Energy Requirements at Rest
At rest, the body is in homeostasis, and therefore the body's energy requirement is also constant
Almost 100% of ATP at rest is produced by aerobic metabolism
- Blood lactate levels are low ( <1.0 mmol/L)
Resting O2 consumption for a 70 kg adult:
- 0.25 L
Rest-to-Exercise Transitions
ATP production increases immediately
Oxygen uptake increases rapidly:
- In transition from rest to light/moderate exercise, O2 consumption reaches steady state within 1-4 minutes
- After steady state is reached, ATP requirement is met through aerobic ATP
Jumping on a Treadmill Example
At onset of exercise, ATP production increases immediately to meet the requirement
However, O2 consumption does not increase immediately, but reaches a steady state in consumption 1-4 minutes after the onset of exercise
The fact that O2 consumption does n
Oxygen Deficit
Lag in oxygen uptake at the beginning of exercise; the difference between oxygen uptake in the first few minutes of exercise and an equal time period after steady state has been obtained
Causes oxygen debt during recovery phase
Comparison of Trained and Untrained Subjects in the Oxygen Defecit
Trained subjects have a lower oxygen deficit
- Better-developed aerobic bioenergetic capacity to allow the aerobic energy systems kick in faster
- Due to cardiovascular and muscular adaptations
Results in less production of lactate and H+ and spare PC
Recovery from Exercise
Oxygen uptake remains elevated above rest into recovery
Oxygen debt: refers to the repayment for O2 deficit that occurred at the onset of exercise
- Term used by AV Hill (father of exercise physiology)
Excess post-ecercise oxygen consumption (EPOC):
- Ter
Oxygen Debt
AKA EPOC; refers to the O2 consumption above rest following exercise
"Rapid" portion of O2 debt (2-3 minutes post-exercise):
- Resynthesis of stored PC
- Replenishing muscle and blood O2 stores
"Slow" portion of O2 debt (can last for greater than 30 minut
EPOC is Greater Following Higher Intensity Exercise
(Initial oxygen deficit is higher)
EPOC is greater following high intensity exercise than low intensity exercise because:
1) Heat production and body temperature are higher
2) Greater depletion of PC
- Additional O2 required for resynthesis
3) Greater blo
Removal of Lactic Acid Following Exervise
Classical theory: majority of lactic acid converted to glucose in liver (used for gluconeogenesis)
Recent evidence:
- 70% of lactic acid is oxidized: used as substrate by heart and skeletal muscle
- 20% converted to glucose (gluconeogenesis)
- 10% convert
General Metabolic Response to Exercise
Short term, high intensity exercise less than 10 secs relays on the anaerobic metabolic pathways
Events longer than 10 to 20 seconds and less than 10 minutes generally produces the needed ATP for muscular contraction via a combination of anaerobic and aer
Metabolic Responses to Short-Term, Intense Exercise
Energy to perform short-term high intensity exercises comes primarily from anaerobic metabolic pathways
First 1-5 seconds of exercise: ATP through ATP-PC system
- Dominates until 20 seconds
Intense exercise longer than 5 seconds: gradual shift to ATP prod
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, moderate-intensity exercise; 2 exceptions to this are exercising in a hot/humid environment, a
Metabolic Responses to Incremental Exercise
Oxygen uptake increases linearly until maximal oxygen uptake (VO2max) is reached
- No further increase in VO2 with increasing work rate
VO2max: maximal capacity to transport and utilize oxygen during exercise
- "Physiological ceiling" for delivery of O2 t
Lactate Threshold
The point at which blood lactate rises systematically (exponentially) during incremental exercise (describes the blood lactate inflection point)
- Appears at about 50-60% VO2max in untrained subjects
- Appears at higher work rtes (65-85% VO2max) in traine
Explanations for the Lactate Threshold
1) Low muscle oxygen (hypoxia)
2) Accelerated glycolysis:
- NADH produced faster than it is shuttled into the mitochondria
- Excess NADH in the cytoplasm coverts pyruvate to lactate
3) Recruitment of fast-twitch fibers:
- Lactate dehydrogenase isoenzyme i
Lactate Dehydrogenase
AKA LDH; catalyzes the conversion of pyruvate to lactate (and NAD+) and the conversion of lactate to pyruvate (and NADH)
Two isoforms (isozymes): M and H
H has highest affinity for lactate -> pyruvate
- Slow-twitch fibers
M has highest affinity for pyruva
Practical Uses of the Lactate Threshold
Prediction of performance
- Combined with VO2max
Planning training programs
- Marker of training intensity
- Choose a training HR based on LT
Frank Shorter and Steve Prefontaine
- Prefontaine: highest VO2max recorded
- Shorter: high LT
Cool Down
Lactate can be used to form glucose as an energy fuel during cool down
Cool down isn't totally necessary in healthy individuals
Does Lactate Cause Muscle Soreness
Lactate production is commonly believed to cause muscle soreness:
- Delayed-onset muscular soreness (DOMS)
- 24-48 hours after exercise
Physiological evidence does not support this claim
- Lactate removal is rapid (within 60 min) following exercise
- Powe
Estimation of Fuel Utilization
Respiratory exchange ratio (RER or R): the ratio of CO? produced to the O? consumed
- Commonly used to estimate the percent contribution of carbohydrate or fat to energy metabolism during exercise, since fat and carb differ in the amount of O2 used and CO
Factors Governing Fuel Selection
Intensity and duration of exercise
-During low intensity, prolonged exercise, there is a progressive increase in the amount of fat oxidized by the working muscles
- Endurance gained subjects use more fat and less carb than less fit subjects during prolong
Exercise Intensity and Fuel Selection
Low intensity exercise (<30% VO2max)
- Fats are primary fuel
High intensity exercise (>70% VO2max)
- Carbs are primary fuel
R increases as exercise intensity increases
Proteins only contribute only a small percentage (about 2%) during exercise of less tha
Crossover Concept
Describes the shift from fat to carb metabolism as exercise intensity increases
Due to:
1) Recruitment of fast muscle fibers:
- As exercise intensity increases, more fast muscle fibers are recruited
- Have an abundance of glycolytic enzymes, but not many
Crossover Point
Work rate (exercise intensity) at which the energy derived from carbs exceeds that of fat
- As intensity increases beyond the crossover point, a progressive shift occurs from fat to carb metabolism
McArdle's Syndrome
Genetic error in muscle glycogen metabolism:
Cannot synthesize the enzyme phosphorylase due to a gene mutation
Results in inability to break down muscle glycogen
- Use more fat as a fuel during submax exercise
Also prevents lactate production
- Blood lact
Is Low-Intensity Exercise Best for Burning Fat?
At low exercise intensities (about 20% VO2max):
- High percentage of energy expenditure (about 60%) derived from fat
- However, total energy expended is low
- Total fat oxidation is also low
At higher exercise intensities (about 50% of VO2max):
- Lower pe
Exercise Duration and Fuel Selection
Prolonged (greater than 30 min), low-moderate intensity (40-59% VO2max) exercise:
- Shift from carbohydrate metabolism toward fat metabolism
- R value decreases
Due to an increased rate of lipolysis (fat break down)
- Triglycerides -> glycerol + FFA by en
At Rest
Most energy comes form burning fat (60%)
Interaction of Fat and Carb Metabolism During Exercise
Fats burn in the flame of carbohydrates"
- A reduction in kreb's cycle intermediates (due to glycogen depletion) results in a diminished rate of ATP production from fat metabolism because fat can be metabolized only via kreb's cycle oxidation
Glycogen is
Carbohydrate Feeding via Sports Drinks Improves Endurance Performance
The depletion of muscle and blood carb stores contributes to fatigue
Ingestion of carbs can improve endurance performance
- During sub maximal (<70% VO2max), long-duration (>90 minutes) exercise
- 30-60 g of carb per hour are required to enhance performan
Gatorade
Invented by Dr. Robert Cade; introduced in 1967 orange bowl
Harvard Fatigue Laboratory
DB Dill's dog Joe
Original study that carb drinks improve performance was conducted here when studying fatigue in soldiers during WWII
Made first treadmill
Couldn't go <3 hours on the treadmill
Ate carbs
Was then able to go >13 hours on the treadmill
Sources of Carb During Exercise
Carbohydrate is stored as glycogen in both the muscle and the liver
- Glycogen in skeletal muscle: provides a direct source of CHO for muscle energy
- Liver glycogen stores serve as a means of replacing blood glucose via glycogenolysis (controlled by phos
Sources of Fat During Exercise
Most fat is stored in the form of triglycerides, but some is stored in muscle cells as well
Intramuscular triglycerides
- Primary source of fat during higher intensity exercise; contribution declines with increased exercise durations
Plasma FFA
- Primary
General Rule for Carb/Fat Use as Intensity Increases
As intensity increases, the % of fuel coming from fat decreases and the % of fuel coming from carbs increases
Carbs: as intensity increases, carbs contribute progressively more to energy production
- The contribution by muscle glycogen greatly increases w
General Effect of Exercise Duration on Muscle Fuel Source
As duration increases, carbs contribute less and fat contributes more (starts off roughly equal though)
Carbs: as duration increases, the contribution by carbs decreases
- Muscle glycogen decreases
- Blood glucose increases
Fats: as duration increases, th
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 (about 2%) to total energy production during exercise
- May increase to 5-10% late
Lactate as a Fuel Source During Exercise
Can be used as a fuel source by skeletal muscle and the heart
- Converted to pyruvate, transformed to acetyl-CoA and enters the Krebs cycle
Can be converted to glucose in the liver (gluconeogenesis)
- Cori cycle
Lactate shuttle
- Lactate produced in one t
Cori Cycle: Lactate as a Fuel Source
Lactate produced by skeletal muscle is transported to the liver
Liver converts lactate to glucose
- Gluconeogenesis
Glucose is transported back to muscle and used as fuel