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Chapter 5

Chapter 5. Energy Expenditure and Fatigue. Chapter 5 Overview. Measuring energy expenditure Energy expenditure at rest and during exercise Fatigue and its causes. Measuring Energy Expenditure: Direct Calorimetry. Substrate metabolism efficiency 40% of substrate energy  ATP

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Chapter 5

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  1. Chapter 5 • Energy Expenditure and Fatigue

  2. Chapter 5 Overview • Measuring energy expenditure • Energy expenditure at rest and during exercise • Fatigue and its causes

  3. 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

  4. Figure 5.1

  5. Measuring Energy Expenditure:Direct Calorimetry • Pros • Accurate over time • Good for resting metabolic measurements • Cons • Expensive, slow • Exercise equipment adds extra heat • Sweat creates errors in measurements • Not practical or accurate for exercise

  6. Measuring Energy Expenditure:Indirect Calorimetry • Estimates total body energy expenditure based on O2 used, CO2 produced • Measures respiratory gas concentrations • Only accurate for steady-state oxidative metabolism • Older methods of analysis accurate but slow • New methods faster but expensive

  7. Measuring Energy Expenditure:O2 and CO2 Measurements • VO2: volume of O2 consumed per minute • Rate of O2 consumption • Volume of inspired O2− volume of expired O2 • VCO2: volume of CO2 produced per minute • Rate of CO2 production • Volume of expired CO2− volume of inspired CO2

  8. Figure 5.2

  9. Measuring Energy Expenditure:Haldane Transformation • V̇ of inspired O2may not = V̇ of expired CO2 • V̇ of inspired N2 = V̇ of expired N2 • Haldane transformation • Allows V of inspired air (unknown) to be directly calculated from V of expired air (known) • Based on constancy of N2 volumes • VI = (VE x FEN2)/FIN2 • VO2 = (VE) x {[1-(FEO2 + FECO2) x (0.265)] − (FEO2)}

  10. 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

  11. 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

  12. Table 5.1

  13. Measuring Energy Expenditure:Indirect Calorimetry Limitations • CO2 production may not = CO2 exhalation • RER inaccurate for protein oxidation • RER near 1.0 may be inaccurate when lactate buildup  CO2 exhalation • Gluconeogenesis produces RER <0.70

  14. Measuring Energy Expenditure:Isotopic Measurements • Isotope: element with atypical atomic weight • Can be radioactive or nonradioactive • Can be traced throughout body • 13C, 2H (deuterium) common isotopes for studying energy metabolism • Easy, accurate, low-risk study of CO2 production • Ideal for long-term measurements (weeks)

  15. 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

  16. Energy Expenditure at Rest:Basal Metabolic Rate • Basal metabolic rate (BMR): rate of energy expenditure at rest • In supine position • Thermoneutral environment • After 8 h sleep and 12 h fasting • Minimum energy requirement for living • Related to fat-free mass (kcal  kg FFM-1 min-1) • Also affected by body surface area, age, stress, hormones, body temperature

  17. Resting Metabolic Rate andNormal Daily Metabolic Activity • Resting metabolic rate (RMR) • Similar to BMR (within 5-10% of BMR) but easier • Doesn’t require stringent standardized conditions • 1,200 to 2,400 kcal/day • Total daily metabolic activity • Includes normal daily activities • Normal range: 1,800 to 3,000 kcal/day • Competitive athletes: up to 10,000 kcal/day

  18. Energy Expenditure During Submaximal Aerobic Exercise • Metabolic rate increases with exercise intensity • Slow component of O2 uptake kinetics • At high power outputs, VO2 continues to increase • More type II (less efficient) fiber recruitment • VO2 drift • Upward drift observed even at low power outputs • Possibly due to ventilatory, hormone changes?

  19. Figure 5.3

  20. Energy Expenditure DuringMaximal Aerobic Exercise • VO2max (maximal O2 uptake) • Point at which O2 consumption doesn’t  with further  in intensity • Best single measurement of aerobic fitness • Not best predictor of endurance performance • Plateaus after 8 to 12 weeks of training • Performance continues to improve • More training allows athlete to compete at higher percentage of VO2max

  21. Figure 5.4

  22. 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

  23. Energy Expenditure During Maximal Anaerobic Exercise • No activity 100% aerobic or anaerobic • Estimates of anaerobic effort involve • Excess postexercise O2 consumption • Lactate threshold

  24. 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

  25. Figure 5.5

  26. 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

  27. Figure 5.6

  28. 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

  29. 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

  30. 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)

  31. Figure 5.7

  32. Energy Expenditure:Energy Cost of Various Activities • Varies with type and intensity of activity • Calculated from VO2, expressed in kilocalories/minute • Values ignore anaerobic aspects, EPOC • Daily expenditures depend on • Activity level (largest influence) • Inherent body factors (age, sex, size, weight, FFM)

  33. Table 5.2

  34. 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

  35. 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

  36. 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

  37. 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

  38. 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

  39. Figure 5.8

  40. 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)

  41. Figure 5.9

  42. 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

  43. Figure 5.10

  44. 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

  45. 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+

  46. 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

  47. Figure 5.11

  48. 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

  49. Figure 5.12

  50. 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)

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