Muscles contract progressively from points of origin to point of insertion.
The reason you will lift more, for example on a flat db press is because you are taking advantage of the pectorals doing what they are designed to do; bring the arms across the front of the chest.
If you do an incline dumbell press, besides bringing the deltoids into play, you are not calling in to practice all of the pectoralis muscle fibers that you potentially could:
"Not all myofibrils exert force to the same magnitude throughout the length of a sarcolemma; only as a muscle loses its mechanical efficiency-due to the force deviating from a straight line-wherein it increases its girth at the point of full contraction will those myofibrils toward the center of the muscle belly be called upon to work."
If you do 80 pound flat dumbells press for 8 reps, you are exhausting, in accordance with the mechanical physics listed about, more muscle fibers than using 60 pound dumbells for 8 reps in the incline position. You aren't just exhausting some (as in using a lower weight with an incline press); you are exhausting maximally what you are capable of doing. Incline DB presses will, to an extent, improve your chest - and make you stronger - but it won't make your upper chest bigger, in my opinion. Regarding hypertrophy, muscles grow as a whole, not selectively.
The reason you will lift more, for example on a flat db press is because you are taking advantage of the pectorals doing what they are designed to do; bring the arms across the front of the chest.
If you do an incline dumbell press, besides bringing the deltoids into play, you are not calling in to practice all of the pectoralis muscle fibers that you potentially could:
"Not all myofibrils exert force to the same magnitude throughout the length of a sarcolemma; only as a muscle loses its mechanical efficiency-due to the force deviating from a straight line-wherein it increases its girth at the point of full contraction will those myofibrils toward the center of the muscle belly be called upon to work."
If you do 80 pound flat dumbells press for 8 reps, you are exhausting, in accordance with the mechanical physics listed about, more muscle fibers than using 60 pound dumbells for 8 reps in the incline position. You aren't just exhausting some (as in using a lower weight with an incline press); you are exhausting maximally what you are capable of doing. Incline DB presses will, to an extent, improve your chest - and make you stronger - but it won't make your upper chest bigger, in my opinion. Regarding hypertrophy, muscles grow as a whole, not selectively.