Aerobic Versus Anaerobic 

Aerobic training focuses on developing an athlete’s cardiorespiratory endurance and primarily engages the aerobic energy system. The term ‘aerobic’ translates to ‘with oxygen’, highlighting the body’s capacity to absorb, transport and utilise oxygen for energy production during sustained activities.

In contrast, anaerobic training is highly versatile and tailored to specific session goals. The term ‘anaerobic’ means ‘without oxygen’, and this type of training predominantly utilises the glycolytic and ATP-PCr energy systems. Anaerobic training can target various fitness components, such as strength, power, speed, muscular endurance and the body’s ability to clear lactate.

Energy systems are the foundation of muscular movement, providing the energy required for all physical activities. All energy originates from sunlight, which plants convert into chemical energy through photosynthesis. Humans then consume plants or animals that feed on plants, transferring stored energy into our bodies. This energy is stored in the form of carbohydrates, fats or proteins.

The human body relies on its ability to extract chemical energy (ATP) from the breakdown of these nutrients to fuel activities such as running, walking, liftin weights and maintaining bodily functions. Once food is digested, the nutrients are either stored or converted into ATP for immediate use as fuel. Without this continuous energy supply, the body would cease to function and sustain life.

Think of the body as a machine, much like a car that requires fuel to operate. Without fuel, a car remains stationary; with the right fuel, it powers up and moves efficiently. Similarly, the human body requires energy in the form of ATP to convert chemical energy into mechanical energy for movement and function.

In scientific terms energy can be defined as ‘the ability to perform work’. Energy can be categorised into various forms eg. heat, light, electrical, nuclear, chemical, mechanical.

For the purposes of human movement, the focus is on the transfer of chemical energy (derived from food) into mechanical energy (muscle contraction). 

Energy is measured in kj = kilojoules. 

Foods contain different energy values.

There are three main systems that make energy available: the ATP/PCr system, the Glycolytic (Lactic Acid) system and the Aerobic system (oxygen). 

ATP-PCr system makes ATP production available via anaerobic breakdown of phosphocreatine. 

Glycolytic system makes ATP production available via breakdown of glucose (oxygen dependent). 

Aerobic Energy System makes ATP production available via breakdown of carbohydrates (CHO) and/or fat (oxygen dependent). 

It is important to remember that the energy systems are not used in isolation from each other, but are all used at the same time to produce ATP. At particular times within sports, a particular system will be dominant. 

An example of this is in soccer throughout the game the aerobic energy system is dominant, particularly during times of lower intensity, such as when a ball goes out and everyone is jogging into position. At other times the Glycolytic (Lactic Acid) system will become dominant, such as when a full back sprints forward in an attack and then needs to sprint back to defend. There will also be times when the ATP-PCr system is dominant, such as jumping to complete a header or a short sharp sprint to beat a player to the ball.

To understand how food is converted to fuel and becomes available as a usable form of energy the production of ATP and its role in supplying energy can be examined.

The energy that is released in the breakdown of food is not directly used by the body, rather it is used to make a chemical compound called ATP.

ATP is an energy rich chemical compound which serves as the immediate source of energy for muscle contraction.

An ATP molecule is made up of:

A: Adenosine + Pi: (tri) Phosphates =  ATP/Adenosine Triphosphate (is Adenosine and x 3 Phosphates)

Now ATP has only two phosphates attached. It is now referred to as Adenosine Diphosphate, a powerless chemical in this state. The total quantity of ATP within the muscles is very small and it is only enough to provide energy for a few seconds of maximal or ‘all out’ work.

= ADP/Adenosine Diphosphate is Adenosine and x 2 Phosphates

ATP is not produced continuously, it must be replenished and recycled or re-synthesised (rebuilt from ADP back into ATP). Fortunately, the body has the ability to do this almost as quickly as it is broken down. There are three energy yielding processes for the production of ATP, the energy systems.

ATP-PCr (Anaerobic) Energy System

Source of Fuel: The ATP-PCr (Adenosine Triphosphate/Phospho-Creatine) energy system uses the ATP that is immediately available within the muscle cell (myocyte). As the ATP is broken down into ADP and P, the ADP reacts with the PC (without the presence of oxygen) in the myocyte to produce another ATP and C (1 ATP per PC).

Efficiency of ATP Production: The ATP-PCr energy system has a very fast rate of ATP production, but has a very limited store of fuel. The PC runs out quickly resulting in this system no longer being available until it has begun to recover.

Duration that the system can operate: The ATP-PCr energy system does not last very long due to the limited fuel source and fast ATP production. This system will deplete its fuel in 8 seconds when used at maximal intensity, but can take as long as 12 seconds if used at a lower intensity. 

Cause of fatigue: Fatigue in the ATP-PCr system is caused by the depletion of fuel. Once the immediate stores of ATP and PC run out the system needs to recover before it can be used again.

By-products of energy production: The ATP-PCr energy system has no by-products other than heat, which is a by-product of every energy system.

Process and rate of recovery: The ATP-PCr energy system recovers as the creatine in the cell connects to the free phosphates again, storing them as PC to be used when they are needed again. This process takes up to 2 minutes for complete recovery, but can be half restored at around the 30 second mark.

Examples: Due to the speed of ATP production and the short duration of the fuel source the ATP-PCr energy system is the dominant system in activities such as a 100m sprint, discus, javelin, high jump and other sports of very short duration. The ATP-PCr energy system is also used to provide brief periods of high intensity within many other sports. Examples include kicking a ball during soccer or rugby league or a short sprint where maximal effort is needed, but lasts only 5-10 seconds.

Glycolytic (Lactic Acid) Energy System

Source of Fuel: The Glycolytic (Lactic Acid) energy system uses carbohydrates (CHO) as its only source of fuel and relies on anaerobic glycolysis for its production of ATP. Glycolysis is the breakdown of glucose to produce ATP. In anaerobic glycolysis the glucose (sourced from glycogen in the muscle or glucose in the blood) is turned into lactic acid as it produces ATP.

Efficiency of ATP Production: This system produces ATP at a fast rate and can produce a lot of ATP. The lactic acid system produces 2 ATP for each glucose molecule it breaks down, however, it also produces lactic acid in the process.

Duration that the system can operate: The glycolytic (lactic acid) system lasts between 30 seconds and 3 minutes depending on the intensity. The less intense the activity the longer it will last, because it will be producing lactic acid at a slower rate at the lower intensity levels.

Cause of fatigue: The cause of fatigue in the glycolytic system is the buildup of pyruvic acid in the muscle. Pyruvic acid is made up of two molecules; pyruvate and a hydrogen ion (H+). Without oxygen the body converts the pyruvate and two H+ to lactate. This helps to reduce the acidity of the muscle and allows anaerobic glycolysis to last longer, as the lactate is removed from the muscle and taken to the liver where it is converted to a useful fuel source such as glucose. In continued high intensity activity the lactate cannot be removed fast enough, which results in a buildup of pyruvic acid. It is specifically the build up of the H+ within the muscle that causes fatigue. It does this by increasing the acidity of the muscle and causing the enzymes needed for anaerobic glycolysis to slow down.

By-products of energy production: The main by-product of the glycolytic system is pyruvic acid (pyruvate and H+). This by-product is then converted to lactate and transported out of the muscle to the liver to be converted to glucose.

Process and rate of recovery: The process of recovery once fatigue has occurred requires oxygen. Pyruvic acid in the presence of oxygen will be converted to acetyl coenzyme A, which is then broken down through the Krebs cycle to produce more ATP. Without oxygen it is converted to lactate and removed from the muscle and taken to the liver to be converted into glucose. This process can take anywhere between 30 and 60 min.

Examples: The Glycolytic Energy system is the dominant system in sports, which requires a high intensity for longer than 10 seconds. Sports such as 200m or 400m run or 50m and 100m swim are highly reliant on the lactic acid system. Other times when it is used would include repeated high intensity activities during other sports such as tennis running back and forth with small breaks in-between, repeated tackles in rugby or an extended piece of high intensity in any other sport such as a full-back going forward in an attack and then having to retreat in soccer.

Aerobic Energy System

Source of Fuel: The Aerobic System can use carbs (CHO), fats and protein as its source of fuel though protein is used sparingly. The aerobic system uses aerobic glycolysis, the Krebs Cycle and the electron transport chain in its production of ATP. It is the presence of oxygen, which allows this energy system to use these various fuel sources.

Efficiency of ATP Production: The Aerobic System is very efficient in producing ATP. It produces 38 ATP molecules per glucose, but the rate of production is medium and cannot cope with the higher intensity levels.

Duration that the system can operate: This energy system can produce ATP continuously for well over an hour. In fact, it may not have a limit as long as fuel sources can be found (you will die if this energy system cannot be used). Muscle glycogen will deplete after about an hour of exercise, which will result in an increased need for oxygen as fats become the dominant fuel source and use more oxygen per ATP produced than CHO.

Cause of fatigue: Though this system does not need to stop, a reduction in intensity will occur when CHO stores deplete. Since fats require more oxygen to produce ATP than CHO, an athlete will normally decrease their intensity when their main fuel source switches from CHO to fats. This is often called hitting the wall. If it is possible for the athlete to transport oxygen at a faster rate than they are when their CHO runs out, then their body will adjust and bring the extra oxygen to the muscle. This will mean an increase in respiration and possibly an increase in heart rate and cardiac output, but it will allow the athlete to continue to perform.

By-products of energy production: The Aerobic System produces water and carbon dioxide as by-products in its production of ATP. Water can build up in the muscle and cause stiffness and a sort of ‘swelling’ if exercise is continued at a high enough intensity for long enough, but generally it is transferred out of the muscle and into the blood as water is being lost through sweat during exercise. The carbon dioxide is taken out of the muscle and expired by the lungs into the atmosphere. If carbon dioxide is not removed it can cause fatigue.

Process and rate of recovery: Recovery for the Aerobic System is about restoring fuel stores to their pre-exercise levels. This requires the ingestion, digestion and transportation of the fuel and can take between 12 and 48 hours depending on the intensity and duration of the aerobic performance.

Examples: The Aerobic System is the dominant system for any sport or activity that lasts more than 3 minutes. This includes most team sports such as netball, soccer, rugby and AFL as well as many individual sports such as 1500m swimming, marathon running, cycling, triathlons, tennis and ironman competitions.

Please refer to the image below.