If you’ve ever looked in the mirror and felt like your shoulders look a bit "flat" from the side, you aren't alone. Most lifters obsess over the front delts because that’s what we see in the mirror. We smash overhead presses. We do endless front raises. But the back of the shoulder? It’s usually an afterthought. That is exactly why the lying rear delt fly is such a game-changer.
It’s simple. It's effective.
But honestly, most people do it completely wrong. They swing the weights. They use their traps. They turn a shoulder exercise into a messy back workout. If you want those "3D" shoulders everyone talks about, you have to master the posterior deltoid, and the lying version of the fly is arguably the most "honest" way to do it.
Why Gravity Changes Everything with the Lying Rear Delt Fly
When you stand up and do a rear delt fly, the resistance curve is a bit wonky. You’re fighting gravity, sure, but your body has a million ways to cheat. You can use momentum from your hips. You can shrug your shoulders up to your ears.
By lying down—whether that's prone on a bench or on your side—you take the legs out of the equation. You’re locked in.
The lying rear delt fly forces the small, stubborn muscles of the posterior shoulder to do the heavy lifting. Specifically, the posterior deltoid, the infraspinatus, and the teres minor have to work in unison. According to research published in the Journal of Strength and Conditioning Research, horizontal abduction (the movement of pulling your arms apart) is the primary function of the rear delt. When you are lying face down on a bench, gravity is pulling that weight directly toward the floor, making the peak of the movement incredibly difficult.
It’s a different kind of burn. It’s localized. You’ll feel it right in that little pocket of muscle on the back of your shoulder that usually stays quiet during heavy rows.
The Setup: Prone vs. Side-Lying
There are two main ways to handle this. Most people go for the prone version. You lay chest-down on an incline bench. Set the bench to about a 30-degree or 45-degree angle. If the bench is too high, you start turning it into a row. Too low, and your dumbbells hit the floor.
Let your arms hang. Keep a slight bend in the elbows.
Now, here is the secret sauce: don't pull with your hands. Think about leading with your elbows. If you focus on the dumbbells, your wrists will likely take over, or you’ll start using your triceps.
Then there is the side-lying version. This one is underrated.
You lie on your side on a flat bench. You hold a single dumbbell in your top hand. You start with the weight across your body and fly it up toward the ceiling. Why do this? Because it changes where the tension is heaviest. In a standard prone fly, the hardest part is the top. In a side-lying fly, there is significant tension right at the start of the move.
Mixing these two variations is how you actually build complete muscle density.
Common Mistakes That Kill Your Gains
Stop shrugging. Seriously.
The biggest mistake I see in the gym—and I mean everywhere—is people trying to use 40-pound dumbbells for a lying rear delt fly. Your rear delts are small. They are not designed to move massive weight in isolation. When you go too heavy, your brain says, "Help!" and recruits the trapezius and the rhomboids.
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Your shoulder blades should stay relatively still. If you’re squeezing your shoulder blades together like you’re doing a heavy row, you’ve missed the point.
Keep the pinkies up. Or at least, try a neutral grip.
A lot of experts, including guys like Jeff Cavaliere (Athlean-X), suggest that rotating your thumbs downward or keeping a neutral grip can help better isolate the posterior head of the deltoid while minimizing the involvement of the lateral delt. It’s a subtle shift, but it works.
Another thing? Range of motion. You don't need to bring the weights way past your torso. Once your arms are parallel to the floor, the rear delt has done its job. Going higher just puts unnecessary stress on the rotator cuff and moves the tension to the mid-back.
Keep it controlled. Lower the weights slower than you lift them.
The Science of the "Rear" Shoulder
Let’s talk about the posterior deltoid for a second. It’s one of the three heads of the deltoid muscle. While the anterior delt gets hammered during any pressing movement (bench press, shoulder press), the posterior delt is often neglected.
This leads to "internal rotation."
That’s the "caveman" posture where your shoulders roll forward. It looks bad, but it feels worse. It leads to impingement and chronic pain. Strengthening the rear delt through the lying rear delt fly acts as a counterbalance. It pulls the shoulders back. It opens up the chest.
In a study by Schoenfeld et al., it was noted that muscle hypertrophy is often maximized when a muscle is worked in its lengthened position under load. The lying fly provides this, especially if you use a slight incline. It’s not just about aesthetics; it’s about joint integrity. A strong rear delt stabilizes the glenohumeral joint during heavy benching.
If your bench press has plateaued, believe it or not, hitting your rear delts might be the fix.
High Reps or Heavy Weight?
Go for the pump.
Since the rear delt is heavily composed of Type I (slow-twitch) muscle fibers, it responds exceptionally well to higher volume. I usually recommend people stay in the 12 to 20 rep range.
If you can only do 6 reps, the weight is too heavy. You’re cheating. You’re using momentum. You’re shrugging.
Try this: do 15 reps of the lying rear delt fly, but at the top of every rep, hold it for a one-second count. Squeeze. If you can't hold it, you don't own the weight.
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Sample Integration into Your Split
How do you actually fit this in? Don't just tack it on at the end of a long workout when you're exhausted.
- Push Day: Use it as a finisher to balance out all the pressing.
- Pull Day: Group it with your rows and lat pulldowns. This is my favorite way. Your back is already warm, and the rear delts are primed to fire.
- Dedicated Shoulder Day: Do it early. Before you’re too tired to maintain form.
Sometimes I like to superset these with face pulls. It’s a brutal combination. You hit the delts from two different angles, and the blood flow is insane.
Actionable Steps for Your Next Workout
Don't just read this and go back to your old routine. Try these three specific tweaks next time you grab the dumbbells:
- The Head Rest: If you’re doing the prone version on a bench, let your forehead rest on the top of the bench. This prevents you from craning your neck and helps keep your spine neutral. It also makes it much harder to "body swing" the weight up.
- The 2-Second Eccentric: Count "one-one-thousand, two-one-thousand" on the way down. This slow descent is where the most muscle damage (the good kind) happens.
- Lighten the Load: Drop your usual weight by 5 or 10 pounds. Focus entirely on feeling the muscle "knot" at the back of your shoulder contract.
The lying rear delt fly isn't a flashy move. You won't break any powerlifting records with it. But if you want shoulders that look complete—and a back that stays healthy—it’s non-negotiable. Stop neglecting the muscles you can't see in the mirror. They are usually the ones that need the most love.
Get on the bench. Stay strict. Watch your shoulders actually start to grow.
Next Steps for Better Shoulders:
To get the most out of your rear delt training, start by incorporating the prone lying fly twice a week. Begin with 3 sets of 15 reps using a weight that feels "too light" for the first 8 reps, then focus on a hard peak contraction for the final 7. After two weeks, add the side-lying variation to your routine to challenge the muscle at a different point in the strength curve. Monitor your posture over the next month; you should notice your shoulders sitting more naturally in a "back and down" position during daily activities.