Dumbbell Rear Delt Flye: Why Your Shoulders Still Look Flat From The Side

Most people treat the dumbbell rear delt flye as an afterthought. They’ve finished their heavy presses, their lateral raises, and maybe some rows. Then, they grab the lightest weights in the rack, flap their arms like a dying bird for twelve reps, and call it a day. It’s no wonder their rear delts look like they don't even exist. Honestly, it’s frustrating to watch because this specific muscle—the posterior deltoid—is exactly what gives your shoulder that 3D, "cannonball" look that pops when you stand sideways.

If you want that look, you've got to stop treating this movement like a casual warm-up.

The rear delt is a small, stubborn muscle. It’s biologically designed to assist in pulling, but it’s easily overshadowed by the massive muscles of the back like the latissimus dorsi and the trapezius. When you do a dumbbell rear delt flye, your body naturally wants to use those bigger muscles to move the weight. It’s more efficient. Your brain doesn't care about hypertrophy; it just wants to get the dumbbell from Point A to Point B. To actually grow, you have to outsmart your own nervous system.

The Biomechanics of Why You're Failing

Let's get technical for a second. The posterior deltoid originates on the spine of the scapula and inserts into the humerus. Its primary jobs are horizontal abduction and external rotation. When you perform a dumbbell rear delt flye, you are trying to isolate horizontal abduction.

The problem? Most people pull too far back.

If your shoulder blades are pinching together at the top of the movement, you’ve stopped working your delts and started working your rhomboids and mid-traps. The rear delt’s range of motion is actually quite short. It's done its job long before your hands are level with your spine. Dr. Mike Israetel from Renaissance Periodization often highlights that "mind-muscle connection" isn't just bro-science; it’s about positioning your body so the target muscle has to do the work. If you feel a massive squeeze in your upper back, you’re doing a back exercise, not a shoulder exercise.

Stop Using Your Ego

Stop picking up the 35s. Seriously. Put them back.

Most guys—and it’s usually the guys—try to heavy-load the dumbbell rear delt flye. This leads to a rhythmic "bounce" at the bottom of the rep. They use momentum to kick the weight up, and their lower back starts doing a weird dance. Because the rear delt is so small, it can really only handle a fraction of the weight you’d use for a side lateral raise.

If you can’t hold the weight at the top of the rep for a split second, it is too heavy. Use the 10s. Use the 5s. Heck, use the 2.5lb plates if you have to. Your ego might take a hit, but your delts will actually grow.

The "Pinky Up" Myth

You’ve probably heard someone tell you to turn your hands so your pinkies are up, like you're pouring out two pitchers of water. While this can increase posterior delt activation by forcing more internal rotation, it can also lead to shoulder impingement for some lifters. It narrows the subacromial space.

Instead of obsessing over the "pour," try a neutral grip. Palms facing each other. This is often more comfortable for the rotator cuff and allows for a more natural path of motion. Some lifters find that a slight internal rotation helps, but it shouldn't feel like you're grinding your shoulder joint into dust.

Positioning: Seated, Standing, or Incline?

How you stand—or sit—changes everything.

👉 See also: Plyo Box for Working Out: Why Most People Are Doing It Wrong

The standing bent-over dumbbell rear delt flye is the classic version. It’s also the easiest to mess up. Your hamstrings get tired, your lower back starts to ache, and you start uprighting the movement until it becomes a crappy upright row.

If you’re serious about isolation, use an incline bench. Set it to about 30 or 45 degrees. Lay face down (prone) on the bench with your chest supported. This takes your legs and lower back out of the equation entirely. You can’t cheat. You can’t swing. It’s just you, the dumbbells, and your rear delts screaming for mercy.

• The Chest-Supported Method: Laying face-down on a bench. Gravity is most intense at the bottom and mid-range.
• The Seated Version: Sit on the edge of a flat bench, lean forward until your chest touches your thighs. This is a great middle ground if the incline bench is taken.
• The Standing Version: Only do this if you have great core stability and can keep your torso parallel to the floor.

The Secret is in the "Sweep"

Don't think about lifting the weights up. Think about pushing them out.

Imagine there are two walls on either side of you. Your goal isn't to hit the ceiling; it's to touch those walls with the sides of the dumbbells. By focusing on a wide, sweeping motion, you naturally limit the involvement of the traps. Keep a slight bend in your elbows—locked arms can irritate the joint, but too much of a bend turns the movement into a row.

Think of your arms as hooks. The elbows should be the highest point of the movement.

Dealing with the Traps

Your trapezius muscles are bullies. They want to take over every "pull" movement. To shut them up during the dumbbell rear delt flye, try to protract your shoulder blades slightly. Instead of pulling your shoulders back, imagine you’re reaching forward and then out. This rounds the upper back slightly (in a controlled way), which "unlocks" the rear delts and keeps the traps from squeezing the shoulder blades together.

How Many Reps?

Rear delts are largely slow-twitch dominant. They respond incredibly well to high volume and metabolic stress. Doing sets of 5 or 8 reps is basically useless here. You want to be in the 15 to 25 rep range.

You want the burn.

You want to reach a point where you can only move the weights halfway up because the lactic acid is so thick. That "partial range" at the end of a high-rep set is actually gold for rear delt growth. Even when you can't get a full rep, keep pulsing. Those partials keep the tension on the muscle when it's at its weakest.

Common Mistakes You’re Probably Making

1. The Head Bob: Stop looking at yourself in the mirror. When you look up, you strain your neck and change the pull angle. Keep your neck neutral, looking at the floor about three feet in front of you.
2. The T-Grip vs. The V-Grip: If you bring the weights back toward your hips, you’re hitting your lats. The weights should stay in line with your shoulders or even slightly forward.
3. The Speed: If your reps look like a hummingbird on caffeine, slow down. Two seconds up, a pause, and two seconds down. The eccentric (lowering) phase is where a lot of the muscle damage—and thus growth—happens.

Real-World Programming

Don't put these at the end of your workout when you're exhausted. If your rear delts are a weak point, do them first. Start your shoulder or back day with a dumbbell rear delt flye variation to "wake up" the muscle.

A great technique used by pro bodybuilders like John Meadows (the "Mountain Dog") involves "hang and swings." You take a heavy pair of dumbbells, let them hang at the bottom, and just swing them out about 10-15 inches. It looks like you're doing nothing, but the constant tension on the rear delt is brutal. It’s a great way to finish a session.

Why Science Cares About This

It’s not just about aesthetics. Weak rear delts are a recipe for shoulder pain. Most of us spend our days hunched over keyboards or phones, which leads to internal rotation of the humerus. This stretches and weakens the posterior muscles while tightening the pecs.

By strengthening the rear delt through the dumbbell rear delt flye, you’re creating a "counter-pull." You’re helping pull the shoulders back into a healthy, neutral position. This can actually increase your bench press over time because it provides a more stable base for the shoulder joint to press from. It’s structural integrity disguised as vanity.

Actionable Steps for Your Next Workout

Kinda ready to stop sucking at these? Here is exactly what you should do during your next session:

1. Find an incline bench and set it to 30 degrees.
2. Grab weights that feel embarrassingly light (5s or 10s).
3. Lay chest-down and let your arms hang. Turn your palms toward your feet (pronated) or toward each other (neutral).
4. Initiate the move by pushing the weights out to the walls, not up to the ceiling.
5. Stop the rep when your arms are parallel to the floor. Do not go higher.
6. Slow down. Control the weight for a 3-count on the way down.
7. Aim for 20 reps. If you don't feel a deep, localized burn in the back of your shoulder by rep 15, your form is off or the weight is too heavy (yes, too heavy, because you're likely using traps to compensate).

The dumbbell rear delt flye is a masterclass in discipline. It requires you to ignore the heavy weights, ignore the urge to "power through," and instead focus on a tiny, specific patch of muscle. But once you nail the form, you’ll start seeing that shoulder separation that sets an advanced physique apart from a beginner one. Stick with the high reps, keep the traps out of it, and be patient. Those rear delts will grow; they just need you to stop ignoring them.

Next Steps for Implementation

• Assess your posture: If your shoulders roll forward, add 4 sets of rear delt flyes twice a week, regardless of your split.
• Video your set: Record yourself from the side. If your torso is moving more than an inch, drop the weight immediately.
• Vary the angle: Every few weeks, switch from a neutral grip to a pronated grip to hit the fibers from a slightly different perspective.