Understanding Shoulder Dynamics

By Stefanie Pusateri | January 2020

The shoulder is a fascinating and complex marvel of biomechanics. Many of us take this marvel for granted and get frustrated when injuries or changes to range of motion slow us down. Understanding the shoulder components and mechanics increases awareness and helps to prevent and recover from injuries.

There are three basic parts of the shoulder: the bones, joints and the muscles. An understanding of these three elements are necessary in order to get a full picture of this essential part of our bodies. In general when people think of the shoulder they have the shoulder joint in mind. We’re going to focus on the shoulder girdle because there are some important stabilizing structures outside of the ball and socket joint that are key for successful shoulder movement.

Let’s start with defining the bones:

  • Humerus – This is the long bone that forms our upper arm.
  • Scapula – The scapula, or shoulder blade is the triangular shaped bone on the back side of our shoulder.
  • Clavicle – Commonly known as the collar bone, this thin bone sits at the top of the shoulder.
  • Sternum – The breast bone, or sternum is another critical stabilizer of the shoulder girdle.

More important than the bones themselves is understanding the way that these bones fit together. The shoulder girdle joints are designed in a way that provides an extreme range of motion and flexibility. Most people think of the shoulder as being one ball and socket joint, but there are two other joint complexes that are critical to the function of the shoulder.

  • Sternoclavicular joint – The clavicle attaches to the main skeleton at the sternum. The clavicle is the key piece to the shoulder girdle. Without it, the shoulder wouldn’t have a stabilizing base and would just free float and make movement impossible. That’s the magic of the skeletal system; if we didn’t have bones for muscles to attach to, we wouldn’t be able to move at all.
  • Acromioclavicular joint – This is by far my favorite joint in the body because it is tiny yet powerful. This joint is formed by the junction of the collarbone and the scapula. The scapula has a little hook at the top of it that reaches over to join with the clavicle. This joint is the only place where the scapula is attached to the skeleton! The rest of the scapula is held in place by ligaments and muscles.
  • Glenohumeral joint – The ball at the head of the humerus muscle fits in to the socket formed by the clavicle and scapula. This ball and socket joint is what makes it possible for us to get such a wide range of motion with our arm movements. The ball joint allows our arm to roll freely, but it’s also necessary for our shoulder blade to float freely. If our scapula was attached to our spine instead of being stabilized by muscles, we would be able to move our arms around like a windmill, but pulling our arms across our bodies would be considerably limited.

There are a surprising number of muscles involved in shoulder joint movement. I’m going to group these into two sections, the movers and the stabilizers:


The movers are the larger muscle groups that are designed to provide movement and strength. These are the muscle groups that lie close to our skin and do most of our heavy lifting, so to speak.

  • Deltoids – The anterior, middle and posterior deltoids sit at the top of the humerus and more or less cover the ball and socket joint. When we lift our arms up to put deodorant on in the morning, the deltoids are one of the main muscles involved in that movement.
  • Rhomboids – These are the muscles that attach the long edge of the scapula to the spine and the place where 94% of the human population frequently have “knots”. You know that burn you get between your shoulder blades after too much time at the desk? That’s the rhomboids. These muscles are used in rowing motions or when you’re bringing your arm back to pull start your snowblower.
  • Pectoralis – Pectoralis major and minor serve as the big movers at the front of the chest. These muscle groups attach to the sternum and the humerus and are the main reason we can move our arms across the front of our bodies. Without our pectoralis muscles, we would have to twist our entire upper body in order to put our seat belts on.


Stabilizers are the smaller muscles that are snuggled under the big movers. The stabilizers are the wee helper muscles that are responsible for fine motor movements. The stabilizers are on constant guard and trigger the big movers to initiate movement or change direction as needed.

  • Supraspinatus – The supraspinatus is found sitting at the top of the back side of the scapula. It’s job is to help the deltoid muscle group with moving the elbow above the main ball and socket joint of the shoulder.
  • Infraspinatus – This muscle sits under the supraspinatus and covers the bottom back half of the scapula. It is the one that’s responsible for the motion you make when you have someone sitting to your right at the movie theater and they ask you to pass them the popcorn bucket (using your right arm)
  • Subscapularis – Since the scapula is only attached to the skeleton at the acromioclavicular joint, it’s free floating and has muscles on the front and back sides. The subscapularis is the muscle that sits on the front side of the scapula (between the scapula and the ribs) and is responsible for the movement you make when you turn your arm in to check your watch/Fitbit/Apple watch.
  • Teres – The teres major and minor attach to the bottom outside of the scapula and the humerus. It’s one of the muscles that helps pull your shoulders back and down when you realize that you’ve been hunched over your keyboard like a vulture for the past hour. Hypothetically speaking.

Note: The supraspinatus, infraspinatus, subscapularis and teres muscles are known as the SITS muscles and often referred to as the rotator cuff muscles. In addition to helping the big movers, these small but mighty tissues are the main bits that keep the humeral head from popping out of the ball and socket glenohumeral joint.

  • Serratus Anterior – This subtle, brilliant muscle group attaches the ribs to the scapula. It’s the usually tender section of muscle that lies about at the intersection of the arm and side seams of your tshirt, just under your armpit. This muscle helps with raising your arm and also assists during the breathing process.

There is a lot of information here, but without the background it’s difficult to appreciate and more importantly understand shoulder dysfunctions. The bottom line, at least from a massage therapy standpoint, is that muscles are a collaborative bunch. When one muscle gets tired or injured, all the others chip in to help out. When the muscles are distracted from their main jobs the entire system has to find a way to compensate in order to follow through on the messages the brain is sending. For instance, if one of the big movers is injured or weakened, all those little SITS muscles have to divide their time between stabilizing and kicking in on the big movements and they weren’t built for that. They are smaller and can’t keep up. That’s where tears can happen and if that tear isn’t caught soon enough and rehabilitated with the help of a physical therapist, surgery can happen. So don’t ignore those aches and pains, especially if they don’t go away or keep coming back. Your little stabilizers could be giving you an important message and trying to do their part to keep you happy, healthy and wise.

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