DPT-217-A&P: Energy Systems

You must first complete DPT-216-A&P: Endocrine System before viewing this Lesson

The Three Energy Systems

Chapter 5                                                                  The Energy Systems 

Muscles require energy in order to contract and create movement. Sometimes they work at low intensity and demand a slow, steady supply of energy. At other times those same muscles must contract with maximum speed and power, so stores of explosive energy have to be available instantly. 

The body has three inter-related ‘systems’ for supplying energy across this broad range of intensities, rather like a car has gears for driving at different speeds. The three systems are:

  1. Aerobic
  2. Lactic acid (anaerobic)
  3. Creatine phosphate (anaerobic)

Each system will contribute more or less to the total demand for energy depending upon intensity, and to a lesser extent on exercise duration, fuels available, and fitness level.

All three energy systems have this in common: they all re-synthesize a molecule called adenosine triphosphate(thankfully abbreviated to ATP)

Adenosine Triphosphate (ATP)

ATP is a molecule stored in small quantities within muscle cells ready to be broken down and liberate its chemical energy for muscular contraction. The adenosine base has three phosphate groups attached (hence triphosphate). Energy is stored in the bonds between the phosphate groups and, when a bond is broken, energy is released. 

When exercise starts these small stores of ATP are quickly broken down and used up, so they must be re-synthesised again almost immediately for energy supply to continue. 

The continuous cycle of ATP re-synthesis is summarised in the following diagram.

When ATP is utilised, liberating energy, it breaks down into adenosine diphosphate (ADP) molecule and a separate phosphate (P) molecule. The muscle cell must then re-combine ADP and P together in order to re-synthesise ATP. This process needs input of energy – and this is the function of the three ‘systems’. 

The three systems each supply energy to re-synthesise ATP. 

The ATP is then utilised for muscle contraction.

Aerobic System

The aerobic energy system utilises fats, glucose (from stored glycogen,a giant polymer of individual glucose molecules all joined together) and amino acids as fuels. These nutrients, together with oxygen (O2) are delivered to specialised cells within the muscles called mitochondria. There are many thousands of mitochondria inside our muscle fibres, each one specialised at metabolising fuels in the presence of oxygen to liberate energy. Mitochondria are particularly plentiful inside type 1 aerobic muscle fibres.

The process of energy production within a mitochondrion is called the ‘Citric Acid Cycle’ or the ‘Krebs Cycle’. At this level it is not necessary to understand all the details of this complicated chain of energy transfer. Instead, a summary of the process is enough.

Firstly, glycogen, fats and amino acids are transported and processed so that they can enter the mitochondrion. For example, stored muscle glycogen is converted into glucose, which is then converted to pyruvic acid (also called pyruvate). Assuming that oxygen is present, pyruvic acid then enters the mitochondrion. 

Once inside the mitochondrion there is a series of chemical conversions involving many different enzymes. Each step liberates energy that can be used to re-synthesise large quantities of ATP. At the end of the process the main waste products are molecules of carbon (C) and ions of hydrogen (H+). However, because oxygen is present, these molecules combine to make carbon dioxide (CO2) and water (H2O). Both of these molecules are then carried away from the muscle. The CO2travels in the blood to the lungs where it is exhaled. The H2O is re-used, re-absorbed or excreted in sweat or urine depending upon the body’s state of hydration.

Aerobic energy production will continue indefinitely, so long as fuels and oxygen are present. However, the rate of energy production is limited. This is partly because of the time it takes to mobilise and transport the fuels, but also because of the finite capacity to deliver oxygen to the mitochondria and to carry the carbon dioxide away again. Thus, the aerobic system predominates in low-to-moderate intensity activities like walking, jogging, swimming, cycling and aerobics classes.

The three fuels – fats, glycogen and amino acids – are not used in equal proportions. As exercise begins, glycogen contributes the largest proportion because it is the most locally available fuel (it is stored within the muscle itself). Then as exercise continues, the balance will gradually shift to obtain more energy from fat stores, as the process of transporting and metabolising them gets up to speed. The body prefers to use fats if possible because these form the most plentiful energy stores in the body. Tens of thousands of kilocalories are stored as fat, even on lean athletes, whereas glycogen storage is limited to about 1500 – 2000 kilocalories. 

Amino acids contribute least to energy supply. In fact, they are only utilised in significant quantity when carbohydrate reserves run low. This could be because a person has been exercising for a long time, running a marathon for example, or it could be that the individual is following a low carbohydrate diet and therefore has depleted glycogen stores in the first place. Note that the body does not have a ‘store’ of amino acids as such and that muscle protein must be broken down and converted in the liver to liberate glucose needed in the mitochondria for energy production (a process known as gluconeogenesis). Therefore, depleted glycogen stores can lead to a catabolic(muscle wasting) effect.

One interesting feature of aerobic metabolism is that fat cannot be used alone as a fuel for ATP generation. There must always be a mixture of fat and glucose. This is because glucose breakdown provides an important intermediate compound that is essential for effective fat metabolism. Without glucose this compound is unavailable and efficient fat metabolism ceases. Hence the saying:

‘Fat burns in the flame of carbohydrate’

Glucose should normally be present if there is sufficient carbohydrate in the diet to keep glycogen stores topped up, or, as previously described, limited glucose can be made in the liver from amino acids via gluconeogenesis. 

In the absence of glucose the human body is remarkably adaptable and does still manage to metabolise fats, but in a very inefficient way, and at the expense of producing ‘ketones’ that start to accumulate in the blood. Ketones are important because they are the only other fuel apart from glucose that can be used by the brain and nervous system. However, ‘Ketosis’ or ‘ketoacidosis’ can be potentially toxic to the body if it continues for any length of time (Wootton, 1989). 

It is unwise to severely restrict carbohydrate intake when undertaking any programme of high intensity training.

Lactic Acid System

Complete metabolism of fats, glucose and amino acids must happen inside the mitochondria and needs the presence of oxygen. The aerobic system does this very efficiently and yields large quantities of ATP. However, it is a relatively slow process because of the large number of steps involved in the transport of fats, oxygen and carbon dioxide, and the conversion via the citric acid cycle to useful energy.

If energy is needed more quickly, then the muscles use a second system that does not employ mitochondria or use oxygen. Muscle glycogen is converted to glucose, which is converted to pyruvic acid, which can then be broken down partially, in the absence of oxygen, to release energy rapidly. This is known as anaerobic glycolysis

However, the absence of oxygen creates a problem. In the aerobic system we saw that the hydrogen ions (H+) produced were able to combine with oxygen to form water (H2O) – a neutral by product that the body can easily deal with. But without oxygen, the Hionsremain and must be temporarily ‘stored’ in the form of lactic acid (also called lactate). As anaerobic glycolysis progresses the lactic acid/ Hionsaccumulate in the blood, causing the familiar burning sensation, fatigue, and eventually inhibiting any further muscle contraction. 

The lactic acid system therefore has a strictly limited duration before blood acidity becomes intolerable. Typically between 1 – 3 minutes of high intensity exercise is all that is possible. In these circumstances, it would be necessary to decrease the intensity of the workout, or stop altogether, so that more energy demand can be met from the aerobic system again. Accumulated lactic acid/Hionsare then gradually dispersed from the muscles and taken to other parts of the body for further processing. For example, liver cells can convert lactic acid back to glycogen for storage. During this recovery period breathing rate and heart rate are still elevated. The body is said to be in a state of ‘oxygen debt’as the aerobic system pays back the energy that has just been ‘borrowed’ from the anaerobic system to meet demand.

The lactic acid system predominates in high intensity activities such as weight training, circuit training, 400 – 800m running, and interval training.

Creatine Phosphate System

Maximal intensity ‘explosive’ activities such as sprinting, weight lifting, jumping, throwing and kicking, are all supplied by a third energy system. Within each muscle there is a small store of a high-energy compound called creatine phosphate (CP). Although CP cannot be used directly to bring about muscle contraction, it can easily donate its phosphate group to ADP, re-synthesising ATP in just one simple step.

The simplicity of the single-step process makes it very rapid at liberating energy. However, there is a limited capacity to store CP within the muscles. On average there is sufficient for about 6 – 10 seconds of maximal exertion. 

The creatine phosphate system is also sometimes referred to as the ‘phospho-creatine system’ or the ‘phosphagen system’.

CP is naturally synthesised in the body and stored in the muscles ready for use. Once used up, this store must be replenished again, ultimately drawing the energy to do so from the aerobic system. But this can only be done if exercise intensity is reduced or stopped completely. If complete rest is possible then roughly fifty percent of CP stores will replenish in about 40 seconds, then seventy five percent by 80 seconds and so on. Thus a recovery time of two to three minutes will be enough to replace most of the original CP reserves(Fleck & Kraemer, 1987). This makes a reasonable guideline for rest time between heavy weight training sets, or training sprints. Note that full recovery will take more prolonged rest.

It has been possible for many years now to buy a supplement called creatine monohydrate (it’s legal and can be bought in most supermarkets). In principle, strength and power athletes may find supplementation helpful in maintaining creatine levels inside muscles and speeding up CP re-synthesis. This means the athlete may be able to perform more intervals in the same amount of time. However, creatine supplementation is unlikely to be effective as a performance enhancer for less intense activities. Anyone thinking of trying creatine monohydrate supplementation should first take time to research any negative side effects they might experience.

Relative Contribution of Each Energy System

The relative contribution of each energy system to total energy usage depends upon several factors:

  • Intensity
  • Duration
  • Fitness level
  • Fuels available


The body will always favour the aerobic system as the ‘default’ because it is the most efficient at producing ATP from available fuel, because it can utilise plentiful fat stores (even on very lean individuals), and because it has no time limit. Only when immediate intensity demands increase beyond the capacity of the aerobic system will the anaerobic systems be called upon. The following table summarises this principle:

Type of ExerciseMain Energy SystemMain Storage Fuel Used
Maximal short bursts lasting less than 6 secondsATP – CPATP and CP
High intensity lasting up to 30 secondsATP – CP – Lactic acidATP, CP and muscle glycogen
High intensity lasting up to 15 minutesLactic acid – aerobicMuscle glycogen
Moderate-high intensity lasting 15 – 60 minutesAerobicMuscle glycogenAdipose tissue (fat)
Moderate – high intensity lasting 60 – 90 minutesAerobicMuscle glycogenLiver glycogenBlood glucoseIntra-muscular fatAdipose tissue (fat)
Moderate intensity lasting longer than 90 minutesAerobicMuscle glycogenLiver glycogenBlood glucoseIntra-muscular fatAdipose tissue (fat)

(Bean, 2003)


If even moderate exercise begins abruptly then for the first few seconds the aerobic system may not be able to meet demands. Thus the two anaerobic systems must contribute until breathing rate, heart rate, blood flow and fuel mobilisation can catch up with needs. When this happens you may experience the feeling of ‘second wind’ when the initial fatigue of starting exercise subsides, and a more comfortable steady pace can be established. A sensible way to avoid this initial fatigue is to build up intensity gradually over the first few minutes of training; in other words, use a warm up.

Fitness Level

A fit individual will be able to perform aerobic exercise at a higher level of intensity compared to an unfit person, who would have to also use their anaerobic energy systems to keep up, and who would inevitably become fatigued and have to stop sooner.

Fuels Available

Stores of glycogen are derived mainly from carbohydrates in the diet. This is important because glycogen is a critical fuel for any intense exercise – particularly for the lactic acid system where it is the only fuel. Any depletion of glycogen stores must shift the emphasis from anaerobic energy supply on to aerobic supply, with a corresponding decrease in exercise intensity.

In many sports all three systems play a significant role. For example, in football it is necessary to sprint, jump and kick a ball. These are all explosive activities requiring the creatine phosphate system. At other times it is necessary to maintain a high rate of work running about the field and drawing on anaerobic energy from the lactic acid system. But ultimately each player must be able to last for a full ninety minutes – relying upon their aerobic fitness. Other examples of sports that use a mix of all three systems include hockey, rugby, tennis, squash, modern pentathlon, and boxing.

Aerobic and Anaerobic Thresholds

In working muscles, both the aerobic and anaerobic systems act simultaneously all of the time. It is the relative contribution that they each make to ATP production that changes. Hence at low intensity aerobic will dominate, with only a very small contribution from the lactic acid system, and tiny background quantities of lactic acid being produced as a by product. Whereas at high intensity anaerobic will dominate, and large quantities of lactic acid build up in the muscles. This implies that, as exercise intensity gradually increases, there must be some point at which there is a change from mostly aerobic to mostly anaerobic. This is termed the ‘anaerobic threshold’. For active people, this will occur at a pace somewhere between a jog and a fast run. For sedentary people anaerobic threshold can often be exceeded by a brisk walk (ADNFS, 1992).

Technically the anaerobic threshold occurs when the muscles are producing lactic acid more quickly than they can clear it. This typically occurs when blood lactate exceeds a concentration of about 4mmol/l(mmol/l = ‘millimoles per litre’ – a scientific measure of the concentration of a chemical). Lactic acid then starts to accumulate, and the rising acidity causes burning, fatigue and eventually inhibition of muscular contractions. Anaerobic threshold is also referred to as the onset of blood lactate accumulationor OBLA. On a Borg 6 – 20 scale, OBLA typically occurs at around 16/17, and is accompanied by a ‘respiratory break point’ where breathing rate rapidly increases.

Practically speaking, OBLA is a useful parameter to know because it represents the maximum intensity at which the aerobic system can function. This sets the maximum pace that is sustainable for, say 20 – 30 minutes. If an athlete trained hard over a number of weeks to increase their OBLA this would mean they could sustain a faster pace – a useful advantage for a 5k or 10k runner.

OBLA can also be used to set the work and rest intensity for interval training. For example, the work interval can be performed slightly above OBLA, so long as the recovery interval that follows is below the OBLA intensity to give aerobic recovery.

Another useful parameter to know is the ‘aerobic threshold’. This happens at a lower level than the anaerobic threshold and is usually defined as the intensity at which blood lactate reaches a concentration of 2mmol/l. On a Borg 6 – 20 scale, aerobic threshold typically occurs at around 13/14. It represents the shift from ‘moderate’ to ‘vigorous’ intensity.

This threshold is useful because it seems to be a good indicator of fuel usage. Below this level, fat is the predominant fuel. Above this level, glycogen is the largest contributor. This knowledge is important for long-distance events such as a marathon where it is important to utilise fat and conserve limited reserves of glycogen. So whereas 5k and 10k races are run near OBLA pace, a marathon is completed at a more modest aerobic threshold pace. 

Aerobic threshold: Blood lactate concentration of 2mmol/l

Anaerobic threshold: Blood lactate concentration of 4mmol/l

Improving the Capacities of the Three Energy Systems

The capacity of all three energy systems can be improved in the long term with a consistent programme of specific training. 

Aerobic System

Specific training for the aerobic system involves rhythmic, continuous exercise that utilises the large muscles of the legs and torso. Examples include: walking, jogging, swimming, cycling and aerobics classes. Intensity of training should be moderate-to-high. Anaerobic threshold gives an indication of the ceiling to training intensity.

Adaptations to the aerobic energy system include the following:

  • The number of capillaries supplying oxygenated blood to the muscle, and removing deoxygenated blood from the muslce, increases, particularly those associated with type 1 fibres.
  • The size and number of mitochondria within the type 1 muscle fibres increases. This improves aerobic energy production.
  • The quantity of aerobic enzymes (proteins that speed up metabolic reactions) inside the mitochondria increases. This facilitates better energy production.
  • There is an improved ability of the muscle to use fat as a fuel. The fitter aerobic energy system is therefore ‘glycogen sparing’.

(Sharkey, 1990)

Lactic Acid System

Specific training for the lactic acid system involves high intensity exercise above the anaerobic threshold. This is typically achieved with interval training. Work intervals of one to three minutes duration, performed at a level just above OBLA, are sufficient to place great demands on this energy system. Resistance training sets of eight repetitions or more, with a limited rest period, will also overload this energy system. Regular anaerobic training of this type leads to several long-term adaptations.

The lactic acid energy system depends on availability of muscle glycogen, and regular anaerobic training depletes those limited glycogen stores. Glycogen replenishment after training therefore becomes a critical factor. The enzyme glycogen synthetasefacilitates this replenishment, and both the amount and the activity of this enzyme increase in athletes who regularly exceed their anaerobic threshold during training.

  • The number of capillaries to the muscle increases, particularly those associated with type 2 fibres. This improves the capacity of the muscles to flush out and clear lactic acid.
  • The ‘buffering’ capacity of the blood improves. The natural mechanisms within the body that control blood pH levels become better adapted at neutralising high levels of lactic acid/H+ions.
  • The individual becomes more familiar with the burning, fatiguing sensation of high blood lactate levels. Psychologically they become more able to tolerate higher levels of blood acidity.

(Sharkey, 1990)

Creatine Phosphate System

Specific training for the creatine phosphate system involves bursts of maximal intensity exercise, each performed for only a few seconds, followed by several minutes of rest. Examples include sprint intervals, heavy resistance training (Olympic lifting, power lifting, etc.), or plyometric jump training. Regular anaerobic training of this type leads to several long-term adaptations: 

  • The capacity of the muscles to store creatine phosphate increases. This means thatmaximum intensity can be generated for a slightly longer duration. This is important to the power athlete. For example, in a 100m sprint the ability to maintain top speed for the second half of the race is critical and makes the difference between winning and losing (Brenkus, 2010).
  • There is increased activity of the enzyme creatine kinase. This enzyme facilitates the conversion of creatine phosphate to ATP + P and vice versa. In other words it speeds up the energy release from the creatine phosphate system, and also reduces the time it takes to replenish levels within the type 2 muscle fibres during recovery. 

Energy Systems Summary

The table on the following page summarises the main points about energy systems:

Back to: Diploma in Personal Training (NVQ) > DipPT - Module 2 (universal-teacher)