The incline angle shifts stress to the clavicular (upper) head of the pectorals. The fly pattern maximizes stretch-under-load at the bottom of the arc, where pec fibers are at their longest length — research consistently shows the stretched position is the strongest hypertrophy stimulus.
Stretch-under-load 12/15 — at the bottom of the fly with arms wide and inclined, the upper pec is near-maximally lengthened while resisting the dumbbells' gravitational pull.
Lever ratio 6:1 — same pec insertion geometry as exercise 1, but the dumbbell fly has a wider arc, creating peak torque at the bottom where the moment arm is longest.
Clavicular head emphasis — the 30–45° incline changes the line of pull to favor upper pec fibers that the flat cable lateral in exercise 1 loads less intensely.
Placed second to complement exercise 1: where the cable lateral provides constant tension and inner pec emphasis, this fly provides maximum stretch and upper pec emphasis. Together they cover different pec regions through different lever geometries.