1. Chapter 4 Exercise Metabolism
EXERCISE PHYSIOLOGY
Theory and Application to Fitness and Performance, 6th edition
Scott K. Powers & Edward T. Howley

2. 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

3. 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

4. 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

5. The Oxygen Deficit
Fig 4.1

6. Differences in VO2 Between Trained & Untrained Subjects
Fig 4.2

7. 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)

8. Oxygen Deficit and Debt During Light-Moderate and Heavy Exercise
Fig 4.3

9. Removal of Lactic Acid Following Exercise
Fig 4.4

10. Fig 4.5

11. 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

12. 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

13. Upward Drift in Oxygen Uptake During Prolonged Exercise
Fig 4.6

14. 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

15. Changes in Oxygen Uptake With Incremental Exercise
Fig 4.7

16. 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

17. Identification of the Lactate Threshold
Fig 4.8

18. Mechanisms to Explain the Lactate Threshold
Fig 4.10

19. 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

20. Effect of Hydrogen Shuttle and LDH on Lactate Threshold
Fig 4.9

21. 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

22. 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

23. 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

24. Illustration of the “Crossover” Concept
Fig 4.11

25. 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

26. Shift From CHO to Fat Metabolism During Prolonged Exercise
Fig 4.13

27. 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

28. 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

29. Effect of Exercise Intensity on Muscle Fuel Source
Fig 4.14

30. Effect of Exercise Duration on Muscle Fuel Source
Fig 4.15

31. The Cori Cycle: Lactate As a Fuel Source
Fig 4.16

32. Chapter 4 Exercise Metabolism