Back muscles
Rear Delts: the small muscle that finishes a shoulder from behind
The back third of the shoulder, and the difference between an outline that drops straight into the arm and one that goes out, over, then down.
What is the rear delts?
The rear delts — the posterior deltoid — are the rearmost of the three deltoid heads, sitting on the back corner of the shoulder above the armpit. They pull the upper arm backward and outward. Trained, they add a visible round cap behind each shoulder and carry the width of a back photo out to its edges.
| Anatomical name | Posterior deltoid |
|---|---|
| BackMaxx zone | Rear Delts |
| What it changes | Rounded shoulders from behind — the detail that finishes a back. |
| How it responds | Small muscle, high frequency. Light weight, strict form, more sets than you think. |
| Exercises tagged | 16 of 77 |
Where the rear delt sits, and why it is easy to miss
The deltoid has three heads. The front one presses, the side one lifts the arm out, and the rear one — posterior deltoid — sits on the back corner of the shoulder and pulls the upper arm backward and outward. On a back photo it occupies the small area above the armpit, outside the shoulder blade, where the shoulder meets the arm.
It is small, it is at the outer edge of the frame, and it is the only muscle on the back you cannot see in a mirror without turning your head. That combination is why it gets skipped, not any difficulty in training it.
Its line of pull is what separates it from everything else on this list. Rows drive the elbow backward past the ribs. The rear delt is loaded when the arm travels away from the body’s midline while staying roughly at shoulder height — out and back, not down and back.
What a developed rear delt changes in a photo
The app’s payoff line for this muscle: rounded shoulders from behind, the detail that finishes a back. That word rounded is literal, and it is about a silhouette.
Without rear delt development, the outline of your shoulder runs from the trap slope out and then straight down into the arm. One continuous fall. With it, there is a distinct bump at the corner — a cap — and the outline goes out, over, and then down.
It also does something to apparent width. Your shoulder blades stop and your arm begins, and the rear delt fills the join. Fill it and the back reads wide all the way to the edge of the photo; leave it empty and a wide set of lats still ends in a narrow shoulder line.
Small muscle, high frequency, more sets than you think
That is the app’s training note, and every part of it is doing work.
Small muscle means the absolute load is never impressive. A pair of dumbbells you could curl for twenty reps is often too heavy for a strict Rear Delt Fly, because the rear delt’s moment arm at shoulder height is terrible and the traps will simply take over the moment the weight wins.
High frequency follows from the small load. Nothing here creates the kind of fatigue that needs four days to clear, so three or four exposures a week beats one big session. Rear delts tolerate being trained the day after they were trained.
More sets than you think is the part people ignore. Two sets of flys at the end of a pull day is not a stimulus. Volume is how this muscle grows, and volume is cheap when the weight is light.
Notice what the corpus quietly confirms: of the 16 exercises that touch Rear Delts, 13 are rated beginner and not one is rated advanced. There is no advanced rear delt movement. There is only more of the simple ones, done strictly.
Rear Delts is a zone of its own
Most muscles in BackMaxx share a zone with two or three neighbours. Rear Delts gets a whole zone to itself — one of the six, with the payoff line rounded shoulders from behind, the detail that pops.
That is a deliberate call. The rear delt does not act on the shoulder blade like the rhomboids and mid-traps do, so folding it into Upper Back would have hidden it inside a score it does not really contribute to. Kept separate, a low Rear Delts score points at exactly one kind of movement.
Eight exercises carry Rear Delts as their primary zone, the smallest primary pool in the app alongside Traps. Eight is enough because they are variations on two patterns. The Rear Delts zone list holds all of them, and Traps & Rear Delts is the three-week block that leads with them instead of tacking them on.
Reading a back photo for a missing rear delt
Shoot from directly behind with your arms hanging naturally. Then look only at the top corners of the image.
| What the photo shows | What it means |
|---|---|
| Shoulder outline drops straight from trap to arm | No rear delt development yet |
| A shallow hollow between the trap slope and the triceps | The delt is there, undersized |
| Visible round cap at the corner, separate from the trap | Developed |
| Cap only appears with the arm raised or flexed | Some size, not enough at rest |
A second check works better than the silhouette for lean and heavy backs alike: raise both arms out to the sides to shoulder height. Held there, an underdeveloped rear delt shows as a flat plane behind the shoulder joint, with the trap and the triceps meeting almost directly. A trained one shows a clear third mass between them.
How many sets, and how light
Three or four sets per session, two to four sessions a week, and a weight you could handle for five more reps than you are doing. The corpus defaults are honest about the rep ranges this muscle likes: 3 sets of 12 to 15 on the flys and machines, 3 sets of 15 to 20 on the band work. Nothing in the rear delt list is prescribed under ten reps.
The intensity cue that matters more than any number: the weight should be light enough that you can stop the movement dead at the top and hold it for a beat without your torso moving. If you have to throw it, you are training momentum. The elbow angle should also stay fixed from start to finish — the moment it opens and closes you have turned a fly into a poor row.
Band work deserves a specific mention. Tension rises as the band stretches, which puts the hardest part of the rep exactly where the rear delt is strongest, at full arm separation. That is why the three band options in the group carry the highest rep prescriptions of any rear delt work in the app.
The movements that load the rear delts most directly
Two patterns cover it. Reverse flys, where the arms sweep out and back at shoulder height, and face pulls, where the hands finish beside the ears with the elbows high.
| Movement | Equipment | Prescription | What it does that the others don’t |
|---|---|---|---|
| Face Pull | Cable | 3 x 12–15 | Adds external rotation at the top, so it also tags Infraspinatus |
| Reverse Pec Deck | Machine | 3 x 12–15 | Fixed arc removes torso cheating entirely; the easiest strict rep in the group |
| Bent-Over Rear Delt Raise | Dumbbell | 3 x 12–15 | Free weight at 90 degrees of hinge, hardest at the top where the delt peaks |
| Band Pull-Apart | Resistance Band | 3 x 15–20 | Standing, no setup, tension peaks at full separation |
| Cable Rear Delt Fly | Cable | 3 x 12–15 | Crossed cables keep load on through the whole range, including the stretch |
| Reverse Snow Angel | Bodyweight | 3 x 8–12 | Sweeps the arm through the full arc from hip to overhead with zero equipment |
Rear delt work is the least equipment-hungry category on the back. Its 16 movements come from just six of the ten equipment types — no pull-up bar, no kettlebell — and five of them need nothing but the floor. Eight of the 16 also tag Infraspinatus & teres minor, because rotating the arm outward and sweeping it back overlap heavily.
What people get wrong training rear delts
Going too heavy, which is the same error as going too fast. Once the dumbbell is heavy enough to require a swing, the traps and the momentum split the work and the delt takes what is left.
Hinging too little. Standing nearly upright turns a rear delt raise into a lateral raise. The torso needs to be close to parallel with the floor for the arm’s path to actually cross the rear delt’s line of pull.
Pulling the elbows past the body. In a fly the hands should stop roughly in line with the shoulders. Driving further back hands the rep to the rhomboids and mid-traps, which is useful work but is not this.
Assuming rows cover it. Three of the 16 rear-delt-tagged exercises are rows — Chest Supported Row and the two upright row variations — and Rear Delts is only a secondary zone in all three. Rows contribute. They do not build the cap.
Training it last, always. It is the same list of movements every week at the end of the session when you are tired and bored. Putting rear delt work first once a week is the cheapest fix available, and it is the logic behind how the rear delt back-view guide sequences a session.
Best rear delts exercises
All 16 movements in the BackMaxx library that load the rear delts, grouped as the library groups them.
Browse the full 77-exercise library →
Frequently asked questions
What does the rear delt do?
The posterior deltoid pulls the upper arm backward and outward, away from the body’s midline, while the arm stays near shoulder height. That out-and-back path is what separates it from rowing, where the elbow drives down and back past the ribs. It is the only one of the three deltoid heads visible in a photo taken from behind.
What are the best rear delt exercises?
Two patterns cover the muscle. Reverse flys, where the arms sweep out and back at shoulder height: Rear Delt Fly, Reverse Pec Deck, Cable Rear Delt Fly, Band Pull-Apart. And face pulls, where the hands finish beside the ears with the elbows high. Face Pull is the pick if you want one movement, since it also tags the infraspinatus.
How do I know if my rear delts are lagging?
Look at the top corners of a photo taken from directly behind. If the outline falls straight from the trap slope into the arm with no bump at the corner, the rear delt has no size yet. Raise both arms to shoulder height and check again: a flat plane where the trap and the triceps nearly meet is the clearest tell.
How often should I train rear delts?
Three or four times a week, three or four sets each time. The loads are light enough that the muscle is ready again the next day, so frequency is the cheapest way to accumulate volume. BackMaxx prescribes 12 to 15 reps on the flys and machines and 15 to 20 on band work — nothing in the group goes under ten.
How heavy should rear delt flys be?
Light enough to stop the weight dead at the top, hold it for a beat, and lower it without your torso moving. If you have to swing it up, the traps and the momentum are splitting the work. Keep the elbow angle fixed from first rep to last; the moment it starts opening and closing, the fly has become a bad row.
Do rows train the rear delts?
Partly. Three of the 16 rear-delt-tagged exercises in BackMaxx are rows — Chest Supported Row, Upright Row and Dumbbell Upright Row — and Rear Delts is a secondary zone in all three, never the primary one. They contribute. None of them builds the cap on the back corner of the shoulder, because a row never takes the arm out wide at shoulder height.
Is your rear delts the problem?
BackMaxx scores eight back muscles individually from one photo, so you find out whether the rear delts is actually what is holding your back back — or whether it is something else.
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