http://standardbredhorses.yuku.com/topic/465
The Hind End Part I - Haunches and Power
The hind end assembly from the top, starts at the final part of the lumbar span (the front of the lumbar span is the lumbar sacral joint). Above this final support is the croup. From the croup starts the pelvic bone. The "go" muscles of the engine are all located around the pelvis. Big pelvis = big power. Big pelvis is about how wide it is side to side, how long it is front to back, and how deep it is top to bottom.
Note that of all these three measurements the least important to power is the width (side to side). It's more important for mares (foaling).
Second in importance is depth (top to bottom). Depth is important for sprint (take off) or pulling (first gear work). Think race horses and draft horses.
The most important is length (front to back). The length of the pelvic bone is the most important criteria for estimating power potential. You measure a horse's engine power by measuring the pelvic length (hip to point of buttock) and compare it in percentage to the overall body length (point of shoulder to point of buttock). The higher the percent, the bigger the engine of the horse. Think thoroughbred again. They can have pelvic lengths of 35% or higher of their total body length. Anything under 29% and you are driving a Cadillac with a lawnmower engine. The pelvis gets longer as the foal grows and the ratio increases.
The gluteal muscles cover the "rump" from croup to the start of the hamstring muscle at the top of the tail. It is a key "go" muscle. If the horse looks flat (or God forbid concave) from the croup to tail, this muscle is underdeveloped. You want a nice convex "can't wait to grab it with your hands" kind of shape. Directly below the glute reaching down to the stifle is the quadriceps muscle. You will see some horses that look "gaunt" through this area, they should look "full". The quads are responsible for drawing the hind leg forward. Remember one of the signs of a strong loin is the fullness of the "loin girth". Skinny waisted horses with gaunt quads do not coil their loins and step under themselves when they move.
Another motor muscle is the hamstring and it runs down the back of the leg to the hock. Sometimes the hamstring is so prominent, that in addition to it looking huge, there is a vertical "crease" between it and the bicep muscle. While there is nothing wrong with having the crease, the hamstring should not be prominent over the other power muscles and it shouldn't be like iron to the touch. If a horse moves with a stiff back, pushing himself instead of carrying himself, the hamstring is the only power muscle that can develop.
That 'crease' by the way, used to be called a "thoroughbred" line, no need for explanation there. I have heard it more recently called a "poverty line".
Next to pelvic length is pelvic slope. You will remember that slopes are determined against the horizon. A horse can have a more upright pelvis or a flatter pelvis. Flatter pelvis is found in carriage horses. Think the older style Morgan, or when people say a Friesian is more "carriage" style.
When you look at a horses engine, look for the skeletal structure first, then the muscling. See if the skeletal structure and the muscle development "agree" with each other. If the skeletal structure is good, and the muscling doesn't "agree" you need to ask yourself why. Is this horse out of condition (unused or otherwise lame)? Is it being condition in an incorrect way (other conformational issues, or rider error)? On the other side of the coin, if the skeletal structure is poor, I would bet the farm that the muscle development will be also.
Hind End Assembly - Part II - the length of the hind leg.
This is one of the most difficult things to just "see". You need to draw a lot of lines until it becomes clear.
First of all, the hind leg of the horse is incredibly connected and interdependent to its parts. The horse cannot change the angle of the stifle, without changing the angle of the hock. The horse cannot change the angle of the pelvis without changing the angle of the stifle. This is different than how our legs work. I can bend my knee and not change my ankle or my hip joint angles. The horse cannot. Watch all of those joints on your horse the next time your farrier lifts a hind leg up, forward or back.
There are 5 bony segments in the hind leg. If you measure them all, and add them together you will have the length of the hind leg. Because the hind leg is more angulated and not more straight like the front leg, you can't just measure a horse at the croup and know how long the back leg is.
The five segments are:
1) Iliac pelvis (this is from the top of the croup (not the point of the hip) to where the pelvis joins the femur, (not the point of buttock).
2) Femur
3) Gaskin
4) Hind cannon
5) Hind pastern and hoof.
When the horse's leg is long in comparison to his croup (or wither) height, he is said to have "crooked" legs.
When the horse's leg is short in comparison to his croup (or wither) height, he is said to have "straight" legs.
The longer a horse's leg is, the easier he will find it to bring his legs under himself. (dressage)
There is such a thing as TOO long and TOO short.
The longer a horse's leg is, the more predisposed he is to being crooked, camped out, sickle hocked or over angulated.
The shorter a horse's leg the more predisposed he is for sticking stifles.
So measure all those bones and come up with a total. Measure the actual height of the croup from the ground and come up with a percentage. A horse with a straighter hind leg will have a leg that is around 105% - 114% the height of the croup. A horse with a crooked hind leg will be over 112% . Note there is overlap here just because there is no perfect hind leg and everything is a functional trade off (i.e. a horse is the sum of his parts).
EDITED: I'm going to use this skeleton picture that Masquerade found, to show the lines where the THL (total hind leg) is measured. In this artist's rendition, the THL is 114.16% of the croup height.
Part III - Femurs and Gaskins
Short femurs (and as we've noted, long pelvis) are required in horses like race horses and draft horses. Horses that only need one gear, be that FAST or SLOW.
Long femurs go along with horses that need more than one gear. Think polo, reining, dressage, hunter-jumper. These horses need SLOW, MEDIUM, and FAST.
You never want a long gaskin. The longer they are, the skinnier they are, like cannon bones. Ideally the gaskin should be equal to the femur as a rule of thumb.
So looking at the 2 pictures above - which horse is built to have "one gear", which one has multiple gears? Any thoughts about the gaskins?
