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Muscle Cramps and Spasms:
The Electrolyte Misconnection

Muscles need sufficient electrolytes--sodium, magnesium, potassium and chloride—in proper balance to function properly. The body manipulates the balance of these minerals inside and outside of muscle cells in order to get the muscles to contract and relax. An imbalance or deficiency of these electrolytes can cause problems with the body’s electrical impulses and lead to muscle cramps and/or muscle spasms. Low levels of any of these minerals can allow the muscle to contract, but prevent it from relaxing.
Electrolyte imbalances can occur due to deficiencies in the diet, sweat, urination, diarrhea, medication side effects, from consuming diuretics, and from problems with absorption. Electrolyte deficiencies can also be caused by increased demand for minerals in the body such as in the case of pregnancy or healing. Muscle cramps often occur in middle-aged and older people and are common in athletes. Some researchers believe a mineral imbalance can negatively affect blood flow to the muscles and that a deficiency of some minerals, like potassium, can interfere with the muscles’ ability to use glycogen, a sugar that is the muscles’ main source of energy.
Long-distance runners and cyclists, even individuals who exercise regularly, are prone to cramps. Often, these individuals have electrolyte deficiencies or imbalances because they lose critical electrolytes in sweat

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http://www.eletewater.com/elpdf/electrolytes_cramps.pdf

electrolytes - on howstuffworks.com
http://health.howstuffworks.com/question565.htm