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Scott K. Powers Edward T. HowleyScott K. Powers Edward T. Howley

Theory and Application to Fitness and PerformanceSEVENTH EDITION

Chapter

Copyright 2009 The McGraw-Hill Companies, Inc. Permission required for reproduction or display outside of classroom use.

Exercise Metabolism


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Chapter 4

Copyright 2009 The McGraw-Hill Companies, Inc. All Rights Reserved.

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 O2debt is greater following

intense exercise when compared to the O2debt

following light exercise.


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Chapter 4


Energy Requirements at Rest

Almost 100% of ATP produced by aerobic

metabolism

Blood lactate levels are low (


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Chapter 4


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

Copyright 2009 The McGraw-Hill Companies, Inc. All Rights Reserved.Figure 4.1

The Oxygen Deficit



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Chapter 4


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



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Chapter 4


Differences in VO2Between Trained andUntrained Subjects

Figure 4.2



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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.



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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



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



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


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



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Chapter 4


Blood Lactate Removal FollowingStrenuous Exercise


Figure 4.4


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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


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



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Chapter 4


Upward Drift in Oxygen Uptake DuringProlonged Exercise


Figure 4.6


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Chapter 4



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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Changes in Oxygen Uptake DuringIncremental Exercise


Figure 4.7


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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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Changes in Blood Lactate ConcentrationDuring Incremental Exercise


Figure 4.8


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Effect of Hydrogen Shuttle on Lactic AcidFormation


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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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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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.


Ch t 4 Estimation of Fuel Utilization During Exercise


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Chapter 4


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


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



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Chapter 4



Figure 4.11

Illustration of the Crossover Concept



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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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p


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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Rate of Fat Metabolism at DifferentExercise Intensities



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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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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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Influence of Exercise Intensity on Muscle

Fuel Source


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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



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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



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The Cori Cycle: Lactate As a Fuel

Source

Figure 4.17



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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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