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Scott K. Powers Edward T. HowleyScott K. Powers Edward T. Howley
Theory and Application to Fitness and PerformanceSEVENTH EDITION
Chapter
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Exercise Metabolism
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Chapter 4
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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
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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 O2debt is greater following
intense exercise when compared to the O2debt
following light exercise.
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Chapter 4
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Energy Requirements at Rest
Almost 100% of ATP produced by aerobic
metabolism
Blood lactate levels are low (
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Chapter 4
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Rest-to-Exercise Transitions
Rest-to-Exercise Transitions
ATP production increases immediately
Oxygen uptake increases rapidly
Reaches steady state within 14 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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Chapter 4
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The Oxygen Deficit
Rest-to-Exercise Transitions
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Chapter 4
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Comparison of Trained and UntrainedSubjects
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
Rest-to-Exercise Transitions
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Chapter 4
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Differences in VO2Between Trained andUntrained Subjects
Figure 4.2
Rest-to-Exercise Transitions
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Chapter 4
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In Summary
In the transition from rest to light or moderate exercise,
oxygen uptake increases rapidly, generally reaching asteady state within one to four minutes.
The term oxygen deficitapplies 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 bodys ATPrequirement is met via aerobic metabolism.
Rest-to-Exercise Transitions
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Chapter 4
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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 O2deficit at onset of exercise
Excess post-exercise oxygen consumption (EPOC)
Terminology reflects that only ~20% elevated O2
consumption used to repay O2deficit Many scientists use these terms interchangeably
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Chapter 4
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Recovery From Exercise: Metabolic Responses
Oxygen Debt
Rapid portion of O2debt
Resynthesis of stored PC
Replenishing muscle and blood O2stores
Slow portion of O2debt
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
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Recovery From Exercise: Metabolic Responses
EPOC is Greater Following HigherIntensity 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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Chapter 4
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A Closer Look 4.1
Removal of Lactic Acid FollowingExercise
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 lightexercise in recovery
Optimal intensity is ~3040% VO2 max
Recovery From Exercise: Metabolic Responses
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Chapter 4
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Blood Lactate Removal FollowingStrenuous Exercise
Recovery From Exercise: Metabolic Responses
Figure 4.4
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Chapter 4
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Metabolic Responses to Exercise: Influence of Duration and Intensity
Metabolic Responses to Short-Term,Intense Exercise
First 15 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
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Metabolic Responses to ProlongedExercise
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 orat high intensity
Upward drift in oxygen uptake over time
Due to body temperature and rising epinephrine andnorepinephrine
Metabolic Responses to Exercise: Influence of Duration and Intensity
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Chapter 4
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Upward Drift in Oxygen Uptake DuringProlonged Exercise
Metabolic Responses to Exercise: Influence of Duration and Intensity
Figure 4.6
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Chapter 4
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Metabolic Responses to Exercise: Influence of Duration and Intensity
Metabolic Responses to IncrementalExercise
Oxygen uptake increases linearly unti l maximal
oxygen uptake (VO2 max) is reached No further increase in VO2with increasing work rate
VO2 max
Physiological ceiling for delivery of O2to 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
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Changes in Oxygen Uptake DuringIncremental Exercise
Metabolic Responses to Exercise: Influence of Duration and Intensity
Figure 4.7
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Chapter 4
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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 ~5060% VO2 max in untrained subjects
At higher work rates (6580% 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
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Changes in Blood Lactate ConcentrationDuring Incremental Exercise
Metabolic Responses to Exercise: Influence of Duration and Intensity
Figure 4.8
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Chapter 4
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Effect of Hydrogen Shuttle on Lactic AcidFormation
Metabolic Responses to Exercise: Influence of Duration and Intensity
Figure 4.9
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M t b li R t E i I fl f D ti d I t i t
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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
Ch t 4 M t b li R t E i I fl f D ti d I t it
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Chapter 4
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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 systematicallyduring graded exercise is termed the lactate threshold oranaerobic threshold.
Controversy exists over the mechanism to explain the
sudden rise in blood lactic acid concentrations duringincremental exercise. It is possible that any one or acombination of the following factors might provide anexplanation for the lactate threshold: (1) low muscle
oxygen, (2) accelerated glycolysis, (3) recruitment of fastfibers, and (4) a reduced rate of lactate removal.
The lactate threshold has practical uses such as inperformance prediction and as a marker of training
intensity.
Metabolic Responses to Exercise: Influence of Duration and Intensity
Ch t 4 Estimation of Fuel Utilization During Exercise
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Chapter 4
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Respiratory exchange ratio (RER or R)
R for fat (palmitic acid)
R for carbohydrate (glucose)
C16H32O2+ 23 O216 CO2+ 16 H2O
VCO2
VO2
=R =16 CO2
23 O2= 0.70
VCO2
VO=R =
6 CO2
6 O2 = 1.00
C6H12O6+ 6 O26 CO2+ 6 H2O
VCO2
VO2
R =
Estimation of Fuel UtilizationDuring Exercise
Estimation of Fuel Utilization During Exercise
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Chapter 4 Estimation of Fuel Utilization During Exercise
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Chapter 4
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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 onlyduring steady-state exercise are the VCO2and VO2
reflective of metabolic exchange of gases in tissues.
Estimation of Fuel Utilization During Exercise
Chapter 4 Factors Governing Fuel Selection
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Exercise Intensity and Fuel Selection
Low-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
Factors Governing Fuel Selection
Chapter 4 Factors Governing Fuel Selection
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Factors Governing Fuel Selection
Figure 4.11
Illustration of the Crossover Concept
Chapter 4 Factors Governing Fuel Selection
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Factors Governing Fuel Selection
A Closer Look 4.2
The Regulation of Glycogen BreakdownDuring 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
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p
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g
Clinical Applications 4.1
McArdles Syndrome: A Genetic Error inMuscle Glycogen Metabolism
Cannot synthesize the enzyme phosphorylase
Due to a gene mutation
Inabili ty 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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A Closer Look 4.3
Is Low-Intensity Exercise Best forBurning 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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Copyright 2009 The McGraw-Hill Companies, Inc. All Rights Reserved.Figure 4.14
Rate of Fat Metabolism at DifferentExercise Intensities
Chapter 4 Factors Governing Fuel Selection
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Exercise Duration and Fuel Selection
Prolonged, low-intensity exercise
Shift from carbohydrate metabolism toward fatmetabolism
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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Interaction of Fat and CHO MetabolismDuring 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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The Winning Edge 4.2
Carbohydrate Feeding via Sports DrinksImproves Endurance Performance
The depletion of muscle and blood carbohydrate
stores contributes to fatigue Ingestion of carbohydrates can improve endurance
performance
During submaximal (90 minutes) exercise
3060 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
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Sources of Fat During Exercise
Intramuscular triglycerides
Primary source of fat during higher intensity exercise
Plasma FFA
From adipose tissue lipolysis
Triglyceridesglycerol + FFA
FFA converted to acetyl-CoA and enters Krebs cycle
Primary source of fat during low-intensity exercise
Becomes more important as muscle triglyceridelevels decline in long-duration exercise
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Copyright 2009 The McGraw-Hill Companies, Inc. All Rights Reserved.Figure 4.15
Influence of Exercise Intensity on Muscle
Fuel Source
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Chapter 4 Factors Governing Fuel Selection
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Sources of Protein During Exercise
Proteins broken down into amino acids
Muscle can directly metabolize branch chain aminoacids and alanine
Liver can convert alanine to glucose
Only a small contribution (~2%) to total energyproduction during exercise
May increase to 510% late in prolonged-duration
exercise
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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
Chapter 4
A Closer Look 4 4
Factors Governing Fuel Selection
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A Closer Look 4.4
The Cori Cycle: Lactate as a FuelSource
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 asa fuel
Chapter 4
Th C i C l L A F l
Factors Governing Fuel Selection
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The Cori Cycle: Lactate As a Fuel
Source
Figure 4.17
Chapter 4 Factors Governing Fuel Selection
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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 fuelsource during high-intensity exercise.
During prolonged exercise, there is a gradual shift fromcarbohydrate metabolism toward fat metabolism.
Proteins contribute less than 2% of the fuel used duringexercise of less than one hours duration. During
prolonged exercise (i.e., three to five hours duration),the total contribution of protein to the fuel supply mayreach 5% to 10% during the final minutes of prolongedwork.
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Chapter 4
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Study Questions
1. Identify the predominant energy systems used to produceATP during the following types of exercise:
a. Short-term, intense exercise (i.e., less than ten secondsduration)b. 400-meter dashc. 20-kilometer race (12.4 miles)
2. Graph the change in oxygen uptake during the transitionfrom rest to steady-state, submaximal exercise. Label theoxygen deficit. Where does the ATP come from during thetransition period from rest to steady state?
3. Graph the change in oxygen uptake and blood lactate
concentration during incremental exercise. Label the pointon the graph that might be considered the lactate thresholdor lactate inflection point.
4. Discuss several possible reasons why blood lactate begins
to rise rapidly during incremental exercise.
Chapter 4
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Study Questions
5. Briefly, explain how the respiratory exchange ratio is used
to estimate which substrate is being uti lized during exercise.
What is meant by the term nonproteinR?
6. List two factors that play a role in the regulation of
carbohydrate metabolism during exercise.
7. List those variables that regulate fat metabolism duringexercise.
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.
Chapter 4
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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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