Lactic Acid, How it Forms and How it is Disposed of


Let’s explain what lactic acid is and how it forms, to do this I have to make a small digression on the muscular metabolic system, that is, how the muscles produce the energy necessary for their contraction.

The energy is provided by ATP or adenosine triphosphate (our fuel) which splits into ADP (adenosine triphosphate) and P, present in the muscles in limited quantities. That’s why during physical activity it needs to be continuously reformed! The synthesis of ATP occurs through 3 different mechanisms, depending on the duration and intensity of muscle effort.

Lactic Acid and Aerobic System

Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 NADH + 2 pyruvate + 2 ATP + 2 H2O + 2 H+.

It provides energy through oxidation mainly of carbohydrates and lipids, in the presence of oxygen and is used during rest or moderate and prolonged physical activity (from about 4 minutes to several hours), during work the consumption of energy and its production remain in balance.

Final products of this reaction are:

  • carbon dioxide and water, which are eliminated by breathing, sweating or through the kidneys;
  • pyruvic acid;
  • ATP.

Pyruvic acid in the presence of oxygen will undergo cell respiration (krebs cycle) and as a final result of this reaction will have the production of 38 molecules of ATP.

Lactic Acid and Anaerobic Altacid System

Phosphocreatine, which contains active phosphoric groups, is present in large amounts in the muscle. In practice, when the muscle requires immediate energy, phosphocreatine transfers one of its phosphate groups to the ADP, promoting the production of ATP (without the need for the presence of oxygen). This takes place in a very small fraction of a second and the energy stored in the phosphocreatine of the muscle is available instantly for muscle contraction. Even when this energy is used for short explosions of maximum muscle power (system used to develop power peaks of maximum 8-10 seconds)

The Anaerobic Lactic Acid Systhymus

When oxygen is not sufficient, pyruvic acid (derived from the oxidation of glucose) is transformed into lactic acid by lactic fermentation. This is the case, for example, in muscle fibrocells during a sudden or high-intensity effort; ATP molecules (i.e. energy, without oxygen consumption) are formed, which are necessary for muscle contraction, but which cannot be sustained for a long time as lactic acid spreads from the muscle fibre to the blood, causing pH reductions and therefore acidosis, a condition which prevents further muscle contraction.

This formation of lactic acid is useful for energy purposes, especially in the early stages of physical exercise or during intense and maximal or sub-maximal efforts (running of 400-800 m, sports such as football and tennis where you have the possibility of recovery between the various motor acts). The body then reuses the lactic acid by recycling it and transforming it back into pyruvic acid. The pyruvic acid will then be transported to the liver and converted to glucose (chorus cycle) to be used to replenish the glycogen reserves of the muscles.

In practice, during physical activity we initially use the anaerobic energy system and forms lactic acid that accumulates causing acidosis and stimulates aerobic breathing. Once the aerobic system is established, the production of lactic acid stops and its level in the blood is reduced until it returns to resting values.

If the muscular energy requirements increase in such a way that there is no balance between the formation of lactic acid and its disposal, there will be a central stimulation (of the brain) with a consequent reduction in physical performance and energy requirements.

Usually this condition corresponds in well-trained athletes to 90% of the heart rate and in other subjects to about 80%.

Lactic Acid Production in Athletes

Very trained people produce much less lactic acid than those who are starting physical activity or sedentary engages in intense muscle exertion by training the aerobic components: cardiac output and heart rate.

With a constant training the cells are able to adapt slowly and getting used to a certain effort will try to increase the number of enzymes involved in the metabolic pathway.

Lactic acid is not produced exclusively during physical activity. Perhaps not everyone knows that it is also produced in restful conditions:

  • in the red blood cells,
  • retina,
  • muscles.

Lactic acid is disposed of within 60 minutes. It is from this that we can understand that it is not responsible for the delayed onset of muscle pain that we feel the day after training and also the next. These muscle pains are in fact caused by microlacerations of the muscle fibres during high-intensity exercises. This is a pain that we experience in the next 12 -72 hours and can last longer.

As a result of the healing processes, the cells then begin to adapt and to endure increasing efforts. This is why it is important to work gradually and give the muscles time to get used to one load before moving on to the next.

Stretching and warming up muscles are essential, but we will treat them separately.