Energy Expenditure and Fatigue 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 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 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

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, 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

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