1. Exercise Metabolism

2. The use of oxygen by cells is called oxygen uptake (VO 2 ). Oxygen uptake rises rapidly during the first minute of exercise. Between 3 rd and 4 th minute a plateau is reached and VO 2 remains relatively stable. Plateau of oxygen uptake is known as steady rate.

3. Steady-rate is balance of energy required and ATP produced. Any lactate produced during steady-rate oxidizes or reconverts to glucose. Many levels of steady-rate in which: O 2 supply = O 2 demand.

4. 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 (=index of ATP production): – 0.25 L/min – 3.5 ml/kg/min Energy Requirements at Rest

5. Rest-to-Exercise Transitions As muscular exercise increases, so will ATP production From rest to light/ mod exercise  O 2 uptake increases rapidly – Initial ATP production through anaerobic pathways: 1. PC system – 10 sec 2. Glycolysis/ TCA – 3 mins After steady state is reached, ATP requirement is met through aerobic ATP production  O 2 consumption reaches steady state within 1–4 minutes  oxygen supply is meeting the oxygen demand by way of aerobic metabolism

6. The Aerobic System Oxygen Deficit: is the difference between the total amount of oxygen required to perform an activity and the actual amount of oxygen initially available until steady state is reached Oxygen deficit = Lag in oxygen uptake at the beginning of exercise…

7. Oxygen Deficit Steady-rate oxygen uptake during light & moderate intensity exercise is similar for trained & untrained.

8. Comparison of Trained and Untrained Subjects Trained: reach steady-rate quicker, have lower oxygen deficit – Better developed aerobic energy capacity  Due to cardiovascular or muscular adaptations =Results in less lactic acid produced Rest-to-Exercise Transitions

9. Differences in VO 2 Between Trained and Untrained Subjects Rest-to-Exercise Transitions

10. Therefore…  The failure of oxygen uptake to increase instantly at the beginning of exercise = anaerobic pathways contribute to overall production on ATP early in exercise.  After a steady state is reached, the body’s ATP requirement is met by aerobic metabolism. Rest-to-Exercise Transitions

11. 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) – elevated O 2 consumption used to “repay” O 2 deficit Many scientists use these terms interchangeably

12. Recovery From Exercise: Metabolic Responses Importance of 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)

14. Restoring ATP levels: - Constantly restoring ATP by resynthesis – 48/72 hrs to restore to normal. This requires: which in turn requires: Restoring PC: -When energy for ATP resynthesis is requires rapidly (sprinting)  provided by the breakdown of PC The energy provided for the PC resynthesis comes from the breakdown of glucose – therefore making an oxygen demand Glucose Oxygen

15. Recovery From Exercise: Metabolic Responses EPOC is Greater After Higher Intensity Exercise Higher body temperature Greater depletion of PC Greater blood concentrations of lactic acid Higher levels of blood epinephrine and norepinephrine

16. Oxygen Deficit and Debt During Light/Moderate and Heavy Exercise Recovery From Exercise: Metabolic Responses

17. 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 >5 seconds – Shift to ATP production via glycolysis Events lasting >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

18. Summary  During high-intensity, short-term exercise (2-20s) the muscle’s ATP production is dominated by the ATP-PC system.  Intense exercise lasting >20s relies more on anaerobic glycolysis to produce ATP.  High-intensity events lasting >45s use a combination of the ATP-PC system, glycolysis, and the aerobic system to produce ATP for muscular contraction. Metabolic Responses to Exercise: Influence of Duration and Intensity

19. 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 & increasing epinephrine and norepinephrine  Both increase metabolic rate Metabolic Responses to Exercise: Influence of Duration and Intensity

20. Upward Drift in Oxygen Uptake During Prolonged Exercise Metabolic Responses to Exercise: Influence of Duration and Intensity

21. 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 & training Physiological factors influencing VO 2 max: 1. Ability of cardio-respiratory system to deliver O 2 to muscle 2. Ability of muscles to use oxygen and produce ATP aerobically

22. Changes in Oxygen Uptake During Incremental Exercise Metabolic Responses to Exercise: Influence of Duration and Intensity

23. 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

24. Changes in Blood Lactate Concentration During Incremental Exercise

25. The amount of LA accumulating depends on HOW LONG you work above the threshold. This has to be monitored because: 1)It will cause muscle fatigue 2)Lactic Acid can be a useful source of energy

26. 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

27. 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

28. The Cori Cycle: Lactate As a Fuel Source Factors Governing Fuel Selection

29. Metabolic Responses to Exercise: Influence of Duration and Intensity Reasons for Lactate Threshold 1.Low muscle oxygen (hypoxia) = increased reliance on anaerobic metabolism 2.Accelerated glycolysis – NADH produced faster than it is shuttled into mitochondria – Excess NADH in cytoplasm converts pyruvic acid to lactic acid 3.Recruitment of fast-twitch muscle fibers – LDH enzyme in fast fibers promotes lactic acid formation 4.Reduced rate of lactate removal from the blood

30. 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 programmes – Marker of training intensity

31. 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

32. Illustration of the “Crossover” Concept

33. 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 (fats)  glycerol + FFA *By enzymes called lipase  Stimulated by rising blood levels of epinephrine

34. Factors Governing Fuel Selection Shift From Carbohydrate to Fat Metabolism During Prolonged Exercise

35. 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

36. 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

37. 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

38. 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

39. 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