while running, having gained speed, while in full motion
at a run
from a running start(in refer, to very fast motion only) at a dead run.
На этот призыв выходит из толпы парень и с разбега бросается в пламя (Салтыков-Щедрин 1). ( context transl ) At this appeal a lad came forward from the crowd, took a running leap and hurled himself into the flames (1a)