The stay apparatus is a group of ligaments, tendons and muscles which "lock" major joints in the

limbs of the horse 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 simil ...

. It is best known as the mechanism by which

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...

s can enter a

light sleep Non-rapid eye movement sleep (NREM), also known as quiescent sleep, is, collectively, sleep stages 1–3, previously known as stages 1–4. Rapid eye movement sleep (REM) is not included. There are distinct electroencephalographic and other char ...

while still standing up. It does, however, exist in other large land mammals, where it plays a role in reducing fatigue while standing. The stay apparatus allows animals to relax their muscles and doze without collapsing. (Horses are able to sleep lying down as well.) The stay apparatus is an arrangement of muscles, tendons and ligaments that work together so that an animal can remain standing with virtually no muscular effort. The effect is that an animal can distribute its weight on three limbs while resting a fourth in a flexed, non-weight bearing position. The animal can periodically shift its weight to rest a different leg and thus all limbs are able to be individually rested, reducing overall wear and tear. The relatively slim legs of certain large mammals such as horses and cows would be subject to dangerous levels of fatigue if not for the stay apparatus. The lower part of the stay apparatus consists of the suspensory apparatus, which is the same in both front and hind legs, while the upper portion of the stay apparatus is different between the fore and hind limbs. In the front legs, the stay apparatus automatically engages when the animal's muscles relax. The upper portion of the stay apparatus in the forelimbs includes the major attachment, extensor and flexor muscles and tendons. In essence, the accessory check ligaments act as tension bands, they stabilize the carpus (called the "knee" in horses), fetlock and bones of the foot. In the upper portion, the shoulder and elbow joints have several musculo-tendinous structures that keep these joints in passive extension. In the hind limbs, the major muscles, ligaments and tendons work with the reciprocal joints of the hock and stifle,Harris, p. 253 which are a reciprocal apparatus that forces the hock and stifle to flex and extend in unison. The medial patellar ligament "locks" the

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 ...

("kneecap") in place and this prevents flexion in both the stifle and the hock. At the stifle joint, a "hook" structure on the inside bottom end of 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 ...

cups the patella and the medial patella

ligament A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal l ...

, prevents the leg from bending. Cattle have a stay apparatus which allows them to rest individual limbs, but cattle generally do not sleep standing up. Dinohippus leidyanus foot bones

Anatomical structures important in the stay apparatus include: *The suspensory apparatus, including the superficial and deep digital flexor tendons along with the proximal and distal check ligaments. The distal sesamoidean ligaments run from the sesamoid bones to the two

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 ...

bones. *Biceps brachii: originates from the caudal side of the scapula and inserts into the radial tuberosity. Flexes the elbow, and is the part of the stay apparatus that keeps the elbow and shoulder from bending. *Triceps brachii: has three heads which originate and insert into separate places: the caudal side of the scapula and into the lateral & caudal side of the olecranon, from the humerus and into the lateral side of the olecranon, and from the medial side of the humerus and into the medial and cranial side of the olecranon. The triceps brachii is the most important extensor of the elbow. Important part of the stay apparatus to keep the elbow fixed. *Extensor carpi radialis: originates from the humerus, continues distally along the dorsal side of the radius, and inserts on the metacarpal tuberosity. Flexes the elbow, extends the carpus. Also used in the stay apparatus to fix the carpus. *The patellar tendon and patellar ligaments. The most common of the ancient, now-extinct

wild horse The wild horse (''Equus ferus'') is a species of the genus ''Equus'', which includes as subspecies the modern domesticated horse (''Equus ferus caballus'') as well as the endangered Przewalski's horse (''Equus ferus przewalskii''). The Europea ...

species in North America, '' Dinohippus'', had a distinctive passive stay apparatus that helped it conserve energy while standing for long periods. ''Dinohippus'' was the first horse to show a rudimentary form of this characteristic, and its existence provided additional evidence of the close relationship between ''Dinohippus'' and the modern '' Equus''.Florida Museum of Natural History
/ref>

References

* {{cite book, title=The United States Pony Club Manual of Horsemanship: Advanced Horsemanship - B, HA, A Levels, last=Harris , first=Susan E., publisher=Howell Book House, year=1996, isbn=0876059817 Horse anatomy