The bruising on the leg after the collision is caused by subcutaneous capillary bleeding, capillary bleeding can appear in the early stage of small lumps, hard lumps, after a long time the blood is absorbed by the human tissue, can appear bruising, purple, etc., after a period of time the bruising can subside, but if not dealt with, the time will be longer. So cold compresses with ice or cold towels should be applied to the surface after the early collision, which can reduce bleeding and the exudation of surrounding tissue fluid. After 2 days, it is recommended to apply hot towels to speed up the local blood circulation, so that the bruising will dissipate faster.