The human shoulder is the most mobile joint of the body. Since it is a muscle-guided and muscle-stabilized joint, it is susceptible to instability and inflammation. It is essential for doctors to learn about the muscular stabilization of the shoulder joint and its functional anatomy. We can imagine how tiring (if not impossible) a life without a properly functioning shoulder joint would be. The muscle groups of the shoulder are distinguished by location and origin: the muscles of the shoulder and the muscles of the upper arm.

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Muscles-that-Move-the-Humerus

Picture: “Muscles That Move the Humerus” by Phil Schatz. Licence: CC BY 4.0


Rotator Cuff: A Look at the Anatomy

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Picture: Teilansicht: “Muscles That Move the Humerus” by Phil Schatz. Licence: CC BY 4.0

The rotator cuff is a group of muscles and tendons surrounding the shoulder that is essential for mobility and stability of the shoulder joint.

This group of muscles includes four skeletal muscles: the supraspinatus muscle, the infraspinatus muscle, the teres minor muscle, and the subscapularis muscle. All these four muscles can be remembered by the mnemonic “SITS”.

The flat tendons of these muscles are connected to the capsular ligament apparatus stabilizing the shoulder joint; one can even say that they are likely grown together.

Supraspinatus Muscle

The supraspinatus muscle runs from the supraspinatus fossa and inserts into the greater tubercle of the humerus. It is supplied by the suprascapular nerve (C5-C6), that arises from the upper trunk of Brachial plexus.

Functions of the Supraspinatus Muscle

The main functions of supraspinatus muscle include the abduction at the shoulder joint and humeral head depression. It plays an essential role in the scapulohumeral balance of the rotator cuff. It centers the humeral head in the glenoid cavity and, along with the deltoid muscle, prevents downward (inferior) subluxation of humerus.

Characteristics of the Supraspinatus Muscle

The anterior fibers of the supraspinatus tendon are connected to the subacromial bursa, while the deep fibers are grown into fibrous membrane and the surface parts of the Coracohumeral ligament. This allows the muscle to have a significant influence on the many structures of the capsular ligament. Therefore, the muscle reacts to the lesions of the structures to which it is connected. As a result, patients are often diagnosed with a muscle disorder, even though the muscle itself is not what caused the primary problem.

Clinical Significance of the Supraspinatus Muscle

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Picture: “Thoracic Upper Limb Arteries” by Phil Schatz. License: CC BY 4.0

The muscle is often affected by the so-called tendinopathy. Tendinopathy is a primarily non-inflammatory degenerative disease of tendons and tendon insertions. Its genesis can be, among other things, vascularisation disorders.

With respect to the arterial blood flow in muscles, physicians should know the following: tendons are supplied through a branch of the humeral circumflex artery (distal) and through the suprascapular artery.

Motions of the shoulder such as extreme adduction or abduction can lead to increased pressures to the head of humerus. Thus, leading to vascular congestion resulting in decreased blood supply to the structures. This should be taken into consideration if a patient has to spend a long period of time in a fixed position.

Another cause of tendinopathy is the damage of the tendon tissue by an abnormal abrasion at the narrowed subacromial space.

The so-called calcific tendinitis causes calcification in the shoulder joint; however, this phenomenon usually affects the supraspinatus tendon. In some cases, calcium may infiltrate the subacromial bursa or the joint capsule, resulting in pain and restriction of movements.

Infraspinatus Muscle

The infraspinatus muscle arises from the infraspinous fossa of the scapula and attaches to the middle facet of the greater tubercle of the humerus. It is innervated by the suprascapular nerve (C5-C6) that arises from the upper trunk of Brachial plexus.

Characteristics of the Infraspinatus Muscle

The insertion area of the infraspinatus muscle is grown together with the fibers of the teres minor, thus separating the teres major.

Function of the Infraspinatus Muscle

The infraspinatus muscle is the main external (lateral) rotator at the shoulder joint. It can perform both adduction and abduction since its fiber route passes through the axes differently. Additionally, it reinforces the shoulder by connecting to the dorsal capsule, while centering the head of the humerus in the glenoid.

Clinical Significance of the Infraspinatus Muscle

Unfamiliar short-term movements can irritate the infraspinatus muscle and cause pain. Such movements include the interception of a fall or rapidly reaching backward. Patients thus describe pain during extension and lateral rotation or are no longer able to lie on the affected side. A patient with such symptoms should be advised to protect the affected side and find a position of ease when sleeping.

Teres Minor Muscle

The Teres Minor arises from the middle section of the scapula and is inserted into the caudal impression on the great tubercle. It is innervated by the axillary nerve (C5-C6), arises from the posterior cord of the brachial plexus.

 

Muscle Origin Insertion Nerve supply Function
Infraspinatus Infraspinous fossa Middle facet of greater tubercle of humerus Suprascapular nerve (C5) Laterally rotates shoulder joint. Holds head of humerus in glenoid cavity
Teres minor Later border of scapula, middle portion Inferior facet of greater tubercle of humerus Axillary nerve (C6)

Characteristics of the Teres Minor Muscle

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Picture: “Thoracic Upper Limb Veins” by Phil Schatz. Licence: CC BY 4.0

Some of the deep fibers are grown together with the joint capsule just near the point of muscle attachment and are directly connected to the infraspinatus muscle. Thus, they share the functions of extension and external rotation. It forms one of the boundaries of the quadrangular space that serves as an outer opening for the axillary nerve and the posterior circumflex humeral artery with accompanying veins and lymphatic vessels.

Function of the Teres Minor Muscle

Besides helping hold the humeral head in the glenoid cavity of the scapula and stabilizing the shoulder joint, it also performs the external rotation and the adduction of the arm at the shoulder joint.

Clinical Significance of the Teres Minor Muscle

Since Teres minor forms one of the boundaries of the Quadrangular space, any morphological change, trauma or edema can narrow the space and cause irritation of the passing structures. The irritation of the axillary nerve results in the restriction of mobility of the teres muscle and the deltoid muscle affecting the abduction of the arm. Long-term complications include disorders of the skin sensitivity of the lateral shoulder region.

Subscapularis Muscle

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Picture: Teilansicht: “Muscles That Move the Humerus” by Phil Schatz. License: CC BY 4.0

The subscapularis muscle arises from the subscapularal fossa of the scapula and inserts into the lesser tubercle of the humerus.

Innervation to subscapularis is supplied by the upper and lower subscapular nerves (C5-C6).

Characteristics of the Subscapularis Muscle

The name, the origin, and the innervation of the muscle are identical to the medical term “subscapularis” and therefore are easy to understand. Some deep fibers are grown together with the ventral capsular ligament, which provides stability. Another characteristic of this muscle is that it forms an intermediate tendon in its center, from which the muscle fibers arise. This is where the muscle gains its wide flat constitution.

Function of the Subscapularis Muscle

The subscapularis muscle is the only muscle to allow for internal rotation with the rotator cuff. Aside from preventing the displacement of the humeral head and the ventral dislocation, it also fixates the biceps tendon in the intertubercular groove. It also allows for adduction of the arm at the shoulder joint.

Clinical Significance of the Subscapularis Muscle

If the innervation of the muscle is restricted, the upper extremity falls into an unnatural position of external rotation.

Rotator Cuff: A Look at the Pathology

Rotator cuff tear is the most common clinical manifestation of this muscle group. It usually occurs around the ages of 50 and 60 years due to circulatory disorders, chronic inflammatory processes or injuries. About 95% of partial and full-thickness tears affect the supraspinatus muscle. Traumatology distinguishes several types of rotator cuff tear according to the scale of severity and form:

Classification of the Partial tears by Ellman:

Ellman Grade I Thickness < 1/4 oder < 3mm
Ellman Grade II Thickness < 1/2 oder < 3-6mm
Ellman Grade III Thickness > 6mm

Classification of the Full-Thickness Tears by Bateman:

Bateman Grade I < 1cm
Bateman Grade II 1-3cm
Bateman Grade III 3-5cm
Bateman Grade IV > 5cm

Classification of Full-Thickness Tears by Ellman:

Transverse Tear at the insertion location
Crescent Transverse tear with the deformation through SSC/ISP
L-shaped Transverse tear with an expansion into the intervalbetween SSP and ISP
Reverse L Transverse tear with an expansion into rotator interval
Trapezoidal L-shaped + Reverse L
Massive Tear Full-Thickness Rotator Cuff Tears Expansion into the teres minor or into the anterior SSC

Shoulder Muscles at the End of the Humerus

This muscle group includes the skeletal muscles that are attached to the head of the humerus which perform direct and indirect functions of the shoulder joints. However, their origin is found in the osseous structures and they are not to be included in the rotator cuff.

Deltoid Muscle

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Picture: Teilansicht: “Muscles That Move the Humerus” by Phil Schatz. License: CC-BY 4.0

The deltoid originates in three distinct sets of fibers: acromial, clavicular, and spinal.

All three fibers converge toward their insertion on the deltoid tuberosity and are innervated by the axillary nerve (C5-C6). They are all involved in arm abduction and prevent the dislocation of the humeral head. However, the origin and the secondary functions, such as synergistic movements, of the three fibers should be distinguished.

Acromial fibers arise from the lateral acromion, clavicular fibers arise from the lateral and caudal clavicle, and spinal fibers arise from the caudal part of the spine of the scapula.

Muscle Origin Insertion Nerve supply Function
Deltoid Clavicular head: lateral third of clavicle
Acromial head: acromium
Spinal head: spine of scapula
Deltoid tuberosity of humerus Axillary nerve (C5) Clavicular head: flexes and medially rotates shoulder joint
Acromial head: abducts shoulderSpinal head: extends and laterally rotates shoulder joint
Supraspinatus Supraspinous fossa Superior facet of greater tubercle of humerus Suprascapular nerve (C5,6) Initiates and assists deltoid in abduction

Characteristics of the Deltoid Muscle

The deltoid muscle is the reference muscle for the C5 segment.

Function of the Deltoid Muscle

Due to their extensive origin and routes, the three fibers of the deltoid muscle can perform various secondary functions and synergistic movements. Acromial fibers do not have any secondary functions.

Clavicular fibers
  • Internal rotation of the shoulder joint by adduction
  • Flexion in front of the body
  • Adduction in front of the body
Spinal fibers
  • External rotation of the shoulder joint by internal rotation
  • Extension and adduction behind the back
Clavicular fibers + spinal fibers
  • The last third part of the adduction of the shoulder joint
  • Abduction of the shoulder joint
Spinal fibers + Acromial fibers
  • Flexion of the shoulder joint
  • Extension of the shoulder girdle

Clinical Significance of the Deltoid Muscle

If a (neurological) functional examination discovered the atrophy or even paralysis of the muscle, the causes are of neurological nature and should be searched for in C5 segment.

Coracobrachialis muscle

It arises from the apex of the coracoid process under the short head of the biceps brachii. It is inserted into an impression at the middle of the medial surface and border of the body of the humerus between the origins of triceps brachii and brachialis. It is innervated by the musculocutaneous nerve (C6-C7).

Characteristics of the Coracobrachialis muscle

The muscle creates an outlet opening for the musculocutaneous nerve.

Function of the Coracobrachialis muscle

The Coracobrachialis muscle is the weak flexor and adductor of the arm. It is also responsible for preventing the dislocation of the humerus.

Clinical Significance of the Coracobrachialis muscle

The musculocutaneous nerve can get trapped in the outlet opening if the muscle was morphologically impaired. The continuous irritation of the nerves can lead to the motor dysfunction of biceps brachii and brachialis. Patients will experience disturbances in sensation to the skin on the radial part of the forearm.

Pectoralis Major Muscle

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Picture: Teilansicht: “Muscles That Move the Humerus” by Phil Schatz. License: CC-BY 4.0

Like the deltoid muscle, the pectoralis major muscle originates in three distinct sets of fibers: clavicular fibers, sternal fibers, and abdominal fibers. From this extensive origin the fibers converge toward their insertion into the lateral lip of the bicipital groove of the humerus. It receives the innervation by the medial pectoral nerve and the lateral pectoral nerve (C5-Th1). Its functions include flexion, adduction, and internal rotation.

The clavicular fibers arise from the medial end of the clavicle; the sternal fibers arise from the anterior surface of the sternum to as low as cartilage of the sixth rib; the abdominal fibers arise from the ventral leaf of the rectus sheath.

Muscle Origin Insertion Nerve supply Function
Pectoralis major Clavicular head: Anterior surface of medial clavicle
Sternocostal head: sternum and superior six costal cartilages
Lateral lip of intertubercular sulcus Lateral and medial pectoral nerves (C5, C6–clavicular head; C7, C8–sternocostal head) Adducts and medially rotates shoulder joint; pulls scapula anteriorly and inferiorly
Pectoralis minor Ribs 3–5 Coracoid process of scapula Medial pectoral nerves (C8, T1) Stabilises scapula and pulls it anteriorly and inferiorly
Subclavius Rib 1 and sternum junction Middle third of clavicle Subclavian nerve (C5) Stabilises and depresses clavicle
Serratus anterior External surface of ribs 1–8 Medial border of scapula Long thoracic nerve (C6, C7) Protracts scapula; holds scapula against posterior thoracic wall

Characteristics of the Pectoralis Major Muscle

The pectoralis major muscle forms the axilla, and its fibers rotate 180 degrees. When the arm is lifted, the fascial layer loosens and shifts back to neutral when the arm is dropped, to avoid restraining. The biciptal groove is located between the clavical fibers and the sternal fibers and can be identified by palpation.

Function of the Pectoralis Major Muscle

The main functions of pectoralis major are flexion, adduction, and internal rotation of the arm. At the fixed end of the humerus, the pectoralis major muscle pulls the shoulder girdle in the ventral direction. Due to its location, it belongs to the chest(pectoral) muscle group.

Clinical Significance of the Pectoralis Major Muscle

In post-traumatic damage of the brachial plexus, it is possible to partially transpose the pectoralis major muscle to restore lost arm functions . Yet, it is considered low-risk and shows a low rate of complications.

Teres Major Muscle

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Picture: Partial view: “Muscles That Move the Humerus” by Phil Schatz. License: CC BY 4.0

The teres major muscle arises from the oval area on the dorsal surface of the inferior angle of the scapula and inserts into the medial lip of the intertubercular sulcus of the humerus near the insertion area of the latissimus dorsi muscle. It is innervated by the thoracodorsal nerve (C6-C7). It is responsible for the extension, adduction and internal rotation of the arm.

Muscle Origin Insertion Nerve supply Function
Teres major Later border of scapula, inferior portion Medial lip of intertubercular groove Lower subscapular nerve (C6) Adducts and medially rotates shoulder joint
Subscapularis Subscapular fossa Lesser tubercle of humerus Upper and lower subscapular nerves (C6) Medially rotates and adducts shoulder joint. Holds head of humerus in glenoid cavity

Characteristics of the Teres Major Muscle

Because of its location the teres major muscle is often called ‘the younger brother of latissimus’. It is synergistic with the latissimus dorsi muscle.

Function of the Teres Major Muscle

The main functions of Teres Major are the extension, adduction and internal rotation of the arm. At the fixed end of the arm, the teres major pulls the scapula outwards.

Clinical Significance of the Teres Major Muscle

The teres major muscle forms the boundaries of both the quadrangular and triangular axillary spaces. The circumflex scapular artery and the axillary nerve pass through the quadrangular space, while the profunda brachii artery and radial nerve pass through the triangular axillary space.

Latissimus Dorsi Muscle

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Picture: Partial view: “Muscles That Move the Humerus” by Phil Schatz. License: CC-BY 4.0

The latissimus dorsi muscle originates in four distinct sets of fibers: scapular fiber, costal fiber, vertebral fiber and iliac fiber. All four fibers converge toward their insertion at the middle of the medial surface and border of the body of the humerus. They are innervated by the thoracodorsal nerve (C6-C8). The muscle performs extension, adduction and internal rotation.

The scapular fiber arises from the inferior angle of the scapula; the costal fiber originates between the tenth and the twelfth ribs, while the vertebral fiber arises from the thoracolumbar fascia and vertebrae Th7-Th12 and L1-L5.

Muscle Origin Insertion Nerve supply Function
Trapezius Descending part: superior nuchal line and external occipital protuberance
Transverse part: nuchal ligament
Ascending part: C7–T12 spinous processes
Lateral third of clavicle and spine of scapula Spinal accessory nerve (CN XI) and C3, C4 spinal nerves for proprioception Descending part: elevates scapula
Transverse part: retracts
Ascending part: depresses
Descending and ascending: rotate scapula superiorly
Latissimus dorsi T6–12 spinous processes, thoracolumbar fascia and iliac crest Floor of intertubercular groove Thoracodorsal nerve (C6, C7) Extends, adducts and medially rotates shoulder joint

Characteristics of the Latissimus Dorsi Muscle

Together with teres major, Latissimus dorsi rotates 180 degrees from its insertion point.

Function of the Latissimus Dorsi Muscle

The Latissimus dorsi muscle performs extension, adduction and internal rotation of the arm.

At the fixed end of the arm, the scapular fibers pull the scapula outwards. The costal fibers assist as an accessory muscle of inspiration and prevent the rib cage from expanding during coughing. Due to its location, it belongs to a group of pelvic wall muscles.

Clinical Significance of the Latissimus Dorsi Muscle

The Latissimus dorsi muscle is used for the breast reconstruction surgery in the so-called latissimus dorsi flap procedure.

Muscles of the Upper Arm

Compared to the group of shoulder muscles, the group of upper arm muscles is significantly smaller. It consists of ventral and dorsal muscle groups, each represented by only one muscle – the biceps brachii muscle as a flexor and the triceps brachii muscle as an extensor, respectively. The muscles of the upper arm are named from their origin on the humerus. However, this can quickly lead to confusion, since the brachioradialis muscle, for example, also originates on the humerus but belongs to the group of the radial muscles of the forearm. Literature provides different information, so medical students should learn from the latest books and muscle tables to avoid misunderstanding and confusion.

Biceps Brachii Muscle

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Picture: Partial view: “Muscles That Move the Humerus” by Phil Schatz. License: CC-BY 4.0

The biceps brachii is a bifurcated skeletal muscle with fibers of two different origins. The longer head arises from the supraglenoid tubercle, while the shorter head arises from the coracoid process.

One tendon inserts into the radial tuberosity, while the other one, the bicipital aponeurosis, attaches to the antebrachial fascia. It is supplied by the musculocutaneous nerve (C5,6). Both heads perform flexion and supination of the elbow.

Characteristics of the Biceps Brachii Muscle

Both bifurcated heads converge near the insertion of the deltoid muscle at the deltoid tuberosity.

Function of the Biceps Brachii Muscle

Besides their primary functions – flexion and supination of the elbow – both heads can separately perform (or synergize) different movements.

Muscle Origin Insertion Nerve supply Function
Biceps brachii Long head: supraglenoid tubercle
Short head: coracoid process
Tuberosity of radius and forearm fascia via bicipital aponeurosis Musculocutaneous nerve (C6) Supinates forearm, and once supinated flexes elbow joint. Flexes shoulder
Brachialis Distal half of humerus Coronoid process and tuberosity of ulna Musculocutaneous nerve (C6) Flexes elbow joint
Coracobrachialis Coracoid process Middle third of humerus Musculocutaneous nerve (C6) Flexes and adducts shoulder joint

Clinical Significance of the Biceps Brachii Muscle

The most common injuries of the biceps brachii are the tears in the muscles. Two types of ruptures are distinguished: the proximal rupture and the less common distal rupture. Proximal rupture can be surgically repaired. The torn tendon is sewed or fixated to the brachialis muscle.

Triceps brachii muscle

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Picture: Partial view: “Muscles That Move the Humerus” by Phil Schatz. License: CC BY 4.0

The triceps brachii is a three-headed skeletal muscle with fibers of three different origins.

The long head arises from the infraglenoid tubercle of the scapula.

The lateral head arises from the dorsal surface of the surgical neck of the humerus up to the deltoid tuberosity, from the lateral radial sulcus and the lateral intermuscular septum.

The medial head arises from the dorsal surface of the humerus, from the medial and distal radial sulcus to the olecranon fossa and from the medial and lateral intermuscular septum.

All three heads converge toward their insertion onto the olecranon process of the ulna and are innervated by the radial nerve (C6-C8). Their primary function is the extension of the elbow.

 

Muscle Origin Insertion Nerve supply Function
Triceps brachii Long head: infraglenoid tubercle
Lateral head: posterior humerus, superior to radial groove
Medial head: posterior humerus, inferior to radial groove
Olecranon Radial nerve (C7, C8) Extensor of elbow joint
Long head also extends shoulder joint
Anconeus Lateral epicondyle of humerus Olecranon (lateral) Radial nerve

Characteristics of the Triceps brachii muscle

The long head of the triceps brachii is a part of various axillary spaces. The biggest part creates the quadrangular space, which transmits the axillary nerve, and the triangular space, through which the radial nerve and the deep artery of the arm (profunda brachii artery) pass.

Function of the Triceps brachii muscle

The primary function of triceps brachii muscle is the extension of the elbow.

Along with its primary function, the long head of the triceps brachii is synergistic with the latissimus dorsi and teres major, which act as adductors and extensors of the arm at the shoulder joint. It, however, is also capable of the external rotation of the shoulder joint.

Clinical Significance of the Triceps brachii muscle

Similar to the biceps brachii, the triceps is also prone to tears, which must be surgically treated.

The Characteristics of the Upper Extremities: Common Exam Questions

Keys are located below the source of references.

1. Which of the following muscles do not belong to the rotator cuff?

  1. The supraspinatus muscle
  2. The infraspinatus muscle
  3. The teres minor muscle
  4. The subscapularis muscle
  5. The biceps brachii muscle

2. Which of the following muscles is most frequently affected by the rotator cuff tear?

  1. The supraspinatus muscle
  2. The infraspinatus muscle
  3. The teres minor muscle
  4. The subscapularis muscle
  5. The biceps brachii muscle

3. Which of the following functions does the biceps brachii muscle not perform?

  1. Flexion of the elbow
  2. Supination of the elbow
  3. Extension of the shoulder joint
  4. Flexion of the shoulder joint
  5. External rotation of the shoulder joint
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