| Upper limb | |
|---|---|
 Front of right upper extremity. | |
 Back of right upper extremity. | |
| Details | |
| System | Musculoskeletal |
| Identifiers | |
| Latin | membrum superius |
| MeSH | D034941 |
| TA98 | A01.1.00.019 |
| TA2 | 138 |
| FMA | 7183 |
| Anatomical terminology | |
Theupperlimbs orupper extremities are theforelimbs of anupright-posturedtetrapodvertebrate, extending from thescapulae andclavicles down to and including thedigits, including all themusculatures andligaments involved with theshoulder,elbow,wrist andknuckle joints.[1] Inhumans, each upper limb is divided into theshoulder,arm,elbow,forearm,wrist andhand,[2][3] and is primarily used forclimbing,lifting andmanipulating objects. Inanatomy, just asarm refers to theupper arm,leg refers to thelower leg.
In formal usage, the term "arm" only refers to the structures from the shoulder to the elbow, explicitly excluding theforearm, 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.
In thehuman body, the muscles of the upper limb can be classified by origin, topography, function, or innervation. While a grouping by innervation revealsembryological andphylogenetic 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]
The shoulder girdle[6] or pectoral girdle,[7] composed of theclavicle and thescapula, connects the upper limb to theaxial skeleton through thesternoclavicular joint (the only joint in the upper limb that directly articulates with the trunk), a ball and socket joint supported by thesubclavius muscle which acts as a dynamicligament. While this muscle prevents dislocation in the joint, strong forces tend to break the clavicle instead. Theacromioclavicular joint, the joint between theacromion process on the scapula and the clavicle, is similarly strengthened by strong ligaments, especially thecoracoclavicular 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. Thepelvic 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]
Theglenohumeral joint (colloquially called the shoulder joint) is the highly mobile ball and socket joint between theglenoid 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 therotator 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 andlatissimus 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 thedeltoid andsupraspinatus must cancel the adduction component and theteres minor andinfraspinatus 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]
The bones forming the human upper limb are
The arm proper (brachium), sometimes called the upper arm,[6] the region between the shoulder and the elbow, is composed of thehumerus with theelbow joint at its distal end.
The elbow joint is a complex of three joints — thehumeroradial,humeroulnar, andsuperior radioulnar joints — the former two allowing flexion and extension whilst the latter, together with itsinferior namesake, allows supination and pronation at the wrist.Triceps is the major extensor andbrachialis andbiceps 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]
The forearm (Latin:antebrachium),[6] composed of theradius andulna; 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 thelateral ormedial 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 arebrachioradialis — a strong elbow flexor — andpalmaris longus — a weak wrist flexor which mainly acts to tense thepalmar 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) andpronator teres andpronator 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 bysupinator. Because biceps is much stronger than its opponents, supination is a stronger action than pronation (hence the direction of screws).[10]
Thewrist (Latin:carpus),[6] composed of thecarpal bones, articulates at the wrist joint (orradiocarpal joint) proximally and thecarpometacarpal joint distally. The wrist can be divided into two components separated by themidcarpal 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]
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, andpalmaris 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]
The hand (Latin:manus),[6] themetacarpals (in the hand proper) and thephalanges of the fingers, form themetacarpophalangeal joints (MCP, including theknuckles) andinterphalangeal joints (IP).
Of the joints between the carpus and metacarpus, thecarpometacarpal 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 theflexor digitorum profundus (FDP) andextensor 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]
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 thebrachial plexus:[14]
Collateral branches of the brachial plexus:[14]
Arteries of the upper limb:
ulnar,nutrient andmuscular branches of thebrachial artery.
Veins of the upper limb:
As for the upper limb blood supply, there are many anatomical variations.[15]
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The skeletons of allmammals are based on a commonpentadactyl ("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), andplantigrade (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]
Inprimates, the upper limbs provide a wide range of movement which increases manual dexterity. The limbs ofchimpanzees, compared to those of humans, reveal their different lifestyle. The chimpanzee primarily uses two modes of locomotion:knuckle-walking, a style ofquadrupedalism in which the body weight is supported on the knuckles (or more properly on the middle phalanges of the fingers), andbrachiation (swinging from branch to branch), a style ofbipedalism 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 thegibbons, have very reduced thumbs and inflexible wrists.[16]
Inungulates 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. Theodd-toed ungulates, such as thehorse, use a single third toe for weight-bearing and have significantly reduced metacarpals.Even-toed ungulates, such as thegiraffe, 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 thehippopotamus, have maintained four digits.[16]
In species in the orderCarnivora, some of which areinsectivores rather thancarnivores, thecats 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. Thesloth bear uses their digits and large claws to tear logs open rather than kill prey. Other insectivorous species, such as thegiant andred pandas, have developed largesesamoid bones in their paws that serve as an extra "thumb" while others, such as themeerkat, uses their limbs primary for digging and havevestigial first digits.[16]
Thearborealtwo-toed sloth, aSouth American mammal in the orderPilosa, 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]
