How is lactic acid produced in the human body?

I believe many people have had this experience, that is, they have not exercised for a long time, and then suddenly did a lot of exercise one day, after which they are prone to muscle soreness. The most common soreness is in the leg muscles. This is caused by lactic acid produced in the muscles after exercise. Excessive lactic acid accumulation can cause pain, but many people do not know how lactic acid is produced in the human body, so let’s learn more about it below.

How is lactic acid produced in the human body?

Lactic acid has long been considered by sports coaches, physical education teachers, athletes, and the general public to be the main cause of muscle fatigue, muscle soreness, muscle cramps, anaerobic threshold, and oxygen debt. In fact, the idea that all special physiological reactions during or after exercise are attributed to lactic acid has long been denied by exercise physiology researchers.

In addition, the argument that lactic acid is closely related to "muscle soreness" is often raised, and this wrong information is often seen even on the Internet. "A large amount of pyruvate is converted into lactic acid, and the accumulation of lactic acid causes muscle soreness after climbing)." "Muscle soreness caused by lactic acid accumulation." "The reason for muscle soreness after long-distance running is that the body is in a state of hypoxia, producing a large amount of lactic acid)." "People who rarely exercise usually have lower aerobic metabolic capacity. When they exercise occasionally, the blood supply to the muscles is insufficient, which will produce some acidic substances (such as lactic acid). Since these substances cannot be excreted from the body or oxidized in time, their accumulation will stimulate the nerve endings in the muscles or cause changes in osmotic pressure, allowing more water to penetrate into the muscle fibers and cause the muscles to swell. These phenomena will lead to muscle soreness... As long as you exercise diligently, you will not feel muscle soreness so easily after exercise."

In fact, the accumulation of lactic acid during intense exercise does cause a decrease in muscle pH, resulting in a decrease in muscle enzyme activity, and then causing muscle activity fatigue (acute muscle soreness). However, the delayed onset muscle soreness (DOMS) phenomenon 1 to 3 days after exercise has no significant correlation with the formation of lactic acid.

There are two reasons why lactic acid has nothing to do with DOMS.

First, the blood lactate concentration returns to the resting level 1 to 2 hours after intense exercise. For athletes who experience delayed-onset muscle soreness 1 to 2 days after exercise, their blood lactate concentration does not increase significantly at all. Second, among a group of runners participating in a running competition at the same time, it is very likely that some of the fewer participants will have symptoms of delayed-onset muscle soreness.

The other part does not have this problem. It seems that the main cause of delayed onset muscle soreness is the sudden and sharp increase in exercise volume and intensity, and the large amount of eccentric muscle contraction exercises that cause slight rupture injuries to muscle fibers.

In the early days, lactic acid was considered to be a product of intense anaerobic exercise, and muscles did not produce lactic acid during slow oxidative metabolism. Under light-load exercise conditions, the human body will metabolize glucose into pyruvate, which is then metabolized and decomposed into the Krebs cycle and oxidized to produce a large amount of energy. In fact, this statement is not correct. When the human body is resting, lactic acid is produced in the digestive system, skeletal muscles, red blood cells and liver. The blood lactate is eliminated through the circulatory system, synthesized into glucose in the liver and kidneys, or directly oxidized to produce energy. The reason why blood lactate can be maintained at 1mM/L when resting is that the production and excretion of lactate are balanced. Since lactic acid is a product of glycolysis (produced quickly) and has a small molecule that can easily pass through cell membranes, it is a very important intermediate product in the metabolic reaction of glucose. Moreover, after eating, the concentration of lactic acid in the human blood will increase, achieving the dual functions of reducing insulin secretion and transporting carbohydrates.