Sudden soreness and weakness of the arm is usually related to over-fatigue, brachial plexus nerve injury, cerebrovascular stenosis and other factors, as follows. 1. Excessive fatigue: If you have recently done strenuous exercise, such as jumping rope, playing basketball, etc., it may lead to excessive fatigue of the arm, which is easy to cause weakness, and the symptoms will be relieved after resting for some time. 2. Brachial plexus nerve injury: it may be due to pulling injury or fracture caused by brachial plexus nerve injury, after brachial plexus nerve injury, it is easy to cause sensory abnormality, and the symptoms such as arm soreness and weakness may appear. 3. Cerebrovascular stenosis: often associated with atherosclerosis, is the main cause of ischemic stroke, patients will often dizziness, headache, and will be accompanied by limb numbness, aching, weakness and other symptoms. Sudden weakness in the arms may also be caused by calcium deficiency, electrolyte disorders and other factors, and should go to the hospital in a timely manner, and standardized treatment under the guidance of the doctor.