I don't know if anybody heres noticed buy everytime after you run and then u go swimming right after, when u come out your stomache just for some reason seems flatter than before? is it becasue swimming strethces your belly?
I notice this whenever I work out. The core is pretty much involved with any activity. Anytime it is worked... you will feel more supported and it will appear firmer. This is usually noticed more by people with slimmer or toned bodies though.
It's always good to do a few plank poses before a day at the beach if ya know what I mean