![Horse anatomy](https://upload.wikimedia.org/wikipedia/commons/2/21/Horse_anatomy.svg)
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
A limp is a type of asymmetric abnormality of the gait. Limping may be caused by pain, weakness, neuromuscular imbalance, or a skeletal deformity. The most common underlying cause of a painful limp is physical trauma; however, in the absen ...
, or movement at an abnormal gait. Injuries to and problems with horse legs can be relatively minor, such as stocking up, which causes swelling without lameness, or quite serious. Even leg injuries that are not immediately fatal may still be life-threatening to horses, as their bodies are adapted to bear weight on all four legs and serious problems can result if this is not possible.
Limb anatomy
Horses are
odd-toed ungulates, or members of the order Perissodactyla. This order also includes the extant species of
rhino
A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species o ...
s and
tapir
Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inh ...
s, and many extinct families and species. Members of this order walk on either one toe (like horses) or three toes (like rhinos and tapirs). This is in contrast to
even-toed ungulates
The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...
, members of the order Artiodactyla, which walk on cloven hooves, or two toes. This order includes many species associated with
livestock
Livestock are the domesticated animals raised in an agricultural setting to provide labor and produce diversified products for consumption such as meat, eggs, milk, fur, leather, and wool. The term is sometimes used to refer solely to animal ...
, such as sheep, goats, pigs, cows and camels, as well as species of
giraffes,
antelopes and
deer
Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the re ...
.
According to evolutionary theory, equine hooves and legs have evolved over millions of years to the form in which they are found today. The original ancestors of horses had shorter legs, terminating in five-toed feet. Over millennia, a single hard hoof evolved from the middle toe, while the other toes gradually disappeared into the tiny vestigial remnants that are found today on the lower leg bones. Prairie-dwelling equine species developed hooves and longer legs that were both sturdy and light weight to help them evade predators and cover longer distances in search of food. Forest-dwelling species retained shorter legs and three toes, which helped them on softer ground. Approximately 35 million years ago, a global drop in temperature created a major habitat change, leading to the transition of many forests to grasslands. This led to a die-out among forest-dwelling equine species, eventually leaving the long-legged, one-toed ''
Equus'' of today, which includes the horse, as the sole surviving
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of the
Equidae
Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, asses, and zebras, and many other species known only from fossils. All extant species are in the genus '' Equus'', ...
family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
.
Legs
![Equine-dist-forelimb-bones](https://upload.wikimedia.org/wikipedia/commons/5/56/Equine-dist-forelimb-bones.png)
Each forelimb of the horse runs from the
scapula
The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...
or shoulder blade to the
navicular bone
The navicular bone is a small bone found in the feet of most mammals.
Human anatomy
The navicular bone in humans is one of the tarsal bones, found in the foot. Its name derives from the human bone's resemblance to a small boat, caused by th ...
. In between are the
humerus (arm),
radius
In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
(forearm), elbow joint,
ulna
The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
(elbow),
carpus
In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carpal ...
(knee) bones and joint,
large metacarpal (cannon), small metacarpal (splint),
sesamoid,
fetlock
Fetlock is the common name in horses, large animals, and sometimes dogs for the metacarpophalangeal and metatarsophalangeal joints (MCPJ and MTPJ).
Although it somewhat resembles the human ankle in appearance, the joint is homologous to the ba ...
joint,
first phalanx (long pastern), pastern joint, second phalanx (short
pastern
The is a part of the leg of a horse between the fetlock and the top of the hoof. It incorporates the long pastern bone (proximal phalanx) and the short pastern bone (middle phalanx), which are held together by two sets of paired ligaments to fo ...
), coffin joint, outwardly evidenced by the coronary band, and the
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
The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...
(thigh),
patella
The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as ...
,
stifle
The stifle joint (often simply stifle) is a complex joint in the hind limbs of quadruped mammals such as the sheep, horse or dog. It is the equivalent of the human knee and is often the largest synovial joint in the animal's body. The stifle jo ...
joint,
tibia
The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...
,
fibula
The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...
,
tarsal (hock) bone and joint,
large metatarsal (cannon) and small metatarsal (splint) bones. Below these, the arrangement of sesamoid and phalanx bones and joints is the same as in the forelimbs.
[Harris, p. 226.] When the horse is moving, the distal interphalangeal joint (coffin joint) has the highest amount of stresses applied to it of any joint in the body, and it can be significantly affected by
trimming and
shoeing
Shoe-throwing, or shoeing, showing the sole of one's shoe or using shoes to insult are forms of protest in many parts of the world. Shoe-throwing as an insult dates back to ancient times, being mentioned in verse 8 of Psalm 60 and the simila ...
techniques. Although having a small range of movement, the proximal interphalangeal joint (pastern joint) is also influential to the movement of the horse, and can change the way that various shoeing techniques affect tendons and ligaments in the legs. Due to the horse's development as a
cursorial
A cursorial organism is one that is adapted specifically to run. An animal can be considered cursorial if it has the ability to run fast (e.g. cheetah) or if it can keep a constant speed for a long distance (high endurance). "Cursorial" is often us ...
animal (one whose main form of defense is running), its bones evolved to facilitate speed in a forward direction over hard ground, without the need for grasping, lifting or swinging. The ulna shrank in size and its top portion became the point of the elbow, while the bottom fused with the radius above the radiocarpal (knee) joint, which corresponds to the
wrist
In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carp ...
in humans. A similar change occurred in the fibula bone of the hind limbs. These changes were first seen in the genus ''
Merychippus
''Merychippus'' is an extinct proto-horse of the family Equidae that was endemic to North America during the Miocene, 15.97–5.33 million years ago. It had three toes on each foot and is the first horse known to have grazed.
Discovery and nami ...
'', approximately 17 million years ago.
The anatomy of the forelegs begins at the scapula. This is the shoulder in which provides the ease of movement as it is connected to various bones surrounding it such as the cervical vertebra (a section of the spine). The next bone is the humerus which leads onto the radius below. The radius is then connected to the bones of the knee. The carpus is located at the front of the knee and the pisiform is the back of the knee. Below the knee is the cannon bone which is also known as the 3rd metacarpal. 55 million years ago when the Eohippus existed, the cannon bone used to be the 3rd toe of the foot. Its fusion took place in order to increase height and power of the limb. Behind the cannon bone are the splint bones. The splint bones are also known as the 2nd and 4th metacarpal and fused 25 - 35 million years ago during the time of the Miohippus. They provide extra strength and support of the cannon bone and used to be the 2nd and 4th toes of the foot. Below the cannon bone is the fetlock joint in which lays a structure of many bones. Firstly are the sesamoid bones that act as part of the system that allows the leg to drop as pressure is applied and spring back up as pressure is released. Below this is the proximal phalanx also known as the long pastern which is followed by the middle phalanx (short pastern). Below these bones are the navicular bone and the distal phalanx. the distal phalanx can be known as the coffin bone or the pedal bone. Below the navicular bone is the navicular bursa.
There are three main muscle groups of the forelimb. The
triceps muscle
The triceps, or triceps brachii (Latin for "three-headed muscle of the arm"), is a large muscle on the back of the upper limb of many vertebrates. It consists of 3 parts: the medial, lateral, and long head. It is the muscle principally responsib ...
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
In anatomy, extension is a movement of a joint that increases the angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extendin ...
, while the flexion of the lower leg joints is achieved through movement of the
flexor muscles
Anatomical terminology is a form of scientific terminology used by anatomists, zoologists, and health professionals such as doctors.
Anatomical terminology uses many unique terms, suffixes, and prefixes deriving from Ancient Greek and Latin. ...
. 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 gluteal muscles, often called glutes are a group of three muscles which make up the gluteal region commonly known as the buttocks: the gluteus maximus, gluteus medius and gluteus minimus. The three muscles originate from the ilium and sa ...
, 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
The quadriceps femoris muscle (, also called the quadriceps extensor, quadriceps or quads) is a large muscle group that includes the four prevailing muscles on the front of the thigh. It is the sole extensor muscle of the knee, forming a large ...
group of muscles on the front of the femur, while the muscles at the back of the hindquarters, called the
hamstring
In human anatomy, a hamstring () is any one of the three posterior thigh muscles in between the hip and the knee (from medial to lateral: semimembranosus, semitendinosus and biceps femoris). The hamstrings are susceptible to injury.
In quadrupe ...
group, provide forward motion of the body and rearward extension of the hind limbs. Extension of the hock is achieved by the
Achilles tendon
The Achilles tendon or heel cord, also known as the calcaneal tendon, is a tendon at the back of the lower leg, and is the thickest in the human body. It serves to attach the plantaris, gastrocnemius (calf) and soleus muscles to the calcaneus ( ...
, 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 of the horse, both when standing and while moving, and prevents the fetlock joint from hyperextending, especially when the joint is bearing weight. During movement, the apparatus stores and releases energy in the manner of a spring: stretching while the joint is extended and contracting (and thus releasing energy) when the joint flexes. This provides a rebound effect, assisting the foot in leaving the ground.][ This ability to use stored energy makes horses' gaits more efficient than other large animals, including ]cattle
Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult ma ...
. 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.[Harris, pp. 251–253.]
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 the hind limbs consists of the major muscles, ligaments and tendons, as well as the reciprocal joints
Reciprocal may refer to:
In mathematics
* Multiplicative inverse, in mathematics, the number 1/''x'', which multiplied by ''x'' gives the product 1, also known as a ''reciprocal''
* Reciprocal polynomial, a polynomial obtained from another pol ...
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
The navicular bone is a small bone found in the feet of most mammals.
Human anatomy
The navicular bone in humans is one of the tarsal bones, found in the foot. Its name derives from the human bone's resemblance to a small boat, caused by th ...
, 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.
Remnants of the "lost" digits of the horse can be found on the hoof.
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, usually around 60 percent, with exact percentages depending on speed and gait. Movement adds concussive force to weight, increasing the likelihood that a poorly built leg will buckle under the strain. At different points in the gallop
The canter and gallop are variations on the fastest gait that can be performed by a horse or other equine. The canter is a controlled three-beat gait, while the gallop is a faster, four-beat variation of the same gait. It is a natural gait pos ...
, all weight is resting on one front hoof, then all on one rear hoof.[ In the sport of dressage, horses are encouraged to shift their weight more to their hindquarters, which enables lightness of the forehand and increased collection. While the forelimbs carry the weight the hind limbs provide propulsion, due to the angle between the stifle and hock. This angle allows the hind legs to flex as weight is applied during the stride, then release as a spring to create forward or upward movement. The propulsion is then transmitted to the forehand through the structures of the ]back
The human back, also called the dorsum, is the large posterior area of the human body, rising from the top of the buttocks to the back of the neck. It is the surface of the body opposite from the chest and the abdomen. The vertebral column runs ...
, where the forehand then acts to control speed, balance and turning. The range of motion and propulsion power in horses varies significantly, based on the placement of muscle attachment to bone. The muscles are attached to bone relatively high in the body, which results in small differences in attachment making large differences in movement. A change of in muscle attachment can affect range of motion by and propulsion power by 20 percent.
"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
Cutting is the separation or opening of a physical object, into two or more portions, through the application of an acutely directed force.
Implements commonly used for wikt:cut, cutting are the knife and saw, or in medicine and science the scal ...
, in which quick starts, stops and turns are required, will be shorter and more thickly built than those of a Thoroughbred
The Thoroughbred is a horse breed best known for its use in horse racing. Although the word ''thoroughbred'' is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thoroughbred breed. Thoroughbreds are ...
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
A limp is a type of asymmetric abnormality of the gait. Limping may be caused by pain, weakness, neuromuscular imbalance, or a skeletal deformity. The most common underlying cause of a painful limp is physical trauma; however, in the absen ...
. 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.[Harris, pp. 265–266.] Poor conformation and structural defects do not always cause lameness, however, as was shown by the champion racehorse Seabiscuit
Seabiscuit (May 23, 1933 – May 17, 1947) was a champion thoroughbred racehorse in the United States who became the top money-winning racehorse up to the 1940s. He beat the 1937 Triple Crown winner, War Admiral, by four lengths in a two-horse ...
, who was considered undersized and knobby-kneed for a Thoroughbred
The Thoroughbred is a horse breed best known for its use in horse racing. Although the word ''thoroughbred'' is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thoroughbred breed. Thoroughbreds are ...
.[
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 they 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-hocked), too straight (straight behind) or having an inward deviation (]cow-hocked
Cow-hocked (adj.) or cow hocks ( n.) describes a defect in the conformation of four-legged animals, primarily of livestock and horses, but also of dogs and cats.
Description
An animal is cow-hocked when its hock is set inward, resulting in a s ...
). Feral horse
A feral horse is a free-roaming horse of domesticated stock. As such, a feral horse is not a wild animal in the sense of an animal without domesticated ancestors. However, some populations of feral horses are managed as wildlife, and these ...
s 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 and injuries
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 digestive, circulatory
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
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
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...
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.
There are numerous issues that can occur with horses' legs that may not necessarily cause lameness. Stocking up is an issue that occurs in horses that are held in stalls for multiple days after periods of activity. Fluid collects in the lower legs, producing swelling and often stiffness. Although it does not usually cause lameness or other problems, prolonged periods of stocking up can lead to other skin issues. Older horses and horse with heavy muscling are more prone to this condition. A shoe boil is an injury that occurs when there is trauma to the bursal sac of the elbow, causing inflammation and swelling. Multiple occurrences can cause a cosmetic sore and scar tissue, called a capped elbow, or infections. Shoe boils generally occur when a horse hits its elbow with a hoof or shoe when lying down. Windpuffs, or swelling to the back of the fetlock caused by inflammation of the sheaths of the deep digital flexor tendon, appear most often in the rear legs. Soft and fluid-filled, the swelling may initially be accompanied by heat and pain, but can remain long after the initial injury has healed without accompanying lameness. Repeated injuries to the tendon sheath, often caused by excessive training or work on hard surfaces, can cause larger problems and lameness.
Leg injuries that are not immediately fatal still may be life-threatening because a horse's weight must be distributed evenly on all four legs to prevent circulatory
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
problems, laminitis
Laminitis is a disease that affects the feet of ungulates and is found mostly in horses and cattle. Clinical signs include foot tenderness progressing to inability to walk, increased digital pulses, and increased temperature in the hooves. Seve ...
, and other infections. If a horse loses the use of one leg temporarily, there is the risk that other legs will break down during the recovery period because they are carrying an abnormal weight load. While horses periodically lie down for brief periods of time, a horse cannot remain lying in the equivalent of a human's "bed rest
Bed rest, also referred to as the rest-cure, is a medical treatment in which a person lies in bed for most of the time to try to cure an illness. Bed rest refers to voluntarily lying in bed as a treatment and not being confined to bed because of ...
" because of the risk of developing sores, internal damage, and congestion.
Notes
References
*
* {{cite book, title=The United States Pony Club Manual of Horsemanship: Advanced Horsemanship – B, HA, A Levels, author=Harris, Susan E., publisher=Howell Book House, year=1996, isbn=978-0876059814
External links
Lameness from The Horse
Equine Anatomy and Physiology: The Forelimb
Horse anatomy