Module One: Anaerobic and Aerobic Pathways

Energy Metabolism: Anaerobic Metabolism

• Different pathways of generating ATP exist in order to meet energy demands.
• Run faster, lift more weight, and run longer periods of time by optimizing the available energy pathways.

Anaerobic Pathway

• Muscle glycogen converts via Glycolysis to Pyruvic Acid.
• When O2 is not present, Glycolysis produces ATP.
• Glycolysis occurs in the muscle fibre.
• The anaerobic pathway happens during contraction.

Aerobic Pathway

• Fatty acids are converted via oxidative phosphorylation to H2O and CO2.
• ATP is produced when O2 is present in oxidative phosphorylation.
• The aerobic pathway happens during contraction.

ATP Resynthesis

• Creatine phosphate is converted to creatine, which then resynthesizes ATP during rest.

Types of Exercise & Energy Generation

• Different types of exercise place demands on the different pathways for energy generation,
• Glucose molecules undergo Glycolysis, producing Pyruvate
• Pyruvate molecules are converted into Acetyl-CoA
• Acetyl-CoA enters the KREBS cycle, eventually producing ATP

Anaerobic Energy Systems

• Supply energy in the absence of oxygen.
• ATP stores last only a few seconds.
• ATP breaks down into ADP + Pi + energy, catalysed by ATPase.
• Creatine phosphate (CP) replenishes ATP.

Creatine Phosphate

• ADP + CP becomes ATP + C, catalysed by Creatine kinase.
• Anaerobic glycolysis immediately replenishes ATP.
• Glucose undergoes a number of steps to turn into Lactate + 2 ATP without the presence of O2.
• Advantages of anaerobic systems include not requiring oxygen to produce ATP
• Anaerobic systems provide immediate energy and it is stored where it is needed.

ATP - Creatine Phosphate System

• Cells store 4-6 times more CP than ATP.
• It produces ATP at a high rate but has limited duration at maximal rates.
• CP has a 1/2 time of recovery 20-60s, and full recovery takes 3-6 minutes.
• It is important for brief maximal exercise (10s), recovery, and repeated intense interval exercise.
• CP depletion leads to reduced power during intense exercise.
• CP resynthesis during recovery is important to power output during subsequent intervals.

Glycolysis Overview

• Glucose to Glucose-6-phosphate consumes 1 ATP
• Fructose-6-phosphate to Fructose-1,6-bisphosphate consumes 1 ATP
• Glyceraldehyde to 1,3-Bisphosphoglycerate produces NADH
• 1,3 Bisphosphoglycerate to Glycerate-3-Phosphate produces ATP
• Phosphoenolpyruvate to Pyruvate produces ATP

What Happens During Exercise?

• In short duration (10s-2min), high intensity exercise, pyruvate is used as the external electron acceptor

Blood Lactate Concentrations

• Exercise of short duration and high intensity produces the largest increases in blood lactate concentration.
• Anaerobic glycolysis is a sustem in short duration of intense exercise
• Blood lactate measures about 323W after 8 mins

Anaerobic Glycolysis - Summary

• System only uses CHO (glycogen, glucose, glucose-6-phosphate (G6P)).
• The pathway has a moderate to high rate of ATP provision and peaks from 20-30 seconds, most by 40-60s; 2-3 min full capacity evident.
• It supplies 50% of ATP for a 30s all-out effort test (Wingate test).
• In light-moderate exercise, sufficient oxygen is available to cells (Review Aerobic glycolysis and oxidative phosphorylation).
• No appreciable blood lactate accumulates under steady state aerobic conditions.
• In strenuous exercise, where energy demands exceed oxygen supply, pyruvate is the final electron acceptor - anaerobic glycolysis.
• System helps maintain the cytoplasmic REDOX potential (NAD+/NADH) of the cell.
• Provides NAD+ for glyceraldehyde-3-phosphate reaction in glycolysis again.
• Produces Lactate.
• Occurs in the presence of O2 and at rest, however: Mass action effect: pooling of pyruvate shifts reaction to Lactate – increased Lactate concentration.

Aerobic Metabolism

• Energy production during short duration, high intensity exercise.
• Anaerobic glycolysis has a fast rate of energy production
• Produces lactate.

Oxidative phosphorylation

• Oxidative phosphorylation is an aerobic energy pathway
• Fats, carbohydrates and proteins can be used as substrates in this system
• This system is used in aerobic metabolism
• It has high ATP yield
• This system Requires OXYGEN for the reactions to occur to produce ATP

Oxidative Phosphorylation

• In the cytosol, Macronutrients are converted via glycolysis into pyruvic acid.
• In the mitochondrial matrix, Kreb's cycle leads to oxidative phosphorylation, requiring the electron transport chain to produce a high yield of ATP (32 ATP).
• ADP is phosphroylated to make ATP in presence of oxygen

Low Intensity, Long Duration Exercise

• Transfer of electrons generates energy needed to rephosphorylate ADP to ATP.
• Proton pump generates energy for rephosphorylation of ADP to ATP.
• Oxygen is the driving force behind these reactions because it is the FINAL electron acceptor therefore termed AEROBIC metabolism.

Oxygen (O2) Consumption (VO2)

• Muscle oxygen uptake = oxygen consumption = VO₂.
• VO₂ measures the amount of oxygen that is used by the working muscles during exercise.
• Oxygen usage = energy production.
• The amount of oxygen consumed is an indirect representation of the amount of energy one can produce.
• Measured absolutely (L.min-1) or relative to body weight (mL.kg-1.min-1).
• Increased oxygen uptake takes between 1 and 4 minutes to reach "steady-state" when exercise begins.

Oxygen Consumption

• Oxygen consumption (VO2) increases with increasing exercise intensity
• Steady state is reached in a similar amount of time regardless of exercise intensity
• The point that is reached where oxygen consumption cannot increase any more with an increase in exercise intensity

Oxygen Consumption (VO2) in Athletes

• Once steady state VO₂ is reached, depending on the training state of an individual, one can maintain this rate of aerobic metabolism for lawn mowing/running a marathon/ ultramarathon.
• Endurance athletes can last in endurance events because they possess a high capacity of the central circulation to deliver oxygen to working muscles, as well as high capacity of the exercised muscles to use available oxygen.

Measurement of Oxygen Consumption (VO2)

• Because production of energy ultimately relies on oxygen use, one can indirectly measure energy expenditure by measuring oxygen consumption (VO2)
• This can be done via direct or indirect calorimetry
• Calorimetry forms the basis of many forms of exercise testing

Indirect Calorimetry

• Applications include measurements of maximal cardiorespiratory and muscular endurance, accumulated O2 deficit, metabolic rate, and energy expenditure.
• Limitations include that it is that it requires a whole body assessment in order to use sophisticated and expensive equipment. It is sensitive to measurement error.
• It can only be accurately used for metabolic intensities, economy, efficiency, and energy expenditure during steady state exercise and requires subjects to wear apparatus on face or in mouth.

Indirect Calorimetry: Closed-Circuit

• Subject breathes 100% oxygen provided by system.
• Soda lime absorbs CO2.
• Difference between initial and final volumes of O₂ indicates oxygen consumption.
• Bulky, resistant to large breathing volumes and CO₂ absorption lags production. Measurement errors are likely.

Open Circuit Indirect Calorimetry

• Simple method for the measurement of VO₂ during exercise.
• Assesses the metabolic intensity of the exercise.
• Person inhales ambient air, which contains: 20.93% 02, 0.03% CO2 and 79.04% N2.
• Changes in O2 and CO2 % in expired air compared to O₂ and CO2 % in inspired air indirectly reflect energy metabolism.
• The volume of oxygen consumed (VO2) by the body is equal to the difference between the volumes of inspired and expired oxygen.
• The volume of carbon dioxide produced (VCO2) by the body is equal to the difference between the volumes of expired and inspired carbon dioxide.

Calculations

• VO2 = V₁F₁O2 - VEFEO2 where:
  • VO₂ = Oxygen consumption (per unit time)
  • V₁ = Volume of air inspired (per unit time) - don't know this
  • VE Volume of air expired (per unit time) - we can measure this!
  • F₁O₂ = Fraction of oxygen in inspired air - we know this!
  • FEO₂ = Fraction of oxygen in expired air - we can measure this!

What is the definition of maximal exercise capacity?

• Maximal exercise capacity refers to the point when increasing exercise intensity no longer results in increased oxygen consumption
• VO2 max depends almost exclusively on the functional capacity of the cardiovascular system and is a fundamental measure of physiologic functional capacity for exercise.

Maximal Oxygen Consumption

• Highest rate of oxygen transport and use that can be achieved at maximal physical exertion
• The most widely recognised measure of aerobic fitness, dependent upon the cardiovascular system.
• The highest reported VO2max was during the swedish cyclist Oskar Svendsen, which measured 96.7 ml.min-1.kg

Sport and VO2max Summary

• Indirect calorimetry is used to measure expired gas to calculate oxygen consumption (VO2).
• VO2 is the difference in the volume of oxygen inspired and the volume of oxygen expired.
• VO2 indirectly measures energy production through the aerobic pathway.
• VO2 relies on the ability of the cardiovascular system to deliver O₂ as well as the muscles to use O₂.
• Maximal oxygen consumption (VO2max) is an predictor of individual performance in endurance sports.

Energy Continuum

• The energy continuum illustrates the contributions of different energy systems (ATP stores, ATP-PCr system, Glycolytic system, Aerobic System) at various durations of activity from 2 seconds to 2+ hours.

Recap: Open Circuit Indirect Calorimetry

• Collection of expired gases and measurement of VO2.
• Requires Aerobic endurance exercise at steady state
• VO₂ in steady state where O₂ demand = O₂ supply

What Happens in Aerobic Exercise of Increasing Intensity

• O₂ supply eventually cannot meet the demand.
• The energy to continue exercising has to come from somewhere.

The Metabolic Process

• Overall oxidative metabolism is decreased when mitochondrial NADH/NAD+ level is elevated.
• Glycolysis is then used to reduce pyruvic acid, which then turns into plasma lactate
• Glucose-6-phosphate at rest turns into pyrovic acid which then turns into lactic acid in order to achieve physical exercise

Lactate Transportation

• Lactate is transported out of the muscle cell via the monocarboxylate 1 (MCT 1) transporter and is co-transported with H+.
• Results in a reduction in plasma pH, as more blood becomes acidic.

Lactate Threshold

• Lactate concentration can be measured in blood.
• When the level of lactate increases appreciably - lactate threshold is surpassed.
• LT indicates the point at which energy production (ATP) goes from being aerobic to anaerobic.

VO2 and Increasing O2 Supply

• Some mechanisms include Additional muscle groups needed, changes in exercise mechanics, Acidaemia shifts unloading of O₂ from haemoglobin, Progressive muscle vasodilation due to local metabolites
• Known as non-steady state VO₂.
• Seen for exercising workloads that are accompanied by lactic acidosis.
• Following the accumulation of lactate in the plasma, exercise ceases shortly after
• In strenuous exercise, where energy demands exceed oxygen supply, oxygen is not available to be the final electron acceptor.
• In strenuous exercise, hydrogen atoms at the start of the electron transport chain begin to accumulate
• With excess glycogen levels, anaerobic glycolytic systems supplement the energy provision at higher exercise intensities.

Aerobic Metabolism - RER and RQ

• VO2 = Volume of Oxygen consumed
• VCO₂ = Volume of Carbon Dioxide produced
• The amount of substrate used during exercise is calculated
• Volume of oxygen consumed (VO2) by body is equal to the difference between volumes of inspired and expired oxygen to measure energy expenditure
• The volume of carbon dioxide produced (VCO₂) by the body is equal to the volumes of expired and inspired carbon dioxide.

Respiratory Quotient (RQ)

• CHO and fat require different amounts of O₂ for oxidation to their end products (CO2 and water).
• Carbon dioxide produced per unit of oxygen consumed gives an indication of the type of substrate being utilized – known as the respiratory quotient (RQ).
• RQ = CO2 produced / O₂ consumed.
• Indicates reliance on either fat or CHO as fuel substrate during intense exercise at the cellular level.

Respiratory Exchange Ratio (RER)

• It is difficult to measure respiration at the cellular level.
• RER assumes that the exchange of CO2 and O₂ at the lungs reflects what is happening at the cellular level.
• RER reflects the pulmonary exchange of CO2 and O2 under differing physiologic and metabolic conditions i.e. Hyperventilation and exhaustive exercise.
• RER = VCO2/VO2

Substrate Oxidation

• Oxidation of carbohydrates (CHO) requires less oxygen than oxidation of fats.

Importance of RER

• Provides an indication of substrate use during exercise.
• Indicates near-maximal intensity – closer to 1.
• Is only relevant if measured during steady state exercise (respiratory gases).
• Is an estimate of RQ during steady-state.