1. Scott K. Powers Edward T. Howley Theory and Application to Fitness and Performance SEVENTH EDITION Chapter Copyright ©2009 The McGraw-Hill Companies, Inc. Permission required for reproduction or display outside of classroom use. Exercise Metabolism

2. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Objectives 1.Discuss the relationship between exercise intensity/duration and the bioenergetic pathways that are most responsible for the production of ATP during various types of exercise. 2.Define the term oxygen deficit. 3.Define the term lactate threshold. 4.Discuss several possible mechanisms for the sudden rise in blood-lactate concentration during incremental exercise.

3. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Objectives 5.List the factors that regulate fuel selection during different types of exercise. 6.Explain why fat metabolism is dependent on carbohydrate metabolism. 7.Define the term oxygen debt. 8.Give the physiological explanation for the observation that the O 2 debt is greater following intense exercise when compared to the O 2 debt following light exercise.

4. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Outline  Energy Requirements at Rest  Rest-to-Exercise Transitions  Recovery from Exercise: Metabolic Responses  Metabolic Responses to Exercise: Influence of Duration and Intensity Short-Term, Intense Exercise Prolonged Exercise Incremental Exercise  Estimation of Fuel Utilization During Exercise  Factors Governing Fuel Selection Exercise Intensity and Fuel Selection Exercise Duration and Fuel Selection Interaction of Fat/ Carbohydrate Metabolism Body Fuel Sources

5. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Energy Requirements at Rest Almost 100% of ATP produced by aerobic metabolism Blood lactate levels are low (<1.0 mmol/L) Resting O 2 consumption: –0.25 L/min –3.5 ml/kg/min Energy Requirements at Rest

6. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Rest-to-Exercise Transitions ATP production increases immediately Oxygen uptake increases rapidly –Reaches steady state within 1–4 minutes –After steady state is reached, ATP requirement is met through aerobic ATP production Initial ATP production through anaerobic pathways –ATP-PC system –Glycolysis Oxygen deficit –Lag in oxygen uptake at the beginning of exercise

7. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Figure 4.1 The Oxygen Deficit Rest-to-Exercise Transitions

8. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Comparison of Trained and Untrained Subjects Trained subjects have a lower oxygen deficit –Better-developed aerobic bioenergetic capacity –Due to cardiovascular or muscular adaptations Results in less production of lactic acid Rest-to-Exercise Transitions

9. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Differences in VO 2 Between Trained and Untrained Subjects Figure 4.2 Rest-to-Exercise Transitions

10. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. In Summary  In the transition from rest to light or moderate exercise, oxygen uptake increases rapidly, generally reaching a steady state within one to four minutes.  The term oxygen deficit applies to the lag in oxygen uptake in the beginning of exercise.  The failure of oxygen uptake to increase instantly at the beginning of exercise suggests that anaerobic pathways contribute to the overall production on ATP early in exercise. After a steady state is reached, the body’s ATP requirement is met via aerobic metabolism. Rest-to-Exercise Transitions

11. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Recovery From Exercise: Metabolic Responses Recovery From Exercise Oxygen uptake remains elevated above rest into recovery Oxygen debt –Term used by A.V. Hill Repayment for O 2 deficit at onset of exercise Excess post-exercise oxygen consumption (EPOC) –Terminology reflects that only ~20% elevated O 2 consumption used to “repay” O 2 deficit Many scientists use these terms interchangeably

12. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Recovery From Exercise: Metabolic Responses Oxygen Debt “Rapid” portion of O 2 debt –Resynthesis of stored PC –Replenishing muscle and blood O 2 stores “Slow” portion of O 2 debt –Elevated heart rate and breathing =  energy need –Elevated body temperature =  metabolic rate –Elevated epinephrine and norepinephrine =  metabolic rate –Conversion of lactic acid to glucose (gluconeogenesis)

13. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Recovery From Exercise: Metabolic Responses EPOC is Greater Following Higher Intensity Exercise Higher body temperature Greater depletion of PC Greater blood concentrations of lactic acid Higher levels of blood epinephrine and norepinephrine

14. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Oxygen Deficit and Debt During Light/Moderate and Heavy Exercise Recovery From Exercise: Metabolic Responses Figure 4.3

15. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. A Closer Look 4.1 Removal of Lactic Acid Following Exercise Classical theory –Majority of lactic acid converted to glucose in liver Recent evidence –70% of lactic acid is oxidized Used as a substrate by heart and skeletal muscle –20% converted to glucose –10% converted to amino acids Lactic acid is removed more rapidly with light exercise in recovery –Optimal intensity is ~30–40% VO 2 max Recovery From Exercise: Metabolic Responses

16. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Blood Lactate Removal Following Strenuous Exercise Recovery From Exercise: Metabolic Responses Figure 4.4

17. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Contributing to EPOC Figure 4.5 Recovery From Exercise: Metabolic Responses

18. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Metabolic Responses to Exercise: Influence of Duration and Intensity Metabolic Responses to Short- Term, Intense Exercise First 1–5 seconds of exercise –ATP through ATP-PC system Intense exercise longer than 5 seconds –Shift to ATP production via glycolysis Events lasting longer than 45 seconds –ATP production through ATP-PC, glycolysis, and aerobic systems –70% anaerobic/30% aerobic at 60 seconds –50% anaerobic/50% aerobic at 2 minutes

19. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. In Summary  During high-intensity, short-term exercise (i.e., two to twenty seconds), the muscle’s ATP production is dominated by the ATP-PC system.  Intense exercise lasting more than twenty seconds relies more on anaerobic glycolysis to produce much of the needed ATP.  Finally, high-intensity events lasting longer than forty-five seconds use a combination of the ATP-PC system, glycolysis, and the aerobic system to produce the needed ATP for muscular contraction. Metabolic Responses to Exercise: Influence of Duration and Intensity

20. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Metabolic Responses to Prolonged Exercise Prolonged exercise (>10 minutes) –ATP production primarily from aerobic metabolism –Steady-state oxygen uptake can generally be maintained during submaximal exercise Prolonged exercise in a hot/humid environment or at high intensity –Upward drift in oxygen uptake over time –Due to body temperature and rising epinephrine and norepinephrine Metabolic Responses to Exercise: Influence of Duration and Intensity

21. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Upward Drift in Oxygen Uptake During Prolonged Exercise Metabolic Responses to Exercise: Influence of Duration and Intensity Figure 4.6

22. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Metabolic Responses to Exercise: Influence of Duration and Intensity Metabolic Responses to Incremental Exercise Oxygen uptake increases linearly until maximal oxygen uptake (VO 2 max) is reached –No further increase in VO 2 with increasing work rate VO 2 max –“Physiological ceiling” for delivery of O 2 to muscle –Affected by genetics and training Physiological factors influencing VO 2 max –Maximum ability of cardiorespiratory system to deliver oxygen to the muscle –Ability of muscles to use oxygen and produce ATP aerobically

23. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Changes in Oxygen Uptake During Incremental Exercise Metabolic Responses to Exercise: Influence of Duration and Intensity Figure 4.7

24. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Metabolic Responses to Exercise: Influence of Duration and Intensity Lactate Threshold The point at which blood lactic acid rises systematically during incremental exercise –Appears at ~50–60% VO 2 max in untrained subjects –At higher work rates (65–80% VO 2 max) in trained subjects Also called: –Anaerobic threshold –Onset of blood lactate accumulation (OBLA) Blood lactate levels reach 4 mmol/L

25. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Changes in Blood Lactate Concentration During Incremental Exercise Metabolic Responses to Exercise: Influence of Duration and Intensity Figure 4.8

26. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Metabolic Responses to Exercise: Influence of Duration and Intensity Explanations for the Lactate Threshold Low muscle oxygen (hypoxia) Accelerated glycolysis –NADH produced faster than it is shuttled into mitochondria –Excess NADH in cytoplasm converts pyruvic acid to lactic acid Recruitment of fast-twitch muscle fibers –LDH isozyme in fast fibers promotes lactic acid formation Reduced rate of lactate removal from the blood

27. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Effect of Hydrogen Shuttle on Lactic Acid Formation Metabolic Responses to Exercise: Influence of Duration and Intensity Figure 4.9

28. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Mechanisms to Explain the Lactate Threshold Metabolic Responses to Exercise: Influence of Duration and Intensity Figure 4.10

29. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Metabolic Responses to Exercise: Influence of Duration and Intensity Practical Uses of the Lactate Threshold Prediction of performance –Combined with VO 2 max Planning training programs –Marker of training intensity

30. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. In Summary  Oxygen uptake increases in a linear fashion during incremental exercise until VO 2 max is reached.  The point at which blood lactic acid rises systematically during graded exercise is termed the lactate threshold or anaerobic threshold.  Controversy exists over the mechanism to explain the sudden rise in blood lactic acid concentrations during incremental exercise. It is possible that any one or a combination of the following factors might provide an explanation for the lactate threshold: (1) low muscle oxygen, (2) accelerated glycolysis, (3) recruitment of fast fibers, and (4) a reduced rate of lactate removal.  The lactate threshold has practical uses such as in performance prediction and as a marker of training intensity. Metabolic Responses to Exercise: Influence of Duration and Intensity

31. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Respiratory exchange ratio (RER or R) R for fat (palmitic acid) R for carbohydrate (glucose) C 16 H 32 O 2 + 23 O 2  16 CO 2 + 16 H 2 O VCO 2 VO 2 = R =R = 16 CO 2 23 O 2 = 0.70 VCO 2 VO 2 = R = 6 CO 2 6 O 2 = 1.00 C 6 H 12 O 6 + 6 O 2  6 CO 2 + 6 H 2 O VCO 2 VO 2 R = Estimation of Fuel Utilization During Exercise

32. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Estimation of Fuel Utilization During Exercise

33. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. In Summary  The respiratory exchange ratio (R) is the ratio of carbon dioxide produced to the oxygen consumed (VCO 2 /VO 2 ).  In order for R to be used as an estimate of substrate utilization during exercise, the subject must have reached steady state. This is important because only during steady-state exercise are the VCO 2 and VO 2 reflective of metabolic exchange of gases in tissues. Estimation of Fuel Utilization During Exercise

34. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Exercise Intensity and Fuel Selection Low-intensity exercise (<30% VO 2 max) –Fats are primary fuel High-intensity exercise (>70% VO 2 max) –Carbohydrates are primary fuel “Crossover” concept –Describes the shift from fat to CHO metabolism as exercise intensity increases –Due to: Recruitment of fast muscle fibers Increasing blood levels of epinephrine Factors Governing Fuel Selection

35. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Figure 4.11 Illustration of the “Crossover” Concept

36. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection The Regulation of Glycogen Breakdown During Exercise Dependent on the enzyme phosphorylase Activation of phosphorylase –Calmodulin activated by calcium released from sarcoplasmic reticulum Active calmodulin activates phosphorylase –Epinephrine binding to receptor results in formation of cyclic AMP Cyclic AMP activates phosphorylase

37. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Figure 4.12 The Regulation of Muscle Glycogen Breakdown During Exercise

38. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection McArdle’s Syndrome: A Genetic Error in Muscle Glycogen Metabolism Cannot synthesize the enzyme phosphorylase –Due to a gene mutation Inability to break down muscle glycogen Also prevents lactate production –Blood lactate levels do not rise during high-intensity exercise Patients complain of exercise intolerance and muscle pain

39. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. A Closer Look 4.3 Is Low-Intensity Exercise Best for Burning Fat? At low exercise intensities (~20% VO 2 max) –High percentage of energy expenditure (~60%) derived from fat –However, total energy expended is low Total fat oxidation is also low At higher exercise intensities (~50% VO 2 max) –Lower percentage of energy (~40%) from fat –Total energy expended is higher Total fat oxidation is also higher Factors Governing Fuel Selection

40. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Figure 4.14 Rate of Fat Metabolism at Different Exercise Intensities

41. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Exercise Duration and Fuel Selection Prolonged, low-intensity exercise –Shift from carbohydrate metabolism toward fat metabolism Due to an increased rate of lipolysis –Breakdown of triglycerides  glycerol + FFA By enzymes called lipases –Stimulated by rising blood levels of epinephrine

42. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Shift From Carbohydrate to Fat Metabolism During Prolonged Exercise Figure 4.13

43. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Interaction of Fat and CHO Metabolism During Exercise “Fats burn in the flame of carbohydrates” Glycogen is depleted during prolonged high-intensity exercise –Reduced rate of glycolysis and production of pyruvate –Reduced Krebs cycle intermediates –Reduced fat oxidation Fats are metabolized by Krebs cycle

44. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Carbohydrate Feeding via Sports Drinks Improves Endurance Performance The depletion of muscle and blood carbohydrate stores contributes to fatigue Ingestion of carbohydrates can improve endurance performance –During submaximal ( 90 minutes) exercise –30–60 g of carbohydrate per hour are required May also improve performance in shorter, higher intensity events Factors Governing Fuel Selection

45. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Sources of Carbohydrate During Exercise Muscle glycogen –Primary source of carbohydrate during high-intensity exercise –Supplies much of the carbohydrate in the first hour of exercise Blood glucose –From liver glycogenolysis –Primary source of carbohydrate during low-intensity exercise –Important during long-duration exercise As muscle glycogen levels decline

46. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Sources of Fat During Exercise Intramuscular triglycerides –Primary source of fat during higher intensity exercise Plasma FFA –From adipose tissue lipolysis Triglycerides  glycerol + FFA –FFA converted to acetyl-CoA and enters Krebs cycle –Primary source of fat during low-intensity exercise –Becomes more important as muscle triglyceride levels decline in long-duration exercise

47. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Figure 4.15 Influence of Exercise Intensity on Muscle Fuel Source

48. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Figure 4.16 Effect of Exercise Duration on Muscle Fuel Source

49. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Sources of Protein During Exercise Proteins broken down into amino acids –Muscle can directly metabolize branch chain amino acids and alanine –Liver can convert alanine to glucose Only a small contribution (~2%) to total energy production during exercise –May increase to 5–10% late in prolonged- duration exercise

50. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Factors Governing Fuel Selection Lactate as a Fuel Source During Exercise Can be used as a fuel source by skeletal muscle and the heart –Converted to acetyl-CoA and enters Krebs cycle Can be converted to glucose in the liver –Cori cycle Lactate shuttle –Lactate produced in one tissue and transported to another

51. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. A Closer Look 4.4 The Cori Cycle: Lactate as a Fuel Source Lactic acid produced by skeletal muscle is transported to the liver Liver converts lactate to glucose –Gluconeogenesis Glucose is transported back to muscle and used as a fuel Factors Governing Fuel Selection

52. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. The Cori Cycle: Lactate As a Fuel Source Factors Governing Fuel Selection Figure 4.17

53. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. In Summary  The regulation of fuel selection during exercise is under complex control and is dependent upon several factors, including diet and the intensity and duration of exercise.  In general, carbohydrates are used as the major fuel source during high-intensity exercise.  During prolonged exercise, there is a gradual shift from carbohydrate metabolism toward fat metabolism.  Proteins contribute less than 2% of the fuel used during exercise of less than one hour’s duration. During prolonged exercise (i.e., three to five hours’ duration), the total contribution of protein to the fuel supply may reach 5% to 10% during the final minutes of prolonged work. Factors Governing Fuel Selection

54. Chapter 4 Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved. Quantifying Body Fuel Sources Factors Governing Fuel Selection