Name the substance whose build up in the muscles during vigorous physical exercise may cause cramps.

Lactic acid, which is formed in muscles during vigorous physical exercise, may cause cramps. When a person runs fast, lactic acid builds up in the muscles, causing painful cramps. The extra oxygen taken while breathing reacts with the lactic acid in the muscles, breaking it down to make carbon dioxide and water. As the lactic acid breaks down the cramps will begin to disappear.