The Rear Leg

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 the hamstring muscles (biceps femoris, semitendinosus, semimembranosus; Figure 2). The gluteus medius is a huge muscle, the biggest in the body, which forms the swell or round of the croup. As you can see in the figure, it lies above the hip joint while the hamstrings lie behind it. In both cases, their contraction causes the leg to swing backwards, rotating around the hip joint as the center.

An interesting point is to be made here. When retraction begins, the foot is in the air; it swings backward to contact, impact, the ground. The velocity of the foot, swinging downward and backward, is equal to the velocity of the body moving forward. That means that at the moment of impact the horse's leg only has to deal with forces directed straight down the leg and not with forces parallel to the ground. This is generally true of all angular motion over the ground. For instance, a point on a wheel of a car is rotating down and backward at the moment that particular point contacts the ground.

You do the same thing when you walk or run. Try to walk without your foot swinging down and back; there will be quite a jolt!

Once the foot impacts with the ground, the powerful gluteus and hamstrings continue to contract. The foot is now in contact with and "stuck to" the ground; the muscular work, then, pushes the horse's body forward.

Let's say now that we hook a draft load to our horse, a wagon full of hay or whatever, and ask him to pull it. The horse starts forward and cannot move it. Being willing he tries again, this time dropping the croup down and closing up the angles at the hip, stifle, and hock (Figure 2). Now he can move the load. Exactly the same thing happens when a horse prepares to jump over a fence; he squats down in order to close up those joint angles. In this position he can move the several joints through a greater range of angular motion and, so, is able to generate more force against the ground and push itself, and the load, forward. (There is more to it than that, but that is generally true, if oversimplified.)

This position, squatted down, is essentially the same thing as a horse that, genetically, has a sickle hock type of conformation. Since this position is best suited for pulling loads, man's selection of draft horses over the years--breeding those horses best at pulling loads--has resulted in many sickle hocked draft horses.

But, you may ask, what about the Thoroughbred jumper? He certainly has to squat down in order to generate enough force to get over a jump, but Thoroughbreds on average have straight hocks, not sickle hocks. Here, we can make an important point. A horse can be sickle hocked to start with, as the draft horse, or, because of the work it does, it uses the rear leg in a sickle hocked manner. That is, it squats down. Thoroughbreds are bred for speed--that is, for a straighter and, so, longer, rear leg. When we shift the horse to jumping, he is not, by conformation, bred to jump. The horse must adjust to this new work by squatting down in order to generate the force needed.

Clearly, there are horses, other than Thoroughbreds, used for jumping--the European warmbloods, for example. Many of them are bred for that purpose. Ergo, we tend to see more sickle hock conformation in those horses.

One final note for this time. In the older days of harness racing, horses had to pull quite heavy drivers, and race two, three, or more heats in an afternoon. Those Standardbreds tended, on average, to be sickle hocked. Today, with all the emphasis on speed, velocity, and single heat or dash racing, the Standardbred has been selected more for the straighter rear leg. As a result, the speed records for the mile have decreased quite markedly over the years since dash racing replaced heat racing.

Next time we shall continue with the rear leg and, specifically, this sickle hock conformation, and see what it means in terms of lameness.

About the Author

James R. Rooney, DVM

The late James R. Rooney, DVM, was Professor Emeritus of the Gluck Equine Research Center, Department of Veterinary Science, at the University of Kentucky. Rooney was a 1949 graduate of Dartmouth College with a bachelor's degree in English drama; a 1952 graduate of New York State Veterinary College at Cornell University; and a Diplomate, Emeritus, of the American College of Veterinary Pathologists. Rooney authored more than 100 articles and books on diseases and locomotion of horses, including: Biomechanics of Lameness in Horses, The Lame Horse, Clinical Neurology of the Horse, Autopsy of the Horse, and Mechanics of the Horse.

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