The posterior shoulders comprise the rear deltoid and rotator cuff muscles, specifically the infraspinatus and teres minor. Each of these muscles are responsible for external rotation of your shoulders. The rear deltoid also allows you to move your arms towards the rear. You can do many exercises to build mass and increase the strength of your posterior shoulders. Depending on your specific goal though, you'll need to adapt to a certain set and rep scheme.
Rear Deltoid Exercises
Among the most effective exercises you can do for your rear deltoids are the bent-over rear delt raise and the machine reverse fly. This is because these exercises mimic the aforementioned main actions of your posterior shoulders. To do rear delt raises, hold a dumbbell in each hand, bend forward so your back is parallel to the ground and then raise your arms sideways until your arms are parallel to your shoulders. Keep a slight bend in your elbows. For machine reverse flys, sit on the machine, grasp each handle with your arms slightly bent and then extend the handles out behind you until your arms are parallel with your shoulders.
Working Your Rotator Cuff
You should do one exercise for your posterior rotator cuff for every two rear deltoid exercises. This is important for maintaining a healthy and strong shoulder complex. You can train your infraspinatus and teres minor by doing side-lying dumbbell external rotations. This exercise allows you to directly target these muscles in primary fashion so you can directly strengthen them. Also, the exercise lets you use the full range of motion of these muscles. To do dumbbell external rotations, grab a dumbbell in your right hand and lie on your right side. Bend your right elbow and put it up against the right side of your body. Keep your right elbow by your right side at all times. Rotate the dumbbell externally, or away from you, as much as you can, then return it toward your right side. Repeat with your left arm after you finish with your right arm.
Sets and Reps
If your main goal is to increase muscle mass, you should follow a rep range of between 10 to 12 reps. This is the range used by most bodybuilders because it is effective for building muscle. But if you want to focus on increasing strength, it is best you use heavier weights and keep your rep range between six and eight for rear deltoid exercises. The heavier weights will force your central nervous system and muscles to adapt and get stronger in the long run. However, always do 10 to 12 reps for your rotator cuff, because these are smaller muscles that can be easily injured if you pile on the weights. As for your set count, do three to four sets per exercise no matter what your goal is.
When to Train
You can easily include posterior shoulder exercises in your back and shoulder workouts. You will be working your posterior shoulders, especially your rear deltoids, during most back exercises. You can therefore do the aforementioned exercises at the end as finishing movements. But, if you want to do the exercises during your shoulder workouts, you can include them at the beginning, middle or end of your routine.