The reason for the hard lump on the arm is usually the following: 1. local scar growth, when the arm has been traumatized will lead to the formation of local hard lumps, generally do not worry too much, with the extension of time scar can gradually soften, if necessary, you can apply local scar cream or go to the medical aesthetic department for treatment. 2. there may be a swelling on the arm caused by, for example, sebaceous cysts, lipomas, fibroids, or even intramuscular leiomyosarcoma. The company’s main business is to provide a wide range of products and services to the public. There are other causes, such as swollen lymph nodes. In short, if you have a persistent lump on your arm, you need to go to the hospital promptly in order to clarify the cause.