Does muscle soreness really come from “lactic acid accumulation”?

After climbing several floors in a row, there was a soreness in my thighs. Is this lactic acid accumulating? On the second day after exercise, the muscles are so painful after a little effort, why the lactic acid has not “goed”?

“Lactic acid accumulation causes muscle soreness” is “common sense” among many people. Fitness coaches say this, and similar explanations can be found everywhere on the Internet and in books. But in fact, lactic acid is a “backbone man”.

Lactic acid was once believed to be the main cause of muscle soreness
When lactic acid was first discovered, it had nothing to do with muscles. In 1780, a Swedish medicinal chemist discovered it in yogurt, hence the name “lactic acid”.

Over the next 100 years, scientists have found through a study that after multiple contractions of a single muscle, lactate levels increase, pH decreases (acidification), and muscle strength decreases. These changes are even more pronounced when oxygen is insufficient. Obviously; when the human body reaches a state of fatigue during exercise, it also has similar changes and feels muscle soreness; the higher the lactate level, the more severe the muscle soreness and weakness; the lactate concentration in some people who are sick will also increase ; Lactic acid is very acidic (pH﹦3.86), and strong acidity can cause soreness.

In the 1920s, some researchers put these results together and put forward the hypothesis: when muscles contract in an environment with insufficient oxygen, sugars are forced to break down to produce metabolic waste products such as lactic acid, which leads to acidification; lactic acid will only make people produce Feelings of muscle soreness and fatigue are not helpful.

This hypothesis put the “pot” of muscle soreness on the head of lactic acid, and made it “notorious” in the next century.

Muscle contraction produces “lactate” instead of “lactic acid”
The traditional concept is that when muscles contract, sugar breaks down to produce lactic acid, which is immediately decomposed into lactate and hydrogen ions, which causes acidification of the environment and makes the muscles sore and weak. However, in 1983, a scientist put forward another point of view, which was affirmed by more researchers after 2004, that is, the word “lactic acid” itself is not accurate, because no studies have confirmed that muscle contraction produces “lactic acid”. What is produced is not lactic acid, but “lactate”. This decomposition process not only does not directly produce acid, but may also consume hydrogen ions and alkalinize cells.

In addition, muscles do acidify during exercise, but the source of hydrogen ions is still under debate. At present, most people believe that lactic acid is not a direct source, and hydrogen ions may mainly come from the ATP hydrolysis process.

Although muscle soreness and weakness do occur at the same time as the increase in lactate levels during exercise, and the degree is related, the causal relationship between them cannot be determined. In fact, muscle soreness and fatigue are accompanied by many other physiological changes, such as acidification and decreased ATP levels.

Lactate alone does not cause muscle soreness
In addition to theoretical analysis, the actual test results do not support the claim that lactate alone causes muscle soreness during exercise.

In a 2014 study, researchers directly injected lactate into the palm of a person’s palm muscles and found that even if the concentration of the solution is significantly higher than the level that can be achieved by exercise, it will not cause significant pain. The other two common metabolites during muscle contraction-ATP and hydrogen ions, hardly cause pain when injected alone, but injection of these three substances at the same time can cause significant muscle pain. This test eluted the “primary culprit” of muscle soreness for lactate. On its own, no matter how high the concentration is, it is not enough to “ignite” your muscles.

It is currently believed that among all metabolites, the three main substances that cause muscle pain are lactate, ATP and hydrogen ions. When the three exist alone or when only the two are superimposed, the effect is relatively weak, but when the three appear at the same time, there will be a synergistic effect that enhances each other, which significantly enhances pain signals. When the concentration of these three is low, it will bring a feeling of warmth, and when the concentration increases, it will make people feel pain and burning.

In addition, there are many substances that increase in concentration during muscle contraction and may increase pain, including bradykinin, potassium, and a variety of cytokines.

Therefore, the muscle soreness caused by exercise is the result of the combined forces of many substances. Although lactate is important, this “pot” should not be carried by it alone.

The soreness on the second day of exercise has nothing to do with lactate
As for the muscle soreness on the second day after exercise, the relationship with lactate is even less, because as early as the end of exercise, lactate has been metabolized.

At rest, the concentration of lactate in the blood is 0.5 to 2.2 mmol/L. After starting exercise, the lactate level usually rises gradually, and the greater the exercise intensity, the faster the increase rate. After stopping exercise, the lactate concentration in the blood generally continues to rise, reaching a peak (10-30 mmol/L) 5 minutes after the end of exercise, and then gradually decreases, and usually falls back to the resting level within 1 hour. Level.

Since the concentration has already recovered, the muscle soreness on the second day after exercise has nothing to do with the accumulation of lactate. For this soreness that begins 6-12 hours after exercise and reaches its peak at 48-72 hours, researchers still have not ascertained the mechanism. Currently, it is believed that it is mainly related to skeletal muscle mechanical damage and inflammation, rather than lactate. Caused by.

Lactate is actually very useful
Lactate has assumed the reputation of causing muscle soreness for many other substances, but its important role in the human body is rarely mentioned. In recent years, research has gradually discovered that lactate is not a metabolic waste but an important fuel. It is also a signal molecule, and it is a signal molecule that integrates a variety of sensors and has the function of transmitting signals.

When people need to get a lot of energy in a short time, sugar is broken down in the cell plasma to produce lactate. These lactates can enter the mitochondria to oxidize and provide energy, or they can be transported to the outside of the cell to be taken up by nearby skeletal muscle, heart, brain, liver, kidney and other cells to oxidize and supply energy or re-synthesize sugar. In some cases, lactate is even a better energy source than glucose.

There is also an extreme case to verify the role of lactate, which is to observe how the state of patients who cannot produce lactate changes during exercise. Such patients are unable to break down sugar into lactate due to lack of muscle phosphorylase. However, avoiding the accumulation of lactate does not enhance their exercise capacity, but makes fatigue come earlier.

In short, lactate is not a metabolic waste that “persecutes” muscles. Next time you feel sore, don’t blame it again.