Rav Moshe Feinstein used to stand perfectly straight with much humbleness when he would daven. He told people that he learned how to stand this way after a Russian guard was yelling at him and threatened to kill him. Rav Moshe said that if he stood this way because he feared his life from a human, he certainly should stand that way when praying for his life while standing in front of the Ribono Shel Olam.