Fatigue And How The Body Recovers From Exercise

Fatigue

Depletion of energy sources

Creatine phosphate

Depletion of Creatine phosphate is found at the muscles. Creatine phosphate is synthesised in the liver and then goes to the skeletal muscles to be stored. Creathine phosphate is used to form ATP from ADP. It is also important for intense exercise for up to ten seconds like 100 metres maximum power cannot be maintained.

Muscle and liver glycogen    

Muscle and liver glycogen can occur despite that there is enough oxygen and ATP through metabolic pathways. Glycogen is a readily available source of energy under both aerobic and anaerobic conditions. When glycogen stores are depleted muscles stop contracting because the body is unable to use fat as a sole source of fuel. Fatty acids from fat stores cannot be fully oxidised to release all their energy if there is insufficient breakdown of carbohydrates at the same time.     

Effects of waste products

Blood lactate accumulation

Muscle lactate is disposed by oxidation at first then to pyruvate and then turned to carbon dioxide and water. Some blood lactate is taken in by the liver which reconstructs it to glycogen. The remaining blood lactate diffuses back into the muscle or other organs to be oxidised. This oxidation forms carbon dioxide which is later excreted by the lungs.    

Carbon dioxide

During exercise carbon dioxide increases the level of blood activity. Carbon dioxide is carried in chemical combinations in the blood. In red blood cells enzymes speed up the reaction of carbon dioxide and water to form carbonic acid. 

Increased blood acidity

When the carbonic acid breaks down it breaks down into hydrogen ions and bicarbonate ions. The hydrogen ion is responsible for the increased blood acidity.  

Neuromuscular fatigue

Depletion of acetylcholine

Acetylcholine is neurotransmitter that is released to stimulate skeletal muscles and the parasympathetic nervous system. Its effect are short because it is destroyed by acetlchilinesterase an enzyme which are realised into the sarcolemma of muscle fibres which then prevent muscle contractions in the absence of additional nervous stimulation.  

Reduced calcium-ion release

Calcium ions are realised allowing actin and myosin to couple and form actomyosin. During relaxation the calcium ions are removed and the muscle returns to its resting state. If the store of calcium ions is reduced the ability of the actin and myosin to couple is compromised which prevents continued muscle contraction.

Recovery

(EPOC) Exercise, post, oxygen, consumption

The two major components of EPOC are fast components and slow components. Fast components are the amount of oxygen required to synthesise and restore ATP and creatine phosphate. Slow components are the amount of oxygen required to remove lactic acid from the muscle cells and blood. After light exercise recovery to a resting state takes place quickly and without realising it. EPOC defines the excess oxygen uptake above the resting level in recovery  

Oxygen debt

When you have a short intense burst of exercise like sprinting you generate energy for this anaerobically or without oxygen, when you stop exercise you are still breathing heavily, this is your body taking in extra oxygen to repay the oxygen debt. The difference between the oxygen the body required and what it actually managed to take in during a sprint is called oxygen deficit. When you stop sprinting and start to recover you will actually need more oxygen to recover than your body. This is called excess post exercise oxygen consumption.

Fast components

Muscle phosphogen stores

Alatacid oxygen debt represents that portion of oxygen used to synthesise and restore ATP and creatin phosphate stores which have been almost completely exhausted during high intensity exercise. During the first three minutes of recovery EPOC restores almost 99 % of the ATP and creatine phosphate used during exercise.

Slow components

Removal of lactic acid

The slow component of EPOC is concerned with the removal of lactic acid from the muscles and the blood. This can take several hours depending on the intensity of the activity. Around half of lactic acid is removed after 15 minutes and most removed after an hour. Lactacid recovery converts most of the lactic acid to pyruvicacid which is oxidised by the kerbs cycle to create ATP  

Replenishment of myoglobin

Myoglobin is an oxygen storage protein found in muscle it forms a loose combination with oxygen while the oxygen supply is full and stores it until the demand for oxygen increases. During exercise the oxygen form myoglobin is quickly used up after exercise additional oxygen is required to pay back any oxygen that has been borrowed from myoglobin stores.

Replacement of glycogen

The replenishment of muscle and liver glycogen stores depends on the type of exercise. Short distance high intensity exercise may take two to three hours and long endurance activates may take several days. Replenishment of glycogen stores is most rapid during the first few hours after training is complete. To restore glycogen stores quicker you can accelerate this with high carbohydrates.  

How The Cardiovascular, Respiratory, Neuromuscular, Energy And Skeletal Systems Adapt To Chronic Exercise

Chronic exercise

Long-term exercise is also known as chronic exercise.  Chronic exercise programme is one which the exercise period is not less than eight weeks. This chronic exercise will affect the body in many different ways allowing it to cope with greater stresses. These adaptations will allow you to exercise at higher intensities for longer. Responses to long term exercise include changes to the heart, lungs and muscles, although the extent of the changed depends on the type and intensity of exercise undertaken.

Cardiovascular endurance

The main adaptations that occur to the cardiovascular system through endurance training are concerned with increasing the delivery of oxygen to the working muscles.

Cardiac hypertrophy                                                                                                                                                                                        The cardiac muscle surrounding the heart hypertrophies, resulting in thicker, stronger walls and therefore increases in heart volumes. The more blood pumped around the body per minute, the faster Oxygen is delivered to the working muscles. Regular aerobic exercise stimulates the increase in both the thickness of the muscle fibres and the number of contractile elements contained in the fibres. An increase in heart size indicates the adjustments of a healthy heart to exercise training. It is the left ventricle that adapts the greatest extent as well as the chamber size increasing. These changes are reversible when you discontinue aerobic training.

Stroke volume

Stroke volume is measured in milliliters per beat. During physical activity, stroke volume can increase anywhere from 60 to 110 milliliters, depending on intensity. the stroke volume of the trained person will be higher than that of the non-trained person. This increase in stroke volume also leads to a higher cardiac output than the non-trained exerciser. This increase in cardiac output during your exercise can help your body meet its demands for oxygen to your exercising muscles. Long term exercise increases the size of the heart. This increase in size increases the athlete’s stroke volume. As a result of increased stroke volume and cardiac hypertrophy the resting heart rate decreases   

Cardiac output

The heart rate decreases after long term exercise because of an increased stroke volume and cardiac hypertrophy. The heart becomes bigger and can pump more blood per beat it and doesn’t have to beat as often when the body is at rest. But cardiac output decreases only slightly following long term exercise. During maximal exercise on the other hand, cardiac output increases significantly. This is a result of an increase in maximal stoke volume as maximal heart rate remains unchanged with training.

Heart rate

The resting heart rate decreases in trained individuals due to the more efficient circulatory system. The heart rate decreases after long term exercise because of a increased stroke volume and cardiac hypertrophy. Getting fitter causes a decrease in resting pulse rate. Some top athletes resting pulse rate between 30 and 40 beats per minute. As the heart becomes bigger it takes less effort to pump blood around the body and resting heart rate is reduced. Resting heart rate can decrease significantly following training in a previously sedentary individual. During a 10 week exercise program, an individual with an initial resting heart rate of 80 beats per minute can reasonably expect to see a reduction of about 10 beats per minute in their resting heart rate.

Blood volume, Vessels and Pressure                                                                                                                                                                   The long term effect of aerobic exercise is an approximately 20 percent increase in blood volume. An increase in blood volume means your body can deliver more oxygen to your working muscles. Your body will also be able to better regulate your body temperature during exercise. Blood volume is the amount of blood circulating in the body. Blood volume increases because of capillarisation during long term exercise. When the blood volume increases there is more blood to circulate which allows greater supply of oxygen to skeletal muscles. Arterial walls become more elastic which allows greater tolerance of changes in blood pressure. Blood plasma is also increased which changes the ratio of red blood cell volume to total blood volume this will lower blood viscosity making the blood flow more easily and reduce blood pressure. The number of red blood cells increases, improving the body’s ability to transport Oxygen to the muscles for aerobic energy production. Arterial walls become more elastic which allows greater tolerance of changes in blood pressure. Long-term aerobic exercise improves the elasticity of your blood vessels, or the ability of your vessels to expand and contract. The improved elasticity delivers more oxygen and glucose to your muscles at a faster rate. A long-term adaptation to aerobic exercise is a decrease in both your systolic and diastolic blood pressures during rest and during sub-maximal exercise

Capillarisation                                                                                                                                                                                               Long term exercise can lead to the development of a capillary network to a part of the body. The amount of capillaries and the capillary density increase which improves the cardiac and skeletal muscle cappillarisation because of aerobic training. The density of the capillary beds in the muscles and surrounding the heart and lungs increases as more branches develop. This allows more efficient gaseous exchange of Oxygen and Carbon Dioxide. An increase in the number and diameter of capillaries surrounding the alveoli leads to an increase in the efficiency of gaseous exchange.

Capillaries surround small air sacs, called alveoli, inside your lungs that capture the oxygen you breathe in. Your lungs adapt to regular exercise by activating more alveoli. More alveoli can supply more oxygen to working muscles and tissues throughout your body. Pneumonia occurs when fluids in your lung prevent alveoli from exchanging gases. Pneumonia is an inflammation of the lung tissue affecting one or both sides of the chest that often occurs as a result of an infection. Having more alveoli can suppress the effects of pneumonia by reducing the proportion of alveoli that are affected by this disease. Emphysema occurs when alveolar walls break down and gradually reduce the exchange of oxygen and carbon dioxide in your lungs. Regular exercises may help slow the progression of emphysema by increasing the number gas-exchanging alveoli.

Respiratory adaptations

Minute ventilation

Minute ventilation depends on breathing rate and tidal volume. In regular adults they can generally achieve 100 litres per minute however in a trained athlete minute ventilation can increase over time by 50 per cent to 150 litres per minute.

Respiratory muscles

Your diaphragm is a broad band of muscle that sits under your lungs and forms the base of a region known as the thoracic cavity by attaching to the lower parts of your ribs, sternum and spine. The intercostals form the muscle tissue in between individual ribs. The long-term effect of exercise is to build the endurance of these respiratory muscles, allowing deeper, fuller and more efficient breaths.Long term exercise can make the external intercostal muscles get stronger which make a greater degree of contraction so while the internal intercostals muscles relax during inspiration it means more airs forced into the lungs. During expiration the greater degree of contraction of the internal intercostals and the relaxation of external intercostals allows you to breath out a greater volume of air. This results in larger respiratory volumes, which allows more Oxygen to be diffused into the blood flow which is V02 max.

Resting lung volumes

Long term exercise will increase the surface area of the lungs which will allow a greater volume of deoxygenated blood to access to the sites of gaseous exchange within the lungs. The increased ability of the blood to take on more oxygen due to the increased surface area of alveoli, aids trained athletes a lot.  Vital capacity slightly increases along with tidal volume during maximal exercise the increased strength of the respiratory muscles are responsible for this change

Exercise exposes your lungs to stronger rushes of airflow. Aerobic exercise in particular exposes your lungs to strong and constant rushes of air. This activity helps clear mucus in your lungs. Mucus build up can diminish your lung capacity and lead to bacterial infections. “According to a 1997 "European Respiratory Journal" article by the University of Ulsan's Wong Don Kim, excessive mucus in your lungs is associated with higher mortality, may obstruct airflow and increases your risk of infections. Regular exercise can help offset these conditions by preventing mucus from building up in your lungs” This shows exercising regularly for a long time can help prevent risk of infections because exercise prevents mucus from building up in the lungs. 

Oxygen diffusion rate                                                                                                                                                                                                An increase in the number and diameter of capillaries surrounding the alveoli leads to an increase in the efficiency of gaseous exchange. An increase in diffusion rates in tissues favours oxygen movement from the capillaries to the tissues and carbon dioxide from the cells to the blood. Long term exercise causes these rates to increase allowing both oxygen and carbon dioxide to diffuse more quickly Capillaries are the smallest blood vessels in your body. Oxygen seeps out of thin capillary walls as carbon dioxide seeps in during respiration. Exercise stimulates vasodilation, which increases the diameter of blood vessels in your body, including the capillaries. Your body adapts to long-term exercise by increasing the size and number of capillaries, including alveolar capillaries. This adaptation makes the exchange of carbon dioxide and oxygen more efficient.

Neuromuscular adaptations

Hypertrophy

Long term exercise increases the cross sectional size of existing muscle tissue this is because of the increase in the number of myofibrils and connective tissue. High intensity training results in hypertrophy of fast twitch muscle fibres. This results in more ATP and PC stored in them and increased capacity to generate them due to more enzymes in the bigger muscles.

Tendon strength

Long term exercise increases tendon strength. Tendons have to adapt to meet increased demands of the skeletal muscles. Tendons connect muscle to bone and come under forces during exercise forcing them to adapt to become stronger to better deal with the forces applied, it also becomes of greater important to adapt and maintain a balance as the muscles they are attached to grow in size, results in a greater force and pull upon the bone, or improve in their ability to move in faster and more repeated bouts.

Myoglobin stores

The amount of myoglobin within skeletal muscle increases, which allows more Oxygen to be stored within the muscle, and transported to the mitochondria. Long term training makes muscles increase their oxidative capacity which is achieved by an increase supply of ATP also the ability of the muscles to store myoglobin. Oxygen delivery to muscles increased by increased myoglobin and capillarisation

Number of mitochondria                                                                                                                                                                                               With training muscles tend to increase their oxidative capacity. This is achieved by an increase in the number of mitochondria in the muscle cells. Increased numbers of mitochondria means an increase in the rate of energy production. Muscles consume more oxygen due to increased size and number of mitochondria therefore glycogen and fats are utilized more effectively. Increased numbers of mitochondria means an increase in the rate of energy production. Long term training increases the quantity of mitochondria for metabolism to take place, this in combination with a greater potential storage capacity for glycogen, fat and Myoglobin results in an increase in the output of ATP, particularly via the Aerobic pathway.

Storage of glycogen and triglycerides

Long term training makes muscles increase their oxidative capacity which is achieved by an increase supply of ATP also the ability of the muscles to store larger amounts of glycogen and the ability to use triglycerides as an energy source can also be stored. Endurance training causes an increase of muscle glycogen. Slow twitch fibres can grow by 22% therefore increasing the amount of glycogen stores so exercise can be prolonged

Neural pathways

Long term exercise makes the neural pathways change these changes are cellular adaptations, modifications of neurotransmitters, alterations in reflex and chemical and biochemical responses. Long term exercise also creates new neural pathways. Exercise gets blood to your brain bringing it glucose for energy and oxygen to soak up the toxic electrons and it stimulates the protein that keeps the neurons connecting. 

Energy system adaptations

Increased anaerobic and aerobic enzymes

Long term exercise makes muscle tissue generate ATP. The increase of the size of mitochondria is usually accompanied by an increase in the level of aerobic system enzymes. These changes account for why athletes can sustain prolonged periods of aerobic exercise as a result of long term exercise.

The anaerobic system increases in enzymes that control the anaerobic phase of glucose breakdown. Aerobic training will increase the number on mitochondria in slow twitch fibres, this allows greater production of energy by producing more ATP through the aerobic energy system.

Fats as an energy source

During exercise fat combustion powers almost all exercise. Fat oxidation increases if exercise extends over an hour. Towards the end of an exercise session fat accounts for 75 per cent of the total energy required. Long term exercise will make athletes burn fat as a fuel than non trained adults. Training will increase the amount of enzymes in our body needed to break down body fats and store it in our muscle tissue so it can be used as an energy source

Tolerance to lactic acid.

As a result of working anaerobically and enduring levels of lactic acid the muscles adapt in order to withstand greater levels of lactic acid in the future, this is coupled with the bodies increasingly improved ability to clear lactic acid and regulate current levels, as a result of a combination of the enhancements and adaptations of the other bodily systems to long term exercise and the body experience at clearing previous bouts of lactic acid. Long term exercise will saturate the muscles in lactic acid which educates your body in dealing with it more effectively. Anaerobic training will help our bodies tolerate lower PH levels. This means more energy can be produced by the lactic acid energy system

Skeletal adaptations

Increased calcium stores

Long term exercise slows the rate of skeletal aging. Athletes who maintain physically active lifestyles have greater bone mass compared to those who take part less. Weight bearing or resistance training will result in us becoming stronger and able to withstand impact better this happens because exercise means mineral content is increased making bones harder.

Tendon strength

Tendons attach muscles to bones or to muscles. Long term exercise increases tendon strength. Tendons have to adapt to meet increased demands of the skeletal. Strength training increases the strength of muscle tendons which makes them less prone to injury

Stretch of ligaments and stronger ligaments

Long term exercise will make ligaments increase their stretch so that they can cope with the body’s gain of muscle. The ligaments adaption occurs when fibroblast secretions increase the production of collagen fibres relative to the training undertaken. Ligaments become slightly stretchier which will help reduce injuries such as joint strains. Our bones are held together by ligaments. When exposed to regular exercise, ligaments will become stronger and more resistant to injury. Because ligaments have no or a very poor blood supply, any adaptations are very slow to develop.

Exercise increases the production of synovial fluid which keeps our joints lubricated and makes them supple. Synovial fluid production increases the range of movement available at your joints in the short term. Exercise increases the range of movement available at our joints as more lubricating synovial fluid is released into them. Mobility exercises such as arm circles and knee bends keep our joints supple by ensuring a steady supply of synovial fluid. Exercise also helps increase the thickness of cartilage in joints and increases the production of synovial fluid this will strengthen joints and make them less prone to injury.

Weight-bearing exercise such as strength training and running put stress through your bones. In response to this stress our bodies produce cells called osteoblasts which build new bone and make our bones stronger and denser. Increased bone density can prevent a condition called osteoporosis which is the weakening of bone and an increased likelihood of suffering fractures. 

How The Cardiovascular, Respiratory, Neuromuscular And Energy Systems Respond To Steady State Exercise.

Cardiovascular System

During exercise your contracting muscles require a continual supply of nutrients and oxygen to support energy production. These requirements of nutrients and oxygen are more than usual so your heart has to beat faster and harder to meet the increased demands. It also beats harder and faster to remove excess carbon dioxide.

Before exercise your heart rate usually increases above its normally resting levels. This is known as the anticipatory heart rate. The sympathetic nervous system controls adrenaline and therefore increases the heart rate before exercise and during it too.  An example is a sprint. If a athlete is about to have a race the athletes heart rate is likely to increase because of the event that is about to take place.   


Exercise is detected by the medulla oblongata. The medulla is the central nervous system located in the brain. The medulla detects if the body is doing exercise and then the chemoreceptors detect carbon dioxide in the blood and sends a signal to the medulla which then sends of a signal to the sympathetic nervous system that controls adrenaline. The sympathetic nervous systems then release blood into the heart which makes it increase its flow in blood and increases in cardiac output/stroke volume.     


Stroke volume is the amount of blood pumped by one of the ventricles of the heart in one contraction. The stroke volume is not all of the blood contained in the left ventricle because the heart does not pump all the blood out.  Stroke volume and heart rate together determine the cardiac output. Stroke volume increase to its highest levels during sub maximal exercise and does not increase further during maximal exercise. Stroke volume achieves its maximum amounts at between 40 and 50 per cent of V02 max. The greatest increase in stroke volume occurs from rest to moderate exercise. During maximal exercise stroke volume does not increase because the left ventricle is already full.    


Cardiac output is the volume of blood being pumped out of the heart in one minute. It is equal to the heart rate multiplied by the stroke volume. When doing exercise stroke volume increases.  An athlete doing exercise blood flow can increase sharply. The increase in blood flow is by an increase in stroke volume which allows more oxygen get to the skeletal muscles. An increase in cardiac output has a huge benefit for trained athletes as they can move more blood to the working muscles.  
Blood pressure is the pressure of the blood against the walls of arteries which is the effect of two forces. The first one is created by the heart as it pumps blood into the arteries and the other is the force of the arteries as they resist the blood flow. During exercise both blood pressure and cardiac output increase but they act to restrict the blood pressure rise and bring it down to an efficient level. Blood pressure has two readings systole and diastole. Diastolic pressure is the lower value and is when the heart relaxes and fills with blood. Systolic pressure is when the heart beats and contracts sending blood into the blood vessels.  During steady state exercise dilation of the blood vessels in the active muscles increases the vascular area for blood flow.  The contractions and relaxation of the skeletal muscles forces blood through the vessels and returns it to the heart.   


Blood flow is controlled by pressure, this is achieved by pressure by the vasoconstriction and vasodilation. Vasodilation is the widening of blood vessels due to the relaxation of muscular vessel walls, particularly in the large and small arterioles and large veins. Vasodilation involves an increase in the diameter of the blood vessels resulting in an increased blood flow to the muscle area supplied by the vessel. Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. Vasoconstriction involves a decrease in the diameter of a blood vessel walls resulting in the reduction of blood flow.

Respiratory system

Respiration is a process of passive and active breathing. The active process of breathing is known as inspiration and is controlled by the external intercostals and diaphragm. Expiration is a passive process and is the product of the diaphragm and external intercostals relaxing.   

Respiratory control centre (RCC) controls breathing (pulmonary respiration) and is located in the medulla. The respiratory control centre has two main systems, inspiratory and expiratory centres.

The inspiration centre takes in air while the expiratory centre takes out air. The inspiratory system is a active process so it contracts. The inspiratory centre sends impulses to the respiratory muscles via the phrenic nerves to the diaphragm and the intercostals muscles. This signal makes the diaphragm and the external intercostals muscles contract. When the external intercostals muscles contract they cause the ribs and the sternum to move upwards and outwards. When the diaphragm contracts it causes the central part of the diaphragm to flatten. When they both contract they increase the area in chest cavity. This increased area in the chest cavity lowers the pressure in the chest which means oxygen rushers in. This means your breathing rate has increased and this increase is due to changes in temperature, chemicals or active muscles.   

The expiratory centre takes out air. This system is a passive process so it recoils. It’s passive at rest but active during exercise. It recoils from the inspiratory contraction, so it’s the product of the diaphragm and external intercostals relaxing. When the diaphragm is relaxed the central part of the diaphragm rises and regains to its dome shape, when the external intercostals muscles relaxed so that means the ribs and the sternum move downwards and inwards. When all this is happening there is also a recoil. The recoil is like and elastic band or stretching a spring. When the recoil happens the chest cavity shrinks which means there’s greater pressure which causes carbon dioxide to rush out. 

As exercise intensity increases, so does your breathing rate. This increase of you breathing is to meet the demands of muscles which need oxygen. For example a trained athlete at rest might use about 250ml of oxygen per minute but they may require 3,600ml per minute during exercise. The medulla measures intensity by temperature, carbon dioxide levels and blood acidity levels.

When intensity reaches a certain point, expiration becomes more active, this will require the internal intercostals to contract. This is to help with getting rid of the carbon dioxide. When exercise reaches a level where oxygen cannot be taken in, is called your V02 max. If oxygen can be used at a constant rate it is known as steady state exercise.   

An increase in breathing rate prior to exercise is known as anticipatory rise. The anticipatory rise at the beginning of exercise is due to the release of adrenaline. This is controlled by the RCC.  When exercise begins there is an immediate and much greater increase in breathing rate. After seven minutes of aerobic exercise breathing continues to rise but at a much slower rate.

Tidal volume is the air ventilated per breath. Exercise results in an increase in minute ventilation. Minute ventilation is the volume of gas ventilated in one minute.      

Neuromuscular Junction

Neuromuscular junctions connect the end of a motor neuron to a muscle fibre. Neuromuscular junctions are attached to muscle fibres, which transmits nerve inpulses.

For a muscle to contract the central nervous system which is the medulla sends a signal to the motor neuron. The signal is action potential and could be to flex your arm. The motor neuron is a nerve that allows communication between muscles and the brain. Motor neuron attach to fibres so the bigger the muscle the more motor neurons it has. For example the quadriceps will have more motor neuron than the finger. The medullas signal also known as action potential gets sent down through the axon and arrives at the motor neuron. When the neuron gets the signal calcium gets released. Calcium gets released to help encourage the release of acetylcholine in the neuron. Acetylcholine is a neurotransmitter which transmits a signal to the muscle fibre. To transmit a signal it has to cross the synaptic cleft into the muscle fibre. When the muscle fibre gets this signal your arm will flex.    

Energy system

The body takes in chemical energy in the form of food, this is stored in the body in the form of adenosine triphosphate (ATP) which is a high energy compound that is converted into kinetic energy and used to create movement. The movement of muscles require ATP, muscles require a continued supply of ATP.

Skeletal muscles are powered by one compound ATP. The body only stores ATP in small quantities in the cells. It’s only enough to power for a few seconds of all out exercise, so the body has to replace or resynthesise ATP on a continual basis. ATP consists of a base which is called adenine and three phosphates. It is formed by a reaction between an adenosine diphospate (ADP) molecule and a phosphate. When a molecule of ATP is combined with water, the last phosphates groups splits off and energy is released.     

Creatine phosphate or Phosphocreatine or PC system. The PC system can operate with or without oxygen. During the first five seconds of exercise the PC system s relied on almost exclusively. The PC system can sustain exercise for 3 to 15 seconds at a high intensity. If the activity continues then the body must rely on another energy system. An example of a sport that users the PC system are 100m sprinters, they have to sprint intensely for 10 – 15 seconds.

Glycolysis is the breakdown of glucose. The end product of glycolysis is pyruvic acid. This can be used in the kerb cycle or converted into lactic acid. Fast glycolysis is another term used if the final product is lactic acid. Fast glycolysis can produce energy at a greater rate but the end product is lactic acid so it leads muscles to fatigue. This system last for 1-3 minutes. An example of a sport that users the lactic system is 400m runners.      

The aerobic energy system is involved with exercise at a low intensity and gets more important the longer the sport goes on. Only dehydration, lack of fuel and overheating will end exercise using this system. Aerobic systems fuelling varies according to its intensity and duration. In prolonged exercise free fatty fuel acids are used because glycogen stores are limited. For high intensity exercise the system prefers glycogen as fuel.

The kerb cycle is a series of aerobic chemical reactions occurring in the matrix in the mitochondria. The main purpose of the krebs cycle is to provide a continuous supply of electrons to feed the electron transport chain. The electron transport chain is a series of biomechanical reactions during which free energy contained within hydrogen which is derived from the kerb cycle is released so that it can be used to synthesise ATP.   

One of the three energy systems is dominant in contributing the energy required for the resynthesis of adenosine triphosphate (ATP). The contribution of each energy system is dependent on the intensity and duration of the exercise.