That's because most of the energy released by our muscle cells during aerobic respiration (biological cell respiration) is converted into heat energy. Finally, through skin sweating, perspiration and other heat dissipation methods, our muscles will heat up. And the effect is obvious.

There are two ways of cell respiration in biology, one is aerobic respiration and the other is anaerobic respiration.

First, let's talk about the process of aerobic breathing. In the process of aerobic respiration of cells, the premise is that there must be oxygen. Your muscle cells are oxidized and decomposed into two molecules of pyruvate by one molecule of glucose in the cells (this is the first stage). Then pyruvate in the two molecules is further oxidized and decomposed into carbon dioxide, water and energy. But most of the energy is converted into heat energy, which is emitted through the skin, leading to the burning of muscle heat when we exercise.

Then, another kind of anaerobic breathing is similar, but there are differences. Its first stage is the same as aerobic respiration, and the second stage is to decompose two molecules of pyruvate into lactic acid, carbon dioxide and a small amount of energy. Like aerobic respiration, most of this energy is converted into heat energy.

But when we exercise, most of us breathe with oxygen, because ordinary cells live in oxygen, and cells without oxygen are as essential as people without water. This energy is always converted into heat energy. However, it is impossible for our skin to dissipate heat quickly, and all that heat energy is waiting to explode in the skin. Therefore, our skin will burn badly.