The shoulder: joint structure, movements and muscles

The arms are the most active part of our body. We rely on them for most of our basic functions in the world. We use them to perform a range of activities... from something basic like eating to much more sophisticated things such as creating art. This entire universe of movements is enabled by the shoulder joint.

Four distinct articulations can be identified in the shoulder joint: the sternoclavicular, the acromioclavicular, and the scapulothoracic, and the glenohumeral.
 
The small sternoclavicular articulation is where the whole arm attaches to the torso. It must be able to withstand significant load, although it only affords limited movement. This explains why soft-tissue swelling and osteoarthritis are common here.
 
The acromioclavicular articulation, is also a small synovial articulation with limited range of movement susceptible to osteoarthritis. The integrity and movement of the shoulder can be significantly compromised if this joint is enlarged.
 
The nonsynovial scapulothoracic articulation is composed of the scapula and the thoracic cage, which are separated by a large bursa. Its main purpose is to supplement the large articulation of the true shoulder joint.
 
The glenohumeral joint, considered the true ‘shoulder joint, is a shallow ball-and-socket articulation. This makes it extremely mobile but also unstable and vulnerable. As the glenoid is very shallow, were it not protected, it can easily slip inferiorly, resulting in dislocation.
 
The surfaces of the surrounding bones are covered by cartilage. The glenoid labrum is a fibrocartilage rim that deepens the glenoid fossa to reduce the disproportion between the shallow socket and the bigger ball making the joint mobile but unstable.
The surrounding soft tissues protect and support the joint. The tendons of the infraspinatus and teres minor muscles protect posteriorly. Anteriorly, the subscapularis tendon supports. Superiorly, protection comes from the long head of the biceps and the tendons of supraspinatus.
 
These tendons around the humeral head together with their corresponding muscles rotating the humeral head within the glenoid socket form the rotator cuff. Its purpose is to stabilise the shoulder so that the larger surrounding muscles of the deltoid and pectoralis major can exert force efficiently.
 
The acromioclavicular bony arch and the coracoacromial ligament protect the superior aspect of the shoulder. The superior portion of the rotator cuff, the supraspinatus tendon, and the long head of the biceps tendon all pass beneath this protective structure. It is worth noting that the biceps is the only muscle of the rotator cuff that depresses the humeral head.
 
The subacromial space between the acromioclavicular bony arch and the coracoacromial ligament can be narrowed in cases of osteoarthritis or swelling of the soft tissues. This can lead to an impingement syndrome characterised by a painful pinching of the tissues trapped in the subacromial space. 

Ligaments
 
A capsule attached to the circumference of the glenoid cavity and to the anatomical neck of the humerus surrounds the shoulder joint. This capsule is very loose. If there was no atmospheric pressure the humerus could be pulled at least an inch away from the scapula. This feature makes the joint even more mobile.
 
The longitudinal fibres of the anterior capsule are arranged in the form of three flat bands. These are the glenohumeral ligaments. They extend from the anterior border of the glenoid fossa to the anterior surface of the neck of the humerus.
 
The transverse humeral ligament is a continuation of the capsule. It crosses the intertubercular groove between the tubercles of the humerus.
 
The glenoid ligament is a band of strong fibro-cartilaginous tissue attached to the rim of the glenoid cavity. Its main purpose is to deepen the socket.
 
The coracohumeral ligament is a flat band. It extends from the lateral border of the base of the coracoid process to the neck of the humerus near the large tubercle.
 
The coracoacromial ligament is an accessory ligament. It is a strong band of fibres between, as the name suggests, the coracoid and acromion processes. It prevents upward dislocation by arching over the top of the joint.
 
All these ligaments are in reality part of the capsule.
 
The tendons of subscapularis, supra- and infraspinatus are close to the capsule of the joint near their insertions. This further strengthens the joint. The joint is only weak inferiorly as there are no strengthening ligaments there.
 
An extensive synovial membrane lines the whole capsule. It also includes the part of the long tendon of the biceps which is inside the capsule of the joint.

Bursae
 
The subscapular bursa lies underneath the tendon of subscapularis. It is essentially a continuation of the articular synovial membrane communicating with the joint cavity by an opening in the capsule. It protects the tendon from wear and tear during movement.
 
A second smaller bursa can sometimes be found between the capsule and the tendon infraspinatus.
 
The subdeltoid or subcromial bursa, a large bursa underneath the deltoid muscles, does not communicate with the joint cavity. It is inferior to the acromion and the deltoid and superior the joint capsule and the supraspinatus tendon. It can cause shoulder pain when inflamed. 

​Muscles & Movements
 
The shoulder enables the placement of the arm in space. This is accomplished by the scapula and the glenohumeral articulation moving in synchrony, a movement pattern termed the scapulohumeral rhythm.
 
The complexity of possible shoulder movement, makes it difficult to subdivide them according to the four articulations. Usually, all four work in synchrony but this also means that pathology in any one of them is likely to affect the whole joint. 

The shoulder is surrounded by the deltoid, supraspinatus, infraspinatus, teres major and minor, and subscapularis. These muscles together with pectoralis major, biceps brachii, coracobrachialis, latissimus dorsi, triceps brachii act on the joint to produce movements.
 
The shoulder can move across all three planes. Flexion and extension take place in the sagittal plane. Abduction and adduction happen in the frontal plane. Horizontal adduction and abduction occur in the transverse or horizontal plane.
 
Medial and lateral rotation (toward and away from the body), as well as circumduction (flexion/extension and abduction/adduction), are also possible.

Flexion is the movement of the arm forward in the sagittal plane. Think of raising your arm to show you want to speak. It involves the anterior fibres of the deltoid, pectoralis major, coracobrachialis, subscapularis, and both heads of biceps brachii.
 
Extension is the backward movement of the arm in the same plane. Think of raising your torso by pushing your arms back into the bed while lying on your back. The posterior fibres of the deltoid, teres major, infraspinatus, latissimus dorsi and triceps brachii working together make this movement.
 
Abduction is the movement of the arm in the coronal plane, away from the body. Think of raising your arms horizontally parallel to the ground to form the letter T with your body. Infraspinatus initiates the movement for the first 15 degrees and the middle fibres of the deltoid complete it.
 
The movement of the arm in the same plane toward the body is called adduction. Reaching for the seatbelt with your arm closest to it and bringing it to the body is an example. According to most modern sources, the muscles producing this movement are pectoralis major, latissimus dorsi, and teres major. Older texts also mention the role of coracobrachialis, the short head of the biceps, the long head of the triceps and the weight of the limb.
 
Medial rotation is the turning of the arm inwards so that the thumb points toward the body. It is produced by the synchronous action of pectoralis major, latissimus dorsi, subscapularis, teres major and the anterior deltoid.
 
The opposite action is lateral rotation. That is turning the arm outward with the thumb pointing away from the body. It is accomplished by infraspinatus and teres minor. An everyday example of these movements can be the turning of a valve using one’s hand.
 
The shoulder is also capable of a conical or circular movement (circumduction) using a combination of the muscles described above.

Next time you do something with your arms you may want to stop for a second and marvel with gratitude the beautiful architecture and functioning of your shoulder.

Related articles:
The knee: joint structure, movements and muscles
The hip: joint structure, movements and muscles
The ankle: joint structure, movements, muscles
The elbow: joint structure, movements and muscles