Decline presses are for frat boys and gym bros. When's the last time you saw a bodybuilder whose lower pecs were too small for his upper pecs? It just doesn't happen.
According to EMG (i think that's the acronym... were they attach electrodes to muscles to measure a muscles activity/contraction) studies, decline presses involve more "upper" pec fibers than incline presses.
Incline presses, IMO, emphasize more of the anterior delt fibers, & De-emphasize the pec/upper-pec fibers.
If you're looking for a movement that emphasizes the pecs overall, I don't think there's a better movement than decline presses or decline db-flyes.
Training with "angles" isn't so much emphasizing a particular head of a muscle group, but instead you're placing other parts of that muscle in a weaker position to contract.
Triceps are a prime example, as are back movements.
With triceps, sure, doing pushdowns will activate all 3 heads of the triceps, but the majority of the workload gets placed on the lateral head. However, placing the upper-arm overhead, as verticle as possible, will place the lateral and medial heads in a weak position to contract, so the workload gets shifted primarily to the long head.
The further down you bring that upper arm, for example laying triceps extensions on a 45° incline bench & doing "overhead" triceps extensions will put a lil more of the workload back toward the lateral and medial (moreso lateral) heads. So you get more of a 33/33/33 % workload spread.
A favorite of mine is to do tricep extension/pushdown that mimic a 45° laying triceps ext.
You take an overhead pulley with a straightest, but instead of a conventional "push down", you step back a bit (2-3ft), bend forward at The waist 45°, & bring your arms up till your elbows are up about inline with your chin & do the "push down". However, you're not pushing the bar down, you're pushing the bar "out" because you're keeping your upper-arms locked in that position.
The entire movement, if viewing it from the side, would look exactly like a lying triceps extensions ("skullcrusher", either flat, or 30° incline), yet you still have resistance at the peak-contraction because of the cable. With a barbell/ez-bar, at peak contraction/elbows locked, you lose resistance because the weight is supported through the joints.
.. Ill chime in later with back stuff later. Getting tired of typing...
