O2 to muscle
– Affected by genetics and training
• Physiological factors influencing VO2 max
– Maximum ability of cardiorespiratory system to deliver
oxygen to the muscle
– Ability of muscles to use oxygen and produce ATP
aerobically
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Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Changes in Oxygen Uptake
During Incremental Exercise
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Figure 4.7
Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Lactate Threshold
• The point at which blood lactic acid rises systematically
during incremental exercise
– Appears at ~50–60% VO2 max in untrained subjects
– At higher work rates (65–80% VO2 max) in trained
subjects
• Also called:
– Anaerobic threshold
– Onset of blood lactate accumulation (OBLA)
• Blood lactate levels reach 4 mmol/L
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Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Changes in Blood Lactate Concentration
During Incremental Exercise
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Figure 4.8
Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Explanations for the Lactate Threshold
• Low muscle oxygen (hypoxia)
• Accelerated glycolysis
– NADH produced faster than it is shuttled into
mitochondria
– Excess NADH in cytoplasm converts pyruvic acid to
lactic acid
• Recruitment of fast-twitch muscle fibers
– LDH isozyme in fast fibers promotes lactic acid
formation
• Reduced rate of lactate removal from the blood
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Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Effect of Hydrogen Shuttle on
Lactic Acid Formation
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Figure 4.9
Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Mechanisms to Explain the
Lactate Threshold
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Figure 4.10
Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
Practical Uses of the Lactate
Threshold
• Prediction of performance
– Combined with VO2 max
• Planning training programs
– Marker of training intensity
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Chapter 4
Metabolic Responses to Exercise: Influence of Duration and Intensity
In Summary
 Oxygen uptake increases in a linear fashion during incremental
exercise until VO2 max is reached.
 The point at which blood lactic acid rises systematically during
graded exercise is termed the lactate threshold or anaerobic
threshold.
 Controversy exists over the mechanism to explain the sudden
rise in blood lactic acid concentrations during incremental
exercise. It is possible that any one or a combination of the
following factors might provide an explanation for the lactate
threshold: (1) low muscle oxygen, (2) accelerated glycolysis, (3)
recruitment of fast fibers, and (4) a reduced rate of lactate
removal.
 The lactate threshold has practical uses such as in
performance prediction and as a marker of training intensity.
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Chapter 4
Estimation of Fuel Utilization During Exercise
Estimation of Fuel Utilization
During Exercise
• Respiratory exchange ratio (RER or R)
VCO2
R=
VO
2
• R for fat (palmitic acid)
C16H32O2 + 23 O2  16 CO2 + 16 H2O
VCO2
R=
VO
=
16 CO2
23 O2
= 0.70
2
C6H12O6 + 6 O2  6 CO2 + 6 H2O
• R for carbohydrate (glucose)
R=
VCO2
VO
2
=
6 CO2
6 O2
= 1.00
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Chapter 4
Estimation of Fuel Utilization During Exercise
Estimation of Fuel Utilization
During Exercise
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Chapter 4
Estimation of Fuel Utilization During Exercise
In Summary
 The respiratory exchange ratio (R) is
the ratio of carbon dioxide produced to
the oxygen consumed (VCO2/VO2).
 In order for R to be used as an estimate
of substrate utilization during exercise,
the subject must have reached steady
state. This is important because only
during steady-state exercise are the
VCO2 and VO2 reflective of metabolic
exchange of gases in tissues.
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Chapter 4
Factors Governing Fuel Selection
Exercise Intensity and Fuel Selection
• Low-intensity exercise (<30% VO2 max)
– Fats are primary fuel
• High-intensity exercise (>70% VO2 max)
– Carbohydrates are primary fuel
• “Crossover” concept
– Describes the shift from fat to CHO
metabolism as exercise intensity increases
– Due to:
• Recruitment of fast muscle fibers
• Increasing blood levels of epinephrine
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Chapter 4
Illustration of the “Crossover”
Concept
Factors Governing Fuel Selection
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Figure 4.11
Chapter 4
Factors Governing Fuel Selection
The Regulation of Glycogen Breakdown
During Exercise
• Dependent on the enzyme phosphorylase
• Activation of phosphorylase
– Calmodulin activated by calcium released
from sarcoplasmic reticulum
• Active calmodulin activates phosphorylase
– Epinephrine binding to receptor results in
formation of cyclic AMP
• Cyclic AMP activates phosphorylase
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Chapter 4
Factors Governing Fuel Selection
The Regulation of Muscle Glycogen
Breakdown During Exercise
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Figure 4.12
Chapter 4
Factors Governing Fuel Selection
McArdle’s Syndrome: A Genetic Error in Muscle Glycogen
Metabolism
• Cannot synthesize the enzyme phosphorylase
– Due to a gene mutation
• Inability to break down muscle glycogen
• Also prevents lactate production
– Blood lactate levels do not rise during high-intensity
exercise
• Patients complain of exercise intolerance and muscle
pain
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Chapter 4
Factors Governing Fuel Selection
A Closer Look 4.3
Is Low-Intensity Exercise Best for Burning Fat?
• At low exercise intensities (~20% VO2 max)
– High percentage of energy expenditure (~60%)
derived from fat
– However, total energy expended is low
• Total fat oxidation is also low
• At higher exercise intensities (~50% VO2 max)
– Lower percentage of energy (~40%) from fat
– Total energy expended is higher
• Total fat oxidation is also higher
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Chapter 4
Factors Governing Fuel Selection
Rate of Fat Metabolism at Different
Exercise Intensities
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Figure 4.14
Chapter 4
Factors Governing Fuel Selection
Exercise Duration and Fuel Selection
• Prolonged, low-intensity exercise
– Shift from carbohydrate metabolism toward fat
metabolism
• Due to an increased rate of lipolysis
– Breakdown of triglycerides  glycerol + FFA
• By enzymes called lipases
– Stimulated by rising blood levels of
epinephrine
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Chapter 4
Factors Governing Fuel Selection
Shift From Carbohydrate to Fat
Metabolism During Prolonged Exercise
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Figure 4.13
Chapter 4
Factors Governing Fuel Selection
Interaction of Fat and CHO Metabolism
During Exercise
• “Fats burn in the flame of carbohydrates”
• Glycogen is depleted during prolonged
high-intensity exercise
– Reduced rate of glycolysis and production of
pyruvate
– Reduced Krebs cycle intermediates
– Reduced fat oxidation
• Fats are metabolized by Krebs cycle
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Chapter 4
Factors Governing Fuel Selection
Carbohydrate Feeding via Sports Drinks Improves Endurance
Performance
• The depletion of muscle and blood carbohydrate stores
contributes to fatigue
• Ingestion of carbohydrates can improve endurance
performance
– During submaximal (<70% VO2 max), long-duration
(>90 minutes) exercise
– 30–60 g of carbohydrate per hour are required
• May also improve performance in shorter, higher
intensity events
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Chapter 4
Factors Governing Fuel Selection
Sources of Carbohydrate During Exercise
• Muscle glycogen
– Primary source of carbohydrate during high-intensity
exercise
– Supplies much of the carbohydrate in the first hour of
exercise
• Blood glucose
– From liver glycogenolysis
– Primary source of carbohydrate during low-intensity
exercise
– Important during long-duration exercise
• As muscle glycogen levels decline
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Chapter 4
Factors Governing Fuel Selection
Sources of Fat During Exercise
• Intramuscular triglycerides
– Primary source of fat during higher intensity exercise
• Plasma FFA
– From adipose tissue lipolysis
• Triglycerides  glycerol + FFA
– FFA converted to acetyl-CoA and enters Krebs cycle
– Primary source of fat during low-intensity exercise
– Becomes more important as muscle triglyceride levels
decline in long-duration exercise
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Chapter 4
Factors Governing Fuel Selection
Influence of Exercise Intensity on
Muscle Fuel Source
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Figure 4.15
Chapter 4
Factors Governing Fuel Selection
Effect of Exercise Duration on Muscle
Fuel Source
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Figure 4.16
Chapter 4
Factors Governing Fuel Selection
Sources of Protein During Exercise
• Proteins broken down into amino acids
– Muscle can directly metabolize branch chain
amino acids and alanine
– Liver can convert alanine to glucose
• Only a small contribution (~2%) to total
energy production during exercise
– May increase to 5–10% late in prolongedduration exercise
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Chapter 4
Factors Governing Fuel Selection
Lactate as a Fuel Source During Exercise
• Can be used as a fuel source by skeletal
muscle and the heart
– Converted to acetyl-CoA and enters Krebs
cycle
• Can be converted to glucose in the liver
– Cori cycle
• Lactate shuttle
– Lactate produced in one tissue and
transported to another
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Chapter 4
Factors Governing Fuel Selection
A Closer Look 4.4
The Cori Cycle: Lactate as a Fuel Source
• Lactic acid produced by skeletal muscle is
transported to the liver
• Liver converts lactate to glucose
– Gluconeogenesis
• Glucose is transported back to muscle and
used as a fuel
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Chapter 4
Factors Governing Fuel Selection
The Cori Cycle: Lactate As a Fuel Source
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Figure 4.17
Chapter 4
Factors Governing Fuel Selection
In Summary
 The regulation of fuel selection during exercise is
under complex control and is dependent upon
several factors, including diet and the intensity and
duration of exercise.
 In general, carbohydrates are used as the major fuel
source during high-intensity exercise.
 During prolonged exercise, there is a gradual shift
from carbohydrate metabolism toward fat
metabolism.
 Proteins contribute less than 2% of the fuel used
during exercise of less than one hour’s duration.
During prolonged exercise (i.e., three to five hours’
duration), the total contribution of protein to the fuel
supply may reach 5% to 10% during the final minutes
of prolonged work.
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Chapter 4
Factors Governing Fuel Selection
Quantifying Body Fuel Sources
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