According to Webster's Dictionary, conformation is the formation of something by the appropriate proportion of parts or elements assembling the whole. It is the components of the "structure". Conformation is the proportionate components of a donkey's skeletal structure. 

The proportional elements of conformation are the foundation of bone under the muscles and tendons that allow him to stand, walk, trot, and run. Proportion is the length of bone that creates the angle the bones connect at the joints. A balanced proportion of skeletal structure evenly distributes the stress of the donkey's weight on all four legs when he is standing or moving. Whether a donkey is 32 inches tall or 64 inches tall the appropriate proportion of his skeletal structure is the same. 

Diagram A is the skeletal structure of a normal proportioned donkey. The weight of his skull is in proportion to the weight of the heavier bone in his pelvis, thigh and hind leg bones allowing him to counter-balance the weight of his head. The height of his hip is the peak where his loin (lumbar vertebrae) connects to his croup (sacral vertebrae) at his pelvis. When the height of the hip is no more than 5 degrees higher than the height of his withers it reveals his spine has no more than a 5 degree upward slope making it easily possible for him to evenly distribute the weight of his torso between his front legs and hind legs.

Disproportional bone length creates joint angles that shift the weight heavily to one part of his body and can causes friction, impact and compression on those joints leading to excessive wear, tendon and hoof problems, stress related injuries and early onset of arthritis.

Diagram B shows a steep shoulder that increases the impact on the shoulder, elbow, knee, and fetlocks joints affecting the health and soundness of his front legs. The steep hip angle increases the impact at the hip and stifle (knee cap) creating the likelihood of recurring stifle injuries. The disproportionately long upper leg bones (fibula and tibia) create a "sickle hock" which can lead to stress related injuries in the hock and fetlock.

As the degree of disproportion of the skeletal structure increases it increases the stress on the joints.

In Diagram C the disproportion of this donkey example shows obvious dwarf-tendencies of a high hip that is ten degrees higher than the withers showing his spine is forced into an excessive upward slope to his pelvis. This shifts the burden of weight onto the donkey's shoulders, elbows, knees, fetlocks and coffin bones. The backward tilt of the pelvis creates increased stress at the knee cap (stifle). The oversize head adds an additional burden on the already stressed forequarter joints plus increases the likelihood of a cervical (neck) injury.

Diagram D shows the extreme disproportion of the skeletal structure of a dwarf with an oversize head, short ewe neck which does not attach cleanly, excessively short fore cannons with the thick knee and fetlock joints which can be mistaken for heavy "drafty" bone, a long back which is weak in the loin, high hip, excessive upward slope of the spine, and sickle hocks. This poor donkey is a poster child for what you do not want in your breeding herd. She has inherited severe conformation faults of disproportion of her skeletal structure from her sire and dam and their ancestors. She will pass these disproportional faults on to her foals no matter how good the conformation is of the jack she is bred to. She is going to be prone to impact and stress injuries of her joints and it is not likely she will be able to live out her life without arthritic joints plaguing her with pain and recurring bouts of founder.

The most important individual component of a donkey's conformation is his legs. Even if he has appropriate skeletal proportion in the rest of his body without strong, solid legs under him to carry his weight he is going to be prone to injuries related to movement. The legs are the most mechanically complicated element of a donkey's skeletal structure containing 20 bones connected by 6 joints in a single leg. When walking or trotting/pacing his entire weight will alternate between being carried on two legs at a time.

Photograph 1: This donkey is walking with a parallel stride. Her entire weight and the weight of the rider is being carried on legs A and B while legs C and D are in motion.

Photograph 2: This donkey is walking with a diagonal stride. His weight is being carried on legs A and B while legs C and D are in motion.

Photograph 3: This donkey is trotting with a diagonal stride. His weight is being carried on legs A and B while legs C and D are in motion.

Photograph 4: This donkey is pacing with a parallel stride. His weight is being carried on legs A and B while legs C and D are in motion.

These examples show the burden of weight put on a healthy, well-conformed pair of legs every time the donkey takes a step. If one leg of the weight-bearing pair has a conformation fault, the leg paired with the weak leg would suffer an additional burden of weight which can lead to stress and impact injuries. A donkey needs four legs to carry his weight. If one of those legs is unable to carry its share of the load, then the other three legs have to work harder. If a human sprains his ankle or knee, or even has an ingrown toenail that causes pain with every step, you naturally shift additional weight onto your other foot and before long you are hobbling on both legs because of the additional burden on your healthy foot. The same thing happens with a quadrupedal animal such as a donkey. Notice in Diagram A of a normal well-proportioned donkey how much heavier the bone is in the hind legs compared to the front legs. Nature designed the heavier bone in the hindquarters because the hindquarters are the propulsion of the donkey pushing him forward.

Diagram E shows how the spine is attached to the pelvis like an interlocking piece of a puzzle and the pelvis is attached to the femur with a ball and socket joint. The pelvis compares to the transmission of your vehicle. Because it is the foundation of the donkey's ability to propel himself, the angle of the pelvis determines the angle of connection for his power train which are his hind legs. The pelvis of Donkey A in the diagram is at an appropriate angle to allow him to propel himself with little effort. The pelvis of Donkey B in the diagram is at a steep, backward tilt. This effects her ability to reach forward or extend her hind legs in stride. Her femur attachs to her pelvis at a sharper angle which follows through to the connection angle at her knee cap (stifle) which will make her more prone to stifle injuries.

Diagram F is a rear view of the hind legs showing the joint angles in a normal, narrow, wide, and cow hock stance. In a normal stance there is a vertical drop from where the femur connects to the pelvis straight down through the center of the hock to the center of the heel. This direct line of balance in the hind leg avoids unnecessary sideways wobble as the donkey strides forward. In a narrow stance there is stress on the hocks and fetlocks from the donkey's weight as he strides and stands. In a wide stance the stress is shifted to his hip joints. With cow hock the joint stress is to the entire leg from hip, stifle, hock, and fetlock related to motion because there is an outward swing as he brings his leg forward.

Even though the front legs of your donkey bear most of his weight whether he is standing or in motion, they are not connected bone to bone to the spine like the hind legs. The forequarters are suspended in a flexible cradle of the scapula or shoulder blades connecting to the spine with tendons, ligaments, and cartilage. If a donkey has a hindquarter conformation fault it does affect the health of the front legs. A donkey with cowhocks or a wide or narrow hindquarter stance will be more prone to shoulder, knee, and fetlock injuries of the front legs. Just as the angle of the pelvis determines the hindquarter structure; the angle of the shoulder determines the angles of connection for the front leg structure. A steep shoulder puts stress on all of the front leg joints.

Diagram G shows an example of being "over at the knees" a common fault related to a steep shoulder.

Diagram H shows a correct hoof angle.

Diagram I shows a clubfoot, a serious conformation fault of the foot.