Exercise Metabolism

• 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.

• 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.

Energy Requirements at Rest

• Almost 100% of ATP produced by aerobic metabolism

• Blood lactate levels are low (<1.0 mmol/L)• Resting O2 consumption (=index of ATP

production):– 0.25 L/min– 3.5 ml/kg/min

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 sec2. 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

 

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…

Oxygen Deficit

• Steady-rate oxygen uptake during light & moderate

intensity exercise is similar for trained & untrained.

Comparison of Trained and Untrained Subjects

• Trained: reach steady-rate quicker, have lower oxygen deficit – Better developed aerobic energy capacityDue to cardiovascular or muscular adaptations

=Results in less lactic acid produced

Differences in VO2 Between Trained and Untrained Subjects

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

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” O2 deficit

• Many scientists use these terms interchangeably

Recovery From Exercise: Metabolic Responses

Importance of Oxygen Debt• “Rapid” portion of O2 debt– Resynthesis of stored PC– Replenishing muscle and blood O2 stores

• “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)

• 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

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

Oxygen Deficit and Debt During Light/Moderate and Heavy Exercise

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

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 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

Upward Drift in Oxygen Uptake During Prolonged Exercise

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 muscle2. Ability of muscles to use oxygen and produce ATP aerobically

Changes in Oxygen Uptake During Incremental Exercise

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

Changes in Blood Lactate Concentration During Incremental Exercise

• 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 fatigue2) Lactic Acid can be a useful source of energy

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

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

The Cori Cycle: Lactate As a Fuel Source

Reasons for Lactate Threshold1.Low muscle oxygen (hypoxia) = increased reliance

on anaerobic metabolism2.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

Practical Uses of the Lactate Threshold

• Prediction of performance – Combined with VO2 max

• Planning training programmes– Marker of training intensity

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

Illustration of the “Crossover” Concept

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

Shift From Carbohydrate to Fat Metabolism During Prolonged Exercise

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

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

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

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

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