I'm reviewing my understanding of hindquarter conformation at present, in particular I'm looking to better understand the relationship between sacrum, pelvis and lumbar.
In your principles of conformation series volume 2, you compare the loin of different horses and note that some have the lumbosacral joint placed more favourably close to the intersection of a line drawn between the two points of hip.
What I can't seem to 'picture' is what it is structurally that allows for the variation in position of LS joint relative to point of hip. I.e. what is structurally different between a horse with a well placed LS joint and one without? It seems in my mind as if the only way the LS joint could vary in position behind the point of hip would be because of varying size or angle of the pelvis itself.
I'm hoping you may be able to make sense of my confusion!
Dear Shelley: You ask very thoughtful questions, showing that you have read "Principles of Conformation Analysis" in depth. I wonder if you have access to a horse skeleton? If so, your ability to picture what might be going on would be greatly increased.
There are multiple possibilities with regards to moving the position of the L-S joint:
1. Build a horse with longer, wider ilia so that the points of hip lie farther forward.
2. Build a horse with different proportions to the iliac crest, that connects the point of hip to the peak of croup (the tubera coxae and tubera sacrale, respectively). Make it so that the distance between the two points is longer, and then move the sacro-iliac joint farther back.
3. Change the angle of the 'arms' of the sacrum (the transverse processes that articulate with the internal surface of the ilium so as to form the S-I joint). Instead of having the arms go outward from the main body of the sacrum at more or less 90 degrees, have them angle more forward; this sets the S-L joint back relative to the S-I joint but also to the pelvis above it.
The point is to look for horses that have the coupling farther forward, which simultaneously lengthens the effective lever-arm of the sacrum while shortening the lumbar span. Horses that have the L-S joint farther forward generally will have wider couplings also, and typically they are smoother simply because stronger and thus less prone to strain. Horses that have the L-S joint set farther back look as if the back is slung, like a hammock, from the peak of croup.
Write back again if this isn't clear....and if you have not, try to make your way to the nearest natural history museum or University museum where there is a mounted horse skeleton that you can look at: it will really help a lot. Cheers -- Dr. Deb
That has helped me visualise the possible variations. I went ahead and had a look at a range of conformation shots in a sales magazine with your points in mind and I do feel as though I can 'see' the cause for variation a little better now.
I definitely do need to look at a 'live' skeleton some more also to better train myself to see the shapes and positions of the bones when I am thinking about them.