This document discusses various topics related to exercise physiology including aerobic and anaerobic exercise, oxygen consumption, substrate utilization, the lactate threshold, and causes of fatigue. It provides details on: 1) How aerobic exercise relies on oxidative metabolism of carbohydrates and fats requiring oxygen, while anaerobic metabolism results in excess carbon dioxide release from buffer systems. 2) The respiratory exchange ratio can determine the relative metabolic contribution of carbohydrates and fats based on the ratio of carbon dioxide produced to oxygen consumed. 3) Glycogen depletion does not directly cause fatigue but limits endurance exercise as muscle glycogen stores are needed for energy production; carbohydrate intake can delay fatigue by maintaining blood glucose

2. Overview
Aerobic Exercise and Oxygen Consumption
Substrate Utilization
– Respiratory Exchange Ratio
Anaerobic Exercise and the Lactate Threshold
– Bicarbonate Buffer System
– Definition, Possible Causes
Causes of Fatigue
Glycogen Depletion
– Exercise Intensity
– CHO intake

3. Aerobic Exercise and
O2 Consumption

4. Maximal Duration of Energy
System
30
sec
1
min
3
min
5
min
2-3
hr
%Contribution
ATP-PC
Glycolysis
Oxidative
10
sec

5. Aerobic Exercise and
O2 Consumption
Oxidative metabolism of CHO and FAT requires O2,
produces CO2
Indirect calorimetry - calculated energy expenditure
based on gas exchange (VO2 and VCO2)
Must be primarily aerobic to be accurate
– Anaerobic metabolism results in excess CO2 release
from buffer systems
Difference between inspired and expired air

6. Aerobic Exercise and
O2 Consumption
Energy Expenditure (kcals)
Fitness Level
Contribution of CHO
Contribution of FAT

7. Energy Expenditure

8. Fitness Level: VO2max
Maximal Oxygen Uptake
Measure of Aerobic Fitness
Graded Exercise Test
Maximal Effort
VO2max= Inspired O2 – Expired O2

9. Bruce Protocol
Stage Speed Incline
1 1.7 10%
2 2.5 12%
3 3.4 14%
4 4.2 16%
5 5.0 18%
6 5.5 20%

10. VO2max Data

11. Substrate Utilization
Primary fuel source is CHO and Fat.
Protein can serve as a secondary fuel
source.
Fat requires more O2 than CHO
Relative Contribution determined by
the Respiratory Exchange Ratio
(RER)

12. Respiratory Exchange Ratio
(RER)
Non-invasive technique to determine
relative Metabolic Contribution of
Carbohydrate and Fat.
RER =
VCO2
VO2
Also called Respiratory Quotient (RQ)
during Steady State Exercise.
1.0 = 100% CHO, 0.7 = 100% Fat

13. CHO vs. FAT
6 O2 + C6H12O6  6 CO2 + 6 H2O + 32 ATP
CHO (Glucose = C6H12O6):
23 O2 + C16H32O2  16 CO2 + 16 H2O + 106 ATP
FAT (Palmitic Acid = C16H32O2):
Amount of O2 required is proportional to
amount of C in the substrate!
RER = VCO2/VO2 = 6/6 = 1.0
RER = VCO2/VO2 = 16/23 = 0.7

14. Crossover Effect
% of max 
%Utilization
CHO
Fat35-40% of VO2 max

15. Oxygen Consumption
Limitations
Oxygen Deficit
– Beginning of Exercise, Exercise Transitions
Oxygen Debt/EPOC
– End of Exercise
Lactate Production
– High Intensity Exercise

16. Exercise Transition
Stage 1 Stage 2
Oxygen Deficit
OxygenConsumption

17. Stop
Predicted
O2 Deficit
Rest Transition
OxygenConsumption
Start
Actual
O2 Debt or EPOC

18. Possible Explanations for EPOC
Reform ATP, PC, and replace tissue O2
stores.
Removal of Lactic Acid [to liver (Cori Cycle) or
Oxidation]
2Lactate (C3H6O3) + energy (from 16 ATP)  glucose (C6H12O6)
2Lactate (C3H6O3) + 6O2  6CO2 + 6H2O + 619Kcal
Removal of excess CO2
 Body Temp. and Catecholamines

19. Why is Lactate Produced during
aerobic exercise?
Glycolysis
NADH
Mitochondria
Hydrogen
Shuttle
Pyruvate
Lactate

20. Lactic Acid
Metabolic by-product of Anaerobic
Glycolysis.
Immediately hydrolyzed into Lactate and
H+(acid)
Acid portion is removed from active tissue
and buffered in the blood (bicarbonate
system).
Lactate can be reformed into glucose in the
Liver via Cori Cycle (gluconeogenesis).

21. Bicarbonate System
CO2+H2OH+
From Lactic Acid
HCO3+
Bicarbonate
H2CO3
Carbonic Acid

22. Lactate Threshold
Lactic acid accumulates with prolonged,
high-intensity exercise
Lactate Threshold is the systematic rise in
blood lactate concentration
– Production exceeds clearance
Often used as a measure of aerobic
fitness level

23. Lactate Threshold
LT
Exercise Intensity 
BloodLactate

24. Does Lactic Acid Cause Fatigue?
• No, lactic acid DOES NOT directly cause fatigue!
• Acidosis (H+) causes fatigue
• Inhibits PFK (rate limiting enzyme) and energy
production
• Inhibits actin-myosin cross bridges for muscle
contraction
• Benefits of Lactic Acid:
• Maintains cytosolic redox potential
• Can be converted to glucose and used for
energy production (Cori Cycle)

25. Cytosolic Redox Potential
Lactic Acid
Pyruvic Acid
NADH+H+
NAD+
Lactate
Dehydrogenase
Pyruvic Acid accepts H+; is reduced by NADH
forming a molecule of lactic acid.
C3H4O3 + NADH + H+ → C3H6O3 + NAD+
(Pyruvic Acid) (Lactic Acid)

26. Causes of Fatigue
Energy System Failure
– PC Depletion
– Glycogen Depletion
Metabolic By-Products
– Pi (inorganic phosphate)
– Heat and Muscle Temperature
– Acidosis (H+)
Neuromuscular Fatigue
– Peripheral (neural transmission)
– Central (CNS)

27. Quick check
When _________ runs out, endurance exercise
simply can’t continue……
A. Steam
B. Muscle glycogen
C. The trail
….. unless ______ is ingested.
A. Really strong coffee
B. Air
C. Carbohydrates

28. Substrate Use in Prolonged
Exercise
Coggan and Coyle, 1991
Fat: 100,000
kcals
40 kcals
400
kcals
Liver
glycogen:
200 kcals

29. Glycogen Depletion
Muscle Glycogen used for energy
production (glycolysis, oxidative
phosphorylation)
Depletion selective within muscle fiber
: type I to type II (intensity low to high)
Glycogen depletion does not directly
cause fatigue

30. Glycogen Depletion and
Exercise Intensity

31. CHO and Glycogen Storage

32. CHO and Glycogen Storage

33. CHO Loading

34. CHO during Exercise
Delays fatigue by:
– Maintaining blood glucose levels (especially
important for prolonged exercise)
– “Sparing” glycogen stores
– Glycogen synthesis during low-intensity
exercise
6-8% CHO solution is ideal
~16g CHO/hour