Human life activities need energy. The energy of human body comes from three nutrients in the body: sugar, fat and protein. These nutrients are like the "fuel" of the human body, which produces energy through very complicated biochemical processes in the human body. We know that combustion needs oxygen, and nutrients in the human body release energy, which is also the result of interaction with oxygen. There are all kinds of "fuels" stored in the human body. People just eat regularly, but oxygen needs to be inhaled all the time. Under normal circumstances, such as study, jogging, light manual labor, etc. Oxygen supply is sufficient. At this time, nutrients in the human body, such as glucose, need to breathe. 1 g glucose produces about 16 kilojoules of energy, and the products are carbon dioxide and water. Similarly, when doing aerobic exercise, although the human body consumes a lot of energy, it still maintains the level of aerobic metabolism.

When the human body is in strenuous exercise, the energy required is of course greatly increased. At this time, the oxygen supply is insufficient, and anoxic breathing will occur. Hypoxia breathing is also a physiological process of human body, which can quickly generate energy to form a short explosive force (such as 100 meter sprint). Regular anaerobic exercise can tap human potential and improve competitive performance. However, the rapid energy supply of anoxic respiration is at the expense of reducing the utilization rate of "fuel": at this time, 1 g glucose only produces less than 1.5 kJ of energy, and the product is lactic acid. After strenuous exercise, we will feel muscle aches because lactic acid has accumulated in our muscles.