I used to flex the heck out of my Trek 1220 bottom bracket when doing seated sprints. Could visibly see it swing back and forth. Totally nonscientific experience, but if I simply hit the BB area perpendicularly, it was easily clear just how much that bottom bracket could move with just a small amount of force (different load path/type, I know). My current CAAD10 is miles better in this respect, and honestly, I can't really tell when the BB is flexing.