Sorry if you misunderstood me, but try this for me. Video yourself jogging with and exaggerated bouncing motion then video yourself jogging with that same bouncing but try your best to keep the hips higher with less knee flexion. Whenever we sprint our body collapses during ground contact and the only way to lessen that effect is to apply significant vertical force down into the ground to keep the hips from dropping. If a runner does not apply enough vertical force you will see significantly longer ground contacts, low hips, and/or a longer amortization phase. Just push up and stay up/don’t collapse. I think I will post a video because explaining this is a little difficult.
A video might help, but if I do understand you right, we should use a term other than vertical displacement. Vertical displacement implies that there is actual movement in the vertical component, not a vertical resistance that, if lacking, might lead to collapse….it actually means the body is moving up and down with every stride. I don’t see that happening in that particular video and it wasn’t what I expected.