The arm is very painful after knocking can be given local cold compress, you can use ice packs or use ice water mixture external compresses, it is recommended to go to the hospital to do X-ray examination of the arm, to observe clearly whether there is a fracture situation. If the fracture is excluded and only local soft tissue contusion, you can usually go out to reduce swelling and pain, blood circulation and eliminate blood stasis of traditional Chinese medicine, such as Sanhuang San or pain-relieving plasters, etc., can have the effect of reducing swelling and pain. In addition, pay attention to rest, avoid excessive activities, so that the local swelling, pain gradually ease and the injured tissues gradually recover, the pain can be slowly relieved. If the pain is severe, you can also take anti-inflammatory drugs, such as ibuprofen, acetaminophen, etc. You can also take Chinese medicines that activate blood circulation and eliminate blood stasis, such as Yuanhu Pain Relieving Drops Pills, which are also effective in relieving pain.