The point was to fit the human form inside a circle and a square:

Ancient thinkers had long invested the circle and the square with symbolic powers. The circle represented the cosmic and the divine; the square, the earthly and the secular. Anyone proposing that a man could be made to fit inside both shapes was making a metaphysical proposition: The human body wasn’t just designed according to the principles that governed the world; it was the world, in miniature. This was the theory of the microcosm, and Leonardo hitched himself to it early in his career. “By the ancients,” he wrote around 1492, “man was termed a lesser world, and certainly the use of this name is well bestowed, because …his body is an analogue for the world.”

But what should this microcosmic man look like? Vitruvius hadn’t provided illustrations. Artists in medieval Europe, loosely echoing Vitruvius, had come up with visions of their ideal man: Christ on the cross, representing both the human and the divine. But until the late 1400s, nobody had tried to work out exactly how a man with Vitruvian proportions might be inscribed inside a circle and a square. This was the challenge that prompted Leonardo to draw Vitruvian Man.

Vitruvius was a Roman architect who proposed the idea.  Leonardo wasn’t the first to try.

I drew a picture of myself, naked, in the pose above.  I call it Flabuvian Man.  But it is too scary to look at, so I won’t post it.