Cramps, sometimes calle Charley horses, are unpleasant, often painful, sensations caused by contraction or over shortening of muscles. Cramps can be caused by cold, overexertion or low calcium level in blood (especially for adolescents where they need calcium for both blood and bone maturing). Illness or poisoning can also cause cramps, particularly in the stomach, which is referred to as colic if it fits particular characteristics. See also Delayed onset muscle soreness.