Download Fatigue and Its Causes

Energy Expenditure
and Fatigue
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
Figure 5.1
Figure 5.2a
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
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 CO2 + 16 H2O + 129 ATP
– RER = VCO2/VO2 = 16 CO2/23 O2 = 0.70
• Predicts substrate use, kilocalories/O2
efficiency
Table 5.1
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
Figure 5.3
Figure 5.4
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
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
Figure 5.5
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
Figure 5.6
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
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
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)
Figure 5.7
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
Fatigue and Its Causes
• Fatigue: two definitions
– 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
Fatigue and Its Causes
• Complex phenomenon
– Type, intensity of exercise
– Muscle fiber type
– Training status, diet
• Four major causes (synergistic?)
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Inadequate energy delivery/metabolism
Accumulation of metabolic by-products
Failure of muscle contractile mechanism
Altered neural control of muscle contraction
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
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
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)
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
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–
–
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Muscle glycogen insufficient for prolonged exercise
Liver glycogen  glucose into blood
As muscle glycogen , liver glycogenolysis 
Muscle glycogen depletion + hypoglycemia = fatigue
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
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+
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
Fatigue and Its Causes:
Metabolic By-Products
• Lactic acid accumulates during brief, highintensity exercise
– If not cleared immediately, converts to lactate + H+
– H+ accumulation causes  muscle pH (acidosis)
• Buffers help muscle pH but not enough
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–
–
–
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
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)
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
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