The actual questions of the genesis of metaphors and their initial (prae-)forms are never examined and most often omitted in toto. As a rare exception Yu Ren Dong mentioned for the interested teachers, alas: without referring to any sources, that “pull your leg was originally thought to be used to shorten the suffering of a person who was being hanged.” (Boyan Nikolaev; boyannikolaevyahoo.com)