I'm always telling my clients to keep their shoulder blades in their back pockets because #posture. There's an exception to every rule, right? There is an instance in which you want protraction instead of retraction. When? Anytime you are trying to isolate rear delt activation, you want to push the shoulders forward via protraction. Many people lack well rounded 3-D shoulders because (1.) They don't train rear delts or (2.) They train them incorrectly. Most people think they're training rear delts when in actuality, they're hitting mostly the traps and mid back.
Rear delt training isn't sexy. They're in the back and people tend to focus on primarily training the muscles they can see in the mirror. Don't forget your posterior. The rear delts are important for posture, shoulder stability and they give the delts a nice, well rounded shape. Rear delt work takes focus, concentration, a drop of the ego because you've got to use a much lighter weight and it takes a little knowledge because it's not just moving the weight from point A to point B..here are some tips below.
Rear Delt Exercise Example--Bent Over Reverse DB Fly Tips:
1. Hinge over at the hips...glue your chest down because you shouldn't be bouncing up and down with momentum.
2. Protract the shoulders: push the shoulder blades forward and keep them there throughout rear delt isolation exercises to properly isolate the rear delts and to take the scapula retractors (mid traps and rhomboids) out of the movement.
3. Focus on NOT creating momentum: use the intended muscles...the rear delts.
4. Do not go too heavy: use a controlled tempo with a short range of motion. Think lighter weights, more volume. When you take your shoulder blades out of it via protraction, your rear delts will only move your arm, roughly from straight in front of you, 75 degrees to just short of straight out sideways. If you are bringing the weights up past that point at the top of the movement, then you are likely retracting the shoulder blades and involving your traps and rhomboids. This kills rear delt activation. If you can go incredibly heavy, and your ROM is big, then you probably aren't using your rear delts.
5. At the bottom of the movement, don't bring the dumbbells all the way back down to the bottom or clang them together. Notice, at the bottom, I stop short of going all the way down. This keeps constant tension on my working muscles. If I go to the very bottom, it's a resting position. Increase time under tension!
I hope this helps. Part 2 coming later...