1. Energy Expenditure and Fatigue
Chapter 5
Energy Expenditure and Fatigue

2. Measuring Energy Expenditure: Direct Calorimetry
Substrate metabolism efficiency
40% of substrate energy  ATP
60% of substrate energy  heat
Heat production increases with energy production
Can be measured in a calorimeter
Water flows through walls
Body temperature increases water temperature

3. Figure 5.1

4. Figure 5.2a

5. Measuring Energy Expenditure: Respiratory Exchange Ratio
O2 usage during metabolism depends on type of fuel being oxidized
More carbon atoms in molecule = more O2 needed
Glucose (C6H12O6) < palmitic acid (C16H32O2)
Respiratory exchange ratio (RER)
Ratio between rates of CO2 production, O2 usage
RER = VCO2/VO2

6. Measuring Energy Expenditure: Respiratory Exchange Ratio
RER for 1 molecule glucose = 1.0
6 O2 + C6H12O6  6 CO2 + 6 H2O + 32 ATP
RER = VCO2/VO2 = 6 CO2/6 O2 = 1.0
RER for 1 molecule palmitic acid = 0.70
23 O2 + C16H32O2  16 CO H2O ATP
RER = VCO2/VO2 = 16 CO2/23 O2 = 0.70
Predicts substrate use, kilocalories/O2 efficiency

7. Table 5.1

8. Energy Expenditure at Rest and During Exercise
Metabolic rate: rate of energy use by body
Based on whole-body O2 consumption and corresponding caloric equivalent
At rest, RER ~0.80, VO2 ~0.3 L/min
At rest, metabolic rate ~2,000 kcal/day

9. Figure 5.3

10. Figure 5.4

11. Energy Expenditure During Maximal Aerobic Exercise
VO2max expressed in L/min
Easy standard units
Suitable for non-weight-bearing activities
VO2max normalized for body weight
ml O2  kg-1  min-1
More accurate comparison for different body sizes
Untrained young men: 44 to 50 versus untrained young women: 38 to 42
Sex difference due to women’s lower FFM and hemoglobin

12. Anaerobic Energy Expenditure: Postexercise O2 Consumption
O2 demand > O2 consumed in early exercise
Body incurs O2 deficit
O2 required − O2 consumed
Occurs when anaerobic pathways used for ATP production
O2 consumed > O2 demand in early recovery
Excess postexercise O2 consumption (EPOC)
Replenishes ATP/PCr stores, converts lactate to glycogen, replenishes hemo/myoglobin, clears CO2

13. Figure 5.5

14. Anaerobic Energy Expenditure: Lactate Threshold
Lactate threshold: point at which blood lactate accumulation  markedly
Lactate production rate > lactate clearance rate
Interaction of aerobic and anaerobic systems
Good indicator of potential for endurance exercise
Usually expressed as percentage of VO2max

15. Figure 5.6

16. Anaerobic Energy Expenditure: Lactate Threshold
Lactate accumulation  fatigue
Ability to exercise hard without accumulating lactate beneficial to athletic performance
Higher lactate threshold = higher sustained exercise intensity = better endurance performance
For two athletes with same VO2max, higher lactate threshold predicts better performance

17. Measuring Anaerobic Capacity
No clear, V̇O2max-like method for measuring anaerobic capacity
Imperfect but accepted methods
Maximal accumulated O2 deficit
Wingate anaerobic test
Critical power test

18. Energy Expenditure During Exercise: Economy of Effort
As athletes become more skilled, use less energy for given pace
Independent of VO2max
Body learns energy economy with practice
Multifactorial phenomenon
Economy  with distance of race
Practice  better economy of movement (form)
Varies with type of exercise (running vs. swimming)

19. Figure 5.7

20. Energy Expenditure: Successful Endurance Athletes
1. High VO2max
2. High lactate threshold (as % VO2max)
3. High economy of effort
4. High percentage of type I muscle fibers

21. Fatigue and Its Causes Fatigue: two definitions Reversible by rest
Decrements in muscular performance with continued effort, accompanied by sensations of tiredness
Inability to maintain required power output to continue muscular work at given intensity
Reversible by rest

22. Fatigue and Its Causes Complex phenomenon
Type, intensity of exercise
Muscle fiber type
Training status, diet
Four major causes (synergistic?)
Inadequate energy delivery/metabolism
Accumulation of metabolic by-products
Failure of muscle contractile mechanism
Altered neural control of muscle contraction

23. Fatigue and Its Causes: Energy Systems—PCr Depletion
PCr depletion coincides with fatigue
PCr used for short-term, high-intensity effort
PCr depletes more quickly than total ATP
Pi accumulation may be potential cause
Pacing helps defer PCr depletion

24. Fatigue and Its Causes: Energy Systems—Glycogen Depletion
Glycogen reserves limited and deplete quickly
Depletion correlated with fatigue
Related to total glycogen depletion
Unrelated to rate of glycogen depletion
Depletes more quickly with high intensity
Depletes more quickly during first few minutes of exercise versus later stages

25. Fatigue and Its Causes: Energy Systems—Glycogen Depletion
Fiber type and recruitment patterns
Fibers recruited first or most frequently deplete fastest
Type I fibers depleted after moderate endurance exercise
Recruitment depends on exercise intensity
Type I fibers recruit first (light/moderate intensity)
Type IIa fibers recruit next (moderate/high intensity)
Type IIx fibers recruit last (maximal intensity)

26. Fatigue and Its Causes: Energy Systems—Glycogen Depletion
Depletion in different muscle groups
Activity-specific muscles deplete fastest
Recruited earliest and longest for given task
Depletion and blood glucose
Muscle glycogen insufficient for prolonged exercise
Liver glycogen  glucose into blood
As muscle glycogen , liver glycogenolysis 
Muscle glycogen depletion + hypoglycemia = fatigue

27. Fatigue and Its Causes: Energy Systems—Glycogen Depletion
Certain rate of muscle glycogenolysis required to maintain
NADH production in Krebs cycle
Electron transport chain activity
No glycogen = inhibited substrate oxidation
With glycogen depletion, FFA metabolism 
But FFA oxidation too slow, may be unable to supply sufficient ATP for given intensity

28. Fatigue and Its Causes: Metabolic By-Products
Pi: From rapid breakdown of PCr, ATP
Heat: Retained by body, core temperature 
Lactic acid: Product of anaerobic glycolysis
H+ Lactic acid  lactate + H+

29. Fatigue and Its Causes: Metabolic By-Products
Heat alters metabolic rate
–  Rate of carbohydrate utilization
Hastens glycogen depletion
High muscle temperature may impair muscle function
Time to fatigue changes with ambient temperature
11°C: time to exhaustion longest
31°C: time to exhaustion shortest
Muscle precooling prolongs exercise

30. Fatigue and Its Causes: Metabolic By-Products
Lactic acid accumulates during brief, high-intensity exercise
If not cleared immediately, converts to lactate + H+
H+ accumulation causes  muscle pH (acidosis)
Buffers help muscle pH but not enough
Buffers minimize drop in pH (7.1 to 6.5, not to 1.5)
Cells therefore survive but don’t function well
pH <6.9 inhibits glycolytic enzymes, ATP synthesis
pH = 6.4 prevents further glycogen breakdown

31. Fatigue and Its Causes: Lactic Acid Not All Bad
May be beneficial during exercise
Accumulation can bring on fatigue
But if production = clearance, not fatiguing
Serves as source of fuel
Directly oxidized by type I fiber mitochondria
Shuttled from type II fibers to type I for oxidation
Converted to glucose via gluconeogenesis (liver)

32. Fatigue and Its Causes: Neural Transmission
Failure may occur at neuromuscular junction, preventing muscle activation
Possible causes
–  ACh synthesis and release
Altered ACh breakdown in synapse
Increase in muscle fiber stimulus threshold
Altered muscle resting membrane potential
Fatigue may inhibit Ca2+ release from SR

33. Fatigue and Its Causes: Central Nervous System
CNS undoubtedly plays role in fatigue but not fully understood yet
Fiber recruitment has conscious aspect
Stress of exhaustive exercise may be too much
Subconscious or conscious unwillingness to endure more pain
Discomfort of fatigue = warning sign
Elite athletes learn proper pacing, tolerate fatigue