It’s safe to say that external rotation of the shoulder is a great preliminary litmus test to overall shoulder function. Not much can be done in the gym or on the field (at least not for long) without access to this range of motion. But our potential to generate movement into external rotation is greatly outweighed by our ability to generate force in internal rotation. With the Pecs, and Lats both being prime movers into internal rotation, there is a tall task on the three muscles of external rotation to keep balance through the rotational plane at the shoulder.

Made worse, is the fact that of the three muscles that create movement into external rotation (infraspinatus, teres minor, the posterior fiber of deltoid) two are muscles of stability (infraspinatus, teres minor). Leaving only the “rear delt” as a prime mover of external rotation to combat the output potential of the Lats and the Pecs.

The rear delt has become another muscle that more or less gets labeled as being “flashy” or “non-functional” and only given real credit in the realm of aesthetics- 3D delts bro…

But just like it’s fraternity brother, the bicep, the posterior fiber of the deltoid plays a major role in proper shoulder function. And, in something we will touch on later- they actually work synergistically in some compound movements to help stabilize the shoulder. As mentioned above, the rear delt plays a role in external rotation of the shoulder. Which from a posture and muscular balance standpoint it has the merciless task of balance rotational forces in external rotation against the much more capable pec major, and latissimus dorsi which are so dominant in internal rotation.

It’s all in the name…

The biggest issue I see with rear delt training is improper execution of the muscle in isolation. A lot of this comes down to the name of the cornerstone exercise used to train this muscle "The Rear Delt Fly". The majority of people when they think of training the rear delt in isolation they think of the conventional “rear-delt fly”, and insert their implement of choice (dumbbell, cable, machine etc). Now the problem with this verbiage is that when I say “fly” most people automatically, think of the “pec fly” exercise. So when looking to execute the “rear-delt fly” its often times performed as posterior analog movement to that of the pec, but, for this to anatomically correct the rear delt would have to be an anatomical posterior analog to the pec, which isn’t the case.

The pec major attaches onto the midline of the body, as the sternal head has insertions all the way down the lateral border of the sternum. The rear delt, on the other hand, does not have such a mid-line attachment. It spans from the deltoid tuberosity on the humerus and lays to rest at the spine of the scapula, not the actual spine itself.

When people attempt to replicate the fly movement in reverse- they end up with far too much movement of the scapula to train the rear delt in isolation. The movement should only consist of the humerus moving on the scapula and not the scapula moving on the rib cage, if we remember that rule of 1/3’s of end ranges of motion of the shoulder, any end range of motion of the shoulder is going to be reliant on the scapula setting a trajectory for that movement to occur. So if your rear delt fly begins to initiate scapular retraction, cut that range of motion by 33.3% (roughly speaking) and the scapula should no longer be a contributing factor, and the rear delt will now be active in isolation.

Why train muscles, not movements?

Every now and then you’ll come across the crunchy granola-holistic crowd that questions the motivations in training a muscle in isolation. Doubling down on a position of “ well- that’s not functional”. That’s a valid point, and there is merit to training the humerus in integration with the scapula for applications that require it. But as it comes to the relative function of the humerus and the scapula in a movement like pressing. It’s EXTREMELY functional to train those muscles to act from a fixed scapular position. One of the first cues you will learn as a beginner bench presser is “keeping your shoulder blades together”, and if you think of the execution of the barbell bench press ( or any loaded pressing motion in the gym for that matter) you are moving the humerus against the fixed position of the scapula.

Unlike, for example- a more sport specific application of shoulder motion like a pitcher in baseball, or quarterback in football. These motions of throwing are more continuous, and representative of how the shoulder is meant to function. The pronation of the wrist and the end of a throw is a direct manifestation of internal rotation of the humerus, , which is made possible by elevation and protraction of the scapula as well as flexion of the thoracic spine.

Whether your objective outcome of training is isolation or integration, proper execution of movements at the posterior fiber of the deltoid should be a mainstay in your programming for long-term health and performance of your shoulder.

Stay Strong,