Energy Expenditure and Fatigue. Measuring Energy Expenditure: Direct Calorimetry Substrate...

Energy Expenditure and Fatigue

Measuring Energy Expenditure:Measuring Energy Expenditure:Direct CalorimetryDirect 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 5.1Figure 5.1

Measuring Energy Expenditure:Measuring Energy Expenditure:Indirect CalorimetryIndirect 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

Measuring Energy Expenditure:Measuring Energy Expenditure:OO22 and CO and CO22 Measurements 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

Figure 5.2aFigure 5.2a

Measuring Energy Expenditure:Measuring Energy Expenditure:Respiratory Exchange RatioRespiratory 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:Measuring Energy Expenditure: Respiratory Exchange Ratio 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 5.1Table 5.1

Measuring Energy Expenditure:Measuring Energy Expenditure:Indirect Calorimetry LimitationsIndirect 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

Energy Expenditure at Rest and Energy Expenditure at Rest and During ExerciseDuring 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

Energy Expenditure at Rest:Energy Expenditure at Rest:Basal Metabolic RateBasal 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 andResting Metabolic Rate andNormal Daily Metabolic ActivityNormal Daily Metabolic Activity

Energy Expenditure DuringEnergy Expenditure DuringMaximal Aerobic ExerciseMaximal 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

Energy Expenditure DuringEnergy Expenditure DuringMaximal Aerobic ExerciseMaximal 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

Energy Expenditure During Energy Expenditure During Maximal Anaerobic ExerciseMaximal Anaerobic Exercise

• No activity 100% aerobic or anaerobic

• Estimates of anaerobic effort involve– Excess postexercise O2 consumption

– Lactate threshold

Anaerobic Energy Expenditure:Anaerobic Energy Expenditure:Postexercise OPostexercise O22 Consumption 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

Anaerobic Energy Expenditure:Anaerobic Energy Expenditure:Lactate ThresholdLactate 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

Anaerobic Energy Expenditure:Anaerobic Energy Expenditure:Lactate ThresholdLactate 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 CapacityMeasuring Anaerobic Capacity

• No clear, V�O2max-like method for measuring anaerobic capacity

• Imperfect but accepted methods– Maximal accumulated O2 deficit

– Wingate anaerobic test– Critical power test

Energy Expenditure During Exercise:Energy Expenditure During Exercise:Economy of EffortEconomy 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)

Energy Expenditure:Energy Expenditure:Energy Cost of Various ActivitiesEnergy 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)

Table 5.2Table 5.2

Energy Expenditure:Energy Expenditure:Successful Endurance AthletesSuccessful 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 CausesFatigue 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 CausesFatigue 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:Fatigue and Its Causes:Energy Systems—PCr DepletionEnergy 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:Fatigue and Its Causes:Energy Systems—Glycogen DepletionEnergy 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


• 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)


• 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


• 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:Fatigue and Its Causes:Metabolic By-ProductsMetabolic 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+


• 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


• 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:Fatigue and Its Causes:Lactic Acid Not All BadLactic 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:Fatigue and Its Causes:Neural TransmissionNeural 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:Fatigue and Its Causes:Central Nervous SystemCentral 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

Energy Expenditure and Fatigue. Measuring Energy Expenditure: Direct Calorimetry Substrate...

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