Finding the lumbosacral junction (joint) on your Icelandic Horse (Pony) can be done by palpating the back. As your fingers go down the
spine, there will be a place where it feels like there is no bone, more of a
fleshy place than in other spots down the spine.
You can also find this place by taking your thumb and index finger and
slightly digging into the loins which will make the horse hollow or dip his
back, and you'll be looking for the place from which he flexes.... that's
the LS joint.
In general, horses with lumbo sacral junctions:
 located directly over the hip bone,
 long hips (more than 30 % of their bodies) and
 pelvises that do not have a steep slope or a flat one (approximately 20 degrees),
 hip sockets placed toward the center of the pelvis,
 femurs and tibia/fibulas at about the same length,
are inclined to more diagonal gaits with a long, powerful pushing stride behind.
 lumbo sacral junctions placed to the rear of the hip bone,
 short hips (less than 1/4 of their body length), steep pelvic angles (more than 45 degrees) or
 overly horizontal ones,
 hip sockets placed toward the rear of the pelvis,
 short femurs and long tibia / fibulas
are less likely to be able to round their backs and step strongly under themselves, and so are inclined to more lateral gaits, often with a shorter, higher step in back.