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The upper limbs or upper extremities are the of an upright posture tetrapod vertebrate, extending from the and down to and including the digits, including all the and involved with the shoulder, elbow, wrist and knuckle joints. In , each upper limb is divided into the shoulder, arm, elbow, forearm, wrist and
/ref> and is primarily used for climbing, lifting and dexterity objects. In anatomy, just as arm refers to the upper arm, leg refers to the lower leg.
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.
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 muscle 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.
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 brachii is the major extensor and brachialis and Biceps brachii the major flexors. Biceps is, however, the major supinator and while performing this action it ceases to be an effective flexor at the elbow.
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.
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).
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.
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.
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.
Motor innervation of upper limb by the five terminal nerves of the brachial plexus:
Collateral branches of the brachial plexus:
Veins of the upper limb:
As for the upper limb blood supply, there are many anatomical variations.
In , 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 , have very reduced thumbs and inflexible wrists.
In 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.
In species in the order Carnivora, some of which are rather than , the 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 panda and , have developed large 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.
The arboreal two-toed sloth, a 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.
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