The deltoids are an easy muscle to hit... unless you're talking about the rear deltoids. Targeting this muscle group takes more than doing standard dumbbell reverse flyes. Here's what you need to know and do:
I cringe when someone does sets of 50-pound dumbbell reverse flyes, or buries their face into a pec dec hybrid machine to perform horribly executed reverse flyes with the stack.
There's a reason your rear delts are underdeveloped and weak – it's because you're using too much weight to get the job done and other muscle groups are actually completing the rep.
Since the rear delts aren't easy to hit, we need to respect that it won't take much weight to hit them hard. It can, of course, be argued that delt-recruiting exercises like high pulls or cleans can be performed with heavy weight, but the amount of work that's actually done by the upper traps and other back musculature is high.
So much of our rear deltoid training is dedicated to arm abduction, but it neglects rotation. Doing bent-over flyes is all fine and dandy, but it only tackles one function of the rear delts. What you need to do instead is turn your hands so the palms face forward. You get a much fuller, much more targeted contraction.
As a bonus, turning the wrists so the palms face forward on the bent-over reverse fly also helps prevent the shoulder glide that can be incurred from using a neutral or internally rotated grip.
The same holds true for the face pull. Incorporating an external rotation so the hands finish higher up, over the head, can be a game-changer for just how much you feel it in the target muscles. If you're big and wide with long arms, then using two ropes is better than one.
To take things to the next level, adding a "slide" component to a face pull (like a resisted scapular slide) makes the delts responsible for external rotation from a variety of arm angles through the force curve. This is a great progression to standard wall slides.
The good thing about isometrics? You're able to apply maximal forces in ranges where typical reps of an exercise merely pass through.
That means a lifter will typically spend just an instant at full shoulder extension or flexion, with the meat of the rep being felt at basically every other arm angle but those positions.
Working the snot out of shoulders at full flexion and extension end ranges with isometrics can not only be the hidden key to creating more available range of motion for immobile, injury-prone shoulders, but also to help develop dormant muscle groups like the rear delts, which otherwise get little to no play in exercises intended for them.
Using isometrics as a primer to common exercises – or as their own exercise to be paired or grouped with others – is a great way to get the most out of your muscles through the greatest range possible.
This video shows me putting T Nation editor Dani Shugart through a few of these rear deltoid and upper back isometrics.
Every rear delt movement I see has the lifter start with his hands and arms in line with his body, then proceeding to move them outward (think of the rear delt flyes I showed above). There's nothing wrong with this, other than the fact that solely focusing on this kind of path severely limits how much of your muscle you're going to hit.
To change things up, try setting up on a cable machine for constant tension:
This is a very isolated pattern that, when done correctly, absolutely torches the rear delts. It's important to maintain the same elbow angle throughout the entire range of motion. This isn't a triceps exercise; the fulcrum is the shoulder.
Pull the arm all the way across the body as though you're trying to cover your mouth with your biceps. Then aim down toward your outside shoe with your fist. It'll take a few reps to initially feel, but once you're rolling, you'll get an insane pump in the target region.