What study or effort led to Leonardo da Vinci mastering human proportion in his works of art?

Does Tom take the cabin with him in Uncle Tom's Cabin? Why does Ralph smile jeeringly? Who are the characters in the story the tale of the two cities chapter 7 the shoemaker and their characterization? What are the implied meanings of rich and poor leaves in poem THE RAIN? What is the truth behind the nursery rhyme wee willy winky? What has the author Adam P Fraise written? Where did Steinbeck get the title Of Mice and Men? The life and death of Tom Thumb the great? How did Jonas feel about the giver's favorite transmitted memory? What is Huck's admiration for the family's clock? What is the message in the poem my city? What has the author Terry L St Clair written? Did the men remember to say the kaddish for akiba drumer in the book night? What is a mastery of Subject Matter? What has the author David D Busch written? What is the medical term meaning the act of closing a wound or incision by stiching? What places did J.R.R Tolkien travel in his lifetime? On what did tom blame the unsuccessful marble spell? Knights wear this to protect their bodies? Who is farzana in The Kite Runner?

What study or effort led to Leonardo da Vinci mastering human proportion in his works of art?