[Reading Science] Why Muscle Pain Occurs After Exercising After a Long Time

What causes muscle soreness that stops exercise, which you started with great determination, after just one day?
Photo by YouTube Screenshot

[Asia Economy Reporter Kim Jong-hwa] As winter comes to an end, more and more people are starting to exercise due to the weight gained over the cold months. However, many find that after working out very hard on their first day at the fitness center, they can hardly exercise on the second day. This is due to muscle soreness.

The amount of muscle soreness is proportional to the amount of time spent exercising on the first day. The more intensely you move, the greater the pain. Even a little movement can bring excruciating pain, and many have experienced wandering around the exercise equipment without actually working out, then just taking a shower and going home. So why does this muscle soreness occur?

It is commonly known that when muscles are used, 'lactic acid' is produced. This lactic acid is often called a 'fatigue substance.' When you start exercising, oxygen gradually becomes insufficient, and in this state, the body breaks down glucose to produce lactic acid. This process is called 'lactic acid fermentation.' This lactic acid is the cause of muscle soreness.

The lactic acid produced through lactic acid fermentation in the muscles exists in a charged form. This charge makes it hydrophilic and affects osmotic pressure. When two membranes with different concentrations are separated, the solvent on the low concentration side moves to the high concentration side. The pressure exerted on the membrane separating the low and high concentration sides is called osmotic pressure.

When lactic acid accumulates in the muscles, osmotic pressure decreases, causing osmotic phenomena that bring water into the muscles, making them swell. When muscles swell, they press on surrounding nerves, causing pain. While muscle soreness can also result from muscle tissue damage, the muscle soreness we commonly experience is mostly caused by the accumulation of lactic acid, a fatigue substance.

This muscle soreness caused by lactic acid gradually disappears over time. The pain that starts the day after exercising usually disappears within three days if short, or about a week if longer. So why does muscle soreness disappear so quietly?

Scientists explain this by a phenomenon called 'gluconeogenesis.' Gluconeogenesis mainly occurs in the liver, where new glucose is produced from non-carbohydrate substances such as amino acids, glycerol, and lactate (lactic acid). In other words, gluconeogenesis is a method used to prevent blood sugar levels from dropping in the body.

Doing aerobic exercise or stretching before and after workouts can help relax muscles and reduce lactic acid buildup.
[Photo by YouTube screenshot]

To explain again, lactic acid (lactate) transforms into one of the important substances for gluconeogenesis. The lactic acid accumulated in the muscles moves to the liver through the bloodstream, and the liver converts the lactic acid into glucose, thereby reducing the lactic acid accumulated in the body. When the lactic acid accumulated in the muscles is removed, the swelling that was pressing on the nerves subsides, and the muscle soreness disappears.

Gluconeogenesis mainly occurs in the liver and partially in the kidneys. This process does not take place in the muscles themselves; instead, lactic acid moves to the liver or kidneys, where gluconeogenesis occurs, which is why it takes several days for muscle soreness to disappear.

While it is impossible to completely prevent the production of lactic acid, it can be reduced. Doing light aerobic exercise or stretching for about 10 minutes before or after exercising helps loosen the muscles and prevents sudden strain, thereby reducing lactic acid production.

Additionally, massages, warm compresses, or half-body baths can promote smooth blood circulation, helping the lactic acid accumulated in the muscles move more quickly to the liver or kidneys, which can also reduce muscle soreness caused by lactic acid.