Anatomy, Shoulder and Upper Limb, Muscles

Osama Javed; Kenia Maldonado; Roman Ashmyan

Anatomy, Shoulder and Upper Limb, Muscles

Free Review Questions

Introduction

The upper limb comprises many muscles which are organized into anatomical compartments. These muscles act on the various joints of the hand, arm, and shoulder, maintaining tone, providing stability and allowing precise fluid movement.

Axioappendicular groups of muscles arise from the axial skeleton to act upon the pectoral girdle. Scapulohumeral muscles originate from the scapula and insert into the proximal humerus. Included in this category are the rotator cuff muscles which provide stability to the glenohumeral joint. In the arm, the muscles of the anterior compartment are involved in flexion of the forearm, and the posterior comprises of the forearm extensors. Similarly, the anterior compartment of the forearm contains the flexors of the hand and posterior has extensors. The hand is divided into the thenar, the hypothenar, the adductor compartment, as well as the short muscles of the hand.[1][2][3][4]

Structure and Function

Shoulder and Upper limb muscles are specialized to perform functions of pressure and manipulation of objects. Skeletal muscle is formed by myofibers which contain millions of myofibrils, and each of them is formed by sarcomeres. Sarcomeres are the contractile unit, therefore skeletal muscle fibers are completely dedicated to generating force. Skeletal muscle also carries out multiple functions such as voluntary locomotion, the protection of internal organs, generation of heat, and assisting in postural behavior. [5][6]

The muscle mass relies on the balance between synthesis and degradation of proteins, the regulation of this process is sensitive to several factors such as nutritional status, hormone levels, physical activity, underlying diseases, injuries, among others.[7]

Embryology

Mesenchyme (mesodermal in origin) condenses into sets of dermatomes and myotome complexes. Myotomes migrate into the developing limb buds, to give rise to myoblasts. Elongation of the limb buds, along with muscle formation from myoblasts, compartmentalizes the muscles into their respective muscle groups.

Blood Supply and Lymphatics

The arterial supply to the muscles of the upper limb is primarily from the axillary artery (of subclavian artery) and its branches. The brachial artery supplies to the anterior compartment of the arm, and the profunda brachii supplies the posterior. In the arm, the radial artery supplies the lateral forearm and the ulnar is responsible for the medial aspect. Beyond the wrist, the radial and ulnar arteries form the superficial and deep palmar arterial arches. The deoxygenated blood drains into the cephalic vein and the basilic vein. Lymphatics of the right upper limb drain into the right lymphatic duct, and the left drains into the thoracic duct.[4][8][9]

Nerves

The innervation of the muscles of the upper limb is through branches of the brachial plexus which is composed of the ventral rami of C5 through T1 nerve roots.[10][11]

Muscles

Anterior Axioappendicular Muscles (Thoracoappendicular Muscles)[12]

Pectoralis major

Function: flexion, adduction, medial rotation of the humerus.
Origin: clavicular head: medial clavicle anteriorly, sternocostal head: anterior sternum and costal cartilages of ribs 1 to 6 as well as external oblique aponeurosis
Insertion: lateral edge of intertrabecular groove of humerus
Innervation: medial pectoral nerve (C8, T1) lateral pectoral nerve (C5, C6, C7) of brachial plexus

Pectoralis minor

Function: Depression of shoulder, protraction of scapula
Origin: Third, fourth, fifth ribs close to their respective costal cartilages
Insertion: Coracoid process
Innervation: Medial pectoral nerve (C8, T1)

Subclavius

Function: Depression and stabilization of clavicle
Origin: First rib medially
Insertion: Middle of clavicle, inferiorly
Innervation: Nerve to subclavius (C5, C6)

Serratus anterior

Function: Protraction of scapula, rotation of scapula
Origin: Lateral first to the eighth rib
Insertion: anterior scapula, medially
Innervation: long thoracic nerve (C5, C6, C7)

Posterior Axioappendicular Muscles

Superficial layer

Latissimus dorsi

Function: Adduction, medial rotation, extension of humerus
Origin: Spinous processes of seventh to 12th thoracic vertebrae, iliac crest, thoracolumbar fascia, and inferior third and fourth rib
Insertion: Intertubercular groove of humerus
Innervation: Thoracodorsal nerve (C5,C6,C7)

Trapezius

Function: Elevation, depression, and retraction of the scapula, rotation of glenoid cavity
Origin: Superior nuchal line, nuchal ligament, occipital protuberance, spinous processes of C7- T12
Insertion: Spine of scapula, acromion, and lateral clavicle
Innervation: CN XI

Deep Layer

Levator scapulae

Function: Adduction, medial rotation, extension of humerus
Origin: Transverse processes of C1 through C4 vertebrae
Insertion: Scapula at its medial border
Innervation: Thoracodorsal nerve (C5, C6, C7)

Rhomboid major

Function: Retraction of scapula and depression of glenoid cavity
Origin: Spinous processes of T2 through T5 vertebrae
Insertion: Inferior aspect of medial scapula
Innervation: Dorsal scapular nerve (C4, C5)

Rhomboid minor

Function: Retraction of scapula and depression of glenoid cavity
Origin: Nuchal ligament as well as spines of C7 and T1 vertebrae
Insertion: Superior aspect of medial scapula
Innervation: Dorsal scapular nerve (C4, C5)

Scapulohumeral (Intrinsic Shoulder Muscles)

Deltoid

Function: Anterior part: flexion and medial rotation of arm, middle part: abduction of arm, posterior part: extension and lateral rotation of arm
Origin: Lateral clavicle, acromion and scapular spine
Insertion: Deltoid tuberosity
Innervation: Axillary nerve (C5, C6)

Teres major

Function: Adduction and medial rotation of arm
Origin: Posterior surface of scapula at its inferior angle
Insertion: Intertubercular groove on its medial aspect
Innervation: Lower scapular nerve (C5, C6)

Supraspinatus

Function: Initiation of arm abduction
Origin: Posterior scapula, superior to the scapular spine
Insertion: Superior aspect of the greater tubercle
Innervation: Suprascapular nerve (C5, C6)
Part of rotator cuff muscles

Infraspinatus

Function: Lateral rotation of arm
Origin: Posterior scapula, inferior to the scapular spine
Insertion: Greater tubercle of humerus, between supraspinatus and teres minor insertion
Innervation: Suprascapular nerve (C5, C6)
Part of rotator cuff muscles

Teres minor

Function: Lateral rotation of arm
Origin: Posterior surface of scapula at its inferior angle
Insertion: Inferior aspect of the greater tubercle
Innervation: Axillary nerve (C5, C6)
Part of rotator cuff muscles

Subscapularis

Function: Adduction and medial rotation of the arm
Origin: Anterior aspect of scapula
Insertion: Lesser tubercle of humerus
Innervation: Subscapular nerves (C5, C6, C7)
Part of rotator cuff muscles

*Rotator cuff muscles: supraspinatus, infraspinatus, teres minor, subscapularis

Muscles of Anterior Compartment of Arm (Flexors of Arm)

Biceps brachii

Function: Major flexion of forearm, supination of forearm, resists dislocation of shoulder
Origin: Short head originates from the coracoid process. The long head is from the supraglenoid tubercle of scapula
Insertion: Radial tuberosity and forearm fascia (as bicipital aponeurosis)
Innervation: Musculocutaneous nerve (C5, C6)

Brachialis

Function: Flexion of forearm
Origin: Distal anterior humerus
Insertion: Coronoid process and ulnar tuberosity
Innervation: musculocutaneous nerve (C5, C6, C7 small contribution)

Coracobrachialis

Function: Flexion and adduction of arm
Origin: Coracoid process
Insertion: Middle of the humerus, on its medial aspect
Innervation: Musculocutaneous nerve (C5, C6, C7)

Muscles of Posterior Compartment of Arm (Extensors of Arm)

Triceps brachii

Function: Major extensor of forearm, resists dislocation of shoulder
Origin: Lateral head: above the radial groove, medial head: below the radial groove, long head: infraglenoid tubercle of scapula
Insertion: Olecranon process of ulna and forearm fascia
Innervation: Radial nerve (C6,C7,C8)

Anconeus

Function: Extension of forearm, stabilization of elbow joint
Origin: Lateral epicondyle of humerus
Insertion: Olecranon process and posterior ulna
Innervation: Radial nerve (C7, C8, T1)

Muscles of Anterior Compartment of Forearm (Flexors of Forearm)

Superficial layer

Pronator teres

Function: Pronation of radio-ulnar joint
Origin: Coronoid process and medial epicondyle of humerus
Insertion: Lateral surface of the radius
Innervation: Median nerve (C6, C7)

Flexor carpi radialis

Function: Flexion and adduction at the wrist
Origin: Medial epicondyle of humerus
Insertion: Base of second metacarpal
Innervation: Median nerve (C6, C7)

Palmaris longus

Function: Flexion at the wrist, tensing of the palmaris aponeurosis
Origin: Medial epicondyle of humerus
Insertion: Flexor retinaculum
Innervation: Median nerve (C7, C8)

Flexor carpi ulnaris

Function: Flexion and adduction at the wrist
Origin: Medial epicondyle of humerus and olecranon
Insertion: Pisiform, hook of hamate and fifth metacarpal
Innervation: Median nerve (C7, C8)

Intermediate Layer

Flexor digitorum superficialis

Function: Flexion of the proximal interphalangeal joint of the second, third, fourth, and fifth finger. Also has a weaker flexion action on the metacarpophalangeal joints of the same fingers
Origin: Medial epicondyle, coronoid process, and anterior radius
Insertion: Second, third, fourth, and fifth middle phalanges
Innervation: Median nerve (C7, C8, T1)

Deep Layer

Flexor digitorum profundus

Function: Flexion of the distal interphalangeal joint of the second, third, fourth, and fifth finger
Origin: Medial and anterior surface of proximal ulna and interosseous membrane
Insertion: Second, third, fourth, and fifth distal phalanges
Innervation: Ulnar nerve (C8, T1) for the medial part, anterior interosseous nerve (C8,T1) for the lateral

Flexor pollicis longus

Function: Flexion of the interphalangeal joint of the thumb
Origin: Anterior aspect of radius as well as interosseous membrane
Insertion: Base of distal phalanx of thumb
Innervation: Anterior interosseous nerve (C7, C8)

Pronator quadratus

Function: Pronator of forearm
Origin: Anterior aspect of distal ulna
Insertion: Anterior aspect of distal radius
Innervation: Anterior interosseous nerve (C7, C8)

Brachioradialis

Function: Weak flexor of the forearm
Origin: Proximal supracondylar ridge on humerus
Insertion: Lateral surface of distal end of radius
Innervation: Radial nerve (C5, C6, C7)

Muscles of Posterior Compartment of Forearm

Superficial

Extensor carpi radialis longus

Function: Extension and abduction of the wrist
Origin: Proximal supracondylar ridge on humerus
Insertion: Dorsal base of second metacarpal
Innervation: Radial nerve (C6, C7)

Extensor carpi radialis brevis

Function: Extension and abduction of the wrist
Origin: Lateral epicondyle of humerus
Insertion: Dorsal base of third metacarpal
Innervation: Deep branch of the radial nerve (C7, C8)

Extensor digitorum

Function: Extension of the proximal interphalangeal joint of the second, third, fourth, and fifth finger. Also has a weaker extension action on the metacarpophalangeal joints of the same fingers
Origin: Lateral epicondyle of humerus
Insertion: Extensor expansions on dorsal aspect of second, third, fourth, and fifth middle and distal phalanges
Innervation: Posterior interosseous nerve (C7, C8)

Extensor digiti minimi

Function: Extension of the little finger at metacarpophalangeal joint and interphalangeal joint
Origin: Lateral epicondyle of humerus
Insertion: Extensor expansion on dorsal aspect of fifth phalanx
Innervation: Posterior interosseous nerve (C7, C8)

Extensor carpi ulnaris

Function: Extension and adduction of the wrist
Origin: Lateral epicondyle of humerus and posterior ulna
Insertion: Fifth metacarpal base
Innervation: Posterior interosseous nerve (C7, C8)

Deep Layer

Extensor indicis

Function: Extension of the index finger
Origin: Dorsal surface of distal ulna and interosseous membrane
Insertion: Extensor expansion of second finger
Innervation: Posterior interosseous nerve (C7, C8)

Supinator

Function: Supination of the forearm
Origin: Lateral epicondyle and supinator crest of ulna
Insertion: Lateral surface of radius
Innervation: Deep branch of radial nerve (C7, C8)

Abductor pollicus longus

Function: Abduction of the thumb by acting on the carpometacarpal joint and the metacarpophalangeal joint
Origin: Dorsal aspects of proximal radius, ulna, and interosseous membrane
Insertion: Base of first metacarpal
Innervation: Posterior interosseous nerve (C7, C8)

Extensor pollicus longus

Function: Extension of the thumb by acting on the carpometacarpal joint, the metacarpophalangeal joint, and the interphalangeal joint.
Origin: Dorsal aspects of middle ulna and interosseous membrane
Insertion: Distal phalanx of 1st finger
Innervation: Posterior interosseous nerve (C7, C8)

Extensor pollicus brevis

Function: Extension of the thumb by acting on the carpometacarpal joint and the metacarpophalangeal joint
Origin: Dorsal aspects of middle radius and interosseous membrane
Insertion: Distal phalanx of 1st finger
innervation: Posterior interosseous nerve (C7, C8)

Intrinsic Muscles of Hand

Thenar muscles

Opponens pollicus

Function: Opposition of the thumb
Origin: Flexor retinaculum and tubercle of trapezium
Insertion: Lateral aspect of first metacarpal
Innervation: Recurrent branch of median nerve (C8, T1)

Abductor pollicus brevis

Function: Abduction of the thumb at the metacarpophalangeal joint
Origin: Flexor retinaculum and tubercle of scaphoid
Insertion: Lateral aspect of proximal phalanx of first finger
Innervation: Recurrent branch of median nerve (C8, T1)

Flexor pollicus brevis

Function: Flexion of the thumb at the metacarpophalangeal joint
Origin: Flexor retinaculum and tubercle of trapezium
Insertion: Lateral aspect of proximal phalanx of first finger
Innervation: Recurrent branch of median nerve (C8, T1)

Adductor Compartment

Adductor pollicus

Function: Adduction of the thumb
Origin: Second, third metacarpal, and capitate
Insertion: Proximal phalanx and extensor expansion of 1st finger
Innervation: Deep branch of ulnar nerve (C8, T1)

Hypothenar Muscles

Abductor digiti minimi

Function: Abduction of the little finger at the metacarpophalangeal joint
Origin: Pisiform
Insertion: Medial aspect of proximal phalanx of fifth finger
Innervation: Deep branch of ulnar nerve (C8, T1)

Flexor digiti minimi brevis

Function: Flexion of the little finger at the metacarpophalangeal joint
Origin: Flexor retinaculum and hook of hamate
Insertion: Medial aspect of proximal phalanx of fifth finger
Innervation: Deep branch of ulnar nerve (C8, T1)

Opponens digiti minimi

Function: Opposition of the little finger
Origin: Flexor retinaculum and hook of hamate
Insertion: Medial aspect of fifth metacarpal
Innervation: Deep branch of ulnar nerve (C8, T1)

Short Muscles

Lumbricals

Function: Flexion of the metacarpophalangeal joints with extension of the interphalangeal joints
Origin: Arise from tendons of flexor digitorum profundus. First 2 are unipennate, and the third and fourth are bipennate
Insertion: Extensor expansions of second, third, fourth, and fifth finger
Innervation: Median nerve (C8, T1) for the lateral 2 lumbricals, deep branch of ulnar nerve (C8, T1) for the medial 2 lumbricals

Dorsal interossei

Function: Abduction of the second, third, and fourth finger away from the axial line
Origin: Adjacent metacarpals
Insertion: Extensor expansions and proximal phalanges of the second, third, and fourth fingers
Innervation: Deep branch of ulnar nerve (C8, T1)

Palmar interossei

Function: Adduction of the second, third, and fourth finger towards the axial line
Origin: Palmar surfaces of second, fourth, and fifth metacarpals
Insertion: Extensor expansions and proximal phalanges of the second, fourth, and fifth fingers
Innervation: Deep branch of ulnar nerve (C8, T1)

Physiologic Variants

The biceps brachii is one of the most variable muscles in the human body, this muscle can have more than two heads arising from humerus at the insertion of the coracobrachialis or neck of the humerus. Some reports describe supernumerary bicipital heads fluctuating from 3 to 7 in different groups, being the 3 heads variant the most common type.

Palmaris longus variations have also been described. The reversed palmaris longus is a polymorphic variation where the muscle is fleshy distally and tendinous proximally. Other case reports have documented a variation in its distal attachment of the muscle to the tendon of the adductor pollicis brevis, and long flexor; or to the flexor carpis ulnaris muscle, and muscle of the hypothenar eminence; as well as to the fascias of the forearm or the hypothenar eminence, and at the metacarpophalangeal joint. [13]

Surgical Considerations

Upper extremity injuries are usually treated in non-operative modalities such as medications, splints, injection, physical therapy. The surgical consideration of the muscles of the upper limb depends on the underlying condition.

The surgical purpose in upper limb dysfunction due to brain injury is to decrease muscle spasticity, correct joint contractures, as well as enhance the appearance, and function of the extremity. Some of the approaches to increase or decrease the muscle tone imply the peripheral nerve surgery or lengthening hyperactive muscles.

In patients with spinal cord injuries some of the surgical techniques are aimed to restore elbow rectification, and grasp of the upper limb.

Some injuries where fractures and destruction of soft tissues (skin and muscles) are present, a multidisciplinary approach is required in which the orthopedic surgeons work with plastic surgeons to treat open fractures of the extremities, osteomyelitis, or unstable scars correctly.

Clinical Significance

Rotator Cuff Disease

Involves impingement, tendonitis, as well as tearing of the tendons of the muscles of the rotator cuff. Majority of the cases involve the tendon of the supraspinatus muscle. This is thought to be due to its poor blood supply. The patient complains of pain, especially while lying down on the affected arm or when doing overarm activities. In the clinic, it can be tested by the Hawking’s test and Neer test. The drop test is confirmatory. An MRI is also advised to rule out or confirm a tendon tear. Treatment depends on severity. Management involves NSAIDs, physiotherapy, and arthroscopic repair.

Poland Syndrome[14]

This is the congenital ipsilateral absence or hypoplasia of pectoralis major and pectoralis minor muscles with hypoplasia of the corresponding ribs. It is hypothesized to be caused by an in-utero defect of blood supply to the developing chest. Poland syndrome is commonly associated with defects in breast and/or upper limb development.

Winging of the Scapula

Denervation of the serratus anterior muscle causes palsy of the long thoracic nerve. This causes lateral and posterior movement of the scapula, away from the underlying ribs, giving it a wing-like appearance.

Epicondylitis

Lateral epicondylitis (tennis elbow) is caused by a combination of repetitive or sustained contraction of the extensor muscles of the forearm leading to inflammation of the common extensor origin. Medial epicondylitis (golfers elbow) is due to repetitive or sustained contraction of the flexor muscles of the forearm leading to inflammation of the common flexor origin. Patients present with pain and tenderness over the affected epicondyle that worsens with extension (in the case of lateral epicondylitis) or flexion (when suspecting medial epicondylitis). Treatment involves avoiding exacerbating activities, physical therapy, and pain relief.

Other Issues

Traumatic injury, malignancy, infection, or congenital deformity of the upper extremity can lead to amputation. Surgeons have different surgical options to improve the potential of using prosthetic technologies for this group of patients. Targeted muscle reinnervation is a surgical procedure to enhance the control of myoelectric upper limb prostheses, and it helps to prevent and treat painful postamputation neuromas. This technique was originally described for transhumeral amputations and shoulder disarticulations but nowadays it has also been applied in the treatment of transtibial, transfemoral, transradial, and partial hand amputees. [15][16]

(Click Image to Enlarge)

Left Shoulder Anatomy. This image shows the anatomic relationships between the clavicle, scapula, scapular neck, foramen, and superior border, coracoid process, acromion, greater and lesser humeral tubercles, humerus and anatomical neck, tendon of the biceps brachii long head, and the coracoclavicular (with the conoid and trapezoid), coracoacromial, superior acromioclavicular, coracohumeral, and capsular ligaments.