Scott K. Powers • Edward T. Howley
Theory and Application to Fitness and Performance
SEVENTH EDITION
Chapter
Exercise Metabolism
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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
 Estimation of Fuel
Utilization During
Exercise
 Factors Governing
Fuel Selection
Exercise Intensity and
Fuel Selection
Exercise Duration and
Fuel Selection
Interaction of Fat/
Carbohydrate
Metabolism
Body Fuel Sources
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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
• Oxygen deficit
– Lag in oxygen uptake at the beginning of exercise
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Rest-to-Exercise Transitions
Chapter 4
The Oxygen Deficit
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Figure 4.1
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
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Figure 4.2
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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Recovery From Exercise: Metabolic Responses
Chapter 4
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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Recovery From Exercise: Metabolic Responses
Chapter 4
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
Metabolic Responses to Exercise: Influence of Duration and Intensity
Metabolic Responses to Short-Term,
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
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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Estimation of Fuel Utilization During Exercise
Chapter 4
Estimation of Fuel Utilization
During Exercise
• Respiratory exchange ratio (RER or R)
R=
VCO2
VO
2
• R for fat (palmitic acid)
C16H32O2 + 23 O2  16 CO2 + 16 H2O
R=
VCO2
VO
=
16 CO2
23 O2
= 0.70
2
• R for carbohydrate (glucose)
C6H12O6 + 6 O2  6 CO2 + 6 H2O
R=
VCO2
VO
2
=
6 CO2
6 O2
= 1.00
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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
Factors Governing Fuel Selection
Illustration of the “Crossover” Concept
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Figure 4.11
Factors Governing Fuel Selection
Chapter 4
A Closer Look 4.2
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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Factors Governing Fuel Selection
Chapter 4
Clinical Applications 4.1
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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Factors Governing Fuel Selection
Chapter 4
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
Factors Governing Fuel Selection
Chapter 4
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
Interaction of Fat and CHO Metabolism
During Exercise
• “Fats burn in the flame of carbohydrates”
• Glycogen is depleted during prolonged highintensity 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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Factors Governing Fuel Selection
Chapter 4
The Winning Edge 4.2
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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Factors Governing Fuel Selection
Chapter 4
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 prolonged-duration
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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Factors Governing Fuel Selection
Chapter 4
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
Study Questions
1.
Identify the predominant energy systems used to produce
ATP during the following types of exercise:
a. Short-term, intense exercise (i.e., less than ten seconds’
duration)
b. 400-meter dash
c. 20-kilometer race (12.4 miles)
2.
Graph the change in oxygen uptake during the transition
from rest to steady-state, submaximal exercise. Label the
oxygen deficit. Where does the ATP come from during the
transition period from rest to steady state?
3.
Graph the change in oxygen uptake and blood lactate
concentration during incremental exercise. Label the point
on the graph that might be considered the lactate threshold
or lactate inflection point.
4.
Discuss several possible reasons why blood lactate begins
to rise rapidly during incremental exercise.
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Chapter 4
Study Questions
5.
Briefly, explain how the respiratory exchange ratio is used
to estimate which substrate is being utilized during exercise.
What is meant by the term nonprotein R?
6.
List two factors that play a role in the regulation of
carbohydrate metabolism during exercise.
7.
List those variables that regulate fat metabolism during
exercise.
8.
Define the following terms: (a) triglyceride, (b) lipolysis, and
(c) lipases.
9.
Graph the change in oxygen uptake during recovery from
exercise. Label the oxygen debt.
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Chapter 4
Study Questions
10. How does the modern theory of EPOC differ from the
classical oxygen debt theory proposed by A.V. Hill?
11. Discuss the influence of exercise intensity on muscle fuel
selection.
12. How does the duration of exercise influence muscle fuel
selection?
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