A cramp is a sudden, and involuntary muscle contraction or over-shortening; while generally temporary and non-damaging, they can cause mild-to-excruciating pain, and a paralysis-like immobility of the affected muscle(s). Onset is usually sudden, and it resolves on its own over a period of several seconds, minutes, or hours. Cramps may occur in a skeletal muscle or smooth muscle. Skeletal muscle cramps may be caused by any combination of muscle fatigue, a lack of electrolytes (e.g., low sodium, low potassium, or low magnesium). Cramps of smooth muscle may be due to menstruation or gastroenteritis.