Download Chapter 4 Exercise Metabolism

Chapter 4
Exercise Metabolism
EXERCISE PHYSIOLOGY
Theory and Application to Fitness and Performance, 6th edition
Scott K. Powers & Edward T. Howley
© 2007 McGraw-Hill Higher Education. All rights reserved.
Objectives
• Discuss the relationship between exercise
intensity/duration and the bioenergetic
pathways
• Define the term oxygen deficit
• Define the term lactate threshold
• Discuss several possible mechanisms for the
sudden rise in blood-lactate during
incremental exercise
• List the factors that regulate fuel selection
during different types of exercise
© 2007 McGraw-Hill Higher Education. All rights reserved.
Objectives
• Explain why fat metabolism is dependent on
carbohydrate metabolism
• Define the term oxygen debt
• Give the physiological explanation for the
observation that the O2 dept is greater
following intense exercise when compared to
the O2 debt following light exercise
© 2007 McGraw-Hill Higher Education. All rights reserved.
Rest-to-Exercise Transitions
• Oxygen uptake increases rapidly
– Reaches steady state within 1-4 minutes
• Oxygen deficit
– Lag in oxygen uptake at the beginning of
exercise
– Suggests anaerobic pathways contribute to
total ATP production
• After steady state is reached, ATP
requirement is met through aerobic ATP
production
© 2007 McGraw-Hill Higher Education. All rights reserved.
The Oxygen Deficit
Fig 4.1
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Differences in VO2 Between
Trained & Untrained Subjects
Fig 4.2
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Recovery From Exercise
Metabolic Responses
• Oxygen debt or
• Excess post-exercise oxygen consumption (EPOC)
– Elevated VO2 for several minutes immediately following
exercise
• “Fast” portion of O2 debt
– Resynthesis of stored PC
– Replacing muscle and blood O2 stores
• “Slow” portion of O2 debt
– Elevated heart rate and breathing,  energy need
– Elevated body temperature,  metabolic rate
– Elevated epinephrine & norepinephrine,  metabolic rate
– Conversion of lactic acid to glucose (gluconeogenesis)
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Oxygen Deficit and Debt During
Light-Moderate and Heavy
Exercise
Fig 4.3
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Removal of Lactic Acid
Following Exercise
Fig 4.4
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Fig 4.5
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Metabolic Response to Exercise
Short-Term Intense Exercise
• High-intensity, short-term exercise (2-20 seconds)
– ATP production through ATP-PC system
• Intense exercise longer than 20 seconds
– ATP production via anaerobic glycolysis
• High-intensity exercise longer than 45 seconds
– ATP production through ATP-PC, glycolysis,
and aerobic systems
© 2007 McGraw-Hill Higher Education. All rights reserved.
Metabolic Response to Exercise
Prolonged Exercise
• Exercise longer than 10 minutes
– ATP production primarily from aerobic
metabolism
– Steady state oxygen uptake can generally
be maintained
• Prolonged exercise in a hot/humid
environment or at high intensity
– Steady state not achieved
– Upward drift in oxygen uptake over time
© 2007 McGraw-Hill Higher Education. All rights reserved.
Upward Drift in Oxygen
Uptake During Prolonged
Exercise
Fig 4.6
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Metabolic Response to Exercise
Incremental Exercise
VO2 – Ability to Deliver and Use Oxygen
• Oxygen uptake increases linearly until VO2max
is reached
– No further increase in VO2 with increasing
work rate
• Physiological factors influencing VO2max
– Ability of cardiorespiratory system to
deliver oxygen to muscles
– Ability of muscles to use oxygen and
produce ATP aerobically
© 2007 McGraw-Hill Higher Education. All rights reserved.
Changes in Oxygen Uptake
With Incremental Exercise
Fig 4.7
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Lactate Threshold
• The point at which blood lactic acid suddenly
rises during incremental exercise
– Also called the anaerobic threshold
• Mechanisms for lactate threshold
– Low muscle oxygen
– Accelerated glycolysis
– Recruitment of fast-twitch muscle fibers
– Reduced rate of lactate removal from the
blood
• Practical uses in prediction of performance
and as a marker of exercise intensity
© 2007 McGraw-Hill Higher Education. All rights reserved.
Identification of the
Lactate Threshold
Fig 4.8
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Mechanisms to Explain the
Lactate Threshold
Fig 4.10
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Other Mechanisms for the
Lactate Threshold
• Failure of the mitochondrial hydrogen shuttle
to keep pace with glycolysis
– Excess NADH in sarcoplasm favors
conversion of pyruvic acid to lactic acid
• Type of LDH
– Enzyme that converts pyruvic acid to lactic
acid
– LDH in fast-twitch fibers favors formation of
lactic acid
© 2007 McGraw-Hill Higher Education. All rights reserved.
Effect of Hydrogen Shuttle and
LDH on Lactate Threshold
Fig 4.9
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Estimation of Fuel Utilization
During Exercise
• Respiratory exchange ratio (RER or R)
– VCO2 / VO2
Fat (palmitic acid) = C16H32O2
C16H32O2 + 23O2  16CO2 + 16H2O + ?ATP
R = VCO2/VO2 = 16 CO2 / 23O2 = 0.70
Glucose = C6H12O6
C6H12O6 + 6O2  6CO2 + 6H2O + ?ATP
R = VCO2/VO2 = 6 CO2 / 6O2 = 1.00
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Estimation of Fuel Utilization
During Exercise
• Indicates fuel utilization
• 0.70 = 100% fat
• 0.85 = 50% fat, 50% CHO
• 1.00 = 100% CHO
• During steady-state exercise
– VCO2 and VO2 reflective of O2
consumption and CO2 production at the
cellular level
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Exercise Intensity and Fuel
Selection
• Low-intensity exercise (<30% VO2max)
– Fats are primary fuel
• High-intensity exercise (>70% VO2max)
– CHO 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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Illustration of the
“Crossover” Concept
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Fig 4.11
Exercise Duration and Fuel
Selection
• During prolonged exercise, there is a shift
from CHO metabolism toward fat metabolism
• Increased rate of lipolysis
– Breakdown of triglycerides into glycerol
and free fatty acids (FFA)
– Stimulated by rising blood levels of
epinephrine
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Shift From CHO to Fat Metabolism
During Prolonged Exercise
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Fig 4.13
Interaction of Fat and CHO
Metabolism During Exercise
• “Fats burn in a carbohydrate flame”
• 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
© 2007 McGraw-Hill Higher Education. All rights reserved.
Sources of Fuel During
Exercise
• Carbohydrate
– Blood glucose
– Muscle glycogen
• Fat
– Plasma FFA (from adipose tissue lipolysis)
– Intramuscular triglycerides
• Protein
– Only a small contribution to total energy production
(only ~2%)
• May increase to 5-15% late in prolonged exercise
• Blood lactate
– Gluconeogenesis via the Cori cycle
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Effect of Exercise Intensity on
Muscle Fuel Source
Fig 4.14
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Effect of Exercise Duration on
Muscle Fuel Source
Fig 4.15
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The Cori Cycle:
Lactate As a Fuel Source
Fig 4.16
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Chapter 4
Exercise Metabolism
© 2007 McGraw-Hill Higher Education. All rights reserved.