I learned tennis in the 80s and it's funny to see the backhand of players like Lendl--what I learned in tennis camp as the ideal bh seems so different from what even the one-handers hit now. To me, one of the main differences seems to be the way players like Borg, Lendl or Mac bend their knees and step into it; Almagro, Gasquet and Fed are usually completely upright. I take it one of the reasons for this is the game is faster now and the players don't have the time they once did, so even a one handed bh seems to engage the core more rather than the legs. it's interesting how these things have evolved over time.
Check out the difference in Lendl's follow-through, back in the day vs. 2011. The latter looks much more to me like Almagro, Fed, Gasquet, etc.