Great post! I particularly like when they twist the waist to make the back look even wider...
Some of the best videos I remember featuring a (twitching, flexing and whatnot) bare back were from (Dr.) Norah Josephsen, a beautiful Danish all-around athlete, currently devoted to shooting sports. A Little hard to find, I must say.
40-50 seconds of this red-headed beauty testing each and every one of these muscles. A real treat.