The shape of a muscle cannot be changed
One of the most common misconceptions is that a muscle can be formed into a shape by certain exercises. Who does not know the thesis that concentration curls make the biceps higher or close grip bench press builds up the inner chest. The latter should serve as an example here.
The chest muscle starts at the sternum, clavicle and abdomen and attaches at the upper arm as an active component. Imagine the individual muscle fibers as rubber bands in this simplified model. In order to stimulate the muscle to grow, it must be contracted. The intensity of the contraction determines the strength of the stimulus for muscle growth. In order to specifically build up the inner chest, there must be greater muscle growth there than in the other areas of the chest. For this, the muscle fiber segments in the middle of the chest would now have to contract more than in the other areas. This is not possible, because they are one and the same. Imagine it like a rope that you need to tighten. Either you tighten the rope from beginning to end or you can’t tighten it at all. Or try grabbing a rubber band at both ends, stretching it and then allowing it to contract more tightly in different areas. It is physically impossible.
On the other hand, it is possible to specifically build up the upper or lower chest region by means of a stronger contraction of the upper or lower chest muscle fibers, as is achieved by means of different inclinations of a bench press (incline / decline / regular). Here, not individual segments of the fibers are increasingly stressed, but the upper or lower muscle fibers as a whole.
This principle is transferable to every part of the body.
For example biceps: Although you can put more stress into the inner or outer head of the biceps, you will not be able to target the muscle fiber area in the middle of the bicep to stimulate greater muscle growth. The effect mistakenly considered to be the growth of the inner chest or peak of the biceps is nothing more than an increase in muscle mass or an increased definition.