A recent study that examined muscle activation during the bench press following the completion of dumbbell flyes found that when you bench press after doing flyes, the triceps show higher activation (i.e. were forced to work harder),not the pecs. Common “gym wisdom” is that you can do an isolation exercise prior to a compound movement and thereby force the fatigued muscle to work harder. Unfortunately, this wisdom is patently incorrect according to science (as usual).

J Strength Cond Res. 2009 Oct;23(7):1933-40