Surely you've heard of lactic acid?
Well, that's where the problem is, but we'll start from afar.
Muscle, like other tissues in the body, requires large amounts of energy. These nutrients (including oxygen) are broken down into carbon dioxide and water after doing their "nutritional duty.
Let's make an interesting comparison👇🏻
The average person's quadriceps thigh muscle is 2-4 kg - an impressive figure. And if you have two muscles, you get 4-8 kg.
The weight of all the organs of the abdominal and thoracic cavity will not reach even the lower limit of this range.
Therefore, when there is a heavy load on the muscles, they will not have enough oxygen. As a result, plan B is started - instead of carbon dioxide and water, lactic acid is formed.
And the accumulated lactic acid causes that very pain. And since it takes much longer for carbon dioxide and water to be eliminated from the body, the pain stays for hours or even days.