The main reasons for having bruises on the arms include trauma, rupture of blood vessels, and blockage of blood return in blood vessels. If the arm bruises do not disappear for a long time, it is recommended to go to the hospital for further examination and treatment under the guidance of professional doctors. 1. Trauma: When the arm is traumatized by external force, subcutaneous bruising will be formed, resulting in purple bruises on the surface of the arm. 2. Blood vessel rupture: when people’s blood vessel elasticity is poor, the arm is slightly squeezed, which will cause blood vessel rupture and bruising on the surface. It is recommended that people stay away from risk factors. 3. Obstructed blood return: when the arm is under long-term pressure, resulting in obstruction of blood circulation in the arm, the discomfort of localized bruising. It is recommended that people change their body position appropriately and exercise regularly. If the arm is bruised for a long time, you should consult a doctor in time.