flat, incline, and decline all work the whole chest...so do whichever one you like most.

ab work?

i think with most 5 x 5's...you're not going to be doing all 5 sets with the same weight...one day you might stagger like for example...bench press:

200 x 5, 210 x 5, 220 x 5, 230 x 5, 240 x 5

and the other day would probably be a light day.

but doing 5 x 5 with all the same weight for every exercise every day is gonna be painful

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