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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?) – – – – 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 – – – – 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 – – – – 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