[The following read is an article I wrote for IlluminActive, which is a new fitness lifestyle brand/website that I co-started with two close friends. This is the first installment of our myth-debunk series]
YOU CANNOT CHANGE THE SHAPE OF A MUSCLE with training (as in characteristic form or appearance). A muscle can be trained to change its performance and/or size, but not its genetically determined shape. For example: preacher curls will place more tension at the distal insertion of your biceps brachii, creating a feeling like the part of the muscle nearer the elbow is being worked more significantly. Despite this highly localized feeling, the exercise will not lengthen your biceps muscle as a result.
If a muscle grows in size, it will fill out the available space, growing into a more apparent version of its true shape. Look at before/after pictures of anyone who has added significant muscle mass (e.g. professional bodybuilder and other physique athlete). This person's overall size and visual impression may have increased dramatically, however, shape characteristics and genetics remain visible and unchanged. Those with a trained eye for such sports will know better what I'm talking about.
In other words, you get the hand you're dealt. How you play that hand is up to you.
I explain this NOT to discourage, but to encourage a shift of focus to the controllable. To stop chasing ghosts, arm yourself with a mind.