Exercise Metabolism

Oxygen deficit and steady state VO2

Oxygen deficit refers to the delay in oxygen uptake at the onset of exercise.Steady state VO2 is the plateau in oxygen uptake which is attained within 1- 4 minutes of constant exercise.

VO2 response of trained vs. untrained subjects

Trained subjects will reach steady state faster than untrained subjects.This is likely because trained subjects have improved aerobic response to exercise stimulus.

Oxygen deficit and debt/EPOC during light and heavy exercise

The term oxygen debt should not be used in exercise physiology.

Excess Post-exercise Oxygen Consumption (EPOC) is a more correct term for the elevated VO2 seen after exercise.

Oxygen deficitO2 deficit occurs at the onset of exerciseThe workload is greater than aerobic metabolism can supportTakes time to increase aerobic metabolism from rest to exercise loadThe difference between the metabolic need and the aerobic contribution is the O2 deficit

Steady State Steady state is reached once the aerobic metabolism meets the workload demandThe workload must remain constantIf the workload is greater than VO2max, steady state will never be reached

Oxygen DebtFollowing the exercise bout, O2 consumption is elevated for a period of timeThis is referred to as O2 debtPaying back the deficitThe debt is composed of a rapid and a slow component

Potential factors contributing to O2 debt (fast)Resynthesis of PCsynthesized rapidly within minutes Restoration of muscle and blood O2 storesrestored more quickly than PC stores

Potential factors contributing to O2 debt (slow)Lactate removalPossibleElevated body temperaturePossible-may be elevated for prolonged periodPost-exercise elevation in HR and breathingProbably Elevations in hormones (epinephrine, norepinephrine)

Removal of Lactate Following Exercise

Metabolic Response to Short-Term Intense ExerciseHigh-intensity, short-term exercise (0-10 seconds)ATP production through ATP-PC systemIntense exercise longer than 20 secondsATP production via anaerobic glycolysisHigh-intensity exercise longer than 60 secondsATP production primarily through oxidative mechanisms

Metabolic Response to Prolonged ExerciseExercise longer than 10 minutesATP production primarily from aerobic metabolismSteady state oxygen uptake can generally be maintainedProlonged exercise in a hot/humid environment or at high intensitySteady state not achievedUpward drift in oxygen uptake over time

VO2 drift

Metabolic Response to Exercise: Incremental ExerciseOxygen uptake increases linearly until VO2max is reachedNo further increase in VO2 with increasing work ratePhysiological factors influencing VO2maxAbility of cardiorespiratory system to deliver oxygen to musclesAbility of muscles to take up the oxygen and produce ATP aerobically

Incremental exercise and VO2max

Lactate Thresholdthe lactate threshold is an inflection point where concentrations in the blood increase exponentially with an increase in workrate

it will often correspond to the ventilatory threshold where an individuals breathing becomes labored

Incremental exercise and blood lactate concentration

Potential mechanisms to explain the lactate threshold phenomenon

Potential Contributors to the Lactate Thresholdlow muscle oxygenas the individual exercises at a higher intensity, oxygen availability in the muscle is insufficient to meet the aerobic system demandsthis produces a back log and pyruvate must be converted to lactatethis is an unlikely scenario as oxygen delivery and utilization is quite efficient

accelerated glycolysisas the increasing energy demands of intense work cannot be met by aerobic glycolysis-Krebs alone, glycolysis is accelerated in order to produce a little more energy (ATP)

Failure of the hydrogen shuttle to keep pace can increase lactate concentration

recruitment of fast twitch fibersfast twitch vs slow twitch fibersslow twitch used for low intensity aerobic activitiesfast twitch for hi intensity anaerobic activities

slow twitch readily convert lactate back to pyruvate (LDH)fast twitch readily covert pyruvate to lactate (LDH)

Fuel Utilization During Exercise can be Determined by Gas Exchange

RER = VCO2/VO2

that is the ratio of CO2 produced to O2 consumed

CO2 production is an estimate of the relative contribution of glycolysis to energy productionglucose >> pyruvate >> acetic acid + CO2 >>>> Krebs >>> O2 + H20

when fat enters Krebs as acetyl coA no CO2 is produced, BUT CO2 is formed in Krebs

Percentage of fat and carbohydrate metabolized as determined by a nonprotein Respiratory Exchange Ratio (RER)

RER

% Fat

% Carb

.70

100

0

.75

83

17

.80

67

33

.85

50

50

.90

33

67

.95

17

83

1.00

0

100

Intensity vs Fuel Sourceat higher intensity workloads, carbohydrate will become the primary fuel source

conversely, at lower workloads, fats will predominate

The crossover concept of metabolism vs. intensity

Duration vs Fuel Sourcefor short duration or during the early stages of prolonged exercise, carbohydrates will be the primary fuel source

for longer duration fat will become the predominate source

Fuel shifts from CHO to fat during prolonged exercise

Sources of Fuel During ExerciseCarbohydrateBlood glucoseMuscle glycogen

FatPlasma FFA (from adipose tissue lipolysis)Intramuscular triglycerides

ProteinOnly a small contribution to total energy production (only ~2%)May increase to 5-15% late in prolonged exercise

Blood lactateGluconeogenesis via the Cori cycle

Muscle fuel sources in highly trained endurance athletes

Fuel Sources in Highly Trained AthletesLow IntensityPrimary source plasma FFA (from fat stores)Middle IntensityEqual from plasma FFA and muscle glycogenHigh IntensityMuscle glycogen predominate source

Contributions of four energy sources over prolonged time in endurance athletes

Prolonged Exercise in Trained Endurance Athletes (70 % VO2max)Less than 1 hour glycogen is primary single sourceFFA and muscle triglycerides comprise 50 % though

As exercise progressesContribution from glycogen reducedMuscle triglycerides (intramuscular fat) reduced alsoFFA and plasma glucose increaseMust feed to maintain plasma glucose

The Cori cycle: lactate as a fuel source