From fhotd member Apples
http://standardbredhorses.yuku.com/topic/461
The length of a horse's back plays a role in a number of things, movement and saddle fitting are at the top of the list and not necessarily in that order.
A longer back can provide a smoother ride - If you liken it to a car, a long back horse is like a limo compared to a jeep i.e. the "wheel base" is longer. A longer back gives more room to fit a saddle than a short back. A longer back can mean that a horse will find bending through the torso is easier.
Like a suspension bridge, the back finds structural strength at either end: In front, by the withers and in the rear by the lumbar sacral joint. The third place a back finds strength is in the abdominal muscles found within and around the rib cage.
For back strength, look for:
1) withers that are connected well to the neck, and reach into the back, (they don't just "drop off" suddenly at either end).
2) a well-coupled, smooth textured, short loin.
3) a rib cage length that is as long, or longer than the back.
The key to withers that reach back and a well-coupled loin is that there is no sign of damage in how those supports connect to the back, either in the joints themselves or in the soft tissue around those connections.
The rib cage should be the same length as the back or longer. You want as little unsupported area between the last rib and the hip as possible.
The back is proportional to the body length, and the rib cage length. A too long back is one that is proportionally too long for the horse's body, AND proportionally too long for the rib cage.
Proportional Back Length:
Measure the length of the back from the point of wither to the lumbar sacral joint.
Measure the body length line from the point of shoulder to the point of buttock.
Compare those 2 lines to determine the proportional back length: Back length line x 100 / by the body length.
Proportional Rib Cage Length:
The rib cage line is measured from the point of withers to the last rib where it meets the body length line. So start by marking the body length line (a line from the point of shoulder to the point of buttock). Mark where the last rib intersects with that line. Connect that mark with the point of wither and measure the length of it.
Compare the back length line and the rib length line to determine the proportional rib cage length: Rib cage line x 100 / by back length.
Good proportions summary:
If the proportional back length is 45 % of the length of the horse or less.
If the proportional rib cage length is 100 % of the length of the back or more.
Summary: what is too long - and it is good or bad?
If the proportional back length is more than 45 % AND the rib cage length is less than 100% you have an overall long back. It misses both those marks on the proportional analysis. The "too long" will MOST LIKELY occur in the loin area.
If the proportional back length is more than 45% and the rib cage length is MORE than 100% you also have a long back. In general, the "too long" will most likely NOT be occurring in the loin area.
So a "long back" can be either good, if it is properly supported. Or it can be bad, if it is not properly supported. And the bottom line, the back length should fit with the discipline you are doing. And once again, we find that the loin area is critical.
