I suppose it depends. While I love my consoles I have been a PC gamer most my life , pretty hardcore on several MMO's (no, not WOW)and still play the hell out of TF2 today.
That said, while I cannot fathom doing 40 hours straight I have done some ridiculous stints of time, especially on mmo expansion releases in the upwards of 24 hours straight, heck D3 I played just about that.
This too all depends on work schedule and my overall ability to do so, sure I do a lot of sitting but I also get up, walk around, watch a few minutes of tv, grab a bite to eat etc, you know move the muscles a tad.
Clots @40 hours sure, several factors can attribute to this such as that person's weight, position of sitting, type of chair they were sitting on etc.