Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Musculoskeletal System
Muscular System
Muscles of the Upper Extremities

Muscles of the shoulder girdle. The shoulder girdle, which anchors the free upper limb to the trunk, connects to it via a single sternoclavicular joint. The stabilization of the shoulder girdle is provided by the trunk-originating muscles discussed previously. These include the trapezius, pectoralis minor, rhomboideus, serratus anterior, and levator scapulae muscles. Here, we will describe the muscles located directly on the shoulder girdle itself, which mobilize and secure the free upper limb at the shoulder joint. These comprise the deltoid, supraspinatus, infraspinatus, teres minor, teres major, and subscapularis muscles (Atlas Figs. 24-27, 34, 35).

The deltoid Muscle (m. deltoideus), together with the ball-and-socket shoulder joint, gives the human shoulder its rounded contour (Atlas Figs. 24-27, 34). The muscle originates from the acromial end of the clavicle, the spine and acromion of the scapula, and inserts into the deltoid tuberosity of the humerus. A synovial bursa lies beneath the muscle, sometimes communicating with the shoulder joint cavity. When contracted, the anterior fibers of the muscle participate in flexing the arm at the shoulder joint, the posterior fibers in extending it, while the middle fibers and the muscle as a whole abduct the arm to a horizontal position, after which the humerus abuts against the coracoacromial arch, halting further movement in the joint.

The supraspinatus muscle (m. supraspinatus) originates from the supraspinous fossa of the scapula and the dense fascia covering it, inserting into the apex of the greater tubercle of the humerus (Atlas Figs. 35, 41). This muscle acts as a synergist to the deltoid, though it is only capable of abducting an unloaded arm, albeit more rapidly.

The infraspinatus muscle (m. infraspinatus) originates from the infraspinous fossa of the scapula and the dense fascia overlying the muscle, inserting into the greater tubercle of the humerus (Atlas Figs. 35, 41). The muscle laterally rotates the arm.

The teres minor muscle (m. teres minor) lies beneath the preceding one (Atlas Figs. 27, 35, 41). It originates from the lateral border of the scapula, inserts into the greater tubercle of the humerus, and Functions as a synergist to the infraspinatus muscle.

The teres major muscle (m. teres major) originates from the inferior angle of the scapula and inserts, together with the latissimus dorsi, into the crest of the lesser tubercle of the humerus (Atlas Figs. 25, 27, 35, 41). The muscle medially rotates the arm.

The subscapularis muscle (m. subscapularis) originates from the entire costal surface of the scapula and inserts into the lesser tubercle of the humerus (Atlas Fig. 41). A small synovial bursa, which pouches out from the shoulder joint cavity, lies beneath the muscle. The muscle medially rotates the arm.

The supraspinatus, infraspinatus, teres minor, and subscapularis muscles, located in close proximity to the shoulder joint, fuse with its Joint Capsule. By contracting, they tense the capsule and prevent it from becoming impinged.

Muscles of the arm. The arm region contains two muscle groups: the anterior group (consisting of flexors) and the posterior group (consisting of extensors of the arm at the Shoulder and Elbow joints). These muscles are surrounded by the brachial fascia, which forms distinct sheaths around each group separated by intermuscular septa. The latter extend inward from the brachial fascia and fuse with the humerus.

músculos anteriores The anterior group is formed by the coracobrachialis, biceps brachii, and brachialis muscles, while the posterior group comprises the triceps brachii and anconeus.

The coracobrachialis muscle (m. coracobrachialis), originating from the coracoid process, inserts into the anterior surface of the middle third of the arm and flexes the arm at the shoulder joint (Atlas Fig. 41).

The biceps brachii muscle (m. biceps brachii) arises by its short HEAD from the coracoid process, together with the coracobrachialis. Its long head originates within the joint cavity from the supraglenoid tubercle of the scapula (Atlas Figs. 24-27, 41). Having passed through the capsule, the tendon of the long head lies in the intertubercular sulcus, surrounded by an extension of the synovial layer of the capsule, thereby preserving the hermetic seal of the joint. Lower down, the two heads unite. After crossing the elbow joint, the muscle inserts into the radial tuberosity; a synovial bursa is located here between the tendon and the tuberosity. Some tendon fibers blend into the antebrachial fascia, significantly reinforcing it (see Atlas). The muscle flexes the arm at the shoulder and elbow joints and supinates the forearm (Fig. 1.57).

The brachialis muscle (m. brachialis) originates from the lower two-thirds of the anterior surface of the humerus and from the medial and lateral intermuscular septa, inserting into the ulnar tuberosity (Atlas Figs. 27, 41). The muscle flexes the forearm.

The triceps brachii muscle (m. triceps brachii), located on the posterior surface of the arm, acts as an antagonist to the anterior group muscles. Of the muscle's three heads, the long head arises from the infraglenoid tubercle of the scapula; the lateral head (which is more powerful) and the medial head (which is weaker) originate from the posterior aspect of the humerus and the intermuscular septa, flanking the long head. The muscle inserts via a common tendon into the olecranon of the ulna. The triceps brachii extends the arm at the elbow joint, and its long head also extends it at the shoulder joint.

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Fig. 1.57. Muscles producing pronation (A) and supination (B) of the forearm and hand:

1 — pronator teres; 2 — pronator quadratus; 3 — biceps brachii; 4 — supinator

The olecranon remains uncovered by muscles; a subcutaneous synovial bursa is situated between it and the Skin.

The anconeus muscle (m. anconaeus) is small and triangular. Originating from the lateral epicondyle of the humerus, it runs obliquely inward; it is covered by the dense antebrachial fascia, from which it partially arises, and inserts into the posterior border of the ulna (Atlas Fig. 41). Like the triceps, this muscle extends the arm at the elbow joint. By fusing with the capsule of the elbow joint, the muscle pulls it taut.

Muscles of the forearm. Two muscle groups are distinguished in the forearm region: anterior and posterior; the former contains flexors and pronators, while the latter contains extensors and supinators.

All these muscles are covered by the common antebrachial fascia, which forms a dense compartment around them that sends intermuscular septa inward, separating the anterior and posterior groups. The adjacent muscles originate from these septa as well as from the fascia itself. Passing from the forearm to the hand, particularly thickened areas of the fascia form the PALMAR AND DORSAL carpal ligaments (Atlas Fig. 42).

Distal to the palmar ligament, the fascia thickens into the transverse carpal ligament (see Atlas), which attaches to the margins of the carpal arch, converting it into an osteofibrous canal of the hand. Beneath all these ligaments, the tendons of the forearm muscles pass into the hand.

The muscles of the anterior and posterior groups form superficial and deep layers. In the Superficial layer of the anterior group, the muscles lie, starting from the radial border of the forearm, in the following order: pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, and flexor carpi ulnaris (Atlas Figs. 27, 42). All of them originate from the medial epicondyle of the humerus, the fascia, and the medial intermuscular septum.

The pronator teres muscle (m. pronator teres) runs obliquely downward and inserts into the anterolateral surface of the middle third of the radial diaphysis. The muscle pronates the forearm, i.e., rotates the radius and the hand connected to it inward (Fig. 1.57). In doing so, the radius crosses anterior to the stationary ulna, and the palm of the hand faces backward.

The flexor carpi radialis muscle (m. flexor carpi radialis) is situated obliquely and inserts into the Base of the II metacarpal bone. The muscle flexes the hand and partially pronates the forearm.

The palmaris longus muscle (m. palmaris longus) is vestigial and may be absent. It has a small belly and a long tendon that expands into a broad palmar aponeurosis, which fuses with the skin of the palm. The muscle tightens the palmar skin and assists in wrist flexion.

The flexor digitorum superficialis muscle (m. flexor digitorum superficialis) features a broad muscular belly that divides in the lower half of the forearm into four tendons; after passing through the osteofibrous carpal tunnel, each tendon splits and attaches via two slips to the lateral surfaces of the middle Phalanges of digits II–V. This muscle flexes the middle phalanges and contributes to wrist flexion.

The flexor carpi ulnaris muscle (m. flexor carpi ulnaris) wraps its tendon around the pisiform bone and inserts into the base of the fifth metacarpal bone. The muscle flexes the wrist.

In addition to these functions, all superficial forearm muscles flex the elbow joint.

The Deep Layer of the anterior group comprises: the flexor pollicis longus, the flexor digitorum profundus, and the pronator quadratus (Atlas, Fig. 42).

The flexor pollicis longus muscle (m. flexor pollicis longus) lies in the deep layer most laterally. Originating from the anterior surface of the radius, it inserts into the distal phalanx of the thumb, flexing both the distal phalanx and the thumb as a whole.

The flexor digitorum profundus muscle (m. flexor digitorum profundus) originates from the anterior surface of the ulna and the interosseous membrane. Dividing into four tendons, it enters the hand through the osteofibrous carpal tunnel and inserts into the distal phalanges of digits II–V, having previously passed between the tendinous slips of the flexor digitorum superficialis (see Atlas). The muscle flexes the distal phalanges and partially assists in wrist flexion.

The pronator quadratus muscle (m. pronator quadratus) lies in the distal part of the forearm, directly on the bones. It originates from the anterior surface of the ulna and inserts into the anterolateral surface of the radius. This muscle acts as a synergist to the pronator teres.

Notable among the superficial layer of the posterior group are the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis, which run along the radius and form the lateral muscle complex of the forearm (see Atlas).

The brachioradialis muscle (m. brachioradialis) originates from the lateral surface of the humerus, crosses the elbow joint, runs along the radius, and inserts into its styloid process. The muscle supinates the forearm when it is in a pronated position (which is why it was formerly called the long supinator) and pronates a supinated forearm. Consequently, the tension of the brachioradialis places the arm in an intermediate, neutral position with the palm facing the torso. The muscle also flexes the elbow joint.

The extensor carpi radialis longus and brevis muscles (mm. extensores carpi radiales longus et brevis) originate as follows: the first from the radius immediately proximal to the lateral epicondyle; the second from the lateral epicondyle itself. They insert respectively into the base of the second and third Metacarpal bones. These muscles extend the wrist.

The remaining SUPERFICIAL MUSCLES OF the posterior group originate from the lateral epicondyle of the humerus. These include the extensor digitorum and the extensor carpi ulnaris (see Atlas).

The extensor digitorum muscle (m. extensor digitorum), running along the forearm, divides into four tendons directed toward the dorsal aspect of digits II–V, where they expand into broad tendinous hoods (aponeuroses). On the hand, these tendons are interconnected by tendinous links (juncturae tendinum). The muscle extends the fingers and the wrist.

The extensor carpi ulnaris muscle (m. extensor carpi ulnaris) inserts into the base of the fifth metacarpal bone. It extends the wrist.

In addition to their aforementioned functions, the superficial muscles originating from the lateral epicondyle of the humerus also extend the elbow joint.

The homologous flexors and extensors of the wrist, crossing the biaxial radiocarpal joint, can produce radial or ulnar deviation when working together. For instance, the simultaneous contraction of the ulnar flexor and extensor causes medial deviation of the hand. In such cases, antagonists act as synergists.

The deep layer of the posterior group includes the supinator, the thumb muscle complex, and the extensor indicis.

The supinator muscle (m. supinator) originates from the lateral epicondyle of the humerus and a specialized crest of the ulna, runs obliquely across the forearm, and inserts into the lateral and palmar surfaces of the radius. The muscle supinates the forearm and hand, rotating them outward so that the palm faces forward (Fig. 1.57).

Three thumb-moving muscles originate from the distal third of the dorsal surfaces of the radius and ulna, as well as the interosseous membrane (Atlas, Figs. 24, 43). The abductor pollicis longus inserts into the base of the first metacarpal bone and abducts the thumb and wrist; the extensor pollicis brevis inserts into the base of the proximal (first) phalanx of the thumb, extending and abducting it; the extensor pollicis longus inserts into the distal phalanx of the thumb and extends the digit.

The extensor indicis originates from the posterior surface of the ulna and the interosseous membrane; its distal tendon merges with the extensor digitorum tendon destined for the index finger. Thanks to this dedicated muscle, the index finger can extend independently, whereas isolated extension of the other fingers is difficult, especially since their tendons are linked by interconnecting bands.

Muscles of the hand. The Fascia of the hand is particularly well developed in the central part of the palm, where the tendinous fibers of the long muscle intertwine with it to form the palmar aponeurosis. The tendons of the superficial and deep finger flexors pass beneath this aponeurosis. Small lumbrical muscles (mm. lumbricales) arise from the four tendons of the flexor digitorum profundus and insert into the dorsal tendinous expansions of digits II–V (Atlas, Fig. 42). These muscles flex the fingers at the metacarpophalangeal joints and extend them at the interphalangeal joints.

The thenar eminence (lateral palmar elevation) is formed by the short muscles of the thumb (see Atlas): the abductor pollicis brevis, the flexor pollicis brevis, the opponens pollicis, and the adductor pollicis. Thus, the thumb possesses its own intrinsic muscular apparatus in the palm, which significantly enhances and diversifies its movements.

On the medial side of the palm lies a slightly smaller eminence formed by the hypothenar muscles (Atlas, Fig. 42): the abductor digiti minimi, the opponens digiti minimi, and the flexor digiti minimi brevis. These are less developed than the thenar muscles and are occasionally not fully demarcated from one another.

Located in the spaces between the metacarpal bones are the interosseous muscles (mm. interossei): three palmar interossei (which adduct the fingers toward one another) and four dorsal interossei (which abduct the fingers) (Atlas, Fig. 43).

Over the digits, the palmar aponeurosis thickens and fuses with the periosteum of the phalanges, forming the osteofibrous digital sheaths (Atlas, Fig. 42). The tendons of the finger flexors glide within these structures, enveloped by synovial sheaths.