It can happen spontaneously or progress slowly over time. Weakness in the arm may occur on one or both sides of the body, may accompany weakness in other parts of the body, and may occur with a variety other symptoms, including arm pain. If you have arm weakness, you may have difficulty simply moving the affected arm, or you may have difficulty performing daily tasks. In some cases, physical therapy may help to improve arm strength. If arm weakness occurs with pain, pain medication may assist in resolving both symptoms.