Stretching is divided into dynamic stretching and static stretching in training. Static stretching is safer. When stretching the target muscle, keep stretching for 5 to 10 seconds, and you will feel slightly sore. Don't do dynamic stretching, which will cause muscle strain, because there is a knot at the connection point between ligament and bone, which is called golf knot in medicine. When the ligament suddenly stretches, it will send nerve instructions to the ligament, so that the contraction and stretching forces of the ligament are opposite. If the tension is greater than the contraction force, stretching the target muscle for 5- 10 minutes before training is helpful to reduce the probability of training injury, and also has a good effect on muscle contraction and training quality. Stretching after training will reduce the accumulation of lactic acid, relax muscles and maintain muscle elasticity.