The pectoralis can be divided into “heads” based on their origin. The pec major has two heads: One originates along the medial half (inside half) of the clavicle; the sternocostal head originates along the sternum. Gym wisdom says you can effectively target specific areas by changing your grip width.
At full extension, narrow-grip benches allow one to really squeeze the inner pecs. This indicates that the area of the pecs near the sternum is working.
There is no way to divide the pec major in terms of inner or outer. The pec major starts inner and ends outer. Changing grip width during bench presses cannot alter this.
A study comparing varying grip widths on the bench press was performed by measuring pectoralis muscle activity with electromyography. The results indicated that the clavicular and sternocostal heads were activated most by a wide grip.
The data show that a wide grip is most effective at activating both the inner and outer areas of the chest.
For overall thickness, a wide grip is preferred, since it stimulates the most growth. A narrow grip should be used to add variety to your chest workout. – FLEX