When patting the inner side of the arm, there will be subcutaneous bruising with the strengthening of the patting and the increase of the number of patting. Because the human capillary wall is very thin, and the level of superficial veins is also extremely superficial, so after being subjected to external force, the capillary or vein rupture, so that the blood in the blood vessels run under the skin, and cause subcutaneous bruising. However, if the symptom occurs after just a few light taps, it should be taken seriously and a timely visit to the hematology department of the hospital should be made, which is likely to be related to coagulation dysfunction.