exercise physiology 4

© 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

© 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

© 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)

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

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

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

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

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

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

Fig 4.11

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

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

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

Shift From CHO to Fat Metabolism During Prolonged Exercise

Fig 4.13

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

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

© 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

© 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 4Exercise Metabolism