BioenergeticsMeasurement of Work, Power,

and Energy Expenditure

Chapter 3, 6

Bioenergetics• Muscle only has limited stores of ATP• Formation of ATP

– Phosphocreatine (PC) 磷酸肌酸 breakdown– Degradation of glucose and glycogen (glycolysis)– Oxidative formation of ATP

• Anaerobic pathways 無氧代謝– Do not involve O2

– PC breakdown and glycolysis• Aerobic pathways 有氧代謝

– Require O2

– Oxidative phosphorylation

Anaerobic ATP Production

• ATP-PC system– Immediate source of ATP

– Onset of exercise, short-term high-intensity (<5 s)• Glycolysis 醣解作用

– Energy investment phase• Requires 2 ATP

– Energy generation phase• Produces ATP, NADH (carrier molecule), and pyruvate 丙酮酸

or lactate 乳酸

PC + ADP ATP + CCreatine kinase

The Two Phases of Glycolysis

Glycolysis: Energy Investment Phase

Glycolysis: Energy Generation Phase

Oxidation-Reduction Reactions• Oxidation

– Molecule accepts electrons (along with H+)

• Reduction– Molecule donates electrons

• Nicotinomide adenine dinucleotide (NAD)

• Flavin adenine dinucleotide (FAD)

FAD + 2H+ FADH2

NAD + 2H+ NADH + H+

Production of Lactic Acid (lactate)

• Normally, O2 is available in the mitochondria to accept H+ (and electrons) from NADH produced in glycolysis– In anaerobic pathways, O2 is not available

• H+ and electrons from NADH are accepted by pyruvic acid (pyruvate) to form lactic acid

Conversion of Pyruvic Acid to Lactic Acid

• Recycling of NAD (NADH NAD)• So that glycolysis can continue• LDH: lactate dehydrogenase 乳酸去氫脢

Aerobic ATP Production• Krebs cycle 克氏循環 (citric acid cycle, TCA

cycle, tricarboxylic acid cycle)– Completes the oxidation of substrates and

produces NADH and FADH to enter the electron transport chain

– O2 not involved• Electron transport chain

– Oxidative phosphorylation– Electrons removed from NADH/FADH are passed

along a series of carriers to produce ATP– H+ from NADH/FADH: accepted by O2 to form

water

The Three Stages of Oxidative Phosphorylation

The Krebs Cycle

Relationship Between the Metabolism of Proteins, Fats, and Carbohydrates

Bioenergetics of fats

• Triglycerides 三酸甘油酯– Glycerol + 3 fatty acids– Fatty acids converted to acetyl-CoA ( 乙輔酶 A)

through beta-oxidation– Glycerol can be converted to glycolysis

intermediates (phosphoglyceraldehyde) in liver, but only limited in muscle

– Glycerol is NOT an important direct muscle energy source during exercise

Formation of ATP in the Electron Transport Chain

The Chemiosmotic Hypothesis of ATP Formation

• Electron transport chain results in pumping of H+ ions across inner mitochondrial membrane– Results in H+ gradient across membrane

• Energy released to form ATP as H+ diffuse back across the membrane

• O2 accept H+ to form water• O2 is essential in this process

The Chemiosmotic Hypothesis of ATP Formation

Metabolic Process High-Energy Products

ATP from Oxidative Phosphorylation

ATP Subtotal

Glycolysis 2 ATP 2 NADH

— 6

2 (if anaerobic) 8 (if aerobic)

Pyruvic acid to acetyl-CoA

2 NADH 6 14

Krebs cycle 2 GTP 6 NADH 2 FADH

— 18 4

16 34 38

Grand Total

38

Aerobic ATP Tally

Efficiency of Oxidative Phosphorylation

• Aerobic metabolism of one molecule of glucose– Yields 38 ATP

• Aerobic metabolism of one molecule of glycogen– Yields 39 ATP

• Overall efficiency of aerobic respiration is 40%– 60% of energy released as heat

Control of Bioenergetics• Rate-limiting enzymes

– An enzyme that regulates the rate of a metabolic pathway

• Levels of ATP and ADP+Pi

– High levels of ATP inhibit ATP production– Low levels of ATP and high levels of ADP+Pi

stimulate ATP production

• Calcium may stimulate aerobic ATP production

Action of Rate-Limiting Enzymes

Control of Metabolic Pathways

Pathway Rate-Limiting Enzyme

Stimulators Inhibitors

ATP-PC system

Creatine kinase ADP ATP

Glycolysis Phosphofructokinase

AMP, ADP, Pi, pH

ATP, CP, citrate, pH

Krebs cycle Isocitrate dehydrogenase

ADP, Ca++, NAD

ATP, NADH

Electron transport chain

Cytochrome Oxidase

ADP, Pi ATP

Interaction Between Aerobic and Anaerobic ATP Production

• Energy to perform exercise comes from an interaction between aerobic and anaerobic pathways– 水龍頭 不是電燈開關

• Effect of duration and intensity– Short-term, high-intensity activities

• Greater contribution of anaerobic energy systems

– Long-term, low to moderate-intensity exercise• Majority of ATP produced from aerobic sources

Units of Measure 單位

• Metric system– Used to express mass, length, and volume– Mass: gram (g)– Length: meter (m)– Volume: liter (L)

• System International (SI) units– Standardized terms for measurement of:

• Energy: joule (J) 能量 : 焦耳• Force: Newton (N) 力 : 牛頓• Work: joule (J) 做功 : 焦耳• Power: watt (W) 功率 : 瓦特

Work and Power Defined

Work 功 作功

• Lifting a 5 kg weight up a distance of 2 mWork = force x distanceWork = 5 kg x 2 mWork = 10 kgm1 kgm = 9.8 joule1 joule = 0.24 calorie 卡

( 不是 Kcal 大卡 , 千卡 )

Power 功率

• Performing 2,000 kgm of work in 60 secondsPower = work timePower = 2,000 kgm 60 s Power = 33.3 kgm•s-1

1 kgm/s = 9.8 watt

Work = force x distance Power = work time

Measurement of Work and Power• Ergometry: measurement of work output• Ergometer 測功儀 : apparatus or device used

to measure a specific type of work

Measurement of Work and Power

• Bench step– Work = body weight (kg) x distance•step-1 x steps•min-1 x

minutes– Power = work minutes

• Cycle ergometer– Work = resistance (kg) x rev•min-1 x flywheel diameter (m)

x minutes– Power = work minutes

• Treadmill– Work = body weight (kg) x speed (m•min-1) x grade x

minutes– Power = work minutes

Determination of Percent Grade on a Treadmill

Measurement of Energy Expenditure

• Direct calorimetry– Measurement of heat production as an indication

of metabolic rate

• Indirect calorimetry– Measurement of oxygen consumption as an

estimate of resting metabolic rate

Foodstuff + O2 ATP + Heat Cell work Heat

Foodstuff + O2 Heat + CO2 + H2O

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Direct calorimetry chamber

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Indirect calorimetryClosed circuit method

Indirect calorimetryOpen-Circuit Spirometry

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Douglas bags for gas analysis

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Breath-by-breath gas analyzer

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Estimation of Energy Expenditure• Energy cost of horizontal treadmill walking or

running– O2 requirement increases as a linear function of

speed

• Expression of energy cost in METs– 1 MET = energy cost at rest, metabolic equivalent– 1 MET = 3.5 ml•kg-1•min-1

Linear Relationship Between VO2 and Walking or Running Speed

Calculation of Exercise Efficiency• Net efficiency

• Net efficiency of cycle ergometry– 15-27%

% net efficiency = x 100Energy expended above rest

Work output

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Upper limits of energy expenditure

• Well-trained athletes can expend ~1000 kcal/h for prolonged periods of time

• Up to 9000 kcal/d in Tour de France• More than 10,000 kcal/d in extreme long-

distance running• Energy requirements can be met by most

athletes, if well-planned (e.g. 20% CHO solution during exercise)

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Factors That Influence Exercise Efficiency

• Exercise work rate– Efficiency decreases as work rate increases– Energy expenditure increase out of proportion to the

work rate• Speed of movement

– There is an optimum speed of movement and any deviation reduces efficiency

– Optimum speed at power output– Low speed: inertia, repeated stop and start– High speed: friction

• Fiber composition of muscles– Higher efficiency in muscles with greater percentage of

slow fibers

Net Efficiency During Arm Crank Ergometery

Relationship Between Energy Expenditure and Work Rate

Force-velocity relationshippower output-velocity relationship

Effect of Speed of Movement of Net Efficiency

Running Economy

• Not possible to calculate net efficiency of horizontal running

• Running Economy– Oxygen cost of running at given speed– Lower VO2 (ml•kg-1•min-1) indicates better running

economy

• Gender difference in running economy– No difference at slow speeds– At “race pace” speeds, males may be more economical

that females

Comparison of Running Economy Between Males and Females

Estimate O2 requirement of treadmill running

• Horizontal:• VO2 (ml/kg/min) = 0.2 ml/kg/min/m/min x

speed (m/min)• Vertical:• VO2 (ml/kg/min) = 0.9 ml/kg/m/min x

vertical velocity (m/min)• = 0.9 ml/kg/m/min x speed (m/min) x grade

(%)• Total VO2 (ml/kg/min) = horizontal +

vertical + rest (3.5 ml/kg/min)

Estimate energy consumption according to O2 requirement

• ml/kg/min x kg x min• 1 L O2 consumed = 5 kcal

Example

• 50 kg, 30 min• Speed: 12 km/hr, grade 1%• Speed: 200 m/min• H: 0.2 x 200 = 40• V: 0.9 x 200 x 0.01 = 1.8• Total: 40 + 1.8 + 3.5 = 45.3 ml/kg/min• Total O2: 45.3 x 50 x 30/1000 = ? L O2• Total energy: ? X 5 = Kcal