Anaerobic Respiration & Lactic Acid

Anaerobic respiration is when the body produces energy for exercise without oxygen. There are two types of anaerobic respiration that you need to know about. The immediate ATP-PC system and the Lactic Acid system. Here we explain how they work and which types of sport and exercise they are more predominantly used.

The human body generates energy to make muscles move in three ways. These are known as the aerobic energy system which makes energy by burning fuel with oxygen, ATP-PC system and the Lactic Acid system which both make energy for muscles without oxygen. The term anaerobic means without oxygen. Each energy system is used in differing amounts depending on the sort of exercise.

Anaerobic energy systems

The anaerobic energy systems kicks in in the first few minutes of all exercise. This is because when you first start running, for example, there isn’t enough oxygen available at the muscles for aerobic metabolism. It is also used for short fast, powerful bursts of energy. There are two systems within Anaerobic metabolism, which are the ATP-PC system and the lactic acid system.

The ATP-PC system provides an immediate and intense short burst of energy, useful in sports such as 100m sprints, Powerlifting or throwing events such as the Javelin, Shot Put or Discus throw, but is only useful for around 10 seconds. The Lactic Acid system provides energy for up to a minute, then it is also burnt out and oxygen will be needed (you get tired/out of breath).

Definitions of ATP, ADP, and PC

  • ATP: Adenosin triphosphate molecule.
  • ADP: Adenosine diphosphate molecule
  • PC: Phosphocreatine molecule

ATP-PC System

ATP is a high energy molecule that is broken down in the muscles to form ADP and release energy. PC or Phosphocreatine is another high energy molecule, found in the Sarcoplasm of muscle fibres. The breakdown of ATP and the increase in the volume of ADP triggers an enzyme known as Creatine Kinase to initiate the breakdown of PC into Phosphate and Creatine. Being an exothermic reaction, this provides the energy required to resynthesise ATP at a fast rate.

We only have around 120g of Creatine within our bodies and so this repeated breaking down of PC in order to produce energy to resynthesise ATP is temporary and can only last a maximum of 10 seconds. Therefore the ATP-PC system is used mainly for bursts of speed.

Advantages of the ATP-PC system

  • It does not require oxygen
  • Phosphocreatine is stored in the muscle cell itself ready to be used immediately for energy.
  • Being a small compound it reacts quickly to produce immediate energy.
  • There are no byproducts produced by the reaction which cause fatigue (we simply run out of fuel rather than have any substance inhibit it).
  • PC can be quickly resynthesised so we are ready to sprint or throw again after a short recovery period.


  • Only small amounts are stored in the muscle so it runs out quickly (about 8 to 10 seconds).

Lactic Acid System

What is Lactic Acid? It is simply a by-product when muscles produce energy without sufficient oxygen available. Sometimes also known as Anaerobic Glycolysis due to the initial process being the same as aerobic glycolysis (as above), only without oxygen.

So, as before 10 chemical reactions occur within the Sarcoplasm of the muscle which turns Carbohydrate into Pyruvic acid and 2 molecules of ATP. The difference now is the lack of oxygen meaning the carrier molecule NAD+ cannot offload the Hydrogen (H+) by-product of glycolysis causing a build-up in the cell.

Lactic acid & muscle fatigue

To try to prevent an increase in acidity the pyruvic acid accepts the H+, forming Lactic acid. If oxygen was present the H+ would be transported to the Mitochondria for use in the Kreb’s cycle. Lactic acid is thought to interfere with muscle contraction by disrupting the binding of Calcium to Troponin. Acidity also stimulates free nerve endings within the muscle, causing pain. This form of energy production in the body can only be predominant for up to 2 minutes.

Following anaerobic exercise, despite the metabolic process used not requiring oxygen, your body will be in Oxygen Debt and so your respiration rate will be very high.

Anaerobic Respiration Equation:

Glucose = Ethanol + Carbon Dioxide + Energy

C6H12O6 = 2C2H5OH + 2CO2 + Energy

Anaerobic respiration CP molecule

Any sport or event requiring a sustained burst of high-intensity exercise will use the lactic acid system and cause the body to go into oxygen debt. For example 400m Sprinting, Speed Skating, Crossfit competitions & Circuit training.

Advantages of the lactic system

  • There is a huge amount of glycogen stored in the muscles and liver which can be made available for energy.
  • It is more efficient – resynthesises two molecules of ATP as opposed to just one that the ATP-PC manages.
  • Fewer reactions than the aerobic energy system are needed to produce energy so it is faster (but not as fast as the PC system).
  • Provides more sustained high-intensity energy for between 10 and 180 seconds.
  • Can work both anaerobically (without oxygen) and aerobically (with oxygen).


  • It is not as quick as the PC system.
  • Lactic acid is produced which causes fatigue (and discomfort!)
  • Causes pain by stimulating the pain receptors (telling the body to slow down and known as lactic acid build-up).
  • In the end, it stops working because of muscle fatigue and pain.

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