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
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
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
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.
Rest-to-Exercise Transitions
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
Differences in VO2 Between Trained
and Untrained Subjects
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.
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:
Glucose
which in turn requires:
Oxygen
• 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
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
Recovery From Exercise: Metabolic Responses
Oxygen Deficit and Debt During
Light/Moderate and Heavy Exercise
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
Metabolic Responses to Exercise: Influence of Duration and Intensity
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
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
Upward Drift in Oxygen Uptake During
Prolonged Exercise
Metabolic Responses to Exercise: Influence of Duration and Intensity
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
Metabolic Responses to Exercise: Influence of Duration and Intensity
Changes in Oxygen Uptake During
Incremental Exercise
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% 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 fatigue
2) Lactic Acid can be a useful source of energy
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
Factors Governing Fuel Selection
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
The Cori Cycle: Lactate As a Fuel
Source
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
Metabolic Responses to Exercise: Influence of Duration and Intensity
Practical Uses of the Lactate Threshold
• Prediction of performance
– Combined with VO2 max
• Planning training programmes
– Marker of training intensity
Factors Governing Fuel Selection
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
Factors Governing Fuel Selection
Illustration of the “Crossover” Concept
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
Factors Governing Fuel Selection
Shift From Carbohydrate to Fat
Metabolism During Prolonged Exercise
Factors Governing Fuel Selection
Interaction of Fat and CHO Metabolism
During Exercise
• “Fats burn in the flame of carbohydrates”
• Glycogen is depleted during prolonged highintensity exercise
– Reduced rate of glycolysis and production of
pyruvate
– Reduced Krebs cycle intermediates
– Reduced fat oxidation
• Fats are metabolized by Krebs cycle
Factors Governing Fuel Selection
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
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
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
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