Bruising is a normal phenomenon when the legs are bumped. If the force of knocking is relatively small, and bruising, there are three main reasons: First, the human body’s platelet count is reduced, usually to go to the hospital for a blood test routine blood test, to see the number of platelets, because the platelet count is reduced, it will lead to the human body’s coagulation has obstacles, resulting in bleeding site is not easy to stop bleeding, bleeding increased, there are obvious bruises. Second, thrombin and coagulation factors, if the thrombin activity decreased, the clotting time is prolonged, the number of coagulation factors is reduced, will lead to coagulation disorders, resulting in bleeding after bleeding is not easy to stop bleeding, bleeding increased, there will be bruises. Third, hot compresses after the collision, resulting in accelerated local blood circulation, blood vessel rupture place bleeding more, there will be bruises.