Adventist Youth Honors Answer Book/First aid/Cramps
There are two basic causes of cramping. One is inadequate oxygenation of muscle, and the other is lack of water or salt. Cramps from poor oxygenation can be improved by rapid deep breathing, and stretching the muscle. Cramps from lack of salt and water can be treated by stretching the muscle, drinking water and eating salt. Pounding on the muscle can increase soreness.
What happens in a cramp is that lactic acid builds up because of normal anaerobic muscle metabolism. When the muscle burns sugar without enough oxygen, it makes lactic acid. The lactic acid finally becomes concentrated enough to trigger the contraction of the muscle. When the muscle lacks salt, the nerves firing the muscle are unable to recharge properly, causing a similar effect.