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.
In the images below, the first one shows an overlay of the spine of a trotting horse onto the image of an Icelandic Horse (Pony).
The second image shows the spine adjusted with the pelvis tipped more to conform to the conformation of the Icelandic Horse.