Mike Stevens, of Salt Water Creek, asks :-

What causes cramp?

Phil Sheard, a physiologist at the University of Otago's School of Medical Sciences, responded.

Cramp is a powerful unplanned muscle contraction. It is often painful and may last from a few seconds to many minutes. Particularly intense episodes can leave the muscles feeling tender and sore for days afterwards. Cramps are slightly more common in females (especially during pregnancy) than males and are more likely when undergoing strenuous physical activity, especially in a hot environment. Cramps are not usually an indication of any serious underlying medical condition, most people experience cramps at some stage in their life.

Our muscles normally contract to develop force in response to stimulation from the nervous system. The intense involuntary muscle contractions that characterise cramps may have several causes, the most common of which is over-excitability of the nerves that control muscle contraction. What causes the nerves to become over-excitable is a change in the composition of the watery fluid that bathes the nerve fibres where they branch through the muscle. When we perspire a lot (eg strenuous activity, hot weather) we lose both water and salts from the tiny spaces around all our cells. This loss of water and salts leads to a change in the composition of the fluid around nerve fibres causing them to become spontaneously active, generating powerful uncontrolled muscle contractions.

Cramps are usually relieved by stretching the affected muscles, this inhibits the activity of the nerve fibres contacting the cramping muscle thereby bringing the period of muscle stimulation to an end. Most cramps can be prevented by warming up and stretching muscles prior to vigorous activity, and by ensuring proper fluid intake before, during, and after the activity, perhaps by including an electrolyte-containing sports drink.