Since Michelangelo studied human anatomy he knew not only how the outside of the body looks but how the inside looked and how it functioned.  All of his sculptures are focused on the human body, something real, not something abstract he made up. He took a simple slab of marble and made it look like a real life figure. Everyone knows Michelangelo's Sculpture of David.

Michelangelo depicted David before his battle with Goliath.  The slingshot that he carries (that you don't really notice) represents David's victory was of cleverness and not solely of strength.  David character traits are considered more important than his victory over Goliath, which is why Michelangelo depicted him before battle.