In order for the hind end to function properly, certain mechanics have to happen. It is important to understand how the hind end works so that when you are riding your horse you can help keep it working in good order. It is not uncommon for us to train the hind end wrong, meaning we are asking the horse to collect or sit under more when the spine is shortening versus lengthening. If your horse has a "hunters bump" or a dropped loin with a flat croup, then you are training the hind end incorrectly. A hind end that is working correctly lengthens back to the tail and drives the legs under.
To answer these questions, it is helpful to have a general understanding of how the muscles and bones work. To be able to see which direction the bones are moving in and to know wether or not the movement is correct. With this knowlege you will be able to bring out the best in your horse and lessen your chances of injury.
Sacrum and Lumbar Point of Croup
The function of the hind end sould go as follows.
The spine engages the hind end.
The point of croups lift and separate.
The points of hips narrow.
The seat bones drop.
The lumbar lift and lengthen to the tail.
The sacrum bone glides back to the tail.
The pelvis (ilium bones) swivel around on the greater trochanter.
How the sacrum looks from a bottom view How the sacrum looks from a top view