I'm assuming that roughly 1 in 5 women also have huge boobs that can be unwieldy during exercise even when wearing sports bras. I think that's the point of the study. I've heard from huge-breasted female friends that they do experience a certain breast discomfort when exercising. For "normal" size breasted friends, they feel the discomfort once extremely rigorous exercise is concerned (CAT/ROTC training, drills)