What is arm weakness? Arm weakness refers to a loss of strength in the arm and the inability to move an arm because of decreased muscle strength. 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. Optimal treatment for arm weakness is dependent on the underlying cause of the weakness. Usual causes of arm weakness include injury to, or infection of, the arm; muscle wasting, such as from certain muscular disorders or from lack of use; nerve damage or compression at the vertebral column; or certain hereditary conditions. Stroke is a serious and potentially life-threatening cause of sudden arm weakness that appears on one side of the body – a true medical emergency. Temporary arm weakness may be caused by a general infection, such as the common cold. While arm weakness is generally not serious, arm weakness can be a sign of stroke. Seek immediate medical care (call 911) if you experience serious symptoms that may indicate a stroke, such as a sudden, severe headache; loss of consciousness; confusion; sudden numbness or paralysis, especially if it occurs on one side of the body; vision changes, or difficulty speaking. If your arm weakness is persistent or causes you concern, seek prompt medical care.