I think they are called the intercostal muscles, I know you can strengthen them but you cannot isolate them, ill post back if i remember how to work them.

After searching there is nothing to isolate them which i already knew, but they are worked during normal exercises. To get better at taking a punch which i think you want a good ab work out must also strenghten the ribs, a medicine ball will directly target the area and improve the way you can absorb damage.