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 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.
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
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
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
muscle
2. 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 fatigue
2) 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 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
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 highintensity 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