Sorry if this question seems to be from a ignorant point of view.
I've been going to the gym to do bodybuilding. In my workout, my trainers have told me that exercises seem to be assigned opposing categories. For example biceps vs triceps, hamstrings vs quadriceps. There is also some categories I don't know the name in English, but they are VERY common exercises. For example, what is the name of this muscle:
Respectively, in my language, the first one is called "back" and the second one "shoulder".
My last question is if there is a opposite of pectoral exercises. In common butterfly, you are "compressing" between your arms. In wich exercise do you make force to separate the arms? (reversal of butterfly)
In my language, the exercise opposite of butterfly is normally called "rowing".