# Chapter 4 Energy Expenditure and Fatigue.

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chapter 4 Energy Expenditure and Fatigue

Learning Objectives Learn how exercise affects metabolism and how metabolism can be monitored to determine energy expenditure Discover the underlying causes and sites of fatigue in muscles

Measuring Energy Costs of Exercise
Direct calorimetry measures the body’s heat production to estimate energy expenditure Indirect calorimetry calculates energy expenditure from the ratio of CO2 produced to O2 consumed

A Direct Calorimeter for Human Use

Calculating Oxygen Consumption and Carbon Dioxide Production
. . Calculating VO2 and VCO2 requires: Volume of air inspired (VI) Volume of air expired (VE) Fraction of O2 in the inspired air (FIO2) Fraction of CO2 in the inspired air (FICO2) Fraction of O2 in the expired air (FEO2) Fraction of CO2 in the expired air (FECO2) . .

Calculating O2 Consumption and CO2 Production (L/min)
. . . VO2 = (VI x FIO2) – (VE x FEO2) VCO2 = (VE x FECO2) – (VI x FICO2) . . .

Equipment Used to Measure O2 Consumption and CO2 Production in the Lab and the Field
© Tom Roberts Photo courtesy of Cosmed Engineering

Haldane Transformation
3 gases make up inspired air: oxygen (FIO2 = 20.93%) carbon dioxide (FICO2 = 0.04%) nitrogen (FIN2 = 79.03%) Basic Haldane Transformation Equations VI x FIN2 = VE x FEN2 (2) VI = (VE x FEN2) / FIN2 (3) FEN2 = 1 – (FEO2 + FECO2) . . . .

Rewriting the Haldane Transformation Equations to Calculate VO2
. . . . VO2 = (VI x FIO2) – (VE x FEO2) Substitute into equation 2: VO2 = [(VE x FEN2) / (FIN2 x FIO2)] – [(VE) x (FEO2)] Substitute known values for FIO2 and FIN2: VO2 = [(VE x FEN2) / ( x )] – [(VE) x (FEO2)] . . . . . .

Rewriting the Haldane Transformation Equations to Calculate VO2 (continued)
. Substitute equation 3: VO2 = [(VE) x (1 – (FEO2 + FECO2)) x ( / )] – [(VE) x (FEO2)] Simplified: VO2 = (VE) x {[(1 – (FEO2 + FECO2)) x (0.265)] – (FEO2)} . . . . .

Haldane Transformation
Key Points Three gases make up inspired air The Haldane transformation allows us to calculate VI from VE because the nitrogen concentration is constant . .

Respiratory Exchange Ratio
. The ratio between CO2 released (VCO2) and oxygen consumed (VO2) RER = VCO2 / VO2 The RER value at rest is usually 0.78 to 0.80 . . .

RER: Determining Substrate Utilization
Carbohydrate 6 O2 + C6H12O6 → 6 CO2 + 6 H2O + 38 ATP RER = VCO2 / VO2 = 6 CO2 / 6 O2 = 1.0 Fat C16H32O O2 → 16 CO H2O ATP RER = VCO2 / VO2 = 16 CO2 / 23 O2 = 0.70 . . . .

Measurements of Energy Expenditure
Carbon 13 is infused and selectively traced to determine the isotopes’ distribution and movement Doubly labeled water is ingested, and the rate at which the substance leaves the body is monitored in urine, saliva, and blood and is used to calculate how much CO2 is produced, which can be converted into energy expenditure

Calorimetry Key Points
Direct calorimetry involves using a large chamber to directly measure heat production by the body Indirect calorimetry involves measuring O2 consumption and CO2 production RER at rest = 0.78 to 0.80 RER oxidation of fat = 0.70 RER oxidation of carbohydrate = 1.0 Isotopes can be used to determine metabolic rate over long periods of time

Resting Metabolic Rate (RMR)
RMR is the minimum amount of energy required by the body to sustain basic cellular function Fat-free mass Body surface area Ranges from 1,100 to 2,500 kcal/day When activity is added, daily caloric expenditure is 1,700 to 3,100 kcal/day

Factors That Affect RMR
Age: RMR gradually decreases with age, generally because of a decrease in fat-free mass Body temperature: RMR increases with increasing temperature Psychological stress: Stress increases activity of the sympathetic nervous system Hormones: Thyroxine from the thyroid gland and epinephrine from the adrenal medulla both increase RMR

Metabolic Rate During Submaximal Exercise
Metabolism increases in direct proportion to the increase in exercise intensity During exercise at a constant power output (work rate) VO2 increases from its resting value to a steady-state value within 1-2 minutes There is a linear increase in the VO2 with increases in power output (work rate) . .

Increase in Oxygen Uptake with Increasing Power Output
Reprinted, by permission, from G.A. Gaesser and D.C. Poole, 1996, “The slow component of oxygen uptake kinetics in humans,” Exercise and Sport Sciences Reviews 24: 36.

Increase in Oxygen Uptake with Increasing Power Output
Reprinted, by permission, from G.A. Gaesser and D.C. Poole, 1996, “The slow component of oxygen uptake kinetics in humans,” Exercise and Sport Sciences Reviews 24: 36.

Maximal Oxygen Uptake . VO2max: The maximal capacity for oxygen consumption by the body during maximal exertion Single best measurement of cardiorespiratory endurance and aerobic fitness Increases with physical training Generally expressed relative to body weight (ml · kg-1 · min-1) Normally active untrained college-aged students = ml · kg-1 · min-1 VO2max declines in active people after age by ~ 1% per year .

Relationship Between Exercise Intensity and Oxygen Uptake in Trained and Untrained Man

Estimating Anaerobic Effort
O2 consumption requires several minutes to reach the required steady state level at which the aerobic processes are fully functional Oxygen deficit is calculated as the difference between the oxygen required for a given exercise intensity and the actual oxygen consumption Anaerobic effort can be estimated by examining excess postexercise oxygen consumption (EPOC)—the mismatch between O2 consumption and energy requirements

Oxygen Requirement During Exercise and Recovery

Factors Responsible for EPOC
Rebuilding depleted ATP and PCr supplies Clearing lactate produced by anaerobic metabolism Replenishing O2 supplies borrowed from hemoglobin and myoglobin Removing CO2 that has accumulated in body tissues Increased metabolic and respiratory rates due to increased body temperature

Lactate Threshold It is the point at which blood lactate begins to accumulate substantially above resting concentrations during exercise of increasing intensity The rate at which lactate production exceeds lactate clearance Usually expressed as a percentage of maximal oxygen uptake A high lactate threshold can indicate potential for better endurance performance Lactate accumulation contributes to fatigue

Relationship Between Exercise Intensity and Blood Lactate Concentration

Lactate Threshold and Endurance Performance
Lactate threshold (LT), when expressed as a percentage of VO2max, is one of the best determinants of an athlete’s pace in endurance events such as running and cycling. While untrained people typically have LT around 50% to 60% of their VO2max, elite athletes may not reach LT until around 70% to 80% VO2max. . . .

Economy of Effort

Measuring Energy Use During Exercise
Key Points Excess postexercise oxygen consumption (EPOC) is the metabolic rate above resting levels after exercise Lactate threshold is the point at which blood lactate production begins to exceed the body’s ability to clear or remove lactate Individuals with higher lactate thresholds, expressed as a percentage of VO2max, are capable of the best endurance performance Aerobic endurance performance capacity is also associated with a high economy of effort .

Fatigue and its Causes Energy delivery (ATP-PCr, anaerobic glycolysis, and oxidation) Accumulation of metabolic by-products, such as lactate and H+ Failure of the muscle fiber’s contractile mechanism Alteration in the nervous system

Energy Systems and Fatigue
PCr depletion Glycogen depletion (“hitting the wall”) Pattern of glycogen depletion from Type I and II fibers depends on the duration and intensity of the activity Glycogen depletion is selective to the muscle groups involved in the activity Depletion of liver glycogen to increase blood glucose increases muscle glycogen utilization

Decline in Muscle Glycogen
Adapted, by permission, from D.L. Costill, 1986, Inside running: Basics of sports physiology (Indianapolis: Benchmark Press). Copyright 1986 Cooper Publishing Group, Carmel, IN.

Glycogen Depletion in Different Muscle Fibers

Glycogen Depletion in Different Muscle Groups

High Muscle Temperature Impairs Skeletal Muscle Function and Metabolism
Adapted, by permission, from S.D.R. Galloway and R.J. Maughan, 1997, "Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man," Medicine and Science in Sports and Exercise 29:

Metabolic By-Products and Fatigue
Short-duration activities depend on anaerobic glycolysis and produce lactate and H+ Cells buffer H+ with bicarbonate (HCO3–) to keep cell pH between 6.4 (at exhaustion) and 7.1 Intercellular pH lower than 6.9, however, slows glycolysis and ATP production When pH reaches 6.4, H+ levels inhibit glycolysis and result in exhaustion

Changes in Muscle pH During Sprint Exercise and Recovery

Neuromuscular Fatigue
Fatigue may involve: Decreased release or synthesis of acetylcholine Hyperactive acetylcholinesterase Hypoactive acetylcholinesterase Increased threshold for stimulation of the muscle fiber Competition with ACh for the receptors on the muscle membrane Potassium may leave the intracellular space, decreasing the membrane potential below resting values Central nervous system fatigue

Causes of Fatigue Key Points
Fatigue may result from depletion of PCr or glycogen, which impairs ATP production The H+ generated by lactic acid leads to fatigue by decreasing muscle pH, which impairs the cellular processes of energy production and muscle contraction Failure of neural transmission may cause some fatigue The central nervous system may also limit exercise performance as a protective mechanism

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