1. 4 Energy Expenditure and Fatigue chapter

2. 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

3. Respiratory Exchange Ratio The ratio between CO 2 released (VCO 2 ) and oxygen consumed (VO 2 ) RER = VCO 2 / VO 2 The RER value at rest is usually 0.78 to 0.80....

4. RER: Determining Substrate Utilization Carbohydrate 6 O 2 + C 6 H 12 O 6 → 6 CO 2 + 6 H 2 O + 38 ATP RER = VCO 2 / VO 2 = 6 CO 2 / 6 O 2 = 1.0 Fat C 16 H 32 O 2 + 23 O 2 → 16 CO 2 + 16 H 2 O + 129 ATP RER = VCO 2 / VO 2 = 16 CO 2 / 23 O 2 = 0.70....

6. 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

7. 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

8. 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) VO 2 increases from its resting value to a steady-state value within 1-2 minutes There is a linear increase in the VO 2 with increases in power output (work rate)..

9. 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.

10. 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.

11. Maximal Oxygen Uptake VO 2max : The maximal capacity for oxygen consumption by the body during maximal exertion Single best measurement of cardiorespiratory endurance and aerobic fitness (800 or 1600m) Increases with physical training Generally expressed relative to body weight (ml · kg -1 · min -1 ) Normally active untrained college-aged students = 38-42 ml · kg -1 · min -1 VO 2max declines in active people after age 25-30 by ~ 1% per year..

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

13. Estimating Anaerobic Effort Oxygen deficit 氧債 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 O 2 consumption and energy requirements

14. Oxygen Requirement During Exercise and Recovery

15. Factors Responsible for EPOC Rebuilding depleted ATP and PCr supplies Clearing lactate produced by anaerobic metabolism Replenishing O 2 supplies borrowed from hemoglobin and myoglobin Removing CO 2 that has accumulated in body tissues Increased metabolic and respiratory rates due to increased body temperature

16. 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

17. Relationship Between Exercise Intensity and Blood Lactate Concentration

18. Lactate Threshold and Endurance Performance Lactate threshold (LT), when expressed as a percentage of VO 2max, 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 VO 2max, elite athletes may not reach LT until around 70% to 80% VO 2max....

19. Economy of Effort

21. 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 VO 2max, are capable of the best endurance performance Aerobic endurance performance capacity is also associated with a high economy of effort.

22. 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

23. 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

24. 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.

25. Glycogen Depletion in Different Muscle Fibers

26. Glycogen Depletion in Different Muscle Groups

27. 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: 1240-1249.

28. Metabolic By-Products and Fatigue Short-duration activities depend on anaerobic glycolysis and produce lactate and H + Cells buffer H + with bicarbonate (HCO 3 – ) 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

29. Changes in Muscle pH During Sprint Exercise and Recovery

30. Neuromuscular Fatigue Fatigue may involve: 1. Decreased release or synthesis of acetylcholine 2. Hyperactive acetylcholinesterase (break down ACH) 3. Hypoactive acetylcholinesterase (accumulate excessive ACH unable relax) 4. Increased threshold for stimulation of the muscle fiber 5.Competition with ACh for the receptors on the muscle membrane 6.Potassium may leave the intracellular space, decreasing the membrane potential below resting values ( ＜ -70mv) 7.Central nervous system fatigue

31. 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