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Chapter 5

Chapter 5. 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

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Chapter 5

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  1. 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 CO2 + 16 H2O + 129 ATP • RER = VCO2/VO2 = 16 CO2/23 O2 = 0.70 • Predicts substrate use, kilocalories/O2efficiency

  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 DuringMaximal 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 • 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

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