You can train specific parts of a muscle for good shape. Quite often you hear people saying that they're doing a certain exercise to hit a “certain part” of a muscle. For examples Inclined Bench Press for upper part of the chest, or Declined movements for lower part of the chest, or preacher curls for lower part of the biceps or front squats for upper part of the legs etc. This is rubbish. When you train a muscle for the best results or at mass building intensity
, you train the whole muscle. A muscle either contracts or relaxes; you cannot train one specific part of it no matter how hard you try. One of the biggest examples of this myth in action is people saying they're going to train their upper chest by doing more incline benching, or lower chest by doing more decline benching. The chest is made up of two muscles, the pectorals major, and pectorals minor. The pectorals minor virtually can't be seen so it's the major pectoral that you're looking to build up by training. The minor gets the training by itself.
This doesn't means that the all other accessory exercises like pec deck, flyes, pull overs, incline, declined, leg variation exercises, different bench moments etc are not useful. No no!! All these exercises are great for beginners and for intermediate where the intensity of the exercise is just 20%-40% of the 1 repetition capacity. These lucrative exercises are great to do and great to feel. But once you are on the advance track the big monster basic exercises which are done with explosive intensity of 70%-80% capacity.
Once you exert the muscle at an optimum load it exert fully, may be at any angle. When we exert the muscle at different angle it hast to be trainer to low intensity may be 50 tp 60 % of the total one repetition power, for 15 + reps. Once the reps are more the results are more towards fitness rather than size and mass.
Dr. Randhir Hastir