I'd recommend anybody starting out to begin with the incline press (and possibly not to do any flat work at all), as flat bench creates droopy pecs in most trainees (I've seen it time and time again).
On the topic of emg, this is from Charles Poliquin:
Getting back to our question, the pectoralis major is one muscle with two heads, clavicular and sternocostal, that cover the chest and insert on the upper arm bone (humerus). The clavicular head originates at the center portion of the collar bone (clavical), and in lay language this area is referred to as the upper pecs. One of its primary functions is to raise the arm. The sternocostal head originates at the costal cartilage of the first six ribs and the adjoining section of the breast bone (sternum), and is called the lower pecs. One of its primary functions is to lower the arm.
One method of determining how muscles are activated during an exercise is called electromyography (EMG), a diagnostic tool with a history that spans several centuries and includes a working prototype first presented in 1890, at which time the term electromyography was introduced. In its present form, an EMG involves placing small electrodes in the skin with a needle, or on the skin with surgical tape. These electrodes collect information about the electrical activity of the muscle and send this information to a machine that translates this into data representing the degree of muscular contraction.
In researching the answer for this question, I found one paper using EMGs that compared muscle activation between the incline bench press and the decline bench press. The authors concluded that the lower portion of the pectorals was more active during decline presses than during incline presses, but that neither exercise completely isolated these muscles. From this data you can conclude that exercises which position the hands farther from the center of gravity will more strongly work the upper pecs (e.g., incline bench presses), and exercises that position the hands closer to the center of gravity (e.g., bench presses) work the lower pecs. However, this study did not look at the flat bench press, a consideration that turns out to have significant ramifications in exercise selection.
I say this because I then found another study that did EMG measurements of the incline, flat, and decline presses. It found that the maximum recruitment of the upper pecs was with an incline press, and the maximum recruitment of the lower pecs was with a flat, not decline, press. The study also looked at hand spacing, and what’s interesting is that it found that a wider grip did not significantly increase the involvement of the lower pecs in flat bench presses. With this information, I could now confidently answer this pec training question by saying that incline presses would be best for the upper pecs, and flat bench presses would be best for the lower pecs. But there’s more.
I found another EMG study that examined the differences between muscle activation during the barbell bench press, the dumbbell bench press and the dumbbell fly. What the researchers found was that the dumbbell fly had less muscle activation time than either the barbell bench presses or the dumbbell bench presses. With this information, I could expand my answer by saying that the dumbbell fly is an inferior exercise for developing the lower pecs.
Link to full article:
http://www.charlespoliquin.com/ArticlesM..._Tape.aspx