All About Shoulder Biomechanics | Pongo Power

Shoulder Biomechanics & Injury Prevention

Shoulder dysfunctions are prevalent in today's society. The shoulder is one of the most mobile joints in the human body. Because of its complexity that allows for this mobility, the shoulder joint is regularly injured. The conditions that normally affect the joint include impingement, rotator cuff tears, frozen shoulder, dislocations, and muscular imbalances (which can occur as weakness, pain, and limited mobility). Many of these conditions are preventable, with proper exercise program design.

Poor joint mechanics can lead to unnecessary wear and tear on your shoulders. Having a better understanding of how your shoulder functions, will have you better prepared to deal with any current injuries and/or prevent future injuries, down the road. 

Whereas understanding the anatomy and biomechanics of the joint may seem insurmountable if you are not a movement specialist, this type of knowledge is essential for fitness professionals and Medical Exercise Specialists.


The shoulder has four joints: the glenohumeral joint, the acromioclavicular joint, the scapulothoracic joint, and the sternoclavicular joint.

The Joints

The Glenohumeral Joint (GHJ)

The articulation of the humeral head (the humerus is the long bone, in your upper arm) and the glenoid fossa form a shallow ball and socket joint. This is the joint that most people think of as the shoulder joint. Movements within that joint include shoulder flexion, extension, abduction, adduction, internal rotation, external rotation, horizontal abduction, and adduction. To make the joint more stable, the glenoid fossa faces downward and is surrounded by the labrum. The ligaments, muscles, and the capsule provide additional joint stability. Therefore it is important to keep these muscles toned, activated, lubricated, and functioning properly. 

Pain in your shoulder joint can be the result of muscle imbalance, which can cause instability and nerve impingement (among other issues). Pain is your body's way of alerting you to a possible issue. It is important not to ignore shoulder dysfunction, due to the complex nature of its joints.

There are three ligaments that blend with the shoulder capsule along the anterior surface of the joint. Anteriorly, the GHJ support is provided by the pectoralis, subscapularis, and the bicep tendon. Superiorly, the joint is supported by the acromion process, deltoid, coracoacromial ligament, and supraspinous. On the posterior surface, that support is given by infraspinatus, triceps, and teres minor.

There is very little inferior support, hence is the reason there are a lot of dislocations.

scapulothoracic joint

Scapulothoracic Joint (STJ)

This joint is formed by the articulation of the scapula and the thoracic cage. Unlike the GHJ which has a great deal of mobility, the STJ has little mobility, but great stability. The scapula is the anchor to the GHJ during overhead activities.

The muscles that stabilize the scapula against the ribcage are the traps, serratus anterior, and rhomboids. Movements occurring at the STJ are scapular protraction, elevation, and depression, as well as upward and downward rotation.

Learning how to train these muscles to function properly, with a band (such as rows, and pull-a-parts, or external rotation) can be a true game-changer in maintaining the strength and stability of your upper-body. You can also pack the tools you need, to keep your shoulders healthy, when you travel.

Acromioclavicular Joint (ACJ) & Sternoclavicular Joints (SCJ)

The ACJ—the articulation of the clavicle and the scapula through the acromion process; and the SCJ—the articulation of the clavicle and the sternum, allow rotation of the clavicle. It's not a big rotation, nonetheless, very important for shoulder movement.

Anterior view of shoulder joint with muscles. SOURCE: Based on AMuscsk_20131017_v0_001. Used in 3B11978, 14A12041, 14A12041, 82395, 90431.

Shoulder Biomechanics

There must be some upward rotation of the scapula for overhead movement. That rotation happens with the abduction and flexion of the humerus.

The first 45* of humeral abduction comes with some scapula movement so it can find a stable place on the thoracic cage. For every 15* of abduction, there are 10* of movement at the GHJ and 5* of movement at the scapula. This is called Scapulo-humeral rhythm.

The humerus also rotates to allow the greater tuberosity to clear the acromion process. The rotator cuff (supraspinatus, infraspinatus, teres minor, and subscapularis) stabilizes the humeral head in the GHJ and prevents the humerus from migrating upward during overhead activities.

Understanding the biomechanics of the shoulder joint is highly technical. Whereas you may not memorize this knowledge,  you can master the proper movement patterns, that allow you to activate the right muscles at the right times, to prevent injury. 

Contact one of our Medical Exercise Specialists so that we can provide a complimentary movement screening and fitness assessment. Addressing your musculoskeletal alignment, mobility and muscular activation can help to manage and prevent future injuries. 

Meet the Author

Gardy Reglas

Medical Exercise Manager, Core Conditioning Specialist, & ASCM Personal Trainer


As a Medical Exercise Specialist, Gardy designs exercise programming for the clients with medical conditions such as diabetes, hypertension, lower back disorders,joint replacements, stroke patients.

As a personal trainer, his favorite aspect is working one-on-one with clients using functional tools such as Stability and Medicine balls, and tubings

He believes that this way of training is critical and essential for helping his clients to ”move the way they were meant to move.” Gardy is also a big believer that continuing education a must for the fitness professional

“The fitness industry is always revolving, so it is essential that as a fitness professional, I keep abreast of the latest trends in health and exercise science.”

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