Originally Posted by Point
It really doesn't make any difference whether you are sprinting or just cruising on the flats while seated. The energy that flexes the bottom bracket sideways returns when the force is removed.
Not sure if I buy the bit about the BB flex being returned and reused. At the bottom of the pedal stroke, there's a "flat" spot during the transition from one pedal to the other. I think probably the BB just unflexes when the load is removed, and that energy is lost.
If people were machines and the pedal stroke was a perfect circle, then yeah, maybe I could see some of that energy being returned, but I don't think it's very realistic to assume that we're getting any forward assistance out of that on a typical pedal stroke.