So the shoulder does a lot of different movements. It elevates, depresses, rotates, tilts, retracts and protracts, flexes and extends. And it has the ability to do multiple movements at the same time!
Due to the shoulders ability to move so freely, it pays the price of decreased stability. How do you solve for a decrease in stability? You slab on a lot muscle to keep it in it's place.
The shoulder joint.
The shoulder joint itself consists of the upper arm and the glenoid cavity (a cavity on the outside of the shoulderblade). This joint allows for anteflexion and retroflection and adduction and abduction. These movements will take place within the shoulder joint until approximately 90 degrees of movement. In healthy shoulders this can be more and less than 90 degrees, signing some kind of dysfunction in the shoulder girdle muscles. The shoulder girdle muscles get involved as soon as the shoulder joint is not able to move any further without involving the scapula and the collarbone.
The rotator cuff.
The rotator cuff consists of four muscles. As the name suggests they mainly rotate. They're pretty easy to remember. Your first vantage point is the spine of the scapula. Above and under the spine you have the Supra and Infraspinatus muscles. Underneath the Infraspinatus you'll find the Teres Minor (yes as per anatomy rules there is also a Major), those muscles serve as external rotators. When looking at an anatomical drawing you'll see that all three of these muscles insert at the back of the humerus (upper arm). Now, it wouldn't be much of a rotator cuff if it only contained external rotators. The only internal rotator of the rotator cuff is the Subscapularis muscle. Which sits at the front of the scapula and inserts in front of the humerus.
Now this means all external rotators of the shoulder joint will be found inserting at the back of the humerus. All internal rotators will be found inserting at the front of the humerus.
A couple of notable internal rotators are the Teres Major, the Pectoralis Major and the Latissimus Dorsi. The lats have a number of functions, but as it inserts at the inside of the upper arm it also functions as an internal rotator.
Depression and the shoulder.
The shoulder girdle can move up and down as well. To remember all the correct muscles is pretty straight forward. Any muscle that can be found above the scapula and attaches to the scapula will elevate the shoulder girdle. And muscle that can be found below the scapula and is attached to it will depress the shoulder girdle.
Putting it all together.
Now it's impossible to do a singular shoulder girdle. Protraction is the movement where the scapulas glides across away from each other. Muscles that are attached to both the shoulder blade and the front of the ribcage/sternum will cause protraction. Retraction is the movement where the scapulas glide over the ribcage towards each other. Muscles that are attached to the back of the ribcage/spine will cause retraction.Protraction is always mixed with internal rotation and elevation. This means that retraction is always mixed with external rotation and depression. Therefor if you want a improve a slouching posture, it's not just important to work on the retraction of the shoulder blades, but also depression and external rotation.
An exception to that rule are the Lats, that function both as a depressor and as an internal rotator. This is why you see people that struggle with pull ups will start to internally rotate their shoulders to involve more Lats in the movement. This also explains why they play a role in arm wrestling as the upper fibres of the Lats aid in the internal rotation.
I hope you enjoyed this and have gained a better understanding for the anatomy of the musculoskeletal system:)
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