You could be out for a run or drifting off to sleep when it happens: The muscles of your calf or foot suddenly become hard, tight, and extremely painful. You are having a muscle cramp. Sometimes called charley horses -- particularly when they are in the calf muscles -- cramps are caused by muscle spasms, involuntary contractions of one or more muscles. In addition to the foot and calf muscles, other muscles prone to spasms include the front and back of the thigh, the hands, arms, abdomen, and muscles along the rib cage.

Possible Causes of Muscle Cramps
Muscle cramps can have many possible causes. They include:
-- Poor blood circulation in the legs
-- Overexertion of the calf muscles while exercising
-- Insufficient stretching before exercise
-- Exercising in the heat
-- Muscle fatigue
-- Dehydration
-- Magnesium and/or potassium deficiency
-- Calcium deficiency in pregnant women
-- Malfunctioning nerves, which could be caused by a problem such as a spinal cord injury or pinched nerve in the neck or back.