A cable rear delt fly or reverse cable fly is an essential part of a great shoulder workout routine.
Rear delt is a reasonably obvious shortening for rear deltoids, and that's what this exercise does, it works your posterior deltoid muscles as few other exercises can.
If you're looking to improve your shoulder strength and build up those delts, then read on as we walk you through it step by step.
How To Do The Cable Rear Delt Fly
While there are variations which we'll talk about later, to execute the cable rear delt fly properly, you need a cable machine with handle attachments.
If you don't have one at home (most of us don't, you're not alone), then your gym is likely to, and if it doesn't, find a new gym that does.
- First, make sure the pulleys on the cable machine are set to a height just above your head
- The starting position is standing with your feet shoulder-width apart at the center of the cable machine
- You are going to cross your arms for the cable rear delt fly so grab the right handle with your left hand and the left handle with your right hand
- When you are ready to begin, take one step backward to put some tension into each cable
- As with most strength training, form and posture is essential, and you need to keep your core tight
- Next, contract your rear delts to bring your arms slowly forward until you get your shoulder blades together and you feel the pinch
- Now just as slowly return your arms to the starting position
- If you've done it correctly, there should only ever be a very slight bend in your elbows during the motion
- Now return to the starting position, maintain core tightness, posture, form, and repeat
The ideal workout to work those rear deltoids with the cable rear delt fly is around 3-4 sets of 10-12 reps. Don't push yourself too hard too soon.
Form is always more important than rep count. Once you are comfortable with the cable rear delt fly form, feel free to challenge yourself and change things up.
2 Cable Rear Delt Fly Variations
1. Bent-Over Cable Rear Delt Fly
If you prefer or you just want a little variation in your life, you can also perform the rear delt fly in a bent-over position.
Set the pulleys in a low position, bend from the waist and keep your body parallel to the ground.
The motion is more or less the same, and you should still feel a slight pinch when you bring your shoulder blades together at the bottom of the rep.
2. 1-Arm Standing Cable Rear Delt Fly
If your dominant arm is pulling too far ahead in size, strength, and swole, you may need to level the playing field and focus on just one for a session or two.
This variation is perfect for that, and the exercise is much the same, just done unilaterally.
You won't have the telling pinch to know you're doing it correctly, and it's also easy to let your posture and form slip (at least it is for me), so pay close attention.
Muscles Worked by the Cable Rear Delt Fly
Your deltoids are separated into three strands: the front delt (anterior), middle delt (lateral or median), and the rear delt (posterior).
Unsurprisingly the cable rear delt fly focuses primarily on working the posterior deltoid muscle.
In addition to this, the exercise also works the upper back muscles such as the rhomboids and erector spinae and being an effective exercise to strengthen your trapezius.
Finally, we cannot forget all the work our core muscles are putting into keeping our posture straight and secure, and so they'll feel that happy burn too.
3 Common Mistakes
We've all been there in the gym open-mouthed, watching someone butcher an exercise we know and love.
If you're like me, then you'd be too shy to go over and offer assistance, or you assume they're doing some hip funky new variation you don't even know about.
Well, I don't have to worry about that online, so here are the three most common mistakes that will ruin your cable rear delt fly workout.
1. Bending Your Elbows
While you will need a slight bend in your elbows (it's built that way after all), you want to try to keep your arms as straight as possible throughout the cable rear delt fly.
We're here to work out our rear deltoids, and the more your arm bends, the more your biceps will step in to help and lessen the rear delts burn.
2. Eyes Bigger Than Your Rear Delt Can Handle
This is good advice for any new workout, but I've seen it first hand with the rear delt. Start with a small weight and work up gradually as you master the form.
If you put too much weight on too early, you won't complete the entire motion and compromise the workout's benefits as well as risking injury.
Your hands should be in line with your shoulders at the bottom of each rep.
3. Going Low
As we mentioned in the setup, it's essential to have the cables set to a height just above your head to work the rear delts.
If the cables are too low, or if you start pulling down instead of out during the exercise, you will switch from a rear delt workout to your lats.
While this is a great variation, it's not the intended goal for most and usually indicates poor form.
3 Main Benefits
If the tag team of nature and my desk job had their way, I would be a fully-fledged hunchback by now. Building your upper back strength can help stop the shoulder from rounding or slouching forward.
Poor posture still takes a conscious effort to correct, but it becomes much easier through focused exercise like this. 
Some people like to call this an accessory exercise, but you ask Eddie Hall if he missed shoulder day, and I'll bet he'll disagree.
Shoulder strength, while perhaps not directly, has a significant impact on your overall upper body strength. Training can improve everything from your barbell bench press to your lateral raise. 
3. Aesthetic Appeal
If you are working out because you want to look good, and let's admit it, even those of us who aren't still are, it's crucial to stay symmetrical.
Lots of gym bunnies overdo the chest and arms and forget about the shoulders.
Easy at-Home Alternative
The bent-over dumbbell reverse fly is an excellent alternative to the cable rear delt fly if you prefer weights or just don't have access to the equipment.
The exercise is much like the bent-over cable rear delt fly mentioned above, but with weights in both hands instead of cables.
Ensure only to maintain a slight bend in your arms and bring your arms up and straight out to your sides.
Cable Rear Delt Fly: It's Easy When You Try
For a long time, I was a little intimidated by the big hulking machine of cables at my gym.
I was worried I would get tangled up or hurt myself without proper hands-on tuition.
Once I was shown the ropes (or the cables, if you will) though this an indispensable part of my upper body routine.
Using this advice, you too can incorporate this exercise into your routine. Let us know how you get on if you have any handy tips of your own.