4 Energy Expenditure and Fatigue chapter. Learning Objectives Learn how exercise affects metabolism and how metabolism can be monitored to determine energy.

4

Energy Expenditure and Fatigue

chapter

Learning Objectives

bull Learn how exercise affects metabolism and how metabolism can be monitored to determine energy expenditure

bull Discover the underlying causes and sites of fatigue in muscles

Respiratory Exchange Ratio

bull The ratio between CO2 released (VCO2) and oxygen consumed (VO2)

bull RER = VCO2 VO2

bull The RER value at rest is usually 078 to 080

RER Determining Substrate Utilization

Carbohydrate

6 O2 + C6H12O6 rarr 6 CO2 + 6 H2O + 38 ATP

RER = VCO2 VO2 = 6 CO2 6 O2 = 10

Fat

C16H32O2 + 23 O2 rarr 16 CO2 + 16 H2O + 129 ATP

RER = VCO2 VO2 = 16 CO2 23 O2 = 070

Resting Metabolic Rate (RMR)

RMR is the minimum amount of energy required bythe body to sustain basic cellular function

ndash Fat-free massndash Body surface areandash Ranges from 1100 to 2500 kcaldayndash When activity is added daily caloric expenditure is

1700 to 3100 kcalday

Factors That Affect RMR

bull Age RMR gradually decreases with age generally because of a decrease in fat-free mass

bull Body temperature RMR increases with increasing temperature

bull Psychological stress Stress increases activity of the sympathetic nervous system

bull Hormones Thyroxine from the thyroid gland and epinephrine from the adrenal medulla both increase RMR

Metabolic Rate During Submaximal Exercise

bull Metabolism increases in direct proportion to the increase in exercise intensity

bull During exercise at a constant power output (work rate) VO2 increases from its resting value to a steady-state value within 1-2 minutes

bull 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 GA Gaesser and DC Poole 1996 ldquoThe slow component of oxygen uptake kinetics in humansrdquo Exercise and Sport Sciences Reviews 24 36

Increase in Oxygen Uptake with Increasing Power Output

Reprinted by permission from GA Gaesser and DC Poole 1996 ldquoThe slow component of oxygen uptake kinetics in humansrdquo Exercise and Sport Sciences Reviews 24 36

Maximal Oxygen Uptake

bull VO2max The maximal capacity for oxygen consumption by the body during maximal exertion

bull Single best measurement of cardiorespiratory endurance and aerobic fitness (800 or 1600m)

bull Increases with physical trainingbull Generally expressed relative to body weight

(ml kg-1 min-1)bull Normally active untrained college-aged students =

38-42 ml kg-1 min-1

bull VO2max declines in active people after age 25-30 by ~ 1 per year

Relationship Between Exercise Intensity and Oxygen Uptake in Trained and

Untrained Man

Estimating Anaerobic Effort

bull Oxygen deficit 氧債 is calculated as the difference between the oxygen required for a given exercise intensity and the actual oxygen consumption

bull Anaerobic effort can be estimated by examining excess postexercise oxygen consumption (EPOC)mdashthe mismatch between O2 consumption and energy requirements

Oxygen Requirement During Exercise and Recovery

Factors Responsible for EPOC

bull Rebuilding depleted ATP and PCr suppliesbull Clearing lactate produced by anaerobic metabolism

bull Replenishing O2 supplies borrowed from hemoglobin and myoglobin

bull Removing CO2 that has accumulated in body tissues

bull Increased metabolic and respiratory rates due to increased body temperature

Lactate Threshold

bull It is the point at which blood lactate begins to accumulate substantially above resting concentrations during exercise of increasing intensity

bull The rate at which lactate production exceeds lactate clearance

bull Usually expressed as a percentage of maximal oxygen uptake

bull A high lactate threshold can indicate potential for better endurance performance

bull 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 athletersquos 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 Pointsbull Excess postexercise oxygen consumption (EPOC)

is the metabolic rate above resting levels after exercise

bull Lactate threshold is the point at which blood lactate production begins to exceed the bodyrsquos ability to clear or remove lactate

bull Individuals with higher lactate thresholds expressed as a percentage of VO2max are capable of the best endurance performance

bull Aerobic endurance performance capacity is also associated with a high economy of effort

Fatigue and its Causes

bull Energy delivery (ATP-PCr anaerobic glycolysis and oxidation)

bull Accumulation of metabolic by-products such as lactate and H+

bull Failure of the muscle fiberrsquos contractile mechanismbull Alteration in the nervous system

Energy Systems and Fatigue

bull PCr depletionbull Glycogen depletion (ldquohitting the wallrdquo)

ndash Pattern of glycogen depletion from Type I and II fibers depends on the duration and intensity of the activity

ndash Glycogen depletion is selective to the muscle groups involved in the activity

ndash Depletion of liver glycogen to increase blood glucose increases muscle glycogen utilization

Decline in Muscle Glycogen

Adapted by permission from DL 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 SDR Galloway and RJ Maughan 1997 Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man Medicine and Science in Sports and Exercise 29 1240-1249

Metabolic By-Products and Fatigue

bull Short-duration activities depend on anaerobic glycolysis and produce lactate and H+

bull Cells buffer H+ with bicarbonate (HCO3ndash) to keep cell pH

between 64 (at exhaustion) and 71bull Intercellular PH lower than 69 however slows

glycolysis and ATP productionbull When pH reaches 64 H+ levels inhibit glycolysis and

result in exhaustion

Changes in Muscle pH During Sprint Exercise and Recovery

Neuromuscular Fatigue

Fatigue may involve1 Decreased release or synthesis of acetylcholine2 Hyperactive acetylcholinesterase(break down ACH)

3 Hypoactive acetylcholinesterase(accumulate excessive ACH unable relax)

4 Increased threshold for stimulation of the muscle fiber5 Competition with ACh for the receptors on the muscle

membrane6 Potassium may leave the intracellular space

decreasing the membrane potential below resting values( ＜ -70mv)

7 Central nervous system fatigue

Causes of Fatigue

Key Pointsbull Fatigue may result from depletion of PCr or

glycogen which impairs ATP productionbull The H+ generated by lactic acid leads to fatigue by

decreasing muscle pH which impairs the cellular processes of energy production and muscle contraction

bull Failure of neural transmission may cause some fatigue

bull The central nervous system may also limit exercise performance as a protective mechanism