Section 4: Energy Systems

All functions of the body require energy, including muscle contraction to move the skeleton, digestion of food, respiration and repair and growth of tissues.

In order for these key functions to take place, important chemical reactions are continually occurring at cellular level throughout the body.

Adenosine triphosphate

All living organisms need the high-energy molecule, adenosine triphosphate (ATP) to function, much like a car needs petrol or diesel.

ATP is considered the energy currency of cellular life and the only fuel the human body recognizes and uses. Its role is to capture chemical energy, obtained from the breakdown of food molecules, and release it to fuel other cellular processes.

Structure of ATP

ATP is composed of one adenosine molecule bonded with three phosphate molecules. ATP releases its energy when one of its high-energy phosphate bonds is broken and it is converted to adenosine diphosphate (ADP). When this high-energy bond is broken down, energy is released.

There is a very limited store of ATP within the muscles; it only lasts for approximately 1-2 seconds. Once the limited store of ATP has been used up, it has to be remade. Re-synthesis of ATP comes from the breakdown of either phosphocreatine (another chemical in the body) or certain nutrients in the diet, such as carbohydrate, fat and protein, which can be used to replenish stores.

The different energy systems

There are three energy systems, which all use different fuels to convert the ADP back into ATP for use by the cells. These energy systems are: 

Creatine phosphate (CP) or phosphocreatine system

The CP system provides ATP to primarily fuel high-intensity, explosive activity such as a shot put, 100m sprint or power lifting movement. It is also used to initiate most movement, regardless of intensity, as it is readily available in muscle tissue and doesn’t have to wait for the heart to push oxygen to the specific areas.

CP benefits significantly from its chemical makeup, which allows it to regenerate ATP almost immediately by using its phosphate molecule:

Creatine phosphate, like ATP, has a high-energy bond which, when broken down by the enzyme, creatine kinase, releases enough energy to yield an ATP molecule. This chemical reaction is very quick but only lasts for a very short period of time because of limited creatine phosphate stores. The CP system is exhausted after a maximum of ten seconds.

Lactate system

The lactate system is used to predominantly fuel high-intensity exercise that lasts longer than 10 seconds, e.g. a 400m race. It also becomes involved in low-moderate intensity exercise when the demand for oxygen and glycogen stored in the liver cannot be met.

The lactate system taps into stores of glycogen (stored form of carbohydrate) in the muscles to fuel ATP by breaking it down into glucose without the presence of oxygen. The conversion of glucose to lactic acid occurs constantly within the body and only accumulate. If the rates of lactic acid production and removal are equal then there is no problem, but if the production of lactic acid exceeds the muscles’ and cardiovascular system’s ability to disperse it, it will lead to a build-up which will eventually cause the cessation of activity. This is known as the onset of blood lactate accumulation (OBLA), which is associated with certain sensations, such as laboured breathing (or breathlessness), ‘heavy’ limbs and pain (‘the burn’), which usually bring about a need to slow down or stop.

Targeted interval training improves the body’s ability to withstand the build-up of lactic acid and/or the ability to remove it quickly, which delays or prevents accumulation.

Anaerobic training uses up the glycogen stored in the muscle quickly and requires short periods (1-3 minutes) of strenuous activity followed by periods of recovery. Ideally this recovery should be active (e.g. walking between running intervals) to aid the return of blood to the liver. Insufficient recovery, or static rest, might not allow the lactic acid to be dispersed before the commencement of the next interval.

Aerobic system

‘Aerobic’ simply means ‘with oxygen’ and refers to the energy system that produces ATP form the complete breakdown of carbohydrate, fat and protein in the presence of oxygen.

The aerobic energy system is dominant when there is sufficient oxygen in the cells to meet the energy production requirements, e.g. when the body is at rest, or the intensity of the activity is low-to-moderate and enough oxygen can be delivered by the circulatory system.

Fat (fatty acids) and carbohydrate (glucose) are the two main macronutrients that supply energy to cellular aerobic metabolism for the production of ATP. Whether the body is at rest or exercising aerobically, both carbohydrate and fat are required in varying proportions.

The aerobic system produces ATP, carbon dioxide, water (H2O) and heat from the breakdown of fat and carbohydrate. These waste products are easily removed from the body. Carbon dioxide is transported to the lungs via the circulatory system and water is removed via sweat.

Location of ATP production

All three systems produce energy at cellular level, but in different places within the actual cell.

Aerobic energy production occurs in small subcellular structures called mitochondria, which is known as the ‘powerhouse’ of a cell.

The larger and/or more plentiful the mitochondria, the greater the potential for aerobic ATP production of that cell. This indicates that an athlete could run, swim or cycle at a higher intensity for a sustained period of time without fatigue.

Anaerobic energy production (CP and lactate systems) occurs within muscle cells, in the fluid matrix outside of the mitochondria.

The interaction of energy systems during exercise

It is very important to realise that the energy systems do not work independently of each other, in fact, all three systems working harmony to provide the body with energy. However, activities can be categorised by the dominant energy system, depending on their intensity and duration, for example:

  • A golf swing or shot put would predominantly use the CP system.
  • A 400m sprint or a squash or tennis rally would predominantly use the lactate system.
  • A long-distance marathon run would predominantly use the aerobic system.

It is the intensity and duration of the activity or sport that determines which system predominates.

An individual’s fitness level will also dictate the use of energy systems. For example, a trained middle distance runner would be able to utilise the aerobic energy system at a higher intensity for a longer duration than a sedentary individual, who would have to switch to the lactate system earlier, or reduce the intensity of the session.

Summary of energy systems

Energy systems

CP system Lactate system Aerobic system
Oxygen dependency Anaerobic. Anaerobic. Aerobic.
Speed of energy production Very rapid. Rapid. Slow.
Substrate needed

(energy source)

Stored chemical energy


Glycogen. Glycogen and fat.
Amount of energy produced Very limited ATP. Limited ATP. Unlimited ATP.
By-products of energy production No fatiguing waste products. Lactic acid. No fatiguing waste products (only cardon dioxide and water).
Duration of energy production Short duration (0-10 seconds). 1-3 minutes of intense activity. Long duration.
Intensity of activity Very high intensity

(95-100% max effort).

High intensity

(60-95% max effort).

Low-to-moderate intensity

(up to 60% max effort).

Recovery required Quick recovery

(30 seconds – 5 minutes)

20 minutes – 2 hours

(recovery from lactic acid exposure).

Time to eat and drink

(to replenish fuel stores).

Predominant fibre types Type IIb. Type IIa. Type I.