A sudden bruise on the arm is mainly caused by trauma, i.e., accidental bleeding or oozing of blood or tissue fluid under the skin of the arm. The bruising is caused by the absorption of blood clots into the fatty tissue under the skin. Therefore, at the beginning of the trauma, cold compresses with ice or cold towels should be applied to the painful area in a timely manner to effectively reduce bleeding and tissue fluid exudation, and to reduce further expansion of swelling. If the bruise has already appeared, you can use hot towels on the surface, or soak in a hot bath to accelerate the blood circulation of the subcutaneous tissue, which can eliminate the bruise as soon as possible.