Originally Posted by Milton Keynes

Seriously, I'm no physics expert, but it's only logical that if the exertion you're putting into moving your bike across the ground is also causing some flex in the frame, then part of your effort is wasted in bending the frame back & forth. Or, the frame is absorbing some of the force instead of putting it fully into the pedals. I'm sure someone might show some science to prove me wrong, so have at it...