Adenosine Triphosphate (ATP)

• Energy is the capacity to perform work.
• All energy for body functions originates from the breakdown of ATP (adenosine triphosphate).
• The body requires energy for all functions and activities throughout life.

Adenosine Triphosphate

• ATP is the body's chemical energy currency for all cells, including muscle cells.
• ATP powers metabolic activities, including muscle contraction.
• The body's energy systems ensure ATP supply through aerobic and anaerobic processes.
• ATP serves as the immediate energy source for most bodily reactions, including nerve conduction, tissue repair, digestion, and hormone production.

ATP Structure and Bonding

• ATP splits when releasing energy, producing ADP (adenosine diphosphate) and a free phosphate (Pi).
• ATP has three phosphate bonds; breaking them releases energy.
• Electrical nerve impulses trigger enzyme release, breaking the bond between the second and third phosphates.
• This process leaves adenosine diphosphate (ADP) and inorganic phosphate (Pi).
• Muscles have limited ATP storage and create ATP through three energy systems.
• Substrates used in ADP regeneration to ATP depend on exercise intensity and duration, including Phosphocreatine (PCr), glucose, glycogen, lactic acid (LA), fat, or protein.
• The body creates energy (ATP) via energy systems at rest or during exercise.
• Energy systems re-attach free phosphate to ADP, replenishing ATP stores for "work" such as muscle contraction.
• The formula for ATP breakdown and replenishment is ATP → ADP + Pi + Energy.
• There is approximately 100g of ATP in the muscular system
• Approximately 160kg of ATP is formed in the human body each day.

ATP Storage and Transportation

• ATP stores are immediately available in muscles for about 2 seconds of contraction.
• ATP production starts through energy systems after initial stores deplete.
• Rest conditions provide sufficient oxygen for aerobic ATP production; active conditions may require anaerobic systems due to insufficient oxygen.
• At rest, energy demand is low, and ATP is produced aerobically from fat (two-thirds) and glucose/glycogen (one-third) breakdown in the mitochondria.
• Mitochondria is the center for aerobic energy metabolism.
• The body uses the ATP-PC, Lactic Acid, and Aerobic systems in combination.
• The ATP-PC system metabolizes phosphocreatine without oxygen.
• The Lactic Acid system incompletely metabolizes glucose to lactic acid anaerobically.
• The Aerobic system completely metabolizes glucose, fats, and proteins with oxygen.

ATP-PC System

• ATP is produced through phosphocreatine (PCr) breakdown; this is an anaerobic process occurring in the muscle cell.
• PCr splits, allowing Pi to join with ADP to synthesize ATP quickly.
• PCr stores deplete in approximately 8-10 seconds.
• The phosphate energy system provides ATP during powerful efforts.
• It resynthesizes ATP quickly without oxygen, but its ATP yield is small, and CP stores deplete quickly.
• After about 10 seconds of maximal effort, the phosphate system is largely depleted, and the anaerobic glycolysis system begins to become the dominant provider of ATP.
• It relies on muscle stores of ATP and phosphocreatine.
• ATP and phosphocreatine supplies fully restore to pre-exercise levels within 3-5 minutes, but 50 per cent phosphocreatine replenishment can happen in the 30 seconds of rest recovery.
• If the effort was less than 10 seconds, then the recovery time to pre-exercise levels is faster than 3 minutes.

Lactic Acid System

• In the lactic acid system, ATP is produced through the incomplete breakdown of glucose in anaerobic glycolysis.
• This system mainly provides the bulk of ATP production during high intensity, sub-maximal efforts.
• From approximately 10-30 seconds, glycolysis has taken over as the predominant energy system.
• Glycogen is changed into a form of glucose and sent through glycolysis.
• Energy released from the breakdown of glucose into pyruvic acid is the second way that ADP + Pi reforms into ATP.
• Energy from the breakdown of glycogen and glucose then further broken into pyruvic acid, is used to produce ATP
• Pyruvic acid produced is then usually used by the aerobic system in the mitochondria.
• When energy demands are high, or oxygen is not available, more pyruvic acid is produced than can be used by the aerobic system.
• The excess pyruvic acid is converted into lactic acid.

Aerobic System

• This system produces a LOT of energy.
• It uses oxygen to drive the complete breakdown of glucose, fats, and protein through Glycolysis, Lipolysis, Krebs Cycle, and Electron Transport System in the mitochondria.
• It produces the most energy of the three systems.
• Aerobic glycolysis uses carbohydrates, but there can also be aerobic lipolysis that uses fats for the aerobic system to produce energy (lipo = fats; lysis = breakdown).
• The process is identical to the LA system. Fats and proteins can also be used as a fuel source, if oxygen is required.
• By-products are produced, but these are non-fatiguing.

Aerobic Glycolysis

• Occurs in the muscle cell. This only occurs when carbohydrates are used as the fuel.
• Glycogen is broken down into glucose, which is then broken down into pyruvic acid, releasing a small amount of ATP, identical to LA system.
• Pyruvic acid moves to the next stage due to the presence of oxygen.

Aerobic Lipolysis

• When fats are used as the fuel source, this stage is referred to as lipolysis.
• This occurs when triglycerides are broken down to fatty acids which enter the Krebs Cycle in the mitochondria.
• This process is also aerobic.
• Krebs Cycle (Citric Acid Cycle) occurs within the mitochondria.
• Fuel (pyruvic acid) and oxygen enter the Krebs cycle.
• Pyruvic acid (carbohydrate) or fats or proteins can be used as the fuel in this stage.
• Oxygen combines with carbons, producing CO2 as a by-product which diffuses into the blood and is carried away to the lungs and eliminated.
• Hydrogen ions are produced; these move to the next stage.
• Electron Transport Chain occurs within the mitochondria where hydrogen and oxygen mix to form water.
• Heat and water are produced as a by-product; the water diffuses into tissues and blood and may be sweated or urinated out or breathed out as water vapor.
• Heat is transferred into surrounding tissue, including blood.
• The reactions metabolise Glucose completely, and A large amount of ATP is produced.

Rest or Low intensity Exercise

• They body is at rest, or performing low intensity sub-maximal exercise,
• The demand for ATP is low and it is produced aerobically.
• The by-products are usually easily removed; however, the removal of heat and water through sweat can be compromised in conditions of high temperature and/or humidity.
• Fats approximately 2/3 and carbohydrates approximately 1/3 are the two primary fuel sources for ATP resynthesis.
• Fats produce about 12 times more ATP energy than carbohydrates.

Myoglobin and Oxygen Transport

• Muscles demand more oxygen during intense exercise
• Oxygen transport increases as breathing rate and heart rate rise to deliver more oxygen.
• Haemoglobin releases more oxygen into the bloodstream.
• Myoglobin stores and delivers oxygen directly to the mitochondria for aerobic energy production.
• The body shifts toward anaerobic energy systems, producing lactate if oxygen demand exceeds supply.
• Myoglobin is an oxygen binding protein (like haemoglobin) in skeletal muscle cells that attract O2 from the bloodstream into the muscle, specifically to the mitochondria for aerobic energy production.
• The aerobic system requires oxygen in order to function.
• Oxygen is transported on haemoglobin in blood to the capillary beds (tiny blood vessels where oxygen is exchanged) of the muscle where it is released and diffuses into the muscle cells.
• Myoglobin is important for oxygen storage and short-term supply in muscles.
• Haemoglobin is essential for transporting oxygen throughout the body.
• Athletes with high endurance (e.g., marathon runners) have more myoglobin in their muscles.
• The main function of myoglobin is in aiding the delivery (diffusion) of oxygen from cell membrane to the mitochondria where it is consumed.
• Myoglobin acts as a store for oxygen within muscle cells.
• Myoglobin releases its stored oxygen when the oxygen supply from the blood is too slow to meet muscle demands.
• Intermittent exercise causes more frequent oxygen depletion from myoglobin due to repeated high-intensity bursts, while continuous exercise allows a more stable oxygen supply from haemoglobin in the blood.
• Oxygen released from myoglobin for consumption must be replaced to restore muscle oxygen reserves.
• Replenishment occurs when oxygen supply to the muscle exceeds demand.
• During EPOC, the body continues to consume oxygen at a higher-than-resting rate to replenish myoglobin stores, restore haemoglobin oxygen levels, clear metabolic byproducts such as lactate, and support ATP resynthesis and phosphocreatine (PCr) recovery.
• Oxygen supply remains elevated post-exercise as heart rate and breathing rate stay high to facilitate these processes.

Increased Size and Number of Mitochondria

• The mitochondria are the sites of ATP resynthesis and where glycogen and triglyceride stores are oxidised.
• The greater the number and size of the mitochondria located within the muscle, the greater the oxidisation of fuels to produce ATP aerobically.

Increased Myoglobin Stores

• Aerobic training significantly increases the myoglobin content in the muscle and therefore its ability to extract oxygen and deliver it to the mitochondria for energy production.
• Oxygen is carried by haemoglobin in red blood cells.
• Oxygen diffuses from capillaries into muscle cells before myoglobin binds to oxygen inside the muscle cell.
• Myoglobin transports oxygen to the mitochondria, and mitochondria use oxygen for ATP production through aerobic respiration.

Energy Continuum

• As intensity increases, the influence of anaerobic systems (ATP and LA) increases, while duration increases reliance on the aerobic system.
• Aerobic fitness influences the predominant ATP-producing energy system and its percentage contribution.
• Increased aerobic fitness enhances the body's ability to activate the aerobic system due to increased O2 delivery efficiency and more enzymes/molecules needed for aerobic respiration. –An athlete will rely less on their LA system, as they can work aerobically at a higher intensity for longer and recover quicker between successive bouts using the ATP-CP system.

Diet

• Metabolic adaptation can occur with diet manipulation.
• Athletes consuming a high-fat, low-carbohydrate diet can achieve higher fat oxidation rates during submaximal exercise and oxidize fats at a higher intensity.
• Athletes consuming a high-carbohydrate, low-fat diet adapt to metabolize carbohydrates more readily during submaximal exercise.

Muscle Fibres

• Skeletal muscles consist of slow twitch (type 1) and fast twitch (type 2a and type 2b) fibres.
• Slow twitch muscle fibres (type 1) are smaller red muscle fibres with a rich blood supply. They can contract repeatedly but not forcefully, and as such are suited to endurance exercise.
• Fast twitch muscle fibres (type 2) are bigger muscle fibres with a poor blood supply (thus their pale colouring). They can contract forcefully but fatigue quickly. These are suited to high intensity short duration exercise.
• Type 2a muscle fibres are white/red (pink) muscle fibres that have both aerobic and anaerobic capabilities. They can contract at a rapid rate, with force repeatedly.
• Type 2b muscle fibres are bigger, white muscle fibres with a poor blood supply. They can contract rapidly and forcefully for a limited time.
• Every muscle contains a mixture of the different fibre types. Proportions of fibre types are different between individuals and between different muscles of within and individual.
• Genetic inheritance determines speed or endurance potential.

Oxygen Deficit

• The body prefers to supply all its energy aerobically, and if it cannot, it enters O2 deficit.
• O2 deficit occurs when moving from rest to exercise, and the respiratory, circulatory, and cardiovascular systems cannot supply energy demands in time, meaning anaerobic systems supply ATP for performance.
• O2 deficit is the difference between the oxygen required for the task (aerobically) and the amount the body supplies until reaching a steady state.
• Adjustments involve increased respiratory rate, tidal volume, heart rate, and stroke volume.
• Oxygen deficit is, “The amount of energy which has to be supplied by anaerobic metabolic processes in the early minutes following the start of exercise due to the slow increase in O2 uptake.”

Aerobic Steady State

• Aerobic steady state occurs when there is a balance between the energy required for activity and the energy being supplied aerobically by the body.
• This is visually represented as a plateau on graphs tracking heart rate or respiration rate over time.
• The time it takes to reach aerobic steady state depends on factors such as fitness level, exercise intensity, and environmental conditions.
• Elite endurance athletes reach steady state faster than untrained individuals due to cardiovascular efficiency, oxygen transport capacity, capillary density, and mitochondrial function.
• An athlete can achieve an aerobic steady state at any point within their aerobic training zone (60-85% of MHR, although it is considered by many to be most effective in the 65-75% MHR region, where a person can still carry on a conversation).
• An athlete increases exercise intensity and the anaerobic energy system (typically the lactic acid system) is relied on temporarily to provide extra ATP until oxygen demand is met again, at this point, they reach a new steady state, but with a higher total oxygen deficit.
• Aerobic steady state is the state in which oxygen supply equals oxygen demand, and all required ATP is supplied aerobically from 60-85% MHR of working capabilities.
• Athletes can transition between multiple steady states particularly when increasing intensity while staying within the aerobic training zone.

VO2 Max

• Aerobic power, aerobic capacity and maximal oxygen uptake can all be defined as, “The highest rate of oxygen consumption attainable during maximal or exhaustive exercise”
• This is usually about 2-3.5L.min.
• In absolute terms, VO2 max is expressed in litres per minute (L/Min), but this is too different for body sizes, and so it more commonly refer to relative VO2 max figures which take body size into account by simply dividing the persons’ absolute reading by their weight, and by using millilitres (mL/kg/min) instead of litres.
• VO2 should be viewed more as an indicator of a capacity, rather than as a way of predicting success in endurance events.
• Aerobic tests to exhaustion usually last at least 5 – 10 minutes and your VO2 max is measured during the final stages of whole-body exercise (near exhaustion).
• Accurate testing of maximum oxygen uptake requires expensive machinery and gas analysis equipment.

Affecting Factors

• Aerobic Fitness: Better aerobic fitness enhances the body's ability to take in, transport, and utilize oxygen more efficiently.
• Body Size: VO2 max is usually expressed relative to bodyweight because oxygen and energy needs differ relative to size.
• Gender: Women tend to have lower VO2 max scores than men due to less muscle mass and more fat stores.
• Heredity: The extent to which VO2 max can improve with training is genetically determined.
• Age: Untrained individuals' VO2 max can peak as early as 10 years for girls and 16 years for boys, whereas trained endurance athletes may not achieve this until mid to late 20s or into the 30s. VO2 max declines fastest after approximately 50 years due to decreases in cardio-respiratory efficiency, muscle mass and increases in body fat, however, VO2 max is always higher in active people, and the rate of decline is slower in active people of all ages.

Lactate Inflection Point

• The LIP establishes the exercise intensity beyond which a given exercise intensity or power output cannot be maintained.
• Exercise intensities beyond the LIP are associated with a shortened time to exhaustion - the higher the exercise intensity beyond the LIP, the more rapid the onset of fatigue.
• Lactate Inflection Point is: the point beyond which a given exercise intensity cannot be maintained by the athlete.
• LIP values generally occur around 4mM blood lactate concentration, but this varies between individuals of different aerobic fitness.
• LIP signals an increased rate of carbohydrate metabolism to meet new ATP demands.
• LIP represents the highest steady-state exercise intensity an individual can perform for a longer duration (30 min to 2 hours), while VO2 max shows the rate of work performed near exhaustion.
• LIP is a better indicator of performance over VO2 max.
• For the untrained athlete, average value at around 60% MHR and 50-60% of VO2 max.
• For the trained athlete, values can be up to 90% MHR, and 70-80% VO2 max.
• There are two main ways to improve your LIP through training
• One method involves intermittent training, the other involves continuous training.

Buffering and pH

• If H+ in the blood are high, excess hydrogen is removed by combining with bicarbonate ions (a weak acid) to raise the pH.
• If H+ in the blood are low carbonic acid releases H+ to lower the pH.
• Low pH = high amount of H+ (acidic; below 7)
• High pH = low amount of H+ (alkaline or basic above 7)
• This is when the body more effectively uses lactate to assist in buffering/neutralizing, and in the removal of H+ ions from the muscles into the bloodstream.

Acute and Chronic Adaptations

• Acute responses (immediate): only last for the duration of the exercise between cardiovascular, respiratory and muscular responses.
• Chronic adaptations (long-term effects): take a minimum of 6 weeks of training to develop at rest, during submaximal exercise and during maximum exercise.
• Muscular adaptations will vary and depend on what type of training is being performed.

Acute Cardiovascular Responses to Exercise

• Increased heart rate (HR)
• Increased stroke volume (SV)
• Increased cardiac output (Q)
• Increased systolic blood pressure
• Increased blood flow
• Redistribution of blood flow to working muscles
• Increased a-VO2 difference, the arteriovenous oxygen difference
• Decreased blood plasma volume, Because of increased sweating
• Increased blood lactate concentrations
• Blood pH decreases

Acute Respiratory Responses to Exercise

• Increased respiratory rate number of breaths per minute
• Increased tidal volume, the amount of air inhaled and exhaled per breath
• Increased minute ventilation, the amount of air breathed in one minute.
• Increased oxygen uptake (VO2) or volume of oxygen consumed reflect how much work is being done by the body

Acute Muscular Responses to Exercise

• Increased number of muscle contractions to propel the body
• Increased motor unit activation so that more fibres are fired to contract, and the muscles make more forceful contractions
• Increased recruitment of muscle fibres in a motor unit to produce more force
• Increased blood flow to the muscles
• Increased muscle temperature
• Increased muscle enzyme activity
• Increased oxygen extraction at the muscles as myoglobin delivers more oxygen to the working muscles
• Depletion of muscle energy stores

Chronic Circulo-Respiratory Adaptions to Exercise

• Chronic responses or adaptations to exercise are those changes that occur over longer periods of time.
• They remain, after recovery from exercise has been completed.
• Individual athletes’ capacities and genetic factors affect chronic adaptations
• The frequency, duration and intensity of training affect chronic adaptations
• The type and method of training used affects chronic adaptations

Chronic Circulo-Respiratory Adaptions to Exercise: AT REST

• Decreased resting heart rate (HR)
• Cardiac hypertrophy
• Increased stroke volume (SV)
• Unchanged or decreased cardiac output
• Increased blood volume and haemoglobin
• Decreased blood pressure
• Increased capillarisation of the heart muscle
• Increased capillarisation of skeletal muscle
• Decreased lung ventilation

Chronic Circulo-Respiratory Adaptions to Exercise: DURING SUBMAXIMAL EXERCISE

• Decreased heart rate (HR)
• Cardiac hypertrophy
• Increased capillarisation of the heart muscle
• Improved heart rate recovery rates
• Increased stroke volume (SV)
• Decreased blood flow to working muscles
• Decreased blood pressure
• Increased a-VO2 difference
• Unchanged cardiac output (Q)
• Decreased minute ventilation
• Decreased or unchanged VO2 (oxygen consumption)
• Increased LIP

Chronic Circulo-Respiratory Adaptions to Exercise: DURING MAXIMAL EXERCISE

• Cardiac hypertrophy
• Increased capillarisation of heart muscle
• Increased capillarisation of skeletal muscle
• Increased stroke volume (SV)
• Increased cardiac output (Q)
• Increased VO2 max
• Improved heart rate recovery rates
• Increased a-VO2 difference
• Increased/unchanged muscle blood flow
• Increased minute ventilation
• Increased LIP

Chronic Muscular Adaptions to Exercise: ENDURANCE TRAINING

• Increased Oxygen extraction by increased concentrations of myoglobin
• Increased oxygen delivery.
• Increased numbers of energy production sites i.e., size and number of Mitochondria.
• Increased oxidation of fats.
• Increased fuel stores of muscle glycogen & triglycerides.
• Increased size of slow twitch muscle fibres.
• Decreased utilisation of Anaerobic Glycolysis System.

Chronic Muscular Adaptions to Exercise: NON-ENDURANCE TRAINING (ANAEROBIC AND CALLISTHENIC)

• The chronic muscular effects of any Anaerobic/Resistance training program are dependent on the type of training undertaken.
• Increased muscles stores of ATP & PC stores (as a result of muscular hypertrophy from the training type
• Increased levels of enzymes and thus an increases in the capacity of the ATP-PC system
• Increased muscle glycogen stores and glycolytic enzymes & thus increased glycolytic capacity
• Increased storage of glycogen
• Increased size of fast twitch muscle fibres (Muscular Hypertrophy)
• Increased speed & force of contraction
• Increased strength amounts of connective tissue
• Increased numbers of muscle capillaries
• Flexibility increased length of muscles, tendons & ligaments, increased range of joint movement

CARBS TRANSPORT

• Carbohydrates are broken down by the digestive system into glucose for transportation in the blood to all cells, including muscle and liver.
• Glucose is also released from the liver into the blood to allow for transportation of glucose to all cells in the body (including to muscle during exercise)

Carbohydrates: Storage

• Some glucose is stored in the blood.
• Some glucose is stored as glycogen in muscles and liver for ATP production in anaerobic (LA system) and oxygen (aerobic) systems.
• Excess glucose is converted to fat, which is stored in adipose (fat) tissue.

Carbohydrates: Glycaemic Index (GI)

• Carbohydrates are given a glycaemic index (GI) value, this indicates how quickly blood glucose levels rise after carbohydrate consumption, measured out of 100 where a high index means glucose enters the blood quickly.
• High GI foods are important for instant energy during and after an event.
• Low GI foods are important to consume before an event as the slow sustained glucose release will provide energy for a relatively long time into the event.

FATS

• Fats (lipids) are extremely high in energy.
• They are made up of triglycerides and free fatty acids.
• A triglyceride is used for long-term energy storage
• Sources are dietary or result of the fat conversation of excess carbohydrates.

Fats: Transportation

• Fats are broken down and transported in blood as free fatty acids (FFAs)

Fats: Storage

• Fats are largely as adipose tissue, Muscles and liver store triglycerides and FFA's are stored in blood.
• Body prefers carbohydrate as an energy source.
• Fat becomes important fat becomes increasingly important when the stores of carbohydrate are depleted during prolonged, continual physical activity

Protein

• Proteins contain amino acids, they are needed for growth and repair and contribute relatively little to the sources of ATP unless in extreme circumstances.

Protein: Sources

• Proteins derive via dietary sources such as
• And in extreme circumstances, protein can be released from the blood plasma, viscera, muscle if broken down

Protein: Transportation

• Amino acids are transported by the blood to the sites requiring them.

Protein: Storage

• Proteins are not ‘stored’ as such, but form part of tissues including muscles which can be broken down to release the amino acids.
• Amino acids in the blood can be used as a fuel source for the aerobic system
• Excess amino acids are converted to fat for storage in adipose tissue.

Hitting the Wall

• During extended exercise, often marathon running or long-distance cycling, the athlete experiences a relatively sudden fatigue, decrease in power output and the inability to improve power output.
• This is known as “hitting the wall”.
• It happens when liver and muscle glycogen stores become exhausted and as a result fats (rather than carbohydrates) become the primary fuel source used by the aerobic system to produce ATP.
• Oxidation of fat is relatively slow compared to oxidation of glycogen; thus, the production of ATP is slowed and strategies should be put in place to delay glycogen store exhaustion.

Glycogen Sparing

• The aerobic system utilizes carbohydrate, fats and proteins as fuel sources, where Carbohydrates are the major fuel.
• An adaptation to exercise includes an increased capacity of the aerobic system to metabolise fats.
• At any given exercise intensity, a trained individual relies less on glycogen, thereby sparing these stores.
• Glycogen sparing is the process whereby glycogen stores are not used early in an exercise bout due to the increased ability to use triglycerides to produce energy.
• Glycogen Depletion is delayed
• Carbohydrates: Glycaemic Index (GI)
• Carbohydrates include simple or complex sugars and starches
• Diets should include complex and simple forms of carbohydrates.