“There’s a halacha in the SH”A that a man is required to walk in the street with his head bent so low that he can see at the most, 4 amos in front of him.”
It says not to walk with a straight back, not necessarily bent over. The Kaf Hachaim says that it means to not be too upright as not to see the ground in front of you. He also says that one should not be bent over so as not to see what’s ahead. In short, a medium stance.