I'm sure 0311 will chime in with a much better explination, but sufice to say, you cant isolate one part of a muscle. Your either working your pecs, or your not. Outer, inner, upper, lower there is no such thing, lift heavy, build the whole thing, and the lagging parts will come around eventually.

I would concentrate on compounds, leave the flys alone, they dont do much for you.

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