How To Do a Rear Delt Fly: Technique, Benefits, Common Mistakes

Rear delt flys are a great exercise to train, well, the rear delts. I love me an exercise that is named by the muscle it uses, and the action performed… looking at you, Bulgarian Split Squat. Well-developed rear delts help you achieve that “boulder shoulder” look, but more importantly they’re a great exercise for a strong and healthy posture. 

What is a Rear Delt Fly?

 

A “fly” refers to an exercise where the arms get lifted up, out, and away from the body. As opposed to a chest fly or “pec deck,” a rear delt fly places the tension on the muscles on the back of your body, like the rear deltoids.

The rear delts are 1 of 3 parts of the shoulder muscles: the deltoids. We have anterior (front), lateral (side) and posterior (rear) divisions of the deltoid (delts) muscles. As you can imagine the rear delts are on the backside of the shoulder. The job of the rear delt is to move the arm backward. This is important to know because the function of the muscle dictates how we train it.

How to do Rear Delt Flys: Step by Step Instructions

1. Set up a bench on a slight incline. Just high enough so that when laying on it, your arms do not reach the floor. 
2. Lay your chest down at the top end of the bench, with your head off the bench and arms hanging below you. Your palms should be facing each other. 
3. Keep a slight bend in your elbow, and maintain this bend the entire exercise. Your elbows shouldn’t be bending or straightening at any point.
4. Using the backside of your shoulder, begin to raise the arms up and outwards until you are about in line with your torso. 
5. Pause slightly at the top of the movement, and then with control lower your arms back down to the start position. 

Coaching Cues

1. Only raise up as high as the rear delts will produce movement. This is probably not all the way behind your back where your shoulder blades are pinching together. 
   • Note: It wouldn’t be “wrong” to do this, it would simply be a different exercise that targets your upper mid-back muscles, instead of the rear delts.
2. Keep your chest down. There is no need to extend your back and raise your chest up. To that end, also keep your abs engaged so you don’t hyperextend your lower back.
3. Be sure to move the dumbbells out to the side, and not down and back towards your hips. 
   • Again, not necessarily wrong, but a different exercise. 
4. Grip the dumbbells tightly in your hands. Although you won’t be using a ton of weight here, the tight grip in the hands will help keep your elbow more stable.
5. Remember where the rear delt is: on the backside of your shoulder, outside the shoulder blade, sort of “above” your triceps. Generate force from this muscle, and not from anywhere else. 

Common Mistakes

There are a lot of muscles in this area of the shoulder and upper back, and most of them are small and weak on their own. Which is why we generally do not need very heavy weights to train them in isolation.

This also means that our body will adjust and do funny things to help us complete the movement. Remember, your body doesn’t know you’re trying to train your rear delts, it just wants to lift the weight in the most efficient way possible, which would likely not be isolating the rear delts. So if you’re not paying attention, you will make these common mistakes and compensations: 

Raising Your Chest

Raising your chest up off the bench and arching your back to lift the weight simply creates an illusion that you’re moving the weight farther. In reality, you’re probably actually moving it less, and you’re using your body weight and momentum to swing the weight around, rather than the intended muscles. 

Moving Your Hands Down Towards Your Hips

Yes, if you move in this direction with the arms going down and backwards, the rear delts are still involved. However, so are a lot of other muscles. This would be easier to do, and you could probably lift more weight this way, but the exercise is much less specific to the rear delts, so it’s not really accomplishing the same goal. To keep the focus where we want it, keep the direction of the motion moving straight outwards. 

Note: moving the hands down towards the hips isn’t necessarily wrong or bad, it’s simply just a different exercise. 

Raising Your Arms Too Far Back

Remember that the rear delt is on the back side of the shoulder, outside of the shoulder blade. This means that the rear delt does not pull the shoulder blades together. So if you’re focusing on squeezing the shoulder blades together, then that is focusing on not using the rear delts. Just because you CAN move your shoulder there, doesn’t mean for this purpose that it should go there. Only raise as high as the rear delts can move the arms. 

Note: Raising up higher and squeezing the shoulder blades back would not be wrong or bad, again, simply a different exercise, more like a band pull apart. 

Extending Your Elbows

Your elbows should be in a slightly bent position during the rear delt fly, but they shouldn’t be moving at all. More muscles involved means using heavier weight, so it can be tempting. If you find that the elbows are bending and straightening then you’re starting to use the triceps to lift the weight. Pick a slight amount of elbow bend that is comfortable for you, and lock that in for the entire set. 

Benefits of Rear Delt Flys

I’ve spent a lot of time in gyms, and seen a lot of wild things. Nobody has ever bragged about hitting a PR on rear delt flys. I’ve also never seen a legitimately strong person who had weak or under-developed rear delts. 

What I’m getting at is that it’s an important exercise for overall strength, aesthetics, posture, and shoulder health and function; but it’s also not the sexiest thing to train, and that’s why it’s often neglected or under-prioritized.

Overall Strength

The rear delts are involved in a lot of upper body exercises. Essentially every pulling exercise or variation of a row or pull-ups involves the rear delts to some extent. If they’re weak, they will limit your ability to progress at those exercises. 

Even during pressing movements like a bench press, the rear delts need to work to help stabilize the shoulder throughout the movement. Same as during squats and deadlifts, where the rear delts act to keep the bar close to the body. 

Bottom line is that while they’re not the primary force producer for all these exercises, if they’re weak relative to the rest of your body, they will become the limiting factor. 

Aesthetics

I mean, who doesn’t want nice rounded shoulders? Well, round happens on all sides, not just the front and side that are more prominently trained with pressing exercises. So, to achieve that rounded, capped, shoulder look, you can’t ignore the rear delts. 

Posture and Shoulder Function

The technological world that we live in lends itself to a lot of forward-rounded shoulders. You know, that hunched-over look with your shoulders almost turned in towards each other. Two things need to happen to fix this: pull the front of the shoulder back, and pull the shoulder blades together. 

We’ve established that the rear delts don’t pull the shoulder blades together – a band pull apart would be a better exercise for this – but they do pull the shoulder backward. So, use rear delt flys to train the rear delts and not be a slouch. 

This forward-rounded shoulder position (internal rotation) can also cause pain and injury in the shoulder over time. The shoulder isn’t supposed to be stuck in this position. So in order to restore normal function to the shoulder, the rear delts need to pull their weight. 

Bent-Over Flys Vs Chest-Supported

The only benefit of a bent-over variation over a chest-supported variation is convenience. It’s difficult to hold the position which is the bottom of a hip hinge or RDL, which is challenging enough.

Due to this position, and the weight being out in front of you, it can put unnecessary stress on your lower back. Yes, even though the weight is light; because the weight is so far away from your center of gravity, because… physics. So generally stick with the chest-supported version. 

Since your body isn’t stabilized against anything, you have to work WAY harder to prevent it from moving: to prevent your chest from moving up, lower back from arching, legs from basically jumping up, or overall flailing your body around. Not only would you look like a clown, but you’re also accomplishing nothing useful here. 

Laying on the bench provides stability everywhere you need it, and still allows the arms and shoulders to move freely. So that you can focus on the rear delts, you know, the thing that matters here. 

The bent over variation provides no real benefit, and only a number of ways to mess this up. My advice is to avoid it, and take the extra 3 seconds to set up a bench. The exception to this, as mentioned, is convenience. A bent-over rear delt fly with very light weight can be an easy warm-up exercise, or a great exercise to do before a major pulling movement. In fact, we feature it in many of our RFS warm-up programs. Aside from that, stick with the chest-supported rear delt fly.