Fetlock

Fetlock is the common name for the metacarpophalangeal and metatarsophalangeal joint

Joint

A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...

s (MCPJ and MTPJ) of horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

s, large animals, and sometimes dogs. It is formed by the junction of the third metacarpal (forelimb) or metatarsal (hindlimb) bones (common name: the cannon bones) proximad and the proximal phalanx distad (common name: the pastern

Pastern

The pastern is a part of the leg of a horse between the fetlock and the top of the hoof. It incorporates the long pastern bone and the short pastern bone , which are held together by two sets of paired ligaments to form the pastern joint...

 bone). Paired proximal sesamoid bones articulate with the palmar or plantar distal surface of the third metacarpal or metatarsal bones and are rigidly fixed to the proximo-palmar / -plantar edge of the proximal phalanx.

The fetlock is a hinge joint

Hinge joint

A hinge joint is a bone joint in which the articular surfaces are molded to each other in such a manner as to permit motion only in one plane—backward and forward—the extent of motion at the same time being considerable....

 (ginglymus), allowing flexion

Flexion

In anatomy, flexion is a position that is made possible by the joint angle decreasing. The skeletal and muscular systems work together to move the joint into a "flexed" position. For example the elbow is flexed when the hand is brought closer to the shoulder...

 and extension

Extension (kinesiology)

In kinesiology, extension is a movement of a joint that results in increased 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 extending the flexed elbow. Straightening of...

, but minimal rotation

Rotation

A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

, adduction

Adduction

Adduction is a movement which brings a part of the anatomy closer to the middle sagittal plane of the body. It is opposed to abduction.-Upper limb:* of arm at shoulder ** Subscapularis** Teres major** Pectoralis major** Infraspinatus...

, or abduction

Abduction (kinesiology)

Abduction, in functional anatomy, is a movement which draws a limb away from the median plane of the body. It is thus opposed to adduction.-Upper limb:* of arm at shoulder ** Supraspinatus** Deltoid* of hand at wrist...

.

While sometimes the fetlock is colloquially referred to as an "ankle," even by horse experts, that terminology is not correct. The fetlock actually is a metacarpophalangeal joint

Metacarpophalangeal joint

The metacarpophalangeal joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cavities on the proximal ends of the first phalanges, with the exception of that of the thumb, which presents more of the characters of a ginglymoid joint...

 which corresponds to the human upper knuckle

Knuckle

The knuckles are the joints of the fingers and toes, which are brought into prominence when the hand is clenched and a fist is made. The word is cognate to similar words in other Germanic languages, such as the Dutch "Knokkel" or German "Knöchel" , i.e., Knöchlein, the diminutive of the German...

, such as that on the ball

Ball (anatomy)

The ball of the foot is where the toes join with the rest of the foot.Source:...

 of the foot.

Etymology and related terminology

The word fetlock literally means "foot-lock" and originally referred to the small tuft of hair situated on the rear of the fetlock joint.

"Feather

Feathering (horse)

Feathering, or feather, is a term used to describe the long hair on the lower legs and fetlocks of some breeds of horse and pony. On some horses, especially draft breeds, the hair can almost cover the hooves...

" refers to the particularly long, luxuriant hair growth over the lower leg and fetlock that is characteristic of certain breeds.