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Upper limb
Upper limb
Upper limb
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Upper limb
Front of right upper extremity.
Back of right upper extremity.
Details
SystemMusculoskeletal
Identifiers
Latinmembrum superius
MeSHD034941
TA98A01.1.00.019
TA2138
FMA7183
Anatomical terminology

The upper limbs or upper extremities are the forelimbs of an upright-postured tetrapod vertebrate, extending from the scapulae and clavicles down to and including the digits, including all the musculatures and ligaments involved with the shoulder, elbow, wrist and knuckle joints.[1] In humans, each upper limb is divided into the shoulder, arm, elbow, forearm, wrist and hand,[2][3] and is primarily used for climbing, lifting and manipulating objects. In anatomy, just as arm refers to the upper arm, leg refers to the lower leg.

Definition

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In formal usage, the term "arm" only refers to the structures from the shoulder to the elbow, explicitly excluding the forearm, and thus "upper limb" and "arm" are not synonymous.[4] However, in casual usage, the terms are often used interchangeably. The term "upper arm" is redundant in anatomy, but in informal usage is used to distinguish between the two terms.

Structure

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In the human body, the muscles of the upper limb can be classified by origin, topography, function, or innervation. While a grouping by innervation reveals embryological and phylogenetic origins, the functional-topographical classification below reflects the similarity in action between muscles (with the exception of the shoulder girdle, where muscles with similar action can vary considerably in their location and orientation.[5]

Musculoskeletal system

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Shoulder girdle

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Bones of the shoulder girdle
Main article: Shoulder girdle

The shoulder girdle[6] or pectoral girdle,[7] composed of the clavicle and the scapula, connects the upper limb to the axial skeleton through the sternoclavicular joint (the only joint in the upper limb that directly articulates with the trunk), a ball and socket joint supported by the subclavius muscle which acts as a dynamic ligament. While this muscle prevents dislocation in the joint, strong forces tend to break the clavicle instead. The acromioclavicular joint, the joint between the acromion process on the scapula and the clavicle, is similarly strengthened by strong ligaments, especially the coracoclavicular ligament which prevents excessive lateral and medial movements. Between them these two joints allow a wide range of movements for the shoulder girdle, much because of the lack of a bone-to-bone contact between the scapula and the axial skeleton. The pelvic girdle is, in contrast, firmly fixed to the axial skeleton, which increases stability and load-bearing capabilities. [7]

The mobility of the shoulder girdle is supported by a large number of muscles. The most important of these are muscular sheets rather than fusiform or strap-shaped muscles and they thus never act in isolation but with some fibres acting in coordination with fibres in other muscles.[7]

Muscles
of shoulder girdle excluding the glenohumeral joint[5]
Migrated from head
Trapezius, sternocleidomastoideus, omohyoideus
Posterior
Rhomboideus major, rhomboideus minor, levator scapulae
Anterior
Subclavius, pectoralis minor, serratus anterior

Shoulder joint

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Shoulder joint with ligaments

The glenohumeral joint (colloquially called the shoulder joint) is the highly mobile ball and socket joint between the glenoid cavity of the scapula and the head of the humerus. Lacking the passive stabilisation offered by ligaments in other joints, the glenohumeral joint is actively stabilised by the rotator cuff, a group of short muscles stretching from the scapula to the humerus. Little inferior support is available to the joint and dislocation of the shoulder almost exclusively occurs in this direction. [8]

The large muscles acting at this joint perform multiple actions and seemingly simple movements are often the result of composite antagonist and protagonist actions from several muscles. For example, pectoralis major is the most important arm flexor and latissimus dorsi the most important extensor at the glenohumeral joint, but, acting together, these two muscles cancel each other's action leaving only their combined medial rotation component. On the other hand, to achieve pure flexion at the joint the deltoid and supraspinatus must cancel the adduction component and the teres minor and infraspinatus the medial rotation component of pectoralis major. Similarly, abduction (moving the arm away from the body) is performed by different muscles at different stages. The first 10° is performed entirely by the supraspinatus, but beyond that fibres of the much stronger deltoid are in position to take over the work until 90°. To achieve the full 180° range of abduction the arm must be rotated medially and the scapula most be rotated about itself to direct the glenoid cavity upward. [8]

Muscles
of shoulder joint proper[5]
Posterior
Supraspinatus, infraspinatus, teres minor, subscapularis, deltoideus, latissimus dorsi, teres major
Anterior
Pectoralis major, coracobrachialis

Bones of upper limb

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The bones forming the human upper limb are

  • Clavicle
  • Scapula
  • Humerus
  • Radius
  • Ulna
  • Carpal bones
    • Scaphoid
    • Lunate
    • Triquetral
    • Pisiform
    • Trapezium
    • Trapezoid
    • Capitate
    • Hamate
  • 5 Metacarpal bones
  • 14 Phalanges
Upper limb bones with articular cartilage

Arm

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Superficial muscles of the arm
Main article: Arm

The arm proper (brachium), sometimes called the upper arm,[6] the region between the shoulder and the elbow, is composed of the humerus with the elbow joint at its distal end.

The elbow joint is a complex of three joints — the humeroradial, humeroulnar, and superior radioulnar joints — the former two allowing flexion and extension whilst the latter, together with its inferior namesake, allows supination and pronation at the wrist. Triceps is the major extensor and brachialis and biceps the major flexors. Biceps is, however, the major supinator and while performing this action it ceases to be an effective flexor at the elbow. [9]

Muscles
of the arm[5]
Posterior
Triceps brachii, anconeus
Anterior
Brachialis, biceps brachii

Forearm

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Ventral superficial muscles of the forearm
Main article: Forearm

The forearm (Latin: antebrachium),[6] composed of the radius and ulna; the latter is the main distal part of the elbow joint, while the former composes the main proximal part of the wrist joint.

Most of the large number of muscles in the forearm are divided into the wrist, hand, and finger extensors on the dorsal side (back of hand) and the ditto flexors in the superficial layers on the ventral side (side of palm). These muscles are attached to either the lateral or medial epicondyle of the humerus. They thus act on the elbow, but, because their origins are located close to the centre of rotation of the elbow, they mainly act distally at the wrist and hand. Exceptions to this simple division are brachioradialis — a strong elbow flexor — and palmaris longus — a weak wrist flexor which mainly acts to tense the palmar aponeurosis. The deeper flexor muscles are extrinsic hand muscles; strong flexors at the finger joints used to produce the important power grip of the hand, whilst forced extension is less useful and the corresponding extensor thus are much weaker. [10]

Biceps is the major supinator (drive a screw in with the right arm) and pronator teres and pronator quadratus the major pronators (unscrewing) — the latter two role the radius around the ulna (hence the name of the first bone) and the former reverses this action assisted by supinator. Because biceps is much stronger than its opponents, supination is a stronger action than pronation (hence the direction of screws). [10]

Muscles
of the forearm[5]
Posterior
(Superficial) extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, (deep) supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis
Anterior
(Superficial) pronator teres, flexor digitorum superficialis, flexor carpi radialis, flexor carpi ulnaris, palmaris longus, (deep) flexor digitorum profundus, flexor pollicis longus, pronator quadratus
Radial
Brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis

Wrist

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Main article: Wrist

The wrist (Latin: carpus),[6] composed of the carpal bones, articulates at the wrist joint (or radiocarpal joint) proximally and the carpometacarpal joint distally. The wrist can be divided into two components separated by the midcarpal joints. The small movements of the eight carpal bones during composite movements at the wrist are complex to describe, but flexion mainly occurs in the midcarpal joint whilst extension mainly occurs in the radiocarpal joint; the latter joint also providing most of adduction and abduction at the wrist. [11]

3D Medical Animation still shot of Human Wrist
3D medical animation still shot of human wrist

How muscles act on the wrist is complex to describe. The five muscles acting on the wrist directly — flexor carpi radialis, flexor carpi ulnaris, extensor carpi radialis, extensor carpi ulnaris, and palmaris longus — are accompanied by the tendons of the extrinsic hand muscles (i.e. the muscles acting on the fingers). Thus, every movement at the wrist is the work of a group of muscles; because the four primary wrist muscles (FCR, FCU, ECR, and ECU) are attached to the four corners of the wrist, they also produce a secondary movement (i.e. ulnar or radial deviation). To produce pure flexion or extension at the wrist, these muscle therefore must act in pairs to cancel out each other's secondary action. On the other hand, finger movements without the corresponding wrist movements require the wrist muscles to cancel out the contribution from the extrinsic hand muscles at the wrist. [11]

Hand

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Bones of the hand
Main article: Hand

The hand (Latin: manus),[6] the metacarpals (in the hand proper) and the phalanges of the fingers, form the metacarpophalangeal joints (MCP, including the knuckles) and interphalangeal joints (IP).

Of the joints between the carpus and metacarpus, the carpometacarpal joints, only the saddle-shaped joint of the thumb offers a high degree of mobility while the opposite is true for the metacarpophalangeal joints. The joints of the fingers are simple hinge joints. [11]

The primary role of the hand itself is grasping and manipulation; tasks for which the hand has been adapted to two main grips — power grip and precision grip. In a power grip an object is held against the palm and in a precision grip an object is held with the fingers, both grips are performed by intrinsic and extrinsic hand muscles together. Most importantly, the relatively strong thenar muscles of the thumb and the thumb's flexible first joint allow the special opposition movement that brings the distal thumb pad in direct contact with the distal pads of the other four digits. Opposition is a complex combination of thumb flexion and abduction that also requires the thumb to be rotated 90° about its own axis. Without this complex movement, humans would not be able to perform a precision grip. [12]

In addition, the central group of intrinsic hand muscles give important contributions to human dexterity. The palmar and dorsal interossei adduct and abduct at the MCP joints and are important in pinching. The lumbricals, attached to the tendons of the flexor digitorum profundus (FDP) and extensor digitorum communis (FDC), flex the MCP joints while extending the IP joints and allow a smooth transfer of forces between these two muscles while extending and flexing the fingers. [12]

Muscles
of the hand[5]
Metacarpal
Lumbricals, palmar introssei, dorsal interossei
Thenar
Abductor pollicis brevis, adductor pollicis, flexor pollicis brevis, opponens pollicis
Hypothenar
Abductor digiti minimi, flexor digiti minimi, opponens digiti minimi, palmaris brevis

Neurovascular system

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Nerve supply

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Branches of brachial plexus

The motor and sensory supply of the upper limb is provided by the brachial plexus which is formed by the ventral rami of spinal nerves C5-T1. In the posterior triangle of the neck these rami form three trunks from which fibers enter the axilla region (armpit) to innervate the muscles of the anterior and posterior compartments of the limb. In the axilla, cords are formed to split into branches, including the five terminal branches listed below. [13] The muscles of the upper limb are innervated segmentally proximal to distal so that the proximal muscles are innervated by higher segments (C5–C6) and the distal muscles are innervated by lower segments (C8–T1). [14]

Motor innervation of upper limb by the five terminal nerves of the brachial plexus:[14]

Collateral branches of the brachial plexus:[14]

Blood supply and drainage

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Arteries of the upper limb:

ulnar, nutrient and muscular branches of the brachial artery.

Veins of the upper limb.

Veins of the upper limb:

As for the upper limb blood supply, there are many anatomical variations.[15]

Other animals

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Evolutionary variation

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Main article: Forelimb
Upper/front limbs of (top) salamander, sea turtle, crocodile, bird, (bottom) bat, whale, mole, and human

The skeletons of all mammals are based on a common pentadactyl ("five-fingered") template but optimised for different functions. While many mammals can perform other tasks using their forelimbs, their primary use in most terrestrial mammals is one of three main modes of locomotion: unguligrade (hoof walkers), digitigrade (toe walkers), and plantigrade (sole walkers). Generally, the forelimbs are optimised for speed and stamina, but in some mammals some of the locomotion optimisation have been sacrificed for other functions, such as digging and grasping. [16]

Chimpanzees maintain some of the dexterity brachiating gibbons lack

In primates, the upper limbs provide a wide range of movement which increases manual dexterity. The limbs of chimpanzees, compared to those of humans, reveal their different lifestyle. The chimpanzee primarily uses two modes of locomotion: knuckle-walking, a style of quadrupedalism in which the body weight is supported on the knuckles (or more properly on the middle phalanges of the fingers), and brachiation (swinging from branch to branch), a style of bipedalism in which flexed fingers are used to grasp branches above the head. To meet the requirements of these styles of locomotion, the chimpanzee's finger phalanges are longer and have more robust insertion areas for the flexor tendons while the metacarpals have transverse ridges to limit dorsiflexion (stretching the fingers towards the back of the hand). The thumb is small enough to facilitate brachiation while maintaining some of the dexterity offered by an opposable thumb. In contrast, virtually all locomotion functionality has been lost in humans while predominant brachiators, such as the gibbons, have very reduced thumbs and inflexible wrists. [16]

A bush pig, an ungulate with remaining non-weight-bearing digits, and the skeleton of the extinct Malagasy hippopotamus

In ungulates the forelimbs are optimised to maximize speed and stamina to the extent that the limbs serve almost no other purpose. In contrast to the skeleton of human limbs, the proximal bones of ungulates are short and the distal bones long to provide length of stride; proximally, large and short muscles provide rapidity of step. The odd-toed ungulates, such as the horse, use a single third toe for weight-bearing and have significantly reduced metacarpals. Even-toed ungulates, such as the giraffe, uses both their third and fourth toes but a single completely fused phalanx bone for weight-bearing. Ungulates whose habitat does not require fast running on hard terrain, for example the hippopotamus, have maintained four digits. [16]

A grooming lynx and a two-toed sloth "at home"

In species in the order Carnivora, some of which are insectivores rather than carnivores, the cats are some of the most highly evolved predators designed for speed, power, and acceleration rather than stamina. Compared to ungulates, their limbs are shorter, more muscular in the distal segments, and maintain five metacarpals and digit bones; providing a greater range of movements, a more varied function and agility (e.g. climbing, swatting, and grooming). Some insectivorous species in this order have paws specialised for specific functions. The sloth bear uses their digits and large claws to tear logs open rather than kill prey. Other insectivorous species, such as the giant and red pandas, have developed large sesamoid bones in their paws that serve as an extra "thumb" while others, such as the meerkat, uses their limbs primary for digging and have vestigial first digits. [16]

The arboreal two-toed sloth, a South American mammal in the order Pilosa, have limbs so highly adapted to hanging in branches that it is unable to walk on the ground where it has to drag its own body using the large curved claws on its foredigits. [16]

See also

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Notes

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Upper limb

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The upper limb, also known as the upper extremity, is a functional unit of the that extends from the to the hand, comprising the (brachium), (antebrachium), and hand (manus). It enables essential movements such as grasping, reaching, manipulating objects, and providing sensory feedback for and touch. The upper limb consists of 30 bones, including the in the , the and in the , eight carpals, five metacarpals, and 14 phalanges in the hand. These provide and facilitate articulations, anchored to the trunk via the pectoral girdle (scapula and ) for mobility. For detailed skeletal components, see the Anatomy section. Approximately 60 muscles organized into compartments enable precise control and power for these functions. The upper limb is innervated primarily by the (C5–T1) and supplied by branches of the , supporting coordinated motor and sensory activities. Detailed descriptions of muscles and neurovasculature are covered in the Anatomy section.

Overview

Definition

The upper limb, also known as the upper extremity, is the region of the that extends from the pectoral girdle to the distal phalanges of the hand, forming a key component of the . The pectoral girdle, consisting of the and , anchors the upper limb to the at the , while the free portion includes the (), ( and ), (), and hand (metacarpals and phalanges). This structure encompasses 32 bones per side—4 in the girdle and 28 in the free limb—enabling a wide from the to the fingertips. In contrast to the lower limb, which is primarily adapted for and locomotion through stable pelvic attachments and robust skeletal elements, the upper limb is specialized for mobility, prehension, and fine manipulation, reflecting evolutionary priorities for tool use and environmental interaction in and humans. Its basic composition integrates bones with synovial joints for articulation, skeletal muscles for movement, peripheral nerves from the for sensory and motor innervation, and a vascular network derived from the and accompanying veins to support dexterity and endurance. The nomenclature of the upper limb draws from and Greek roots, with "brachium" denoting the (from the to ) and "manus" referring to the hand, terms established in early anatomical texts like those of and later standardized in works by Vesalius. These etymologies underscore the limb's historical recognition as a versatile appendage, distinct in form and function from the lower extremity.

Functions

The upper limb plays a pivotal role in human physiology by facilitating manipulation and prehension, which involve grasping, reaching, and executing fine motor tasks through the hand's exceptional dexterity. This capability is enabled by the thumb's opposability and the coordinated action of the fingers, allowing precise object handling essential for daily activities such as writing or tool manipulation. The structure supports a range of grip types, from power grips for heavy objects to precision grips for delicate tasks, enhancing environmental interaction and productivity. Sensory functions of the upper limb provide critical tactile feedback through receptors and , enabling spatial awareness and accurate movement control. Tactile sensations detect , texture, and via mechanoreceptors in the skin, while proprioceptors in muscles and joints convey information about limb position and motion to the . This sensory integration allows for seamless adjustment during tasks, preventing errors and supporting adaptive responses. In supportive roles, the upper limb contributes to in postures like quadrupedal support or positions, distributing body load through the and for stability. Additionally, it facilitates communication through gestures, such as or , which convey non-verbal information and enhance social interaction. The upper limb's integration with the underscores its significance in tool use and the of human dexterity. This connection has driven evolutionary adaptations, allowing early hominins to manipulate objects and innovate technologies, shaping cognitive and societal development.

Anatomy

Skeletal components

The skeletal components of the upper limb provide the rigid framework essential for support, mobility, and manipulation, consisting of the , , , , and the bones of the hand. These elements articulate to form a flexible capable of a wide range of movements, with specific features like articular surfaces and fossae facilitating stability and motion. The anchors the upper limb to the and includes the and . The , an S-shaped , articulates medially with the manubrium of the at the sternoclavicular and laterally with the process of the at the ; its sternal end is triangular and enlarged, while the acromial end is flattened and oval. The , a flat triangular bone, features the process projecting anteriorly to articulate with the , the serving as an attachment site, and the glenoid cavity—a shallow, pear-shaped articular surface deepened by the for humeral articulation at the glenohumeral . The forms the skeleton of the arm, extending from the to the . Proximally, it includes a rounded head that articulates with the glenoid cavity and two tubercles—the larger laterally and the smaller anteriorly—for attachments. The shaft is cylindrical with a midway. Distally, the humerus features the capitulum (a lateral rounded condyle articulating with the ), the trochlea (a medial pulley-shaped condyle articulating with the ), the medial and lateral epicondyles for attachments, and fossae such as the anterior coronoid and radial fossae (accommodating ulnar and radial processes during flexion) and the posterior olecranon fossa (for the ulna during extension). The forearm comprises the radius and ulna, parallel long bones connected by the , a fibrous sheet that binds their shafts and transmits forces between them. The , lateral and shorter, has a disc-shaped head proximally that articulates with the and the radial notch of the ; distally, it features a styloid process projecting laterally for ligament attachment. The , medial and longer, includes a proximal process and trochlear notch articulating with the humeral trochlea, a radial notch for the , and a distal styloid process medially. The hand skeleton includes the carpals, metacarpals, and phalanges, enabling precise dexterity. The eight , arranged in two rows, are the proximal scaphoid (with a and waist prone to ), lunate, triquetrum, and pisiform (sesamoid-like), and the distal trapezium (saddle-shaped for thumb articulation), trapezoid, capitate, and hamate (with a hook); they form the wrist's concavity for flexor passage. The five metacarpals are elongated bones with bases articulating proximally with the carpals at carpometacarpal joints and heads distally with phalanges at metacarpophalangeal joints; the first () is the shortest and most mobile. The 14 phalanges consist of proximal, middle (absent in ), and distal bones per digit, with bases articulating proximally and heads distally forming interphalangeal joints; the has two (proximal and distal), while fingers have three each. Ossification of upper limb bones begins in utero via primary centers in diaphyses and secondary centers in epiphyses, with fusion occurring postnatally; timelines vary by bone and sex (earlier in females). The clavicle ossifies first intramembranously at 6 weeks gestation in the diaphysis, with a secondary sternal epiphysis appearing at 18-20 years and full union by 20-25 years. Scapular ossification starts at 8 weeks gestation for the body, spine, and glenoid (primary center), with coracoid at 1 year and epiphyses for acromion (15-18 years), inferior angle (16-18 years), vertebral border (18-20 years), and coracoid/glenoid (16-18 years), uniting by 18-25 years. The humerus has a primary diaphyseal center at 6-7 weeks, with epiphyses for head (1-2 years), greater tubercle (2-3 years), lesser tubercle (3-5 years), capitulum (2-3 years), medial epicondyle (5-8 years), and lateral epicondyle/trochlea (11-14 years), fusing by 16-25 years. Radius and ulna diaphyses appear at 7 weeks; radius epiphyses at carpal end (females 8 months, males 15 months) and humeral end (6-7 years), ulna at carpal end (females 6-7 years, males 7-8 years) and humeral end (10 years), with unions by 17-25 years and 17-24 years, respectively. Carpal ossification is delayed: capitate (females 3-6 months, males 4-10 months), hamate (females 5-10 months, males 6-12 months), triquetrum (females 2-3 years, males ~3 years), lunate (females 3-4 years, males ~4 years), scaphoid (females 4-5 years, males ~5 years), trapezium/trapezoid (females 4-5 years, males 5-6 years), pisiform (females 9-10 years, males 12-13 years). Metacarpal diaphyses form at 9 weeks, with proximal first metacarpal epiphysis at 3 years and distal others at 2 years, uniting by 15-20 years. Phalangeal diaphyses ossify at 7-12 weeks (distal row first, then proximal and middle), with proximal epiphyses at 1-3 years and unions by 18-20 years.
BonePrimary Center (Diaphysis)Key Secondary Centers (Epiphyses)Union Timeline
Clavicle6 weeks gestationSternal end: 18-20 years20-25 years
Scapula8 weeks gestation (body/glenoid)Acromion: 15-18 years; Coracoid: 1 year (initial), 16-18 years (final); Inferior angle: 16-18 years18-25 years
Humerus6-7 weeks gestationHead: 1-2 years; Greater tubercle: 2-3 years; Capitulum: 2-3 years; Medial epicondyle: 5-8 years; Trochlea: 11-14 years16-25 years
Radius7 weeks gestationDistal (carpal): Females 8 months, males 15 months; Proximal (humeral): 6-7 years17-25 years
Ulna7 weeks gestationDistal (carpal): Females 6-7 years, males 7-8 years; Proximal (humeral): 10 years17-24 years
CarpalsVariable, postnatalCapitate: Females 3-6 months, males 4-10 months; Pisiform: Females 9-10 years, males 12-13 yearsN/A (no epiphyses)
Metacarpals9 weeks gestationDistal: 2 years; Proximal (1st): 3 years15-20 years
Phalanges7-12 weeks gestationProximal: 1-3 years18-20 years

Joints and ligaments

The upper limb features a series of synovial joints that provide extensive mobility, from the proximal to the distal finger articulations, stabilized by ligaments that prevent excessive translation while permitting multi-planar motion. These joints are classified based on their morphology and function, with ranging from uniaxial to triaxial, enabling precise manipulation and reach. The , also known as the , is a multiaxial ball-and-socket formed by the articulation of the humeral head with the glenoid cavity of the . It allows flexion-extension, abduction-adduction, and internal-external rotation across three rotational axes, providing the widest in the body. The joint is enclosed by a loose fibrous capsule reinforced by , and the —a fibrocartilaginous rim—deepens the shallow glenoid socket to enhance stability. The elbow complex comprises the humeroulnar and humeroradial articulations forming a uniaxial for flexion-extension, combined with the proximal radioulnar enabling pronation-supination. This setup allows two : one for hinging and one for rotation. Stability is augmented by the annular ligament, which encircles the radial head and binds it to the , preventing during rotation. The radiocarpal joint, or wrist joint, is a biaxial between the distal and the proximal carpal row (scaphoid, lunate, and triquetrum), permitting flexion-extension and radial-ulnar deviation. Its stability relies on collateral ligaments, including the radial collateral (from to scaphoid and trapezium) and ulnar collateral (from to triquetrum), which resist lateral deviations. In the hand, the carpometacarpal (CMC) joint of is a unique saddle synovial joint between the trapezium and first metacarpal, allowing opposition and circumduction with three degrees of freedom for enhanced dexterity. The metacarpophalangeal (MCP) joints are condyloid synovial joints between metacarpals and proximal phalanges, supporting flexion-extension, abduction-adduction, and circumduction in two planes. Interphalangeal (IP) joints, including proximal and distal, are uniaxial hinge synovial joints between phalanges, restricted to flexion-extension for precise gripping. Key ligaments throughout the upper limb include the coracoclavicular , which connects the to the , suspending the and preventing acromioclavicular under the weight of the . The transverse humeral spans the intertubercular groove of the , retaining the long head of the within the groove during motion. At the , palmar and dorsal radiocarpal ligaments extend from the to the carpals, limiting excessive flexion and extension while guiding carpal alignment. All upper limb joints are synovial, featuring a cavity filled with secreted by the , which lubricates articular surfaces to minimize friction and provides nutrients to avascular . This fluid's viscous properties absorb shock and facilitate smooth gliding, essential for the limb's repetitive, high-mobility demands.

Muscles

The muscles of the upper limb enable a wide range of movements, from gross shoulder motions to fine hand manipulations, and are organized into functional groups based on their anatomical regions and actions. These muscles vary in architecture, with types like the biceps brachii featuring parallel fibers for greater excursion and speed, while pennate types, such as the unipennate flexor pollicis longus or multipennate subscapularis, have oblique fibers attaching to tendons for enhanced force production through larger physiological cross-sectional areas. Shoulder muscles primarily stabilize and move the glenohumeral joint, with the providing dynamic stability and others effecting abduction, adduction, and . The supraspinatus originates from the of the and inserts on the of the ; it is innervated by the (C5-C6) and primarily abducts the . The infraspinatus arises from the infraspinous fossa and inserts on the ; innervated by the (C5-C6), it laterally rotates the . The teres minor originates from the upper two-thirds of the lateral border of the and inserts on the ; supplied by the (C5-C6), it laterally rotates the and assists in adduction. The subscapularis originates from the subscapular fossa and inserts on the ; innervated by the upper and lower (C5-C7), it medially rotates the . The deltoid originates from the , , and spine of the , inserting on the of the ; innervated by the (C5-C6), it abducts the and assists in flexion, extension, and depending on the fiber portion. The pectoralis major originates from the , , and costal cartilages of ribs 2-6, inserting on the intertubercular groove of the ; supplied by the lateral and medial pectoral nerves (C5-T1), it flexes, adducts, and medially rotates the .
MuscleOriginInsertionInnervationPrimary Action
SupraspinatusGreater tubercle of Abducts
InfraspinatusInfraspinous fossaGreater tubercle of Laterally rotates
Teres minorLateral border of Greater tubercle of Laterally rotates
SubscapularisSubscapular fossa of Medially rotates
Deltoid, , scapular spineAbducts
Pectoralis major, , 2-6Intertubercular groove of Pectoral nervesFlexes, adducts, medially rotates
Arm muscles act mainly at the elbow joint for flexion and extension. The biceps brachii has two heads: the long head originates from the of the , and the short head from the ; both insert on the radial tuberosity and ; innervated by the (C5-C6), it flexes the and supinates the hand. The triceps brachii has three heads originating from the (long head), posterior (lateral and medial heads), inserting on the process of the ; supplied by the (C6-C8), it extends the . The brachialis originates from the anterior distal and inserts on the coronoid process and ; innervated by the (C5-C6), with a minor contribution from the , it flexes the . Regional innervation for arm muscles centers on the for flexors and the for the extensor.
MuscleOriginInsertionInnervationPrimary Action
Biceps brachii (long/short heads)/Radial tuberosityFlexes , supinates hand
Triceps brachii, posterior processExtends
BrachialisAnterior distal Coronoid process of Flexes
Forearm muscles are divided into anterior (flexor-pronator) and posterior (extensor-supinator) compartments, facilitating , hand, and digit movements; many feature long tendons passing through synovial sheaths to reduce friction. Flexors like the flexor carpi radialis originate from the and insert on the bases of the second and third metacarpals; innervated by the (C6-C7), it flexes and abducts the . Extensors such as the extensor digitorum arise from the lateral via the and insert on the extensor expansions of digits 2-5; supplied by the (C7-C8), a branch, it extends the and digits. Pronators and supinators include the pronator teres, originating from the medial and , inserting on the mid-lateral radius; innervated by the (C6-C7), it pronates the . The supinator originates from the lateral , radial collateral ligament, and , inserting on the proximal radius; supplied by the deep branch of the (C5-C6), it supinates the . In the , flexor tendons travel within a common synovial sheath (ulnar bursa) under the flexor retinaculum, extending proximally into the and distally for the , while the flexor pollicis longus has a separate radial bursa; extensor tendons pass under the extensor retinaculum with individual synovial sheaths for each, minimizing friction during extension. Innervation for flexors is primarily , with ulnar for some medial muscles, while extensors rely on branches.
Muscle ExampleCompartmentOriginInsertionInnervationPrimary Action
Flexor carpi radialisAnteriorMedial epicondyleBases of metacarpals 2-3Flexes, abducts
Extensor digitorumPosteriorLateral epicondyleExtensor expansions digits 2-5Extends , digits
Pronator teresAnteriorMedial epicondyle, coronoid processMid-radiusPronates
SupinatorPosteriorLateral epicondyle, ulnaProximal radiusDeep radial nerveSupinates
Intrinsic hand muscles control precise finger and thumb motions, located in the thenar, hypothenar, and central compartments, often with short tendons or direct insertions. The thenar group includes the abductor pollicis brevis, originating from the flexor retinaculum and of the scaphoid, inserting on the proximal of the ; innervated by the recurrent branch of the (C8-T1), it abducts the . Hypothenar muscles like the abductor digiti minimi originate from the and flexor retinaculum, inserting on the proximal of the ; supplied by the deep branch of the (C8-T1), it abducts the . The palmar interossei (unipennate) originate from the medial sides of metacarpals 2, 4, and 5, inserting on the extensor expansions and proximal phalanges; innervated by the (C8-T1), they adduct the fingers toward the midline. Dorsal interossei (bipennate) originate from adjacent sides of metacarpals, inserting similarly; also ulnar-innervated (C8-T1), they abduct the fingers. Lumbricals originate from the tendons of flexor digitorum profundus, inserting on the extensor expansions of digits 2-5; the first two are median-innervated (C8-T1), the latter two ulnar (C8-T1), flexing metacarpophalangeal joints while extending interphalangeal joints. In the hand, digital fibrous and synovial sheaths enclose flexor tendons of digits 2-5, extending from metacarpal heads to the distal phalanges, with a 1-3 cm gap from the common flexor sheath; extensor tendons lack extensive sheaths beyond the but form expansions over the digits. Hand muscle innervation is dominated by the for thenar and lateral lumbricals, and ulnar for hypothenar, interossei, and medial lumbricals.
Muscle Group/ExampleOriginInsertionInnervationPrimary Action
Thenar (abductor pollicis brevis)Flexor retinaculum, scaphoidProximal phalanx of Abducts
Hypothenar (abductor digiti minimi)Pisiform, flexor retinaculumProximal phalanx of digit 5Abducts
Palmar interosseiMedial metacarpals 2,4,5Extensor expansionsAdduct fingers
Dorsal interosseiAdjacent metacarpalsExtensor expansionsAbduct fingers
LumbricalsFlexor digitorum profundus tendonsExtensor expansions digits 2-5/ulnar nervesFlex MCP, extend IP joints

Neurovasculature

The neurovasculature of the upper limb encompasses the intricate network of nerves and blood vessels that provide sensory and motor innervation as well as arterial supply and venous drainage to the region. The primary neural structure is the , which originates from the anterior rami of spinal nerves C5 through T1. These roots emerge from the intervertebral foramina and unite to form three trunks in the : the superior trunk from C5-C6, the middle trunk from C7, and the inferior trunk from C8-T1. Each trunk subsequently divides into anterior and posterior divisions behind the middle scalene muscle, yielding six divisions that rearrange into three cords around the : the (from anterior divisions of superior and middle trunks), the (from posterior divisions of all trunks), and the medial cord (from the anterior division of the inferior trunk). The cords give rise to the major terminal branches, including the (from the ), (from lateral and medial cords), (from the medial cord), (from the ), and (from the ). The nerves of the mediate sensory, motor, and autonomic functions essential for upper limb operation. Sensory innervation follows dermatomal patterns, with C5 covering the lateral , C6 the lateral and , C7 the middle fingers, C8 the medial and , and T1 the medial upper . Motor innervation corresponds to myotomes, where C5 supplies abductors, C6 elbow flexors, C7 elbow extensors, C8 finger flexors, and T1 intrinsic hand muscles. Autonomic components, primarily sympathetic fibers from the T1 via gray rami communicantes, reach the upper limb through the major to regulate tone and sweat glands. Arterial supply to the upper limb begins with the subclavian artery, which continues as the axillary artery from the lateral border of the first rib to the inferior border of the teres major muscle. The axillary artery then becomes the brachial artery in the arm, descending medial to the humerus and bifurcating at the cubital fossa into the radial and ulnar arteries. The radial artery courses laterally along the radius to the wrist, while the ulnar artery travels medially along the ulna; both contribute to the hand's circulation via the superficial palmar arch (primarily ulnar) and deep palmar arch (primarily radial), which anastomose to supply the palmar and digital vessels. Venous drainage parallels the arterial system, divided into superficial and deep components that ultimately converge into the . Superficial veins include the , which drains the lateral hand and along the radial side and ascends laterally to join the , and the , which drains the medial hand and along the ulnar side and pierces the midway up the to form the with the . Deep veins, such as the paired accompanying the and the radial and ulnar veins in the , provide the primary return flow. Lymphatic pathways follow venous routes, with superficial lymphatics from the hand draining laterally via the to infraclavicular nodes or medially via the to supratrochlear nodes, while deep lymphatics accompany arteries to axillary nodes before entering the subclavian lymphatic duct. Anastomotic networks ensure collateral circulation throughout the upper limb. Around the , the connects branches of the subclavian (e.g., suprascapular and transverse cervical arteries) with the axillary artery's subscapular and branches, forming a robust collateral pathway. At the , anastomoses between the brachial artery's profunda brachii, superior ulnar collateral, and inferior ulnar collateral arteries link with radial and ulnar recurrent branches for alternative flow. In the palm, the superficial and deep palmar arches interconnect the radial and ulnar arteries, supplemented by dorsal carpal and metacarpal anastomoses, to maintain perfusion despite occlusions. Clinically, key pulse points for assessing arterial patency are the , palpated in the medial of the arm for blood pressure measurement, and the , located at the wrist's for routine pulse evaluation.

Biomechanics and Movement

Kinematics

The of the upper limb describes the geometric aspects of its motion without considering forces, focusing on the spatial configurations and trajectories achievable through articulations. The upper limb functions as a serial , comprising the complex, , , , and hand, which collectively enable a wide range of positions for the hand in . Motion occurs primarily through synovial s that permit and, in some cases, translation, allowing for precise manipulation and reach. These movements are constrained by morphology and ligamentous structures, which the joints and ligaments section details further. Key joint ranges of motion define the limits of upper limb mobility. At the (glenohumeral ), flexion reaches up to 180° in the , while extension is approximately 60°; these values align with normative standards established by the American Academy of Orthopaedic Surgeons (AAOS). The , a , allows flexion up to 150°, facilitating folding of the toward the upper arm. Wrist deviation includes radial deviation of about 20° and ulnar deviation of 30° in the frontal plane, enabling side-to-side tilting essential for grasping. These ranges vary slightly by individual factors such as age and sex but represent typical healthy adult capabilities. Upper limb motions are organized around three primary anatomical axes corresponding to the cardinal planes of the body. Flexion and extension occur about the frontal (coronal) axis in the , abduction and adduction about the sagittal axis in the frontal plane, and internal/external about the longitudinal (vertical) axis in the . The complex exhibits the broadest mobility, with rotations in all three planes, while the primarily flexes/extends and the pronates/supinates around its long axis. The hand and fingers add further complexity through multi-planar motions at the carpometacarpal and interphalangeal joints. Degrees of freedom (DOF) quantify the independent parameters needed to specify the limb's configuration. The glenohumeral joint possesses 6 DOF, comprising three rotational (flexion/extension, abduction/adduction, internal/external rotation) and three translational components along the x, y, and z axes, owing to its ball-and-socket design that permits both pivoting and sliding. The contributes 2 DOF: one for flexion/extension at the and one for pronation/supination at the proximal radioulnar joint. The adds 2-3 DOF for flexion/extension and deviation, while the hand provides multiple DOF—approximately 20 in total across (for opposition) and fingers—enabling fine dexterity through combinations of flexion, extension, abduction, and adduction. Overall, the upper limb chain offers 27-30 DOF, far exceeding the 6 needed for full spatial positioning, which introduces redundancy for versatile task execution. To model multi-joint upper limb motions, coordinate systems employ , which decompose complex three-dimensional rotations into sequential rotations about fixed or body axes (e.g., Z-X-Y sequence for ). This approach facilitates kinematic analysis of the arm as a chain, where each segment's orientation relative to the previous is calculated using rotation matrices; for instance, elevation combines humeral elevation and rotation via to track end-effector position. Such representations are standard in for simulating reaching or manipulating tasks. Advanced kinematic descriptions incorporate instant centers of (ICR) and to capture non-spherical behaviors. The ICR is the instantaneous point about which a joint segment rotates at any given moment, migrating during motion—for example, in the , the ICR shifts proximally (11-16 cm from the humeral head) during abduction, reflecting coupled glenohumeral and scapulothoracic translations. models joint motion as a "twist" combining about an axis with along the same axis (a screw displacement), providing a unified framework for analyzing helical paths in upper limb joints like the , where flexion involves both pivoting and minor sliding. This underpins precise simulations of limb trajectories, emphasizing the upper limb's capacity for infinite end positions within its workspace.

Muscle actions and coordination

The upper limb's muscles generate forces through coordinated actions that enable precise movements, with prime movers providing the primary for specific actions. For instance, the biceps brachii serves as the prime mover for flexion, exerting via its moment arm, which measures approximately 4-5 cm at 90° flexion and varies with angle to optimize force production during tasks like lifting. This generation relies on the muscle's insertion on the radial tuberosity, allowing efficient force transmission across the . Synergist muscles assist prime movers to refine actions and stabilize joints, while antagonists provide opposition to control deceleration and prevent unwanted motion. The supinator muscle acts as a synergist with the biceps brachii during rapid elbow flexion combined with forearm supination, enhancing overall torque efficiency in pronated starting positions. In contrast, the triceps brachii functions as the primary antagonist to the biceps, co-activating at levels up to 45% during preparatory phases of dynamic movements to maintain joint stability before and after peak flexion. Muscle force output in the upper limb follows the force-velocity relationship, described by Hill's hyperbolic , which quantifies how contractile force decreases with increasing shortening velocity: F=Fmax⁡⋅Vmax⁡−Vk⋅V+Vmax⁡F = F_{\max} \cdot \frac{V_{\max} - V}{k \cdot V + V_{\max}} Here, FF is the force, Fmax⁡F_{\max} the maximum isometric force, VV the velocity, Vmax⁡V_{\max} the maximum unloaded velocity, and kk a constant reflecting muscle properties. In reaching tasks, this relationship limits elbow flexor force during fast extensions, reducing by up to 50% at velocities near Vmax⁡V_{\max}, thereby influencing reach speed and accuracy. Neural coordination patterns ensure smooth upper limb motion through mechanisms like and co-activation. suppresses antagonist activity via spinal during agonist contraction, facilitating efficient flexion by reducing interference and promoting unidirectional torque. Co-activation, conversely, simultaneously recruits agonists and antagonists to enhance , as seen in stabilization during pointing, where it increases stability without sacrificing mobility. In load-bearing activities such as arm elevation, the deltoid and exhibit synergy to distribute forces across the . The middle deltoid initiates glenohumeral abduction, while the upper upwardly rotates the , together generating coordinated torque to elevate the arm against gravity, with peak co-activation around 60-90° to counter compressive loads up to 1.5 times body weight. This interplay prevents scapular winging and optimizes force transmission for sustained overhead tasks.

Development and Embryology

Embryonic origins

The upper limb develops from paired limb buds that emerge during the fourth week of embryonic gestation, arising from the and overlying . These buds form through the proliferation of mesenchymal cells in the somatic layer of the , initiated by signals from the body axis, including fibroblast growth factor 8 (FGF8) and . The limb buds are characterized by two critical signaling centers: the apical ectodermal ridge (AER), a thickened ectodermal structure at the distal tip that promotes outgrowth, and the zone of polarizing activity (ZPA), a mesenchymal region at the posterior margin that establishes asymmetry. Patterning along the three axes occurs concurrently with outgrowth. Proximal-distal patterning is regulated by (FGF) signaling from the AER, which maintains the progress zone of undifferentiated , while (such as HoxA and HoxD clusters) specify segment identity along this axis. Anteroposterior patterning is controlled by Sonic hedgehog (Shh) secreted from the ZPA, creating a gradient that determines digit identity and posterior structures. Dorsoventral axis formation involves Wnt signaling from the dorsal ectoderm, which promotes dorsal characteristics like extensor muscles, in opposition to ventral signals from engrailed-1 (En1) expressing cells. Skeletal elements arise through chondrogenesis, beginning around week 5, where mesenchymal cells condense into precartilaginous models under the influence of transforming growth factor-beta (TGF-β) and bone morphogenetic proteins (BMPs). These condensations form the proximally, followed by the and , and then the carpals, metacarpals, and phalanges distally. Ossification initiates in week 8 via endochondral mechanisms, with primary centers appearing in the diaphyses of long bones like the and /, while digits ossify later. Disruptions in these processes lead to congenital anomalies; for instance, early truncation or failure of the AER, often due to ischemia or teratogens, results in amelia, the complete absence of the upper limb. Similarly, ZPA defects involving reduced Shh signaling can cause or by altering anteroposterior patterning.

Postnatal changes

The postnatal development of the upper limb involves significant growth spurts during childhood and adolescence, characterized by the elongation of long bones through endochondral ossification at epiphyseal plates. These growth plates remain active until fusion occurs, marking skeletal maturity. For instance, the distal epiphysis of the humerus fuses around 12-17 years (earlier in females), while the proximal epiphysis unites between 14 and 21 years (later in males). In a radiographic study of young athletes, the distal epiphysis of the radius fused at a mean age of 16.85 years in males and 16.71 years in females, and the distal ulna at 16.83 years in males and 16.25 years in females. The clavicle, unique in its intramembranous ossification, completes its growth later, with the medial epiphysis fusing around 21-25 years. These fusion timelines vary slightly by sex and population, but generally, upper limb bones achieve full length by late adolescence, supporting increased load-bearing and mobility demands. Bone remodeling in the upper limb continues throughout life, adapting to mechanical stresses in accordance with Wolff's law, which posits that bone architecture and density adjust to the prevailing forces applied to it. In active individuals, repetitive loading from activities like throwing or weightlifting enhances cortical bone density in the humerus and radius, particularly on the dominant side, as seen in athletes where the playing arm exhibits up to 30-40% greater bone mineral density compared to the non-dominant arm. This adaptive response involves osteoblast and osteoclast activity balancing to thicken trabeculae along stress lines, preventing fractures under asymmetric loads typical of upper limb use. In sedentary lifestyles, however, reduced activity leads to decreased bone density, increasing porosity in the proximal humerus and distal forearm by up to 1-2% per decade after age 30. Muscles of the upper limb undergo in response to resistance exercise, increasing cross-sectional area through myofibrillar protein synthesis and cell . This adaptation is evident in the deltoid and biceps brachii, where 8-12 weeks of training can yield 10-20% gains in muscle volume, primarily via enlargement of type II fast-twitch fibers. Fiber type adaptations also occur, with shifting hybrid fibers toward type I slow-twitch profiles for improved fatigue resistance in forearm flexors, while high-intensity strength work promotes type II for power output in shoulder rotators. These changes enhance and reaching precision, but require consistent loading to maintain. Age-related degeneration profoundly affects upper limb function, beginning subtly in mid-adulthood and accelerating after age 60. , the progressive loss of muscle mass and strength, reduces upper extremity lean mass by 25-45% from peak levels, with annual declines of 1-2% in hand and muscles, leading to diminished force generation. commonly develops in joints like the and , eroding and causing subchondral that stiffens the glenohumeral by 20-30% in by age 70. Concurrently, reduced dexterity arises from slower conduction and joint laxity, with fine motor tasks like pinching declining by 15-25% per decade, as correlates inversely with age (r = -0.45). These alterations collectively impair , such as buttoning or lifting. Sexual dimorphism in the upper limb emerges prominently during , driven by hormonal influences on growth and muscle distribution. Males typically develop broader with a 10-15% larger and deltoid mass, enhancing throwing velocity and upper body strength, which averages 40-50% greater than in females by adulthood. In contrast, females exhibit relatively longer forearms and finer hand control, with superior dexterity in precision tasks due to proportionally smaller but more coordinated intrinsic hand muscles, supporting activities like or . These differences, while adaptive, also influence injury susceptibility, with males more prone to shoulder dislocations from expansive leverage.

Clinical Aspects

Common injuries and conditions

Upper extremity injuries and conditions are among the most frequent presentations in clinical settings, often resulting from trauma, overuse, or degenerative processes. In the United States, upper extremity injuries accounted for 28.8% of all visits between 2012 and 2021, with fractures being the leading cause of admissions among these cases. Incidence rates are elevated in athletes due to repetitive stress and in the elderly owing to reduced and balance issues. These pathologies commonly affect mobility and , with mechanisms involving falls, direct impacts, or chronic strain. Fractures represent a major category of upper limb trauma, particularly those of the , humeral shaft, and . , a , typically occurs via a fall on an outstretched hand (FOOSH) mechanism, leading to dorsal displacement and angulation of the distal fragment. Symptoms include immediate pain, swelling, bruising, and a characteristic "dinner fork" , with tenderness over the distal radius. It is the most common forearm fracture, comprising up to 20% of all skeletal injuries and disproportionately affecting postmenopausal women due to . Humeral shaft fractures arise from high-energy direct blows, twisting forces, or low-energy falls, resulting in mid-arm pain, swelling, , and potential causing . These fractures account for 1-5% of all fractures, with an annual incidence of 13-20 per 100,000, more frequent in young males from trauma and older adults from falls. Scaphoid fractures, the most prevalent carpal injury, stem from FOOSH with axial loading and hyperextension, presenting as snuffbox tenderness, pain exacerbated by radial deviation, and swelling without obvious . They constitute 2-7% of all fractures and 60-70% of carpal fractures, predominantly in young active males aged 20-30, with delayed union risk due to precarious blood supply. Soft tissue injuries encompass tendon and nerve pathologies from repetitive microtrauma or acute overload. Rotator cuff tears involve partial or complete detachment of the supraspinatus or other cuff tendons, often from degenerative attrition in older individuals or acute trauma in younger ones via forceful abduction. Symptoms feature pain, particularly at night or with overhead motion, weakness in arm elevation, and limited active . Prevalence reaches 20-30% in asymptomatic adults over 60, rising to 64% in symptomatic cases, with higher rates in overhead athletes. Lateral , known as , results from eccentric overload of the extensor carpi radialis brevis during repetitive wrist extension and grip, causing microtears at the lateral epicondyle. Key symptoms are aching pain over the lateral , worsened by gripping or wrist extension, with possible radiation to the . It affects 1-3% of the general population, peaking in ages 40-50, and up to 50% of recreational players with frequent play. Carpal tunnel syndrome arises from compression within the due to repetitive hand motions, , or anatomical narrowing, leading to nocturnal in the thumb, index, and middle fingers, hand weakness, and thenar atrophy in advanced cases. It has an annual incidence of 1-2 per 1,000, higher in women and manual workers, comprising a significant portion of occupational upper limb disorders. Dislocations disrupt joint stability through traumatic forces, commonly affecting the and joints. Anterior dislocation, accounting for 95-97% of glenohumeral dislocations, occurs via a levering mechanism of abduction, extension, and external , often in contact sports. It presents with severe pain, inability to the arm, a squared-off appearance, and possible injury causing deltoid numbness. The incidence is approximately 0.2 per 1,000 person-years in the general population, with higher rates (up to 2.5 per 1,000) in young males under 25 and athletes. joint dislocations, typically dorsal at the proximal interphalangeal (PIP) or metacarpophalangeal (MCP) joints, result from hyperextension or axial loading in falls or sports impacts. Symptoms include acute pain, swelling, visible deformity, and restricted motion, with potential or . These injuries represent about 9% of sports-related hand traumas, frequent in ball-handling athletes. Inflammatory conditions involve autoimmune or idiopathic processes leading to and synovial changes. profoundly impacts the hand through symmetric of metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints, driven by autoimmune-mediated cytokine release and formation eroding and . Symptoms encompass morning lasting over 30 minutes, warmth, fusiform swelling of fingers, and progressive deformities like ulnar deviation and swan-necking, often with systemic fatigue. It affects 0.5-1% of the population, with hand involvement in 70-90% of cases, more common in women aged 30-50. Adhesive capsulitis, or frozen , features idiopathic synovial inflammation and capsular fibrosis, possibly linked to immobility or endocrine factors, progressing through freezing (pain-dominant), frozen (-dominant), and thawing phases. Primary symptoms are diffuse shoulder aching, night pain disrupting sleep, and global restriction of active and passive motion, with external rotation most limited. Prevalence is 2-5%, higher in women aged 40-60 and diabetics.

Diagnostic and therapeutic approaches

Diagnostic approaches for upper limb disorders typically begin with clinical evaluation, followed by and specialized tests to assess , , and integrity. X-rays are the primary modality for detecting fractures and bony abnormalities in the upper limb, providing quick visualization of alignment and joint spaces. (MRI) excels in evaluating structures such as tendons, ligaments, and muscles, making it essential for diagnosing tears or labral injuries. (EMG) is used to assess function and detect abnormalities in muscle electrical activity, particularly for peripheral neuropathies or injuries. serves as both a diagnostic and therapeutic tool, allowing direct visualization of intra-articular structures in joints like the and to confirm pathologies such as damage or . Non-surgical therapies form the cornerstone of initial management for many upper limb conditions, aiming to reduce , , and promote without invasive procedures. Immobilization using casts, splints, or slings stabilizes fractures or injured , preventing further displacement while allowing recovery; for instance, a coaptation splint supports humeral shaft fractures by accommodating swelling. protocols emphasize controlled exercises to restore function, including passive and active-assisted movements to regain mobility. , such as non-steroidal drugs (NSAIDs) or intra-articular corticosteroids, alleviates and swelling in conditions like adhesive capsulitis or tendinopathies. Surgical interventions are indicated when conservative measures fail or for severe structural damage, focusing on restoring anatomy and function through precise techniques. Rotator cuff repair involves reattaching torn tendons to the humeral head using arthroscopic sutures or anchors, often improving shoulder stability and range of motion. Fracture fixation employs internal hardware like plates and screws to align and stabilize broken bones, such as in distal humerus or clavicle fractures, promoting union while minimizing displacement. Tendon transfers, such as latissimus dorsi to the rotator cuff, address irreparable tears by rerouting functional tendons to restore motion in chronic cases. Rehabilitation following or is critical for optimizing recovery and preventing complications, tailored to the specific injury. Range-of-motion exercises, starting with gentle passive stretches, gradually progress to active movements to restore flexibility and reduce . Strengthening protocols incorporate resistance training for muscles like the or extensors, enhancing endurance and load-bearing capacity post-injury. As of 2025, advances in upper limb care include and enhanced surgical precision. Platelet-rich plasma (PRP) injections promote tendon healing in conditions like lateral epicondylitis by delivering growth factors to stimulate tissue repair, showing improved pain relief and function in clinical studies. Robotic-assisted , particularly in and microsurgery, offers superior precision for procedures like tendon transfers or flap reconstructions, reducing operative time and complications in upper extremity cases.

Comparative Anatomy

Variations in mammals

The upper limb in mammals exhibits remarkable structural diversity, all derived from a conserved pentadactyl (five-digit) blueprint that has been modified to suit varied locomotor and manipulative demands, such as running, , , or . These variations maintain homologous bones—like the , , , carpals, metacarpals, and phalanges—while altering their proportions, fusions, and articulations to optimize function. In quadrupedal mammals like dogs, the is adapted for and high-speed locomotion, featuring an elongated that enhances stride length and mobility. The is broad and massive to support the , which anchors the limb to the , while the is often rudimentary or absent, allowing greater freedom of movement for galloping. These features distribute body weight across the forelimbs and facilitate rapid propulsion, with the and bones oriented more vertically for stability under load. Primates display upper limb modifications emphasizing manipulation and arboreal , with apes possessing opposable formed by a rotatable first metacarpal that enables precise grasping of objects and branches. In , specialized for brachiation (suspensory swinging), the arms are disproportionately elongated relative to the legs, with flexible joints, elongated forearms, and hook-like hands featuring reduced thenar muscles for secure suspension from above. These adaptations prioritize reach and hook-grip strength over ground-based . Aquatic mammals such as whales have transformed their forelimbs into rigid flippers for hydrodynamic control, with carpals often fused into a single bony plate to stiffen the structure and prevent collapse under water pressure. Phalanges are reduced in distinct digit identity but elongated and numerous (hyperphalangy) within each of the five digits, encased in for streamlined steering and stability during swimming. The is shortened and paddle-like, with the and tightly apposed to minimize flexibility. Burrowing species like moles exhibit robust forelimbs optimized for excavating , including strong, well-developed clavicles that articulate directly with the to transmit powerful digging forces from the shoulders. The digits are short and spade-like, with enlarged claws on the third and fourth for and displacing earth, while the features massive deltopectoral crests for enhanced muscle leverage. Overall, the forelimb is compact and , oriented laterally for perpendicular soil penetration. Bone homologues across mammals underscore the pentadactyl foundation, as seen in where embryonic limbs initially form five digit condensations that fuse postnatally: digits I–II and IV–V merge into splint bones, leaving a single enlarged central digit (III) for weight support and speed. This monodactyl condition, with the encasing the fused terminal , exemplifies extreme reduction while retaining traces of the ancestral pattern in vestigial metacarpals.

Evolutionary adaptations

The evolutionary history of the upper limb traces back to the period, when sarcopterygian fish fins began transitioning into limbs around 385 to 360 million years ago, enabling the shift from aquatic to . This transformation involved the development of a robust pectoral girdle, which decoupled from the skull and strengthened to support on land, with endochondral bones enlarging to form the primitive . Fossils like those of Acanthostega illustrate early polydactylous limbs with fin-like features, marking the initial stages of this adaptation for weight support and rudimentary propulsion on substrates. In the synapsid lineage leading to mammals, the pectoral girdle underwent further modifications around 270 million years ago, evolving into a smaller, more flexible structure that facilitated sprawling to parasagittal postures suited for burrowing and scurrying behaviors in early therapsids. This increased shoulder mobility, evident in fossils like those of Dimetrodon and later cynodonts, allowed for greater range of motion and efficiency in terrestrial navigation, setting the stage for mammalian forelimb versatility. Primate evolution, beginning in the Eocene, emphasized arboreal adaptations, with forelimbs developing elongated phalanges and an opposable for precise grasping of branches during locomotion in fine-branch environments. This prehensile configuration, seen in early like Adapis, enhanced stability and maneuverability in trees, diverging from the more generalized mammalian . By the , around 23 to 5 million years ago, the advent of in early hominins freed the forelimbs from locomotor duties, allowing specialization for manipulation as evidenced in fossils of apes like . In human evolution, the precision grip emerged around 2 million years ago in species like Homo erectus, enabling fine motor control for tool-making and use, as indicated by hand bone morphology supporting pad-to-pad opposition of thumb and fingers. This adaptation coincided with brain enlargement, where increased neocortical volume correlated with enhanced manual dexterity across primates, facilitating complex behaviors like crafting Acheulean tools. Fossil evidence from Australopithecus species, such as the scapulae of A. afarensis (e.g., from the "Lucy" skeleton), reveals shoulder blades with ape-like craniocaudal elongation but hints of human-like glenoid positioning, suggesting a transitional role in climbing and emerging prehensility around 3.9 to 2.9 million years ago. Similarly, Neanderthal hand bones from sites like El Sidrón display robust phalanges and carpals adapted for powerful grips, underscoring convergent evolution in dexterity for tool manipulation despite broader upper limb robusticity.

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