From Ergo Log
The incline bench press is almost as popular in gyms as the flat bench press, while the decline bench press is often neglected. And that's a shame: according to a human study done a number of years ago by sports scientists at Wayne State College, the decline bench press is a better exercise for the chest muscles than the incline bench press.
You have two chest muscles: the pectoralis minor and the pectoralis major. The study mentioned here only looked at the pectoralis major.
The pectoralis major in turn consists of two parts: the upper part, which is attached to the collarbone and is called the clavicular portion of the pectoralis major in anatomy text books [above right: C]; and the larger, lower part, attached to the sternum, officially called the sternal portion of the pectoralis major [above right: S].
Bodybuilders who want to focus on developing the clavicular portion of their pecs add the incline bench press [below left] to their workouts. If they want to emphasise the sternal portion of their pecs, then they go for the decline bench press [below right].
The researchers wanted to know for sure whether these two exercises do actually do what the hypertrophic community think they do. So they got 15 students, all of whom had already been doing strength training for at least a year, to do incline and decline bench presses in the laboratory. The researchers measured the electrical activity in the subjects' chest muscles while they were exercising, and used the information to work out how hard the muscles were working.
Decline > incline
The incline bench press stimulated the upper part of the chest muscle just as much as the decline bench press did, as the figure below shows. The exercise therefore doesn't have the effect that bodybuilders expect it to have.
Con = concentric [upwards] movement; Ecc = eccentric [return] movement.
The figure above shows the effect of both exercises on the lower part of the chest muscle. Here you can see that the decline bench press does have the effect that bodybuilders think it has.
Journal of Strength & Conditioning Research, 1997, 11(3), 163-167.