This month we shall begin a discussion of the rear leg: conformation, function, problems. There is at least one good reason to start with the rear rather than the foreleg. It seems to be the case that as man selects certain animals to breed to other animals that changes occur more rapidly in the conformation of the rear legs. Over the long course (55 million years) of horse evolution, the paleontologists, who study such things, believe that changes in the legs occurred earlier or in some way more significantly in the forelegs. That does not, however, seem to be true when man is the driver of horse evolution. There is interesting room for study and conjecture there.
In any case, the rear leg deserves consideration because it is the prime mover for the horse–the propulsive engine. The forelegs do contribute to motion, but to a lesser degree than the rear. Let’s look, first, at how that is done.
In Figure 1, we have a schematic view of the rear leg with some of the muscles indicated. We have discussed this before, but need to review. The leg performs two basic movements: it swings forward, called protraction, and it swings back, called retraction. Protraction begins as the foot leaves the ground at the end of the period of support, the time during which the foot is on the ground. The leg swings forward as the iliopsoas muscle and the quadriceps femoris muscles contract (Figure 1). There are other muscles at work, but we shall only consider the biggest ones; the others simply assist the big ones, in a sense.
This swinging forward is angular motion; the leg rotates forward around the hip joint as the compromise center of rotation. Once fully protracted–and the extent of that will depend upon the gait and velocity–the leg begins to swing back in the other direction, again, angular motion but in retraction. This backward swing is accomplished, primarily, by the gluteus medius muscle and th