Exercise consumes a lot of glycogen accumulated in the body: water, oxygen and glycogen react under the catalysis of enzymes (this is aerobic respiration), generating water and carbon dioxide, and releasing a lot of energy at the same time. The blood vessels of the whole body are connected together, especially those capillaries, which are all over the body. Blood contains a lot of water, and at the same time it is the most faithful porter of nutrients in the body, and water is an expert in absorbing energy. So the heat generated by these energies will be transmitted to the whole body. Of course, some of them will pass through the pores. The intensity of the exercise we do is too strong, and most of it will not be transmitted in time, so it will accumulate in the blood. The higher the temperature, the more blood will expand, even the capillaries that are not usually ignited will expand. Because there are many, it looks red.

The effect of congestion represents the elasticity of muscle, which is very good, which means that there is a large space for muscle growth and the blood nutrition supply is fast and sufficient. What is said here is that muscles are alive and in good shape, while some people practice for half an hour, panting, and look almost the same as when they don't practice, and their muscles don't explode. This is called rigid training.

But it doesn't mean that you can practice less and don't have to work hard, but you should practice more attentively.