Hmm, I've noticed the same thing - in higher efforts where I pull on the bars, seated or standing, it's my abs that enable my connection to the bars to add to the force delivered by my legs, beyond the application of dead weight. I also think this would be a consequence of the "muscle tension workout" Carbonfiberboy talked about in a recent Polarized threads - 50/11, 50 rpm, for numerous minutes. A few years ago Mrs. Road Fan and I went out on "The Hills of Ann Arbor" route, and that pedaling technique got my lardy butt up the hills, but not in a gear anything like a 50/11!!! For my Trek at the time it was more like 39/34 on 700c wheels.