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

Measuring Energy Expenditure: Indirect Calorimetry Estimates total body energy expenditure based on O 2 used, CO 2 produced –Measures respiratory gas concentrations –Only accurate for steady-state oxidative metabolism Older methods of analysis accurate but slow New methods faster but expensive

Measuring Energy Expenditure: O 2 and CO 2 Measurements VO 2 : volume of O 2 consumed per minute –Rate of O 2 consumption –Volume of inspired O 2 − volume of expired O 2 VCO 2 : volume of CO 2 produced per minute –Rate of CO 2 production –Volume of expired CO 2 − volume of inspired CO 2

Figure 5.2a

Measuring Energy Expenditure: Respiratory Exchange Ratio O 2 usage during metabolism depends on type of fuel being oxidized –More carbon atoms in molecule = more O 2 needed –Glucose (C 6 H 12 O 6 ) < palmitic acid (C 16 H 32 O 2 ) Respiratory exchange ratio (RER) –Ratio between rates of CO 2 production, O 2 usage –RER = VCO 2 /VO 2

Measuring Energy Expenditure: Respiratory Exchange Ratio RER for 1 molecule glucose = 1.0 –6 O 2 + C 6 H 12 O 6  6 CO H 2 O + 32 ATP –RER = VCO 2 /VO 2 = 6 CO 2 /6 O 2 = 1.0 RER for 1 molecule palmitic acid = 0.70 –23 O 2 + C 16 H 32 O 2  16 CO H 2 O ATP –RER = VCO 2 /VO 2 = 16 CO 2 /23 O 2 = 0.70 Predicts substrate use, kilocalories/O 2 efficiency

Table 5.1

Measuring Energy Expenditure: Indirect Calorimetry Limitations CO 2 production may not = CO 2 exhalation RER inaccurate for protein oxidation RER near 1.0 may be inaccurate when lactate buildup  CO 2 exhalation Gluconeogenesis produces RER <0.70

Energy Expenditure at Rest and During Exercise Metabolic rate: rate of energy use by body Based on whole-body O 2 consumption and corresponding caloric equivalent –At rest, RER ~0.80, VO 2 ~0.3 L/min –At rest, metabolic rate ~2,000 kcal/day

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

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 Resting Metabolic Rate and Normal Daily Metabolic Activity

Figure 5.3

Energy Expenditure During Maximal Aerobic Exercise VO 2max (maximal O 2 uptake) –Point at which O 2 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 VO 2max

Figure 5.4

Energy Expenditure During Maximal Aerobic Exercise VO 2max expressed in L/min –Easy standard units –Suitable for non-weight-bearing activities VO 2max normalized for body weight –ml O 2  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

Energy Expenditure During Maximal Anaerobic Exercise No activity 100% aerobic or anaerobic Estimates of anaerobic effort involve –Excess postexercise O 2 consumption –Lactate threshold

Anaerobic Energy Expenditure: Postexercise O 2 Consumption O 2 demand > O 2 consumed in early exercise –Body incurs O 2 deficit –O 2 required − O 2 consumed –Occurs when anaerobic pathways used for ATP production O 2 consumed > O 2 demand in early recovery –Excess postexercise O 2 consumption (EPOC) –Replenishes ATP/PCr stores, converts lactate to glycogen, replenishes hemo/myoglobin, clears CO 2

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 VO 2max

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 VO 2max, higher lactate threshold predicts better performance

Measuring Anaerobic Capacity No clear, V ̇ O 2max -like method for measuring anaerobic capacity Imperfect but accepted methods –Maximal accumulated O 2 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 VO 2max –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: Energy Cost of Various Activities Varies with type and intensity of activity Calculated from VO 2, 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)

Table 5.2

Energy Expenditure: Successful Endurance Athletes 1. High VO 2max 2. High lactate threshold (as % VO 2max ) 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 P i 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

Figure 5.8

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 P i : 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 Ca 2+ 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