Crop Tops were originally used by football players who wanted to cover up but reveal their abdomens to help with cooling and aerodynamics. This trend originated for both women and men within sporting companies before slowly fading away. In the nineties the crop top trend came back but more so for women. In prior years I wouldn't have worn a top that showed my abs because usually these shirts will shorten your torso and aren't particularly flattering unless you have a very flat stomach. However, the crop top has recently come back and I think this looser fit is something I would definitely be willing to experiment with. Since the tops these girls are wearing aren't snug there is more room for comfort. Another option is to wear a shirt (or bodysuit) underneath giving the same effect but omitting the showing skin.