I’m almost positive you cannot ever change the shape of your muscle. Your muscle fibers can only grow or shrink as a whole, so, for example, if you have a high biceps insertion, doing preacher curls all day will not make your biceps longer.
However, some compounds like close grip chins may only emphasize the inner or outer bicep, so one head will lag and isolations could be useful for correcting an imbalance.[/quote]
Here’s food for thought…This quote is taken from Christian Thibaudeau’s HSS-100 Chest Specialization article:
"Studies by Seger et al. found that different types of muscle contraction (eccentric vs. concentric) lead to localized muscle damage in specific parts of a muscle group. Eccentric contractions creates more damage in the distal portion (near both insertions) of a muscle group while concentric contractions creates more damage in the proximal (or muscle belly) portion.
Don’t get me wrong, both types of muscle contractions creates damage on the whole muscle, but the relative amount of damage to different portions of a muscle is contraction-specific. That result is an indirect indication that it IS possible to put more training stress on different portion of a muscle group.
Furthermore, more recent physiology research has found that not all muscle fibers run all the way from one insertion to the other; many muscle fibers are actually intermediate fibers that only cover a small portion of the muscle length.
While these fibers do not represent the majority of the fibers within a muscle group, they still form a significant portion of the motor unit pool. If certain muscle fibers cover only a portion of a muscle, it also indicates that it is possible to place more growth stimulation on certain parts of a muscle.
Finally the fact that different parts of a muscle group can get sore depending on the exercises being performed is also an indirect indication that putting more growth stimulation on certain parts of a muscle group is possible."