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Overview
Aerobic Exercise and Oxygen Consumption
Substrate Utilization– Respiratory Exchange Ratio
Anaerobic Exercise and the Lactate Threshold– Bicarbonate Buffer System– Definition, Possible Causes
Causes of Fatigue
Glycogen Depletion– Exercise Intensity– CHO intake
Aerobic Exercise and O2 Consumption
O2
CO2
Maximal Duration of Energy System
30 sec
1 min
3 min
5 min
2-3 hr
% C
on
trib
uti
on
ATP-PC
Glycolysis
Oxidative
10 sec
Aerobic Exercise and O2 Consumption
Oxidative metabolism of CHO and FAT requires O2, produces CO2
Indirect calorimetry - calculated energy expenditure based on gas exchange (VO2 and VCO2)
Must be primarily aerobic to be accurate– Anaerobic metabolism results in excess CO2 release
from buffer systems
Difference between inspired and expired air
Aerobic Exercise and O2 Consumption
Energy Expenditure (kcals)
Fitness Level
Contribution of CHO
Contribution of FAT
Energy Expenditure
Fitness Level: VO2max
Maximal Oxygen Uptake
Measure of Aerobic Fitness
Graded Exercise Test
Maximal Effort
VO2max= Inspired O2 – Expired O2
Bruce Protocol
Stage Speed Incline
1 1.7 10%
2 2.5 12%
3 3.4 14%
4 4.2 16%
5 5.0 18%
6 5.5 20%
VO2max Data
Substrate Utilization
Primary fuel source is CHO and Fat.
Protein can serve as a secondary fuel source.
Fat requires more O2 than CHO
Relative Contribution determined by the Respiratory Exchange Ratio (RER)
Respiratory Exchange Ratio (RER)
Non-invasive technique to determine relative Metabolic Contribution of Carbohydrate and Fat.
RER =VCO2
VO2
Also called Respiratory Quotient (RQ) during Steady State Exercise.
1.0 = 100% CHO, 0.7 = 100% Fat
CHO vs. FAT
6 O2 + C6H12O6 6 CO2 + 6 H2O + 32 ATPCHO (Glucose = C6H12O6):
23 O2 + C16H32O2 16 CO2 + 16 H2O + 106 ATPFAT (Palmitic Acid = C16H32O2):
Amount of O2 required is proportional to amount of C in the substrate!
RER = VCO2/VO2 = 6/6 = 1.0
RER = VCO2/VO2 = 16/23 = 0.7
Crossover Effect
% of max
% U
tiliz
ati
on
CHO
Fat35-40% of VO2 max
Oxygen Consumption Limitations
Oxygen Deficit– Beginning of Exercise, Exercise Transitions
Oxygen Debt/EPOC– End of Exercise
Lactate Production– High Intensity Exercise
Exercise Transition
Stage 1 Stage 2
Oxygen Deficit
Oxy
gen
Co
nsu
mp
tio
n
Stop
Predicted
O2 Deficit
Rest TransitionO
xyg
en C
on
sum
pti
on
Start
Actual
O2 Debt or EPOC
Possible Explanations for EPOC
Reform ATP, PC, and replace tissue O2 stores.
Removal of Lactic Acid [to liver (Cori Cycle) or Oxidation]
2Lactate (C3H6O3) + energy (from 16 ATP) glucose (C6H12O6)
2Lactate (C3H6O3) + 6O2 6CO2 + 6H2O + 619Kcal
Removal of excess CO2
Body Temp. and Catecholamines
Why is Lactate Produced during aerobic exercise?
Glycolysis
NADH
Mitochondria
Hydrogen Shuttle
Pyruvate
Lactate
Failure
Lactic Acid
Metabolic by-product of Anaerobic Glycolysis.
Immediately hydrolyzed into Lactate and H+
(acid)
Acid portion is removed from active tissue and buffered in the blood (bicarbonate system).
Lactate can be reformed into glucose in the Liver via Cori Cycle (gluconeogenesis).
Bicarbonate System
CO2+H2OH+
From Lactic Acid
HCO3+
Bicarbonate
H2CO3
Carbonic Acid
Lactate Threshold
Lactic acid accumulates with prolonged, high-intensity exercise
Lactate Threshold is the systematic rise in blood lactate concentration– Production exceeds clearance
Often used as a measure of aerobic fitness level
Lactate Threshold
LT
Exercise Intensity
Blo
od
Lac
tate
Does Lactic Acid Cause Fatigue?
• No, lactic acid DOES NOT directly cause fatigue!• Acidosis (H+) causes fatigue
• Inhibits PFK (rate limiting enzyme) and energy production
• Inhibits actin-myosin cross bridges for muscle contraction
• Benefits of Lactic Acid:• Maintains cytosolic redox potential• Can be converted to glucose and used for
energy production (Cori Cycle)
Cytosolic Redox Potential
Lactic Acid
Pyruvic Acid
NADH+H+
NAD+
Lactate Dehydrogenase
Pyruvic Acid accepts H+; is reduced by NADH forming a molecule of lactic acid.
C3H4O3 + NADH + H+ → C3H6O3 + NAD+
(Pyruvic Acid) (Lactic Acid)
Causes of Fatigue
Energy System Failure– PC Depletion– Glycogen Depletion
Metabolic By-Products – Pi (inorganic phosphate)– Heat and Muscle Temperature– Acidosis (H+)
Neuromuscular Fatigue– Peripheral (neural transmission) – Central (CNS)
QUICK CHECK
When _________ runs out, endurance exercise
simply can’t continue……
A. Steam
B. Muscle glycogen
C. The trail
….. unless ______ is ingested.
A. Really strong coffee
B. Air
C. Carbohydrates
SUBSTRATE USE IN PROLONGED EXERCISE
Coggan and Coyle, 1991
Fat: 100,000 kcals
40 kcals400
kcals
Liver glycogen: 200 kcals
Glycogen Depletion
Muscle Glycogen used for energy production (glycolysis, oxidative phosphorylation)
Depletion selective within muscle fiber
: type I to type II (intensity low to high)
Glycogen depletion does not directly cause fatigue
Glycogen Depletion and Exercise Intensity
CHO and Glycogen Storage
CHO and Glycogen Storage
CHO Loading
CHO during Exercise
Delays fatigue by:– Maintaining blood glucose levels (especially
important for prolonged exercise)– “Sparing” glycogen stores– Glycogen synthesis during low-intensity
exercise
6-8% CHO solution is ideal
~16g CHO/hour