How to do a proper rear delt fly

Your shoulder, the deltoid, has three heads. The front head drives every pressing movement, so it never lacks work. The side head gives your shoulders their width and gets trained by lateral raises. The rear head, at the back of the shoulder, is the one that finishes off a round, balanced look and helps pull your shoulders into good posture, and it is the one almost everyone under-trains.

The problem is that the rear delt sits right next to powerful muscles. Your mid-back muscles, the rhomboids and mid traps, are right there and love to take over any pulling movement. If you let your shoulder blades squeeze together on a rear delt fly, those muscles do the job and the rear delt coasts. The whole skill of this exercise is keeping the movement at the shoulder, not the shoulder blade, so the small muscle you actually want to grow does the work.

You can do this bent over with dumbbells, seated leaning forward, on a cable, or on a reverse pec deck machine. The machine is often the easiest place to learn it because the seat holds your torso still. Whichever you use, the movement cues are the same, and they all come back to leading with the elbow and keeping the load on the shoulder.

1. Set your torso and let it stay put. Hinge forward at the hips so your chest points toward the floor, or sit into the machine with your chest against the pad. Brace your midsection. From here your torso does not move for the rest of the set.
2. Use a neutral grip if you can. Palms facing each other, or thumbs pointing up, rather than palms fully down. Research on hand position found the neutral grip produces more rear delt activity than a pronated, palms-down grip. On a machine, use the vertical handles.
3. Lead with the elbow. Keep a soft, fixed bend in the elbow and drive your elbows out and back, as if opening your arms wide. Think about moving the elbows, not the hands. When the hands lead, you tend to rotate the shoulder and lose the rear delt.
4. Stop when your arms reach the sides. Bring your arms out until they are roughly in line with your body. Going further just makes your shoulder blades pinch together and hands the work to your mid-back.
5. Lower slowly. Take two to three seconds to return, resisting the weight the whole way and feeling the stretch across the back of the shoulder. Do not just let the arms fall.

Small choices about hand position and where you set up change how cleanly the rear delt gets loaded. None of this is complicated, but the details are the difference between training the muscle you meant to and just moving weight with your back.

A study on hand position during this exact movement, the horizontal abduction on a reverse fly machine, found that a neutral hand position produced meaningfully more activity in the rear deltoid than a palms-down, pronated grip. That same reverse fly setup is a clean way to load the rear delt, because the seat holds your torso still and stops your body swinging the weight. The practical version of all this: turn your hands so they are neutral or thumbs up, and keep the weight light enough to lead with your elbows.

The rear delt responds well to higher reps and frequent work, the same as the side delt. You are chasing a clean contraction and a good stretch, not a heavy top set. Because the load is light, it recovers fast and tolerates being trained often.

• Rep range: aim for roughly 12 to 20 reps per set. If clean form breaks before 12, lighten the weight.
• Sets: 2 to 4 working sets per session covers it for most people.
• Frequency: because it is light and recovers quickly, hitting the rear delts two or three times a week works well.
• Progress by adding a rep or a small amount of weight over time, the same progressive overload that grows any muscle.