Muscles of the Shoulder Complex

Biceps Brachii
Biceps is a powerful supinator, as well as being a flexor of the elbow, particularly if the forearm is supinated.  In addition, it acts to some extent as a flexor of the shoulder.

Long Head of the Biceps
The long head of the biceps tendon attaches to the supraglenoid tubercle, and exits the joint in the bicipital groove in the hiatus between the subscapularis and supraspinatus tendons.  Fibers of the biceps contribute to the posterior and anterior superior labrum.  The long head of the biceps, with the biceps labral complex, centralizes and stabilizes the joint, as does the rotator cuff.  The biceps tendon has a synovial sheath as an extension of the synovial lining of the glenohumeral joint.
Proximally, biceps brachii has two heads, long and short.  The two heads unite in the lower part of the arm. 

Proximal Attachment:

The tendon of the long head of biceps arises within the shoulder joint capsule.  It is attached to the supraglenoid tubercle, with components from the superior glenoid rim, labrum and joint capsule.  The intraarticular part of the tendon passes over the anterior superior aspect of the humeral head, its distal course covered by the coracohumeral ligament, superior glenohumeral ligament, and the margins of supraspinatus and subscapularis tendons.  It then enters the bicipital groove.  The tendon is surrounded by a synovial sheath.  Below the bicipital groove, the tendon is held in place by the tendon of pectoralis major.
The bicipital groove (intertubercular groove or sulcus) lies between the lateral edge of the lesser tuberosity medially and the greater tuberosity laterally.  It is converted to a tunnel by the transverse humeral ligament, a thick band uniting the bony margins.  .

Nerve Supply
Musculocutaneous nerve (C5,6).

Actions:
Shoulder flexion
Elbow flexion
Forearm supination

Subscapular bursa
The subscapular bursa (superior subscapular recess) communicates with the glenohumeral joint between the superior and middle glenohumeral ligaments.  The bursa lies between and separates the anterior neck of the scapula and subscapularis tendon.  It may extend above the tendon to overlie its anterior aspect.  An inferior subscapular recess is sometimes present, and communicates with the joint space below middle glenohumeral ligament.  If the middle glenohumeral ligament is absent, there is a single wide necked recess.

Supraspinatus
Proximal Attachment
Arises from supraspinous fossa of the scapula and from fascia overlying the muscle.  The muscle converges on a tendon which passes under the acromioclavicular arch and above the shoulder joint. 
Distal Attachment
The tendon inserts on the upper facet of greater tuberosity of the humerus, and is separated from acromion, coracoacromial ligament, and deltoid by the subacromial/subdeltoid bursa.
Nerve Supply
Suprascapular nerve (C4,5,6).
Actions

Supraspinatus initiates abduction, and as part of the rotator cuff, helps to hold the humeral head in the socket throughout its range of movement.  If supraspinatus is paralyzed, abduction may be initiated by leaning to the affected side.

Biomechanics of the Supraspinatus
This muscle and tendon unit is important in maintaining stability of the glenohumeral joint, as the dominant part of the rotator cuff. It is affected frequently by degenerative changes near the tendinous insertion into the head of the humerus with advancing age, and so forms a disproportionate part of clinical practice. The supraspinatus acts to elevate, or abduct, the humerus. There are various hypotheses regarding the causes of the degenerative changes. As well as transmitting tensile loads from the muscle fibers to the greater tuberosity on the head of the humerus, the supraspinatus tendon is subjected to compressive loads on the inferior aspect, as it wraps around the head of the humerus. Similarly, it may be subjected to impingement of the superior aspect against the underside of the coracoacromial ligament and adjacent bone. It is likely that shoulder elevation with different positions of rotation of the humerus will cause the supraspinatus and other elements of the rotator cuff to become skewed, resulting in stress concentrations at one or other of the edges of the attachments.

Although the supraspinatus acts to abduct the humerus, it is only a secondary muscle when compared to the deltoid, since the deltoid has a much greater lever arm. The supraspinatus action, however, is essential for normal glenohumeral joint stability. The supraspinatus has been estimated to exert a force of 200 N during abduction of the unloaded arm to 30 degrees, and up to 700 N during abduction with a weight supported in the hand. Since the muscle-tendon unit is an elastic structure, passive tension rises as it is stretched by adduction of the humerus, and so the arm may be supported in an elevated position after supraspinatus repair to release this tension.

Subcoracoid bursa
A large subscapular bursa may extend upwards to lie under the coracoid process.  The upper part is referred to as the subcoracoid bursa.  Similarly, there may be an isolated bursa at this site.

Subclavius
Proximal Attachment
A small muscle which has a tendinous origin from the junction of first rib and costal cartilage, in front of the costoclavicular ligament. 

Distal Attachment
It inserts on the inferior surface of the middle third of the clavicle.

Nerve Supply
The nerve to subclavius (C5,6), a small branch from the brachial plexus, just distal to the junction of C5 and C6 roots. 

Actions
Subclavius stabilizes the clavicle during shoulder activity, steadying the clavicle in the sternoclavicular joint.

Subscapularis
Proximal Attachment
Subscapularis arises from the medial two thirds of the costal surface of the scapula, separated from the neck of scapula by subscapularis bursa. 

Distal Attachment
It passes laterally, forming a broad tendon which inserts on the lesser tuberosity, shoulder joint capsule, and the front of the upper shaft of the humerus.  Some of the superficial fibers blend with the transverse humeral ligament. 

Nerve Supply
Subscapularis is innervated by both subscapular nerves (C5,6,7).

Actions
Subscapularis is an adductor and medial rotator of the humerus.

Infraspinatus
Proximal Attachment
The infraspinous fossa of the scapula, and from the thick fascia overlying the muscle. 

Distal Attachment
The tendon inserts on the middle facet of the greater tuberosity of the humerus.  There is a bursa between infraspinatus and the neck of scapula. 

Nerve Supply
Suprascapular nerve (C4,5,6).

Actions
It is an lateral rotator of the humerus, but it’s more important action is as a member of the rotator cuff.



Teres Minor

Proximal Attachment
Teres minor arises from the upper two thirds of the lateral border of the scapula.

Distal Attachment
The lowermost of the three facets of greater tuberosity of the humerus. 

Nerve Supply
Axillary nerve (C4,5,6).

Actions

Teres minor is an adductor and lateral rotator of the humerus, and helps to oppose upward subluxation of the humerus due to the powerful actions of deltoid, biceps brachii, and triceps.  Its main action is as a member of the rotator cuff.

 

Biomechanics of the Rotator Cuff

The rotator cuff consists of the tendinous insertions of the supraspinatus, infraspinatus, and teres minor muscles, and subscapularis (SITS). These tendons form a hood that surrounds the head of the humerus anteriorly, superiorly and posteriorly. Co-contraction of these muscles stabilizes the glenohumeral joint during normal activities. In particular, abduction in the plane of the scapula will be accomplished principally by action of the deltoid muscle, but this acts nearly vertically when the humerus is still close to the side of the body, and so tends to sublux the head of the humerus superiorly. The rotator cuff muscles act more horizontally, and their tensions combine with that of the deltoid to direct the resultant joint force vector into the superior concavity of the glenoid, which is normally a stable state.  Conversely, rotator cuff deficiency leads to superior subluxation of the head of the humerus, leading to impingement against the coracoacromial arch, accompanied by abduction weakness and loss of motion.

The complex anatomical structure of the rotator cuff means that the reasons for the frequency of cuff degeneration, which is most often in the supraspinatus tendon, are not fully understood. It is known, for example, that there are distinct layers with different histological and mechanical properties, so this can lead to shearing between the layers, and this also applies to the zone of overlap of the supraspinatus and infraspinatus tendons. The tendons are also subjected to compression forces onto their surfaces, either due to the wrapping around the glenoid and head of the humerus, or from superior impingement beneath the acromion. Finally, humeral rotation will tense and slacken the edges of the supraspinatus tendon.

Subacromial bursa
The subacromial bursa extends under the acromion and coracoacromial ligament.  Laterally the bursa lies over the superior surface of the supraspinatus and infraspinatus tendons, and extends beyond the lateral and anterior aspects of the acromion, under the deltoid.  It is generally surrounded by peribursal fat, and in normal individuals represents a potential space only.

The bursa serves as a gliding mechanism between the rotator cuff and coracoacromial arch.  Although communication exists between the subacromial and subcoracoid bursae, there may be no communication between the subcoracoid and subscapularis bursae.

Deltoid
Proximal Attachments
The deltoid is a thick, powerful muscle which covers the shoulder joint and upper humerus. It arises from the anterior border of lateral third of clavicle, the lateral border of acromion and the lower lip of scapular spine, as well as from the fascia over infraspinatus muscle. 

Distal Attachments
It inserts on the V shaped deltoid tuberosity half way down the lateral aspect of the shaft of humerus. 

Nerve Supply
Deltoid is innervated by the axillary nerve (C5,6).

Actions
It is a powerful abductor of the humerus.  Abduction is, however, initiated by supraspinatus. The anterior portion of deltoid contributes to flexion of the humerus, with the posterior portion to extension.  When the supraspinatus is torn, the upward pull of deltoid results in superior subluxation of the humeral head.

The acromial part of the deltoid is multipennate.  Tendinous septa arising from the acromion and the deltoid tuberosity interdigitate, with short muscle fibers extending between the septa.  This gives the muscle a short but powerful pull.

Pectoralis major
Proximal Attachments
Pectoralis major arises from the anterior chest wall by clavicular and sternocostal heads, and forms the bulk of the anterior wall of the axilla. The smaller clavicular head arises from the medial half of the anterior surface of the clavicle.  The sternocostal head arises from the anterior surface of manubrium, sternum, the upper sixth costal cartilages and the anterior aspect of the rectus sheath.

Distal Attachments
The muscle is attached by its tendon to the lateral lip of the intertubercular groove.  The tendon has two laminae.  The thicker anterior lamina is derived from muscle arising from manubrium, sternum and the second to fifth costal cartilages.  The posterior lamina is derived from fibers from sixth costal cartilage, lower sternum and the anterior aspect of the rectus sheath.  These lower fibers twist on themselves, so that those arising inferiorly on the chest wall pass behind the upper fibers, inserting behind them.  This gives a rounded inferior border to the muscle and the anterior axillary fold. 

Nerve Supply
Medial and lateral pectoral nerves (C5,6 to the clavicular part, C7,8, T1 to the sternocostal part).

Actions
Its upper border of pectoralis major is separated from deltoid by the deltopectoral groove (infraclavicular fossa), in which lies the cephalic vein.  It is a powerful adductor and internal rotator of the humerus.  Its sternal fibers extend  the shoulder joint from a flexed position. Its clavicular fibers flex the shoulder joint from an extended position.

Pectoralis major forms the bulk of the anterior wall of the axilla.

Serratus anterior
Serratus anterior is a large flat muscle which covers the medial axillary wall. 

Proximal Attachments
It arises as a series of slips from the lateral aspect of upper eight ribs and fascia, the lower slips interdigitating with external abdominal oblique muscle.  

Distal Attachments
The muscle inserts on the medial aspect of the costal surface of the scapula, the upper slip passing horizontally backwards to the superior angle, the second to forth slips inserting on the medial border, and the lower four slips passing obliquely upwards and backwards, converging to the inferior angle.

Nerve Supply
The long thoracic nerve (C5,6,7), which descends over the superficial surface of the muscle.

Actions
Serratus anterior is a protractor and lateral rotator of the scapula.

It transfers the weight of trunk to upper limb when weight is being taken on the hands, and pulls the shoulder girdle forwards, for example when pushing.  With the lower slips attaching to the angle, the scapula rotates as it is pulled forwards, so that the glenoid faces more superiorly. The serratus anterior is one of the most important stabilizers of the shoulder girdle and acts by hold the scapula in close to the ribcage.  Serratus anterior weakness is associated with a winged scapula.

Sternocleidomastoid
Proximal Attachment
The lateral surface of the mastoid process, and by an aponeurosis to the lateral part of the superior nuchal line of the occipital bone. 

Distal Attachments
The rounded sternal or medial head is attached to the upper anterior aspect of the manubrium sterni.  The clavicular or lateral head is attached to the upper surface of the medial third of the clavicle. 

Nerve Supply
Accessory nerve, with a branch from the ventral ramus of C2. 

Actions
Its actions will produce side flexion of the head to same side, but rotation of  the head to the opposite side. Both muscles act together to produce flexion of the neck and protraction the head. Sternocleidomastoid can also be effective in extension of an already partly extended neck.

Triceps brachii-lateral head
The triceps is a large muscle occupying the extensor compartment of the arm.  It arises from three heads, medial, lateral, and long, from which it gets its name. 

Proximal Attachment
The lateral head of triceps arises by a flattened tendon from the upper border of the radial groove, from the lateral border of the humerus and lateral intermuscular septum.

Nerve Supply
Radial nerve (C6,7,8), with separate branches to each head.

Actions
Triceps is a powerful extensor of the elbow.

Tendon of triceps brachii
The triceps is a large muscle occupying the extensor compartment of the arm.  It arises from three heads, medial, lateral, and long, from which it gets its name. 

Distal Attachments
The muscle converges on the common tendon, which inserts on the upper surface of the olecranon. 

Nerve Supply
Radial nerve (C6,7,8), with separate branches to each head.

Actions
The long head supports the inferior aspect of the shoulder joint capsule when the arm is abducted.

Triceps brachii-long head
The triceps is a large muscle occupying the extensor compartment of the arm.  It arises from three heads, medial, lateral, and long, from which it gets its name. 

Proximal Attachments
The long head of triceps arises from a tendon attached to the infraglenoid tubercle of the scapula. 

The long head supports the inferior aspect of the shoulder joint capsule when the arm is abducted.  

Nerve Supply
Radial nerve (C6,7,8), with separate branches to each head.

Actions
Triceps is a powerful extensor of the elbow.  In addition, the long head produces weak extension of the shoulder and assists in stabilizing the shoulder joint.

Teres major
Proximal Attachment
Teres major arises from a raised oval area at the dorsal aspect of the inferior angle and the adjacent lateral border of the scapula. 

Distal Attachment
It inserts via a broad flat tendon on the medial lip of the intertubercular groove. It is separated from the tendon of latissimus dorsi by a bursa and the lower borders of both tendons are united for a short distance.

Nerve Supply
Lower subscapular nerve (C6,7).

Actions
It extends the shoulder, adducts and medially rotates the humerus.

Teres major, along with the tendon of latissimus dorsi, forms part of the posterior wall of the axilla.

Trapezius
The trapezius is a broad flat triangular muscle which lies superficially at the back of neck and upper trunk, the pair forming the 'trapezium' from which it derives its name.

Proximal Attachments
It arises from the superior nuchal line of occipital bone, the external occipital protuberance, ligamentum nuchae, the spines of seventh cervical and all thoracic vertebrae and the intervening supraspinous ligaments. 

Distal Attachments
The upper fibers pass down and laterally to insert in the posterior aspect of lateral clavicle.  The middle fibers pass horizontally to the medial border of acromion and upper lip of scapula spine.  The lower fibers ascend to the tubercle of scapula spine.  The upper fibers form the posterior border of the posterior triangle of the neck. 

Nerve Supply
Trapezius receives motor innervation from the spinal accessory nerve, which enters from the posterior triangle.  Fibers from the ventral rami of C3 and C4 are thought to be sensory.

Actions
The upper fibers elevate the scapula, for example when carrying a weight.  The middle fibers produce bracing of the shoulder girdle.  The lower fibers depress the medial aspect of the scapula and shoulder, for example using the hands to rise from sitting.

The action of upper and lower fibers together tend to rotate the scapula so that the glenoid fossa points upwards and forwards.

Latissimus dorsi
Latissimus dorsi is a large flat muscle which passes between trunk and humerus, acting on both the shoulder joint and shoulder girdle. 

Proximal Attachments
It arises from spines and supraspinous ligaments of the lower six thoracic vertebrae deep to trapezius, and from thoracolumbar fascia, by which it is attached to the spines of all lumbar and sacral vertebrae.  In addition, it arises from posterior iliac crest and the lower three or four ribs.  It passes upwards and laterally, gaining a small slip from the inferior angle of scapula. 

Distal Attachment
The muscle converges to form a flat 'strap like' tendon, which winds around the inferior border of teres major to reach its humeral attachment in the floor of the intertubercular groove of the humerus.  The lower border of the tendon unites with the tendon of teres major for a short distance.  A bursa lies between the two.

Nerve Supply
The thoracodorsal nerve (C6,7,8).

Actions
Latissimus dorsi retracts the shoulder girdle, and is a powerful extensor of the flexed humerus (for example, climbing).  It also adducts and internally rotates the humerus.  It is an accessory muscle of respiration, particularly violent expiration.

Latissimus dorsi and teres major form the posterior fold of the axilla.

 

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