What causes muscle cramps?

Cramp is caused by a build-up of a waste substance called lactic acid. When a muscle works harder than usual, it starts to break down stored food without using oxygen. This process is called anaerobic respiration. it produces lactic acid as a waste product. As the lactic acid builds upit interferes with muscle action, making the muscle feel tired, until the acid is flushed away by the blood. If too much lactic acid builds up, it makes the muscle contract very sharply and painfully, causing a cramp.