No. Evolution works only on traits produced by genetic mutation, NOT traits acquired through behaviour. This was one of the flaws in early theories of evolution: it was believed that actions of the parent could influence the genetics of the child, which is not the case. The standard example is giraffes: under the incorrect theory, one could say "they developed longer necks because they stretched them to reach high leaves", but the correct interpretation is instead "the ones with longer-than-average necks could feed better, and hence had more children".
The reason for this is that the genetic material passed on through reproduction comes entirely from the cells in your reproductive organs, so no matter how much you train your neck (or stomach, in your case), none of those changes can in any way get passed to your children, because those cells just aren't involved in the process.