One of the causes of muscle soreness: lactic acid accumulation exercise and exercise need energy. If energy comes from aerobic metabolism in cells, it is aerobic exercise; But if energy comes from anaerobic glycolysis, it is anaerobic exercise. During aerobic exercise, glucose metabolism produces water and carbon dioxide, which can be excreted by breathing; However, during anaerobic exercise, grape glycolysis produces a large number of intermediate metabolites, such as lactic acid, which cannot be eliminated by respiration and forms lactic acid accumulation. Too much lactic acid stimulates muscles to increase osmotic pressure, so that muscle tissue absorbs more water, resulting in local swelling, which will make muscles feel sore. Lactic acid accumulation generally occurs in people who lack exercise, or the muscle parts lacking exercise suddenly move to a certain intensity. Yu Jun also has a deep understanding of this aspect. He starts playing badminton again every once in a while, and the muscles in his thighs and buttocks will be sore from the second day to the third day, because the equipment exercise in his thighs and buttocks is less than that in other parts. With the increase of fitness exercise frequency, the phenomenon of lactic acid accumulation after exercise will become less and less, and the degree will become lighter and lighter. So we should keep the good habit of exercising. The second reason of muscle soreness: muscle fiber connective tissue injury For people who often insist on gym exercise, they will often encounter muscle soreness. Why is this? In fact, the accumulation of lactic acid is only one of these people, and another main reason is the slight injury of muscle fibers. When we gradually increase muscle exercise load, it will cause slight damage to local muscle fibers and connective tissue, and even cause spasm of some muscle fibers. But don't be nervous, this kind of injury is acceptable and has positive muscle building significance. In fact, this kind of muscle fiber injury and spasm is local and microscopic, and the muscle can still complete the motor function, but there is a feeling of pain. After soreness, muscle tissue becomes stronger than before after local minor injuries are repaired, and the same load will not be damaged (soreness) in the future. It is worth noting that this kind of repair needs rest and protein nutrition supplement. In a word, muscle pain caused by slight elongation of muscle fibers explains why we need to increase muscle exercise load step by step, which is also the reason why many muscle-increasing enthusiasts pursue the feeling of muscle pain and swelling after exercise.