You’ve probably seen it a thousand times at the local gym. Someone grabs the heaviest dumbbells they can find, hunches over like they're trying to find a lost contact lens, and starts flapping their arms like a distressed bird of prey. They think they're building "boulder shoulders," but honestly? They’re mostly just hitting their traps and using momentum to ego-lift. If you want to actually grow your posterior deltoids, you have to stop treating the rear delt fly like a power lift. It’s a finesse movement.
The posterior deltoid is a small, stubborn muscle located on the back of your shoulder. It’s responsible for horizontal abduction—basically pulling your arms back and away from your chest. While the front delts get hammered by every bench press and overhead press you do, the rear delts often get neglected. This leads to that "rounded shoulder" look that makes you look like you spend twenty hours a day hunched over a laptop. Not great.
The Anatomy of a Perfect Rear Delt Fly
Most people fail because they don't understand where the muscle actually is. Your rear delt is tiny. Compare it to your lats or your traps; it's a David among Goliaths. When you use too much weight, those bigger muscles take over immediately. To do a rear delt fly correctly, you need to isolate.
Start by grabbing a pair of dumbbells that feel almost "too light." Seriously. If you usually grab 25s, try the 10s or 12s. Bend at the hips until your torso is nearly parallel to the floor. This is where most people mess up—they stand too upright, which turns the move into a weird shrug. Keep your back flat. Your arms should hang down with a slight bend in the elbows. Now, instead of thinking about pulling the weights "up," think about pushing them "out" to the side walls.
Stop the movement when your arms are level with your shoulders. If you go higher, you’re just crunching your scapula together using your rhomboids. You want to feel that specific burn right on the back of the shoulder joint. It's a sharp, localized sensation. If you feel it in your neck, you’re shrugging.
Why Your Grip Actually Matters
Check your hands. Are your palms facing each other (neutral grip) or facing behind you (pronated grip)? There’s actually some debate here, but research, including studies often cited by experts like Dr. Mike Israetel of Renaissance Periodization, suggests that a neutral grip might allow for a slightly better range of motion for some, while a pronated grip (palms down) can help some lifters "find" the muscle better.
I personally prefer the pinkies-up approach. Imagine you’re holding two pitchers of water and you’re trying to pour them out as you reach the top of the fly. This slight internal rotation often helps engage the rear head of the delt more effectively. But hey, everyone's shoulder anatomy is a bit different. If your joints click or pop, adjust the angle.
Common Mistakes That Kill Your Gains
Stop swinging. Just stop. If your torso is moving up and down to help you lift the weights, you aren't doing a rear delt fly anymore; you're doing a lower back exercise with extra steps.
- The Scapular Trap: If you retract your shoulder blades (pinch them together) at the start of the move, your traps and rhomboids do the work. Keep your shoulder blades relatively "spread" or protracted throughout the movement to keep the tension on the delt.
- The "T" vs. the "Y": Don't pull your arms straight back like a letter "T" if it hurts your impingement. Sometimes angling the arms slightly forward—more like a wide "Y" shape—feels better on the joint and still torches the rear delt.
- Elbow Positioning: Keep the bend in your elbow consistent. If you start straight and end with a 90-degree bend, you've turned the fly into a row. Rows are great, but they aren't flies.
Machine vs. Dumbbells: Which Wins?
Honestly, the Reverse Pec Deck machine is underrated. While dumbbells are great for stabilizers, the machine provides "constant tension." With dumbbells, there is zero tension at the bottom of the move because gravity is just pulling the weights straight down towards the floor. On a machine, the cam system keeps the weight pulling against you throughout the entire arc.
If you're stuck with dumbbells, try lying chest-down on an incline bench. This eliminates the need to stabilize your lower back, allowing you to focus 100% of your brainpower on the back of your shoulders. It's a game changer for people who struggle with the "mind-muscle connection."
The Cable Variation
Cables are arguably superior to dumbbells for the rear delt fly because of the resistance profile. When you use a cable cross-over machine, set the pulleys to shoulder height. Don't use handles. Just grab the balls at the end of the cables. Cross your arms in front of you and pull back. Because the cable is pulling your arm across your body, the rear delt is stretched under load at the beginning of the rep. Muscles love being stretched under load. It’s one of the primary drivers of hypertrophy (muscle growth).
Programming for Size and Posture
You don't need to do 3 sets of 10. The rear delt responds incredibly well to high volume and metabolic stress. Think more along the lines of 15 to 25 reps.
- Frequency: Hit them 2-3 times a week. They recover fast.
- Intensity: Take your sets close to failure. Since the weight is light, the risk of injury is low, so you can really push the intensity.
- Order: Try doing them at the beginning of your workout. If you wait until the end of a long back or shoulder day, you’ll be too tired to focus on the form, and you'll just go through the motions.
I've seen guys who can bench 315 pounds but have the posture of a cooked shrimp because they never trained their posterior chain. Don't be that guy. Balanced shoulders not only look better—giving you that 3D look from the side—but they also protect your rotator cuffs. A strong rear delt acts as a brake for your arm during throwing or pressing movements.
Actionable Steps for Your Next Workout
To get the most out of your rear delt fly, start your next upper-body session with a "pre-exhaust" set. Grab some light bands or 5-pound plates. Perform 20 slow, controlled reps focusing exclusively on the squeeze at the back.
Next, move to the seated cable or machine version if available. Do 4 sets of 15 reps, pausing for a full second at the peak of the contraction. Avoid the temptation to increase the weight until you can perform every single rep without moving your torso or shrugging your ears toward your shoulders.
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Finally, incorporate "partials" at the end of your last set. When you can no longer do a full range of motion, perform small, pulsing movements in the bottom half of the rep. This floods the muscle with blood and creates the kind of metabolic waste that triggers growth. Focus on the stretch, keep the weight humble, and watch your shoulders transform over the next eight weeks.