Sudden twitching of the hand and arm may be caused by physiological factors, or it may be caused by hypocalcemia, epilepsy and other diseases. 1. Physiological factors: strain injury causes overwork of the arm muscles and increase of lactic acid, leading to twitching of the arm muscles and decrease of muscle strength. This condition can be improved by resting and increasing nutrition. 2. Hypocalcemia: Because calcium ions play a very important role in the process of muscle contraction. When the calcium ions in the blood are lower than normal, the muscles are easily excited, resulting in symptoms of muscle spasm and twitching. 3. Epilepsy: Partial seizures can be characterized by involuntary twitching of one side of the limb, which can usually be diagnosed with the aid of an EEG. If sudden jerking of the hand and arm occurs sometimes, except for physiological factors, it is recommended to seek medical treatment as soon as possible to avoid delaying the condition.