Exercise Physiology 4

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

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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 © 2007 McGraw-Hill Higher Education. All rights reserved.

The Oxygen Deficit

Fig 4.1 © 2007 McGraw-Hill Higher Education. All rights reserved.

Differences in VO2 Between Trained & Untrained Subjects

Fig 4.2 © 2007 McGraw-Hill Higher Education. All rights reserved.

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 © 2007 McGraw-Hill Higher Education. All rights reserved.

Removal of Lactic Acid Following Exercise

Fig 4.4 © 2007 McGraw-Hill Higher Education. All rights reserved.

Fig 4.5 © 2007 McGraw-Hill Higher Education. All rights reserved.

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 © 2007 McGraw-Hill Higher Education. All rights reserved.

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 © 2007 McGraw-Hill Higher Education. All rights reserved.

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 © 2007 McGraw-Hill Higher Education. All rights reserved.

Mechanisms to Explain the Lactate Threshold

Fig 4.10 © 2007 McGraw-Hill Higher Education. All rights reserved.

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 © 2007 McGraw-Hill Higher Education. All rights reserved.

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 © 2007 McGraw-Hill Higher Education. All rights reserved.

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

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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 © 2007 McGraw-Hill Higher Education. All rights reserved.

Effect of Exercise Intensity on Muscle Fuel Source

Fig 4.14 © 2007 McGraw-Hill Higher Education. All rights reserved.

Effect of Exercise Duration on Muscle Fuel Source

Fig 4.15 © 2007 McGraw-Hill Higher Education. All rights reserved.

The Cori Cycle: Lactate As a Fuel Source

Fig 4.16 © 2007 McGraw-Hill Higher Education. All rights reserved.

Chapter 4 Exercise Metabolism

© 2007 McGraw-Hill Higher Education. All rights reserved.