You might work through this system over several weeks, days or hours, but to enhance your learning and enjoyment make sure you break it up into bite-size chunks.
Here are the sections of energy systems:
As you study energy systems you will learn about:
- Foods for energy
- How we store energy
- Factors affecting which fuels we use
- How we replenish energy substrates
- ATP-PC system
- Anaerobic glycolysis
- The aerobic system
Make notes as you study each section, and interact fully with the activities – watch the animations and complete the quizzes.
Take a break at the end of each section– resting your eyes from the computer screen, getting some fresh air or taking a coffee break will improve your ability to focus on your study and take in information.
Give yourself time to think about what you have learned, and time to absorb and understand it.
We take in food to use as energy in the form of three macronutrients - carbohydrates, proteins and fats. Each nutrient offers a certain amount of energy per gram:
- Carbohydrates contain 4 calories per gram
- Protein foods contain 4 calories per gram
- Fats contain 9 calories per gram
These are approximate but commonly used figures when calculating the energy density of food. Most foods are a combination of different macronutrients, and it should also be considered that food often contains fibre and water, none of which provide calorific energy value. Alcohol also provides energy - 7 calories per gram - but it is not a preferred energy source, and must first be oxidized in the liver into fatty acids to be used for energy.
Healthy proportions of nutrients in the diet
These charts show the recommended proportions of caloric intake from carbohydrates, fats and proteins for a sedentary individual and for a sports person. Generally speaking, the more exercise a person does, the more carbohydrate they need to fuel their activity.
Additional protein may be needed to promote lean tissue growth, strength, power and recovery, but additional carbohydrate is still needed in strength sports, to provide energy for the exercise itself. Body fat stores contain a large amount of energy, so only endurance athletes may need to increase the amount of fat they consume.
Although we use all three macronutrients during exercise, carbohydrates and fats are stored for energy, whereas proteins create lean tissue deposits (muscle) rather than energy stores.
ACSM (2000) Healthy Diet (for sportsperson)
COMA (1991) Healthy Diet (for non sportsperson)
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ACSM (2000) Healthy diet (for sportsperson):
- Carbohydrate (60%)
- Fat (25%)
- Protein (15%)
COMA (1991) Healthy diet (for non sportsperson):
- Carbohydrate (50%)
- Fat (30-35%)
- Protein (15-20%)
Energy from food
Foods are composed mostly of carbon, oxygen and hydrogen atoms, with protein molecules also containing nitrogen or sometimes sulphur atoms. The energy in food molecules is released within our cells and stored in the form of a high energy compound called adenosine triphosphate (ATP).
At rest, the energy released from the breakdown of carbohydrates and fats is enough for our energy requirements, but whenever the level of activity increases or the body is put under a strain, we have to generate more ATP quickly to provide the additional energy required. Glucose is our main energy source: it circulates in the blood stream and is stored in the muscles as glycogen. There are three ways in which we replenish our levels of ATP in the cells, and glucose is used in all three.
Glucose from carbohydrate foods: fruit, vegetables, rice, pasta, potatoes, etc..
An amino acid from protein foods: meat, fish, eggs, dairy foods, beans, etc.
A triglyceride from fatty foods: butter, cheese, oils, etc.
Carbohydrate energy - glucose and glycogen
Carbohydrate foods such as potatoes, rice, pasta, beans and vegetables are broken down into monosaccharides during digestion. The most common of these is glucose. We constantly use glucose for energy and need a continuous source of glucose in the blood to fuel the brain. Low blood sugar levels make us feel lethargic, have poor concentration, and we may begin to feel faint. At this time, we crave sweet or carbohydrate-rich foods to elevate our blood glucose levels. Glucose is stored in the liver and the muscles as glycogen, and we maintain a steady blood glucose level by converting stored glycogen back into circulating glucose as required.
We use a hormone called insulin to turn excess glucose into glycogen, and a hormone called glucagon to convert glycogen back into glucose.
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- Glucose (travels through blood stream)
- Insulin turns excess glucose into glycogen
- Glycogen (stored in liver and muscles)
- Glucagon converts glycogen back into glucose
Stored carbohydrate energy
The average person stores approximately 1500-2000 calories as glycogen, enough for an average of 90 minutes of exercise before fatigue begins. At this stage, we would usually stop exercising and re-fuel (eat!). For lower intensity exercise, the main limiting factor is the amount of glycogen stored. As glycogen stores become low, the amount of glucose in our fuel mix reduces and we use more fat for fuel as glycogen levels become depleted. Eventually, glycogen stores become so low that muscular fatigue sets in.
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Total glycogen stored 375-475g
- Blood - 15-20g of glucose
- Liver - 100g of glycogen
- Muscles - 325g of glycogen
Adipose tissue for energy
Fats can be found in oils, butter and margarines, meat and fish, dairy produce such as milk, yoghurt and cheese, nuts and seeds, and also in biscuits, cakes and chocolate. Fats are used during lower intensity and longer duration activity.
Although fat can be used for energy whilst it is circulating in the blood stream, it is stored as adipose tissue, creating a vast energy store of thousands of calories.
Some sports people maintain very low body fat levels, such as those seen in long distance runners and gymnasts, but higher amounts of stored fat can be useful in some sports such as long distance swimming, as it insulates the body as well as providing energy.
Protein for energy
Foods rich in protein include meat and fish, dairy produce, nuts and seeds, soya products, and beans and pulses.
Protein is not stored for energy, although amino acids in the blood stream or lean tissue can be utilized for energy in the absence of carbohydrate or fat. Amino acids are either glucogenic – they can be converted into glucose for fuel in the liver, or ketogenic – used as ketone bodies for fuel. This usually happens following very low calorie diets, low carbohydrate diets, or during very long duration exercise sessions.
Protein is not a preferred energy source, and catabolism (breaking down) of lean tissue to provide energy is generally considered as unfavorable, as this would reduce metabolic rate, strength, power and endurance, and have virtually no benefits for the average exerciser.
Factors affecting with fuel we use to create energy
Whether we use carbohydrates, fats or even proteins to create more ATP is dependent upon several factors:
- Duration of exercise
- Intensity level of exercise
- Type of activity
- Fitness levels and training effects
Each of these factors will affect how much energy we use up, and what type of nutrient is used to create more ATP.
Duration and intensity
The longer the exercise duration, the more energy is required and used up. In addition, when we exercise for longer periods of time we tend to exercise at a lower intensity, and this also affects what type of fuel our body uses.
You will soon be learning about different types of energy systems. Some are anaerobic, meaning without oxygen, and others are aerobic, meaning with oxygen.
Anaerobic activities tend to be very high in intensity, so the harder you exercise, the more you will use your anaerobic energy systems to replenish ATP and create energy. Anaerobic energy systems are fuelled by glucose, so high intensity exercise tends to utilize more carbohydrate.
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Y axis: Percentage in fuel mix, 0 - 80
X axis: Duration in minutes, 30 minute increments to 210 minutes
- red line (lower left to upper right) = fat
- blue line (upper left to lower right) = carbohydrate
Exercise that is lower in intensity requires fuel at a slower rate, so the aerobic energy system can be used, and other substrates such as fat can be utilized. In most lower intensity, longer duration activity, a combination of glucose and fatty acids are used for energy. This graph provides an example of the proportions of fat and carbohydrate in the fuel mix as duration increases. Because we only store approximately 90 minutes to 2 hours worth of glucose in the form of glycogen, as time goes on, the amount of carbohydrate used decreases. This is for two reasons:
- The body has less glycogen left to use.
- As glucose is such an essential fuel source even at rest, when stores begin to get low, the body creates a ‘glycogen sparing’ effect, holding on to the remaining glycogen, and using more fat for fuel.
'Fat burning' exercise
Because of this change in the proportion of carbohydrate and fat used during an exercise session, some activities and levels of intensity have been termed ‘fat burning’ in the past, indicating that we utilize more fat if we exercise at a lower intensity. Whilst it is true that a higher proportion of fat is utilized when exercising at a lower rather than a higher intensity, the actual number of calories used is much less in lower intensity exercise, and so the actual amount (volume) of fat used may not be higher at all, unless the exercise duration is much longer. Take a look at the examples here, and try to decide which type of run is best...
To run, or to jog... that is the question!
So although a higher proportion of fat is used for fuel in lower intensity workouts, because fewer calories are used overall, we may benefit more from a higher intensity exercise session that uses up more calories.
This does not mean that a long steady jog, or similar long duration, lower intensity workout is without its merits, but it may not be the most effective workout for ‘fat burning’ or using up more calories after all!
A variety of different types of exercise, at a range of intensity levels, is the most effective way to achieve long term health and fitness.
Type of activity
The type of exercise we do will affect the fuels used for energy, as the exercise will be either low or high intensity, will be of a certain duration, and will be either aerobic (with oxygen), or anaerobic (without oxygen).
Remember, anaerobic activities such as weight training, shot put or power lifting will use glucose as a primary energy source, whereas longer duration aerobic activities such as cycling, swimming or jogging will use a combination of carbohydrate in the form of glucose and fat.0
Of course, the harder you work at any activity, the more anaerobic it will become as the intensity increases, so exercises such as cycling or swimming become more anaerobic the faster you go.
Level of fitness and training effects
Your level of fitness also affects the amount of each substrate you use during exercise. Those new to exercise, or new to a specific type of exercise find it more difficult, which increases the intensity and can utilize more fuel. As we exercise more often, our body adapts and becomes more adept at providing fuel for any exercise regularly undertaken. For example, regularly engaging in long duration sports such as running marathons or long distance swimming can have the following training effects:
- Storing more carbohydrate as glycogen, although there is an upper limit to how much we can store.
- Storing more adipose tissue adjacent to muscle, allowing for greater utilization of fat for fuel earlier on in the exercise session.
- Using more fat for fuel allows us to preserve more glycogen, enabling the exerciser to have more energy for a longer period of time.
Training adaptations like these occur following any regular exercise routine, and will be specific to the type of exercise undertaken.
It is essential to replenish energy stores after exercise, otherwise exercise performance is detrimentally affected. Many exercisers fail to consume enough carbohydrate to fully replenish stores before their next exercise session, which makes subsequent workouts difficult.
As the body is depleted in stored glycogen (carbohydrate) after exercise, it is more likely to replenish glycogen stores soon after exercise rather than later. Exercisers should consume carbohydrate foods within two hours of finishing an exercise session whilst the muscles’ capacity to refuel is at their greatest. It can take as long as 48 hours to fully replenish glycogen stores, so meals and snacks should continue to be based upon carbohydrates throughout this period.
To replenish glycogen levels, a carbohydrate-rich snack such as a sandwich, a bowl of cereal or a high energy sports bar should be consumed soon after exercising if a carbohydrate meal is not available, and then a balanced meal consumed later on.
Protein foods are usually deemed important for those participating in strength, size or power sports. However, extra protein is also required for endurance sports as it enables recovery of muscles, tendons and connective tissue, and maintains and promotes muscular strength.
After 60 – 90 minutes of exercise, amino acids are likely to be used for fuel and will need replacing. Protein also boosts immune function – particularly important for endurance sports, and provides the protein and micronutrients required for oxygen transport.
However, large amounts of protein consumed in one meal will simply be converted into fatty acids and utilized as energy or stored. Although the general guideline for protein requirements is 45g/day for women and 55.5g/day for men, you can calculate individual protein requirements based upon your body weight and sport.
|Protein per kg body weight daily
|Average person:||1g / kg|
|Endurancy activity:||1.2 - 1.4g / kg|
|Strength activity:||1.2 - 1.7g / kg|
|Maximum:||2g / kg|
If exercise is done on a regular basis, energy requirements are greater, and more food should be consumed. Endurance activities may require more carbohydrate foods in the diet, strength activities may benefit from a little more protein, but otherwise, the proportions of a healthy diet should remain largely the same, with slightly greater amounts of all macronutrients eaten to meet the demands of regular exercise.
With the exception of very long duration activities such as long distance swimming or cross country ski-ing, it is unlikely that additional fat is required in the diet to replenish fat used up for energy, as any excess protein or carbohydrate is converted into adipose tissue and stored.
The energy systems
We have approximately 3 seconds of energy stored in the form of ATP in each cell. Once this is gone, we replenish our ATP stores through three different energy systems. The first two are anaerobic (energy produced without oxygen), and the third is aerobic (energy produced with oxygen).
- ATP – PC system
- Anaerobic glycolysis (lactic acid system)
- Aerobic glycolysis
ATP energy in the cell
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- Body cell
- ATP energy in the cell
Legend: orange circle = ATP
Fuels used to create ATP
Glucose is the key fuel for energy as it is constantly in the blood stream, so it is used in all three energy systems. The glucose molecule is broken down and energy is stored as a high energy compound called adenosine triphosphate. The third energy system, used during longer periods of exercise and for lower intensity activity, utilizes carbohydrate (glucose) and fat for energy, the proportions varying as the exercise session progresses.
Obtaining useful energy from food
Anaerobic energy is used for sudden energy requirements over a short period of time - sprinting to catch a bus for example. During peaks of activity, insufficient energy is provided through the oxidation of glucose in the aerobic process, so the extra energy required is provided through the anaerobic release of energy from stored and regenerated ATP or partial breakdown of glucose creating more ATP. Apart from these higher peaks of intensity when extra energy is required, we use the third, aerobic energy system, aerobic glycolysis.
The first two anaerobic energy systems are useful in activities which require short bursts of energy such as...
- Weight training
- Martial arts/boxing in short rounds
- Shot put or javelin.
ATP – PC system (Adenosine Triphosphate - Phosphocreatine System)
This system is the simplest of the three energy systems, and provides a rapid, but limited supply of energy when required. ATP is made from a base (adenosine) and it is attached to three phosphate groups. The last phosphate group is attached by a very high energy bond which releases energy when it is broken.
The energy for the contraction of muscles is derived from ATP. When adenosine triphosphate is broken down into adenosine di-phosphate + phosphate, this releases energy. About 43% of the energy released is used to produce ATP in the mitochondria and for other cellular activities, the rest is lost as heat.
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Adenosine -- Adenine, Ribose
Adenosine diphosphate (ADP)
Adenosine triphosphate (ATP)
Replenishing the phosphate
The ATP-PC system only provides energy for no more than approximately 15 seconds of activity and during this time, ADP is turned back into ATP by replenishing itself with another phosphate group from a substrate in the muscles called phospho-creatine (or creatine phosphate).
An enzyme called creatine kinase breaks down phosocreatine to release a spare phosphate molecule and energy. The phosphate molecule re-groups with the adenosine di-phosphate to re-create adenosine triphosphate. This cycle continues until the store of phosphocreatine in the muscle fibres is used up. This reaction takes place within each cell of the body. Each muscle cell (myofibril) contains mitochondria which generate the adenosine triphosphate (ATP). Muscle and liver cells are very active cells, and as such, contain a greater number of mitochondria.
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- 3rd phosphate detached from adenosine triphosphate, energy released from high energy bond.
- Phosphate from phosphocreatine is added to adenosine diphosphate to recreate adenosine triphosphate.
- This will re-create ATP for up to 15 seconds.
A (green) = Adenosine
P (yellow) = Phosphate
PC (oragne) = Phosphocreatine
Labels: Adenosine triphosphate, energy, adenosine diphosphate
After the ATP - PC system
Once the available ATP and creatine phosphate is exhausted, the anaerobic glycolysis system takes over. This is also called the lactic acid system as lactic acid is a by-product, and this system is limited to no more than 3 minutes of exercise. A build up of lactic acid is one of the limiting factors of this anaerobic energy system.
Hence, once these two energy systems are exhausted, muscular fatigue sets in. These two energy systems are generally used at the beginning of any activity, or during spurts of high intensity exercise.
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Glucose is partially broken down producing 4 ATP molecules of energy. Lactic acid is an end product.
Anaerobic glycolysis (the lactic acid system)
Anaerobic glycolysis is an anaerobic reaction which yields 2 molecules of ATP for energy and serves high intensity, immediate energy requirements for up to approximately 3 minutes. Anaerobic glycolysis means splitting of glucose without oxygen. Glucose is only partially broken down, and the by-product is lactic acid.
As soon as glucose enters the cell cytoplasm it becomes phosphorylated, combining with a phosphate group to form glucose-6-phosphate. This uses up 1 ATP and traps glucose in the cell so that it cannot diffuse back out. Using energy from another ATP molecule the glucose is split into two 3-carbon molecules of triose phosphate - these are oxidized into two 3-carbon pyruvic acid molecules and 4 ATP molecules are generated. As two ATP are used up, but four ATP are yielded when pyruvic acid is formed, this process produces 2 ATP molecules per molecule of glucose.
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- Energy from 2 ATP activates glucose
- Glucose splits into two 3-carbon sugars (triose phosphate) which are oxidised to pyruvate
- 4 ATPs of energy and 2 NADH (reduced form of nicotinamide adenine dinucleotide) are created
- Pyruvate is reduced by the NADH to make lactic acid. NADH becomes NAD (co-enzyme nicotinamide adenine dinucleotide)
Co-factors NAD and FAD
NAD and FAD are derivatives of vitamins B3 and B2 which act as co-enzymes (helpers) in energy reactions. They are involved in reduction and oxidation (‘redox’ reactions), accepting and releasing electrons and H+ ions to become reduced and oxidized. These reactions can form energy in the form of ATP: each molecule of NADH creates 3 ATP and each molecule of FADH creates 2 ATP.
NAD is nicotinamide adenine dinucleotide, a co-enzyme found in all living cells. NAD+ is involved in redox reactions, carrying electrons from one reaction to another. NAD+ is an oxidizing agent, accepting electrons from other molecules and becoming reduced forming NADH, which can be used as a reducing agent to donate electrons.
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FAD is flavin adenine dinucleotide. It can be reduced to FADH2 by accepting two hydrogen atoms, then re-oxidized to re-form FAD.
A word about glycolysis (glycolytic and oxidative)
Glycolysis is the breakdown of glucose for energy, and may be either anaerobic or aerobic. Without oxygen present, glucose is only partially broken down in anaerobic glycolysis to yield a net gain of 2 molecules of ATP energy. This reaction is called glycolysis, and stops at the lactate-pyruvate stage. Without oxygen lactic acid is formed; with oxygen, acetyl co-enzyme A is formed and the series of reactions continue on to the Kreb’s cycle and produce an additional 36 molecules of ATP for each glucose molecule oxidized. This is complete breakdown of glucose. Production of ATP through aerobic glycolysis is much slower than through anaerobic glycolysis, so anaerobic energy systems are employed when cells have insufficient oxygen to meet the energy demands placed upon them.
Without oxygen, the pyruvic acid acquires extra hydrogen atoms and is reduced to lactic acid, which accumulates in active muscles. Excessive quantities of lactic acid can produce muscle fatigue and may induce cramp, although it quickly diffuses out of the cell into the blood stream and is transported to the liver, where it is converted back into pyruvic acid.
For this process, there is an additional requirement for oxygen, known as the oxygen debt which is met through breathing deeply long after exercise has finished.
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Main text describes breakdown of glucose shown in diagram
Lactic acid and muscular fatigue
The main cause of fatigue during anaerobic exercise is the accumulation of lactic acid in muscle tissue and a reduced muscle pH due to the accumulation of acid. This occurs as the lactic acid splits to form lactate and hydrogen ions. Hydrogen ions increase the acidity in the muscle tissue (lowering the pH), and create a condition known as acidosis. This inhibits enzyme activity in the mitochondria, reducing the amount of energy released.
Normal blood lactate levels are 1 – 2 mmoles/litre, and this level remains stable during aerobic exercise. Due to increased lactic acid production during anaerobic glycolysis, lactate levels increase to 4 mmoles/litre during medium intensity exercise, and up to 30 mmoles/litre with high intensity exercise. During sprints lasting 1 or 2 minutes, the demands on the anaerobic energy system are high, and muscle lactic acid levels can increase from 1 mmol/litre to 25 mmol/litre.
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Normal: 2 mmoles / litre
Medium intensity anaerobic exercise: 4 mmoles / litre
High intensity anaerobic exercise: up to 30 mmoles / litre
Blood lactate levels
An increasing blood lactate level is known as onset of blood lactate accumulation (OBLA) or the lactate threshold. This is often measured in athletes to monitor endurance – the OBLA level is the moment when blood lactate levels reach 4 mmoles/litre. Through training and the physiological adaptations that take place with regular training, the OBLA can be delayed, as our body becomes more adept at removing lactic acid (lactate) from the muscle and delaying fatigue.
Recovery following strenuous exercise restores muscular efficiency in four ways:
- Replacement of ATP and PC
- Removal of lactic acid
- Replenishing myoglobin with oxygen
- Replenishment of glycogen stores
The lactacid debt
The lactacid debt is the amount of oxygen required to remove lactic acid from the muscles. Lactate is taken via the circulation to the liver, where it is converted back into pyruvate and then oxidized, producing carbon dioxide and water. Some lactic acid is converted back into glucose or glycogen, and a small proportion is converted into protein.
Removal of lactic acid may take about one hour. This depends on whether the athlete rests during recovery (passive recovery) or performs light exercise (active recovery). Active recovery, or cool-down, aids the removal of lactic acid by increasing circulation and maintaining a greater supply of oxygenated blood to the muscle tissue.
Aerobic glycolysis (oxidative glycoloysis)
The third energy system fuels most cellular activity and lower intensity exercise when the body has time to take in additional oxygen which cells use for complete oxidation of glucose. This takes more time, but produces more energy. Each glucose molecule is split into two pyruvate molecules which both undergo the same set of reactions, creating 38 molecules of ATP energy in total from each glucose molecule.
This third energy system is made up of four stages:
- Glycolysis (conversion of glucose into pyruvic acid in the cell cytoplasm)
- Formation of acetyl coenzyme A in cell mitochondria (The link reaction)
- Kreb’s cycle (occurs in the mitochondria only during aerobic glycolysis as this step requires oxygen)
- The electron transport system (only in aerobic glycolysis).
Glucose is completely broken down in aerobic glycolysis
C6H12O6 + 6O2 => 6CO2 + 6H2O
Glucose + Oxygen => Carbon dioxide + Water
The Kreb's cycle (citric acid cycle)
After glycolysis, pyruvate is converted into acetyl co-enzyme A, and this enters the Kreb’s Cycle (also known as the Citric acid cycle or Tricarboxylic cycle). It is a series of chemical reactions oxidizing glucose which take place in the mitochondria of a cell. Acetyl coenzyme A enters the Krebs cycle and combines with citric acid to create a 6 carbon molecule. This is broken down to release carbon dioxide, NAD, FAD and ATP.
The H+ ions produced reduce NAD to NADH during glycolysis, the link reaction and Kreb’s cycle, and FAD to FADH in the Kreb’s cycle. These then enter the electron transport chain, releasing ATP energy as they create redox reactions.
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The electron transport chain
In the electron transport chain, NADH and FADH undergo redox reactions creating ATP, and in a final reaction, hydrogen and oxygen are converted to water.
Glucose has been completely oxidised; the stored energy has been released step by step, yielding 38 molecules of ATP from each molecule of glucose. The by-products of aerobic glycolysis are carbon dioxide and water.
Energy released in these reactions has been trapped in three ways:
- By making ATP
- By reducing NAD to NADH and
- Reducing FAD to FADH, both of which go on to produce ATP.
38 molecules of ATP have been created:
- 2 in glycolysis
- 2 in the Kreb’s cycle
- 30 ATP from NADH
- 4 ATP from FADH.
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- NADH loses electron
- NAD (red circle)
- Electron (yellow circle)
- 3 electron carriers, changing to reduced and then oxidised state
The energy continuum
Some activities rely mostly on one energy system such as the ATP-PC system in weight training, anaerobic glycolysis in a sprint, and aerobic glycolysis throughout most of a marathon.
However, many types of exercise include activity at different levels of intensity and use more than one energy system to replenish energy. The energy continuum describes the types of energy system used during various physical activities.
For example, in team sports like football or netball, there are periods of explosive activity and periods of less intense exercise, so the proportion of aerobic and anaerobic respiration will vary.
|Sport||ATP-PC and LA||LA-O2||O2|
Approx % of energy system used
You have completed your study of the energy system.
You should now have a good knowledge and understanding of the anatomy and physiology of the energy system. You should be able to...
- Name the major nutrients that provide us with energy and know which foods these are found in
- Explain how we store and replenish energy in the body
- Describe the terms anaerobic and aerobic
- Explain how each energy system provides us with energy:
- The Adenosine Triphosphate-Phosphocreatine system
- Anaerobic glycolysis (The Lactic Acid system)
- Aerobic Glycolysis