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

2. Energy Requirements at Rest
Almost 100% ATP produced by aerobic metabolism
Resting O2 consumption:
0.25 L/min
3.5 ml/kg/min
Blood lactate levels are low (<1.0 mmol/L)

3. 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

4. The Oxygen Deficit
Figure 4.1

5. Differences in VO2 Between Trained and Untrained Subjects
Figure 4.2

6. Recovery From Exercise Metabolic Responses
Oxygen debt
VO2 elevated above rest following exercise
Excess post-exercise oxygen consumption (EPOC)
“Rapid” portion of O2 debt
Resynthesis of stored PC
Replenishing 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)

7. Oxygen Deficit and Debt During Light/Moderate and Heavy Exercise
Figure 4.3

8. Blood Lactate Removal Following Strenuous Exercise
Figure 4.4

9. Factors Contributing to Excess Post-Exercise Oxygen Consumption
Figure 4.5

10. Metabolic Responses to 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

11. Metabolic Responses to Prolonged Exercise
Prolonged exercise (>10 minutes)
ATP production primarily from aerobic metabolism
Steady state oxygen uptake can generally be maintained during submaximal exercise
Prolonged exercise in a hot/humid environment or at high intensity
Steady state not maintained
Upward drift in oxygen uptake over time

12. Upward Drift in Oxygen Uptake During Prolonged Exercise
Figure 4.6

13. Metabolic Responses to Incremental Exercise
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 the muscle
Ability of muscles to use oxygen and produce ATP aerobically

14. Changes in Oxygen Uptake During Incremental Exercise
Figure 4.7

15. Lactate Threshold
The point at which blood lactic acid rises systematically during incremental exercise
Also called the anaerobic threshold
Explanations for the lactate threshold
Low muscle oxygen
Accelerated glycolysis
Recruitment of fast-twitch muscle fibers
Reduced rate of lactate removal from the blood
Practical uses of the lactate threshold
Prediction of performance
Marker of exercise intensity

16. Changes in Blood Lactate Concentration During Incremental Exercise
Figure 4.8

17. Mechanisms to Explain the Lactate Threshold
Figure 4.10

18. Other Explanations 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 isozyme in fast-twitch fibers favors formation of lactic acid

19. Estimation of Fuel Utilization During Exercise
Respiratory exchange ratio (RER or R)
R for fat (palmitic acid)
R for carbohydrate (glucose)
VCO2
VO2
R =
C16H32O O2  16 CO H2O
VCO2
VO2 =
R =
16 CO2
23 O2
= 0.70
C6H12O6 + 6 O2  6 CO2 + 6 H2O
VCO2
VO2 =
R =
6 CO2
6 O2
= 1.00

20. Estimation of Fuel Utilization During Exercise
Table 4.1

21. Exercise Intensity and Fuel Selection
Low-intensity exercise (<30% VO2max)
Fats are primary fuel
High-intensity exercise (>70% VO2max)
Carbohydrates 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

22. Illustration of the “Crossover” Concept
Figure 4.11

23. The Regulation of Muscle Glycogen Breakdown During Exercise
Figure 4.12

24. Exercise Duration and Fuel Selection
Prolonged, low-intensity exercise
Shift from CHO metabolism toward fat metabolism
Due to an increased rate of lipolysis
Breakdown of triglycerides  glycerol + FFA
Stimulated by rising blood levels of epinephrine

25. Shift From CHO to Fat Metabolism During Prolonged Exercise
Figure 4.14

26. Interaction of Fat and CHO Metabolism During Exercise
“Fats burn in the flame of carbohydrates”
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

27. Body Fuel Sources During Exercise
Carbohydrate
Blood glucose (from liver glycogenolysis)
Muscle glycogen
Fat
Plasma FFA (from adipose tissue lipolysis)
Intramuscular triglycerides
Protein
Only a small contribution to total energy production (~2%)
May increase to 5-10% late in prolonged exercise
Blood lactate
Gluconeogenesis via the Cori cycle

28. Influence of Exercise Intensity on Muscle Fuel Source
Figure 4.15

29. Effect of Exercise Duration on Muscle Fuel Source
Figure 4.16

30. The Cori Cycle: Lactate As a Fuel Source
Figure 4.17

31. Quantifying Body Fuel Sources
Table 4.2