User:Dana boomer/Sandbox3

__NOINDEX__ The limbs of the horse are structures made of dozens of bones, joints, muscles, tendons and ligaments that support the weight of the equine body. They include two apparatus: the suspensory apparatus, which carries much of the weight, prevents overextension of the joint and absorbs shock, and the stay apparatus, which locks major joints in the limbs, allowing horses to remain standing while relaxed or asleep. The limbs play a major part in the movement of the horse, with the legs performing the functions of absorbing impact, bearing weight, and providing thrust. The hoofs are also important structures, providing support, traction and shock absorption, and containing structures which provide blood flow through the lower leg.

Good conformation in the limbs leads to improved movement and decreased likelihood of injuries. Large differences in bone structure and size can be found in horses used for different activities, but correct conformation remains relatively similar across the spectrum. Structural defects, as well as other problems such as injuries and infections, can cause lameness, or movement at an abnormal gait.

Limb anatomy
Each forelimb of the horse runs from the scapula or shoulder blade to the navicular bone. In between are the humerus (arm), radius (forearm), ulna (elbow), carpal (knee), large metacarpal (cannon), small metacarpal (splint), sesamoid, first phalanx (long pastern), second phalanx (short pastern) and third phalanx (coffin or pedal) bones. Each hind limb of the horse runs from the pelvis to the navicular bone. After the pelvis come the femur (thigh), patella, tibia, fibula, tarsal (hock), large metatarsal (cannon) and small metatarsal (splint) bones. Below these, the arrangement of sesamoid and phalanx bones is the same as in the forelimbs. In the forelimbs there are also the elbow, knee, fetlock, pastern and coffin joints. The hind limbs contain the stifle and hock joints in the upper portion, and the same arrangement of fetlock, pastern and coffin joints as the lower forelimbs.

There are three main muscle groups of the forelimb. The triceps muscle straightens the elbow and foreleg, running from the elbow to the bottom of the shoulder blade. The muscles which extend the lower leg are called extensor muscles, while the flexion of the lower leg joints is achieved through movement of the flexor muscles. There are five main muscles and muscle groups in the hind legs. The vastus muscle flexes the hind leg and runs from stifle to hip, while the gluteal muscles, the large muscles in the hip, extend the femur. Forward motion and flexion of the hind legs is achieved through the movement of the quadriceps group of muscles on the front of the femur, while the muscles at the back of the hindquarters, called the hamstring group, provide forward motion of the body and rearward extension of the hind limbs. Extension of the hock is achieved by the Achilles tendon, located above the hock.

There are two apparatus in the limbs of the horse - the suspensory apparatus and the stay apparatus. The fetlock joint is supported by group of lower leg ligaments, tendons and bones known as the suspensory apparatus. This apparatus carries much of the weight, prevents overextension of the joint and absorbs shock. It also helps provide a rebound effect, which assists the foot in leaving the ground. The suspensory apparatus consists of the suspensory ligament, the check ligament, the deep digital flexor tendon, the superficial flexor tendon, the common digital extensor tendon and the sesamoid bones. Horses use a group of ligaments, tendons and muscles known as the stay apparatus to lock major joints in the limbs, allowing them to remain standing while relaxed or asleep. The lower part of the stay apparatus consists of the suspensory apparatus, which is the same in both sets of limbs, while the upper portion differs between the fore and hind limbs. The upper portion of the stay apparatus in the forelimbs includes the major attachment, extensor and flexor muscles and tendons. The same portion in he hind limbs consists of the major muscles, ligaments and tendons, as well as the reciprocal joints of the hock and stifle.

Hoof
The hoof of the horse contains over a dozen different structures, including bones, cartilage, tendons and tissues. The coffin or pedal bone is the major hoof bone, supporting the majority of the weight. Under the coffin bone is the navicular bone, itself cushioned by the navicular bursa, a fluid-filled sac. The digital cushion is a blood vessel-filled structure located in the middle of the hoof, which assists with blood flow throughout the leg. At the top of the hoof wall is the corium, tissue which continually produces the horn of the outer hoof shell, which is in turn protected by the periople, a thin outer layer which prevents the interior structures from drying out. The wall is connected to the coffin bone by sensitive laminae, a flexible layer which helps to suspend and protect the coffin bone. The main tendon in the hoof is the deep digital flexor tendon, which connects to the bottom of the coffin bone. The impact zone on the bottom of the hoof includes the sole, which has an outer, insensitive layer and a sensitive inner layer, and the frog, which lies between the heels and assists in shock absorption and blood flow. The final structures are the lateral cartilages, connected to the upper coffin bone, which act as the flexible heels, allowing hoof expansion. These structures allow the hoof to perform many functions. It acts as a support and traction point, shock absorber and system for pumping blood back through the lower limb.

Movement


A sequence of movements in which a horse takes a step with all four legs is called a stride. During each step, with each leg, a horse completes four movements: the swing phase, the grounding or impact, the support period and the thrust. While the horse uses muscles throughout its body to move, the legs perform the functions of absorbing impact, bearing weight, and providing thrust. Good movement is sound, symmetrical, straight, free and coordinated, all of which depend on many factors, including conformation, soundness, care and training of the horse, and terrain and footing. The proportions and length of the bones and muscles in the legs can significantly impact the way an individual horse moves. The angles of certain bones, especially in the hind leg, shoulders, and pasterns, also affect movement. The forelegs carry the majority of the weight, with exact percentages depending on speed and gait. At one point in the gallop, all weight is resting on one front hoof.

"Form to function" is a term used in the equestrian world to mean that the "correct" form or structure of a horse is determined by the function for which it will be used. The legs of a horse used for cutting, in which quick starts, stops and turns are required, will be shorter and more thickly built than those of a Thoroughbred racehorse, where forward speed is most important. However, despite the differences in bone structure needed for various uses, correct conformation of the leg remains relatively similar.

Structural defects
The ideal horse has legs which are straight, correctly set and symmetrical. Correct angles of major bones, clean, well-developed joints and tendons, and well-shaped, properly-proportioned hooves are also necessary for ideal conformation. "No legs, no horse" and "no hoof, no horse" are common sayings in the equine world. Individual horses may have structural defects, some of which lead to poor movement or lameness. Although certain defects and blemishes may not directly cause lameness, they can often put stress on other parts of the body, which can then cause lameness or injuries. Poor conformation and structural defects do not always cause lameness, however, as was shown by the champion racehorse Seabiscuit, who was considered undersized and knobby-kneed for a Thoroughbred.

Common defects of the forelegs include base-wide and base-narrow, where the legs are farther apart or closer together on the ground then they are when the originate in the chest; toeing-in and toeing-out, where the hooves point inwards or outwards; knee deviations to the front (buck knees), rear (calf knees), inside (knock knees) or outside (bowleg); short or long pasterns; and many problems with the feet. Common defects of the hind limbs include the same base-wide and base-narrow stances and problems with the feet as the fore limbs, as well as multiple issues with the angle formed by the hock joint being too angled (sickle hock), too straight (straight behind) or having an inward deviation (cow hocked). Feral horses are seldom found with serious conformation problems in the leg, as foals with these defects are generally easy prey for predators. Foals raised by humans have a better chance for survival, as there are therapeutic treatments that can improve even major conformation problems. However, some of these conformation problems can be transmitted to offspring, and so these horses are a poor choice for breeding stock.

Lameness
Lameness in horses is movement at an abnormal gait due to pain in any part of the body. It is frequently caused by pain to the shoulders, hips, legs or feet. Lameness can also be caused by abnormalities in the metabolic, circulatory and nervous systems. While horses with poor conformation and congenital conditions are more likely to develop lameness, trauma, infection and acquired abnormalities are also causes. The largest cause of poor performance in equine athletes is lameness caused by abnormalities in the muscular or skeletal systems. The majority of lameness is found in the forelimbs, with at least 95 percent of these cases stemming from problems in the structures from the knee down. Lameness in the hind limbs is caused by problems in the hock and/or stifle 80 percent of the time.