A man when walking has his head in advance of his feet.
A man when walking across a long level plain first leans [rather] backwards and then as much forwards.
[Footnote 3-6: He strides forward with the air of a man going down hill; when weary, on the contrary he walks like a man going up hill.]
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.