Building your muscles means that you need to work it out to have a satisfactory result. Some people choose to work out their biceps and triceps and some work out their abdomen so that they can have flat chest with very firm and tone abs.
If you want to have a flat chest you need to train the core muscles depending on the crunches and sit ups. However, the number of damages or the incompetence of the workouts shows that several individuals who do crunches and sit ups do not understand how to do it in a proper especially if you are working your abs through sit ups.