Normally, a muscle at work contracts, tightening to exert a pulling force, then stretches out when the movement is finished or when another muscle exerts force in the opposite direction. But sometimes a muscle contracts with great intensity and stays contracted, refusing to stretch out again. This is a muscle cramp. Muscles contract or lengthen in response to electrical signals from nerves. Minerals such as sodium, calcium, and magnesium, which surround and permeate muscles cells, play a key role in the transmission of these signals. Imbalances in these minerals, as well as certain hormones, body fluids, and chemicals, or a malfunction in the nervous system itself can disrupt the flow of electrical signals and cause a muscle to cramp. Fatigued muscles and cold muscles are more likely to cramp.