exercise physiology 4
TRANSCRIPT
© 2007 McGraw-Hill Higher Education. All rights reserved.
Chapter 4Exercise 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
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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
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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
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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
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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
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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
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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
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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
Fig 4.11
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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
Fig 4.13
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Interaction of Fat and CHO Metabolism During Exercise
• “Fats burn in a carbohydrate flame”• 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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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 4Exercise Metabolism