A rear deltoid workout is often one of those things that get overlooked in favor of exercises for the front and middle delts. That is a big mistake as you will see.
The shoulder is one of the most delicate joints in the body. It has plenty of range of motion, which makes it vulnerable to injury if not properly balanced. We tend to get incredibly imbalanced between the muscles on the front of our shoulder and the back side. When this happens, the shoulder will become rounded forward, and the muscles of the rear delt and scapula will get weak and overstretched.
In the above video, Jeff Cavaliere from YouTube channel Athlean-X shows you a complete rear delt workout that will help you to add mass and strength to your rear delts that will not only help you build a boulder shoulder but will protect the joint as well.
You need to combat this weakness with a rear deltoid workout that is balanced and helps to build more mass and strength in the muscle. This starts with a quick assessment of your rear delt by looking at the posture of the shoulder in the mirror. If you have a very rounded shoulder, you likely have allowed your tight pecs to create a weakened rear deltoid.
Likewise, if you experience soreness and trigger points in either of your rear deltoid muscles, then you are dealing with a weak muscle that has allowed the spasms to form as a way of creating artificial stability in a joint that is lacking in it.
Once you have determined that you need to work out your rear delts more, you need to know how to best attack the muscle for maximum gains. First, you must hit the muscle head on with a direct exercise that can hit all three function of the rear delt. You want to extend the arm behind the body, horizontally abduct the shoulder and get some external rotation as well. The “W” raise is a shoulder lateral raise variation that will hit the rear delt head on.
Keep in mind that you will want to use lighter weights on this shoulder exercise to make sure that you are not compromising the integrity of the rather small rotator cuff muscles. Aim for 4-5 sets of this exercise done for 12-15 reps with a good squeeze or contraction on every rep.
From here, if you want to add mass to your shoulders you will have to increase the weight that you are using. The use of indirect rear delt exercises comes in so effectively here. Start doing more one arm dumbbell rows, bent rows, and seated cable rows to build massive rear delts. The key, however, is not just doing the exercises but the way you do them. Allow your elbows to drift upwards away from your sides to target the rear delts even more.