Lactic acid is formed when muscles do not receive enough oxygen to create energy during aerobic respiration. Prolonged periods of intense training can lead to the accumulation of lactic acid due to prolonged lack of oxygen in the muscles.
The health and exercise industry is focused on getting the maximum benefit from training. This includes the best ways to rest and allow the body to recover after exertion. Many blogs about training and fitness invariably mention lactic acid, its accumulation in the muscles and its effect on performance.
What is lactic acid and why is it produced only during training?
What is lactic acid?
Everything consists of chemicals – a series of atoms connected to each other to create molecules. Lactic acid is a molecule consisting of carbon, hydrogen and oxygen atoms, like many other organic molecules. It is an acid, which means that it changes the pH of the environment, giving water a proton (H+ ion).
The bacteria and microbes that ferment foods, the same ones that convert sugar into alcohol, are also responsible for the formation of lactic acid. These microbes ferment when there is a lack of oxygen, which means they are anaerobic.
This fermentation is a way for microbes to obtain energy. They eat sugar, and then, as a result of a series of chemical reactions (called metabolic pathways), this sugar is broken down to produce energy. This is similar to how gorenje wood generates thermal energy that can be used for cooking (cooking can be considered as the work done by fire on food).
However, people and, accordingly, our muscles are not anaerobic microbes. We inhale oxygen and exhale carbon dioxide. In this case, how does lactic acid end up in our muscles?
How do cells (muscles) produce energy?
Humans and aerobic organisms need oxygen to produce ATP. When there is enough oxygen in the body, muscles (and other body cells) break down glucose for energy in a process called glycolysis (glyco = glucose and lysis = breakdown). In the process of glycolysis, a certain amount of ATP and a molecule called pyruvate are formed.
However, a cell can get much more ATP by sending a pyruvate molecule into the mitochondria. Mitochondria is not for nothing called the power plant of the cell; it is equivalent to a power plant in the city. One of the main tasks of mitochondria in a cell is to create ATP and supply it to the rest of the cell. It is here, in the mitochondria, that oxygen is involved in the creation of energy.
This whole process, from glycolysis to chemical reactions in mitochondria, is called cellular respiration.
How do muscles produce energy without oxygen?
When you run (on a treadmill or running away from a saber-toothed tiger), your muscles do not have enough oxygen to maintain such intense activity for a long time. However, the body needs energy, so muscle cells launch their backup plan – lactic acid production.
When there is a lack of oxygen, the mitochondria’s energy production slows down, and they cannot produce enough ATP to power overloaded muscles.
Therefore, the cell, instead of sending pyruvate to the mitochondria, converts pyruvate into lactic acid. This transformation does not create ATP directly, but allows the glycolysis that generates ATP to continue.
For those who need details of biochemistry
How does the conversion of pyruvate into lactic acid help the continuation of glycolysis? This is because of a chemical called NAD. During several chemical reactions of glycolysis, NAD (Nicotinamide adenine dinucleotide) takes electrons, turning into NADH. In the presence of oxygen, this NADH enters the mitochondria and helps generate energy. However, with a small amount of oxygen, NADH can do little.
At the same time, the amount of NAD present in the cell decreases. Without NAD, the cell cannot continue to receive electrons during glycolysis, so the process stops. However, the reaction that converts pyruvate to lactic acid converts NADH to NAD, thereby increasing the concentration and allowing glycolysis to continue producing ATP, which the muscle cell can use to work.
Why does lactic acid accumulate in the muscles during training?
With prolonged physical exertion, the muscles continue to lack oxygen, so more and more lactic acid is formed in them. This fast and light form of energy keeps the muscles toned, but also leads to the accumulation of lactic acid in the muscles.
Why is the accumulation of lactic acid in the muscles bad?
Inside the cells there is a delicate balance of chemicals, the violation of which leads to disruption of the functioning of cells (and in some cases to their death). Lactic acid is an acid that can disrupt the normal pH level in the muscles. Now you’ve probably guessed what this leads to. It was believed that such a violation leads to fatigue of muscle tissue, which means that lactic acid disrupts the normal functioning of the tissue.
In contrast, several studies have not found a sufficiently significant link between lactic acid accumulation and muscle fatigue. Some researchers have suggested that the cause of muscle fatigue is potassium ions coming out of muscle cells, while others have suggested that the cause of pain is exercise-induced injuries and subsequent inflammation.
Thus, the question of whether lactic acid plays any role in the muscles and how well they function is still unresolved!