Labelled diagram of the anterior muscles of the human body.
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Formation of lactic acid in muscle tissues

When insufficient oxygen is supplied to muscle tissues:

Glucose Lactic Acid

(Above: *Simple statement; not a complete equation)

Why does this matter ? Accumulation (or 'build-up') of lactic acid in muscles can lead to muscle fatigue.

Short version:

Formation of lactic acid in muscles occurs when insufficient oxygen is supplied to the muscles, resulting in the release of energy via anaerobic cellular respiration instead of by aerobic cellular respiration (see right).

More detail:

Muscles need energy in order to contract. They receive energy when chemical energy is released from certain molecules, e.g. glucose (a type of sugar), that were previously consumed in food. Most of the chemical energy (i.e. the energy released when certain chemical bonds are broken during chemical reactions) is released to muscles when glucose and oxygen react together in a process called aerobic respiration.

The two possible forms of muscle respiration are:

  • Aerobic Respiration (e.g. in muscles at rest)
    • occurs during 'normal' muscle use e.g. while studying or chatting with friends
    • uses oxygen, which is supplied to the tissues by the red blood cells

  • Anaerobic Respiration (e.g. in muscles that are being over-worked)
    • occurs when the muscles are working very hard and requiring more oxygen than the respiratory and cardiovascular systems can supply
    • is the break-down of glucose when there is insufficient oxygen available for aerobic respiration to happen

These processes can be described by the following *simple equations.

Aerobic Respiration

Glucose + Oxygen Carbon Dioxide + Water + Energy

C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy

Anaerobic Respiration

Glucose Lactic Acid + (smaller amount of) Energy

C6H12O6 2 C3H6O3 + (smaller amount of) Energy

Lactic Acid creates an 'oxygen debt'

Lactic acid is a toxic (poisonous) chemical that can cause muscles to stop working.

The presence of the lactic acid is sometimes described as an 'oxygen debt'.

This is because significant quantities of lactic acid can only§ be removed reasonably quickly by combining with oxygen.
However, the lactic acid was only formed due to lack of sufficient oxygen to release the required energy to the muscle tissue via aerobic respiration. Lactic acid can accumulate in muscle tissue that continues to be over-worked. Eventually, so much lactic acid can build-up that the muscle ceases working until the oxygen supply that it needs has been replenished.

To 're-pay' such an 'oxygen debt', the body must take in more oxygen in order to get rid of the additional unwanted waste product lactic acid (C3H6O3).

To do this the body reacts as follows:

  • the medulla oblongata (a 'primitive' part of the brain) responds to the presence of lactic acid by adjusting the pH of the blood due to the concentration of dissolved carbon dioxide (CO2).
  • the heart rate increases
  • the respiratory rate increases

The 'recovery time' during which extra oxygen is taken-in to re-pay the 'oxygen debt' before the body reverts to its usual heart rate and breathing rate is typically about 12 - 16 seconds. (Obviously this depends on the extent of exertion and the person's physical condition incl. fitness.)

Note: The above is a very simple explanation of lactic acid formation leading to oxygen-debt. For more detail about the release of energy for muscle contraction (for more advanced courses) see ways of production of ATP for energy for muscle contraction.

Other related pages include the different types of skeletal muscle fibres, the effects of exercise on muscles, and
the effects of exercise on circulation.

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