Question: what are big muscle groups?
Answer: Deltoids. The deltoids contribute to both the front and back of the upper body and if they are not well developed your entire upper body will suffer. ... 
Chest. An exercise that will help to develop the pectorals is the bench press; the bench press works the entire pectorals. ... 
Traps. ... 
Triceps. ... 
Biceps. ... 
Abs.