So the forces in a sprint and a standing climb never have the bike completely vertical through the pedal stroke. If you look at the attached video of Cavendish sprinting (and not even going full out) the frame is always at an angle as the forces are applied, this seems to me that it would promote more possible flex to the BB area? More so than a bike sitting upright in a trainer as referenced in the original video.
https://www.youtube.com/watch?v=Kb67p8Cb7v0
William
PS: Apologies If I missed it in the running commentary.