Jackson on Instagram: "If you want to train all the major actions of the pec (which you don’t really need to do, but we are talking about it for the sake of this video) all you need to do is understand what each region contributes to. 1. Upper pecs flex the shoulder (the mid pec contributes as well). 2. Everything flexes the shoulder horizontally but the mid region does it best. 3. Lower pecs extend the shoulder (some of the mid pec contributes as well) although the exercise I was doing was much