When we talk of the working of the muscles, it means the muscle tissue during the exercise require complete relaxation and contraction. Which leads to continues exchange of produced waste material in muscles during exercise and fresh energy nutrients during the relaxation face. Which leads to forceful contraction. For building mass the muscles need complete care of the mind for action during the movement. Every repetition for mass building should be under control and complete muscle group should be involve to complete the movement for at least 6-8 repetition in one set. This is based on the principle of mind in muscle building.
If the repetition is very less, one or two, that means the main object of the movement is to use complete body and all muscle groups to complete the heaviest movement. One may be lifting very heavy for one or two reps but may not be bigger. If the repetitions are more than 15, the working muscles are not getting enough attentions of the mind and the muscles are not exerting much against a moderate load. This will lead to smaller muscle mass may be with more repetitive stamina.
So we should perform 6-8 reps in strict form with complete relaxation and contraction of the muscles during the exercise. This will require weight load of 70% - 80% of your one repetition capacity. This will leave to bigger muscle mass, size and hardness.
Dr. Randhir Hastir