Great question, kamilo12, and one that might be really relevant to Olympic athletes! Cramps are generally caused by muscles contracting (suddenly shortening). You usually get them in your legs, but you can get them anywhere. There are lots of different reasons why you might get a cramp, and some are more serious than others, but probably the more common reasons are due to overexerting your muscles, getting dehydrated, and having low levels of potassium or sodium, which is lost through sweating. When you lose these salts, it causes your muscle cells to shrink, which results in a cramp.