Exercise Metabolism

1. Exercise Metabolism

2. The use of oxygen by cells is called oxygen uptake (VO2). • Oxygen uptake rises rapidly during the first minute of exercise. • Between 3rd and 4th minute a plateau is reached and VO2 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: O2 supply = O2 demand.

4. Energy Requirements at Rest • Almost 100% of ATP produced by aerobic metabolism • Blood lactate levels are low (<1.0 mmol/L) • Resting O2consumption (=index of ATP production): • 0.25 L/min • 3.5 ml/kg/min

5. Rest-to-Exercise Transitions • As muscular exercise increases, so will ATP production • From rest to light/ mod exercise  O2uptake 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 • O2 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

9. Differences in VO2 Between Trained and Untrained Subjects

10. Rest-to-Exercise Transitions 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.

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 O2 deficit at onset of exercise • Excess post-exercise oxygen consumption (EPOC) • elevated O2 consumption used to “repay” O2deficit • Many scientists use these terms interchangeably

12. Recovery From Exercise: Metabolic Responses Importance of Oxygen Debt • “Rapid” portion of O2 debt • Resynthesis of stored PC • Replenishing muscle and blood O2stores • “Slow” portion of O2 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. Glucose • 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 Oxygen

15. EPOC is Greater After HigherIntensity 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

17. 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.

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 submaximalexercise • Prolonged exercise in a hot/humid environment or at high intensity • Upward drift in oxygen uptake over time Due to body temperature & increasing epinephrineand norepinephrine Both increase metabolic rate

20. Upward Drift in Oxygen Uptake During Prolonged Exercise

21. Metabolic Responses to Incremental Exercise • Oxygen uptake increases linearly until maximal oxygen uptake (VO2 max) is reached • No further increase in VO2 with increasing work rate • VO2 max: • “Physiological ceiling” for delivery of O2 to muscle • Affected by genetics & training • Physiological factors influencing VO2 max: 1. Ability of cardio-respiratory system to deliver O2 to muscle 2. Ability of muscles to use oxygen and produce ATP aerobically

22. Changes in Oxygen Uptake During Incremental Exercise

23. Lactate Threshold • The point at which blood lactic acid rises systematically during incremental exercise • Appears at ~50–60% VO2 max in untrained subjects • At higher work rates (65–80% VO2 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: • It will cause muscle fatigue • Lactic Acid can be a useful source of energy

26. 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

28. The Cori Cycle: Lactate As a Fuel Source

29. 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. Practical Uses of the Lactate Threshold • Prediction of performance • Combined with VO2 max • Planning training programmes • Marker of training intensity

31. Exercise Intensity and Fuel Selection • Low-intensity exercise (<30% VO2 max) • Fats are primary fuel • High-intensity exercise (>70% VO2 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

32. Illustration of the “Crossover” Concept

33. 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. Shift From Carbohydrate to Fat Metabolism During Prolonged Exercise

35. 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 (<70% VO2 max), long-duration (>90 minutes) exercise • 30–60 g of carbohydrate per hour are required • May also improve performance in shorter, higher intensity events

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. 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. 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