LO7 of Unit 1 – Energy systems and their relation to exercise (2021)

Energy systems and their realation to exercise

Learning outcomes
By the end of this section, you will be able to:

7.1 Describe how carbohydrates, fats and proteins are used in the production of energy

7.2 Explain the use of the three energy systems during aerobic and anaerobic exercise

7.3 Identify the by-products of the three energy systems and describe their role in muscle fatigue

7.4 Describe the effect of endurance training on the body’s use of fuel for exercise

7.5 Define anabolism, catabolism and post-exercise oxygen consumption (EPOC)

How carbohydrates, fats and proteins are used in the production of energy

What is energy?

All human movements such as sprinting, jumping and marathon running, or essential functions such as digestion, blood circulation and tissue building, require energy. We derive our energy from food that we eat in carbohydrates, fats and proteins. The first step in digestion is to break down food n into smaller subunits via digestion: carbohydrates into glucose, fats into free fatty acids and proteins into amino acids.

These subunits are then used, or in glucose and fatty acids, stored in either the liver, muscles, or fat cells for use later. The energy generated and then released in our body is a substance called ATP, which is short for Adenosine Triphosphate; it is very energy-rich. ATP is often referred to as the ‘energy currency’ and is the only molecule that supplies the energy used in muscle fibres’ contraction. ATP synthesis is obtained from the breakdown of foods we eat in our diet in cellular respiration, particularly from carbohydrate and fat.

Adenosine Triphosphate (energy currency)

ATP molecules consist of an adenosine molecule and three inorganic phosphates held together by what is referred to as high-energy bonds. Energy is released when one phosphate bond breaks, resulting in energy release: the energy powers muscle contraction and other essential life functions. 

The ATP molecule no longer contains three phosphate bonds. ATP becomes ADP (adenosine diphosphate, meaning an adenosine molecule plus two inorganic phosphates) plus a phosphate.


  1. Aerobic – oxygen
  2. Anaerobic – ATP-PC and lactic acid


The aerobic energy pathway synthesises ATP in oxygen, called the ‘oxygen system’, whereas the anaerobic energy pathway generates ATP without oxygen. The anaerobic energy pathway consists of two systems: the ATP-phosphocreatine (PC) and the lactic acid system.

Although all three energy systems contribute to ATP synthesis during activity or exercise, exercise intensity drives each activity’s contribution.

The human body only stores a limited amount of ATP in cells so for exercise to continue for any length of time, ATP must quickly and continually be reformed. It is the continuous cycle of breakdown and resynthesis of ATP molecules to ADP molecules plus phosphate. Then ADP molecules synthesised to ATP molecules that ensure energy available for muscle contraction and other critical cellular functions. The energy bonds found within our food are needed to reform ATP from ADP. Energy (ATP) is synthesised from the following pathways:

  1. Aerobic – oxygen
  2. Anaerobic – ATP-PC and lactic acid


The aerobic energy pathway synthesises ATP with oxygen present, and this is called the ‘oxygen system’, whereas the anaerobic energy pathway generates ATP without oxygen. The anaerobic energy pathway consists of two systems: the ATP-phosphocreatine (PC) and the lactic acid system.

Although all three energy systems contribute to ATP synthesis during exercise, each makes the percentage contribution depending on exercise intensity and duration.

During low-intensity activity, the aerobic energy system mainly meets muscles energy needs. However, if physical activity intensity increases, there is a greater and faster demand for energy. Fortunately, energy needs are topped up by the anaerobic energy systems; otherwise, activity intensity would drop and would need to slow down or stop. 

Although the aerobic energy system is very efficient at forming ATP, its formation is a lengthy process.

The three energy systems during aerobic and anaerobic exercise

ATP-phosphocreatine (PC)

The ATP-PC system is the simplest and fastest system providing immediate muscle contraction energy. It functions through the breakdown of ATP-PC. PC stands for phosphocreatine, also known as creatine phosphate, and is a naturally occurring organic compound found in muscle tissues. 

The enzyme creatine kinase facilitates the phosphate’s separation from the creatine molecule. The energy released can then couple the phosphate to an ADP molecule to form ATP.

Energy release is speedy and does not require oxygen (anaerobic), so it is the system of choice for a short burst of high- intensity work lasting approximately 10 seconds or less. Examples of activities using the ATP- PC system include Olympic weightlifting and maximal sprinting.

Phosphocreatine stores are, so if an activity is to be sustained, the body will call on the glycolytic or oxidative system. There are no waste products to disperse from the muscle, so once ATP-PC stores have been used up full recovery from the activity is needed to restore PC stores.

Developing ATP-PC systems require activity intensity to be very high with long rest periods between efforts to recover PC stores. Rest period durations can be anywhere between 2-10 minutes, depending on the activity being performed.

Lactic acid system (anaerobic)

Glucose is the primary or preferred fuel source for the muscles and is readily available under normal conditions as long as CHO-based foods are regularly consumed. The breakdown of glucose to form ATP is termed glycolysis. Glycolysis can be both aerobic or anaerobic, depending on oxygen availability. Anaerobic glycolysis occurs in the cytoplasm of muscle cells (the fluid that fills a cell). On the other hand, aerobic glycolysis involves oxygen and takes place inside bean-shaped organelles called mitochondria found in all cells. Mitochondria have been termed the cell’s powerhouses as they pump out enormous amounts of energy, a bit like an energy power station.

During exercise, the demand for energy (ATP) and glucose increases significantly, and production must keep pace with exercise demand.

How glucose is metabolised depends on the intensity of and the ability to take in, transport and utilise oxygen in the muscle cell’s mitochondria.

As exercise intensity rises, so does the need for oxygen to synthesise ATP. However, there will be a point where oxygen and ATP demand outstrips its supply, resulting in insufficient ATP synthesis and a subsequent drop in performance. Fortunately, ATP synthesis is maintained by breaking glucose down without oxygen through a series of steps in the cells’ cytoplasm to eventually form pyruvic acid and ATP.


There is a cost, however, to exercising at a high intensity. Within a few minutes, lactic acid and another compound called hydrogen atoms will accumulate in the working muscles faster than they can either be shuttled out of muscles or into the muscles’ mitochondria. When this happens, a point will be reached where exercise performance is hindered.


Comparison between mitochondria and a power station

Oxygen system (Aerobic)

The slowest and most complex energy system to synthesise ATP is the oxygen system. As the name implies, it requires oxygen and is aerobic. The oxygen system mainly uses carbohydrates and fats to generate ATP for energy. Through many steps, the result is ATP plus CO2 and H2O.

Aerobic energy metabolism occurs within specialist structures called mitochondria. Mitochondria are often referred to as the cells’ ‘powerhouse’, and unlike the anaerobic systems, generate a high number of ATP molecules. They are found in all cells and muscle cell sarcoplasm. Aerobic metabolism is the primary energy production method for long-term, low-to- moderate-intensity work lasting longer than 3-5 minutes. As long as oxygen is available to meet the activity’s needs, the oxygen system will supply sufficient energy to sustain workout intensity.

Glucose Pyruvic acid

Recall from the section on the anaerobic lactic acid system that glucose is converted into pyruvic acid and that pyruvic acid has two fates: either anaerobic or aerobic.

Cellular respiration in mitochondria

If oxygen is available (depending on the oxygen transport system), pyruvic acid is converted further and enters the muscle cell’s mitochondria. Here, it is subjected to a very complex chemical reaction series involving enzymes, resulting in ATP formation and CO2 and H2O.

As stated at the start of this section, fat also contributes to our cells’ energy needs. As there are only limited amounts of glycogen available for the body’s energy needs, it would quickly run short if there was no alternative. Fortunately, fat stored as triglycerides in muscles and fat cells (even in a lean adult) can meet the continuing energy needs. There is virtually unlimited energy from fat available, so this is a reliable source.

Just like glycogen needs to be broken down to glucose to be used, triglycerides need to be broken down. With the help of enzymes, triglycerides are broken down into individual free fatty acids (FFAs), and it’s the FFAs that are the primary energy source. Once released, FFAs are transported into the muscle cell’s mitochondria; the breakdown follows the same process as glucose and ATP molecules’ production plus CO2, H2O and heat.

Energy systems overview

The by-products of the three energy systems and describe their role in muscle fatigue

Phosphocreatine ATP-PC system

The ATP-PC system requires no oxygen (anaerobic) and therefore anaerobic, or more specifically, anaerobic alactic. Alactic refers to the absence of lactic acid, meaning that lactic acid and associated hydrogen ionsare not the cause of fatigue at maximal exercise intensities. Performance drops due to the ATP-PC system only providing energy for less than 10 seconds when at maximal activity. The ATP-PC system has no fatiguing waste or by-products. The system is rapidly replenished during recovery; it requires about 30 seconds to replenish about 70% of the phosphagens and 3 to 5 minutes to replenish 100%.

Lactic acid – friend or foe?

In the past lactic acid was thought to be the scourge of exercise performance, causing painful sensations and muscle fatigue; a dead-end product of anaerobic metabolism, a substance to be got rid of as a waste product, remaining in the muscles and causing fatigue. However, scientists now believe that lactic acid is very useful in energy synthesis and is more of a by-product of anaerobic metabolism. It is thought that lactic acid accompanies fatigue but does not cause it.

The culprit in fatigue is now thought to be the accumulation of hydrogen atoms as glucose is converted to pyruvic acid (pyruvic acid is converted to lactic acid if oxygen is insufficient). The accumulation of hydrogen atoms creates a very acidic environment inside the muscle. It disrupts its ability to contract with sufficient force, disrupting myofilament inaction where the heads of the myosin filament fail to connect to their active binding sites in filament.

Recovery time depends on the duration and effort level. To recover between bouts, use the following work to rest ratios:


  • Work Time: 60-180 seconds.
  • Work to Rest Ratio: 1:2 to 1:4
  • Recovery Type: Active
  • Work Time: 15-30 seconds.
  • Work to Rest Ratio: 1:3 to 1:5.
  • Recovery Type: Active

The effect of endurance training on the body’s use of fuel for exercise

Physical training improves performance by making more energy available to working muscles. However, the adaptations each energy system makes to performance is dependent on the specificity of training.

For example, training at maximum effort results in changes to the ATP-PC system, improving maximum power output.

Alternatively, training at a lower intensity over a longer time (endurance) is the process by which cells release energy from the chemical bonds of food molecules to provide energy (ATP) for the essential processes of life.

Anaerobic adaptations

  • Increased muscle buffering capacity permitting a higher level of blood lactate levels
  • The improved buffering capacity allows H+ to be removed and neutralised.
  • There is increased strength and coordination of trained versus untrained muscle. Increased strength allows a given task to be performed with less effort, which reduces fatigue. 
  • Possible increase in ATP-PC enzyme activity; but current studies are conflicting.

Aerobic adaptations

  • Increase in circulatory capacity with endurance training
  • Increase store of glycogen in trained muscles compared to untrained muscles
  • Increase store of muscle triglycerides in trained versus untrained muscle
  • A higher number of aerobic enzymes and increased size and number of mitochondria to oxidise fat, which increases the use of fatty acids as an energy store thus sparing glycogen


Endurance training aims to overload the components of oxygen transport and use. There are three primary methods of endurance training.

Anabolism, catabolism and post-exercise oxygen consumption (EPOC)

Anabolic and catabolic reactions

The human body is just a big bag of chemical reactions. The sum of all those reactions is called metabolism. Metabolism or metabolic processes are continually taking place in the body and categorised as either anabolic or catabolic reactions. 

The reactions involved in food breakdown to obtain energy are called catabolic reactions, such as carbohydrate into glucose and protein into amino acids. On the other hand, anabolic reactions use the energy produced by catabolic reactions to synthesise larger molecules from smaller ones, such as when the body forms proteins from amino acids, glycogen from glucose or triglycerides from fatty acids in your food. Another example is ATP into ADP during energy release.



Oxygen consumption during recovery

Post-exercise the bodily process does not immediately return to resting levels. A light physical effort will mostly go unnoticed, resulting in a quick recovery. Conversely, serious efforts such as 1500m running or 200m swimming require much more time for metabolism to return to resting levels. Metabolism stays elevated for some time after exercise has stopped. The phenomena have been referred to as the excess post-exercise oxygen consumption or EPOC for short. EPOC is the amount of oxygen required to help the body restore and return to its pre-exercise state.

The oxygen needed over resting values is often termed oxygen debt—however, a more appropriate term is recovery oxygen consumption. The intensity of exercise appears to be the most significant factor in boosting post-exercise metabolism. 

EPOC effect on recovery:

  • Production of ATP to replace the ATP used during the workout
  • Resynthesis of muscle glycogen from lactate
  • Restore oxygen levels in venous blood, skeletal muscle blood and myoglobin
  • Increased protein synthesis for the repair of muscle tissue damaged during the workout
  • Restore body temperature to resting levels