It is true that the renaissance masters studied nature intensively, but that they respected it most?
As far as I know about Michelangelo he cared more about the old greek idealized statues, following an idealized set of proportions as well as an idealized anatomy - the shapes of the muscles may infact not be derived from nature at all - like many old greek statues - but be designed after what looks the most beautiful.
According to a book I have, all early and a lot of later greek statues was designed following a geometric design, rather than being copied from nature. The quote I posted earlier from Platon seems to suggest the same.
His "truth" is first and foremost geometry, and the other truth comming from Pythagoras school is that of numbers.
So here you have the most serious influences on greek sculpture design, geometry and numbers, studying nature was a much later school of thought. And please don't forget that the greeks pretty much got their whole culture from the egyptians.
As for Raphael, I've read somewhere that he never deviated from the ideal face design of his own master. Look at the tiny mouth in their faces. This is design and taste, not from nature.
I posted this earlier, might as well repost it here
(from the book A world history of art Di Hugh Honour,John Fleming)
Naturalism and idealization
.... A new attitude to the statue as a visual equivalent and not a reduplication of its subject, had emerged out of attempts to invest marbles and bronzes with the appearance of life. Socrates is reported as saying of statues, that "the quality of seeming alive has the strongest visual appeal."
But shortly before the middle of the 4th century BC his follower Plato condemned further developments towards naturalism, drawing a distinction between the art of producing a likeness and the art of producing an appearance with a reactionary preference for the former.
"Artists nowadays care nothing for truth, they incorporate into their images not proportions that really are beautiful, but those that appear to be so."
In another passage he praised the Egyptians, who did not allow painters and sculptors "to make innovations or to create forms other than the traditional ones."