Like other features, the rump was observed with a horse at rest on a horizontal surface, front canon bones being vertical. The shape and length of the croup correspond to the position and size of the pelvic bone. They are assessed from the tip of the hip and up to the point of the buttock. The inclination of the croup is measured by the angle between this line and the horizontal. It can be assessed with a compass of carpenter or with adhesive tape and a protractor, as the shoulder. Its length is measured with a folding ruler.
We consider that it is long if the length of the basin is greater than 35% of the height of the horse at the top of the croup, rather long between 35% and 31%. Rather short between 31% and 27% and short if the length of the basin is less than 27% of the height at the top of the croup.
The rump is a set of levers operated by the muscles of the hindquarters. According to their insertion points, they will pull back the stifle and contribute to vertical propulsion, lower hip, bring the pelvis forward and contribute to hind legs engagement, round the dorso-lumbar region and contribute to horizontal propulsion.
It is a mechanism that is well explained in the treaties of anatomy and whose complexity exceeds the ambitions of this guide, but you should remember several points.
The longer is the croup, the more the femur will be and more efficient the lever action will be at equal muscle tone.
The more inclined is the croup, the more the angle with the femur is open and less effective is the lever action for vertical propulsion.
More it is horizontal and less the levers are effective to enable the hind legs engagement.
It is generally assumed that a rump long and inclined between 25 and 35 ° is desirable. Furthermore, the more the stifle joint will be forward of the line extending from the point of the hip to the point of the hock, the more the lever will be effective for vertical propulsion.
Morphological diversity of the croups observed in high-level performers is very important because of the very large interactions between this part of the body and other features of morphology and locomotion.
Disadvantages of a croup too inclined will be amplified by hocks behind the point of the buttock when the canon bone is vertical ("standing on the rear") but instead offset by a hock at the right place with a good natural hind legs engagement.
Disadvantages of a flat Croup can be corrected by a flexible back and a low position of the tip of the hip that facilitates the deployment of the back. Disadvantages of a short rump can be compensated by the tonus of the muscles that make it work.
A long croup requires a powerful and toned muscles to fully play its role of lever. The rump of a horse must have "the assets of its ambitions", i.e. a muscles adapted to the lever arm and the efforts required.
There again, the work of the rider will play an essential role for developing the efficiency and tonicity of the natural muscles
Behind the complexity of the interactions of the functioning of the hindquarters, a long croup, angled between 25° and 35°, coupled to long femur and tibia is favorable to the quality of the vertical and horizontal propulsion.