by: Les Sellnow
Much has already been stated in this series about the special concerns involving front limb soundness in the horse since 60-65% of the animal's weight is carried in the front end. This does not mean that there are no concerns involving the back legs. Far from it. We can think of equine rear end function in terms of cars and trucks with rear wheel drive. The engine, comprised of muscles fueled by heart and lungs, provides the power, and the back legs are akin to piston-driven rear wheels.
The pressure and torque placed on the "rear wheels" varies with the discipline involved. When walking or jogging across the countryside during a trail ride, the stresses are light and easily handled by a horse with normal back leg conformation. However, if the discipline happens to be cutting or reining with the Western horse or dressage or five-gaited action with the show horse, it is an entirely different matter. Although different in nature, these four disciplines all put high demands on the horse's rear end.
It should also be remembered that, in addition to being the prime source of propulsion, the back legs also serve as the horse's brakes. Again, the stress put on those brakes varies with the discipline. It is vastly different, for example, in a reining horse than it is for one competing in dressage.
We'll take a look at how Nature has designed the rear portion of the horse's anatomy, especially the leg, in an effort to understand why the animal can do what it does. We also will take a look at some of the problems that can develop in improperly conformed legs as a result of these stresses.
Once again, information comes from multiple sources. An excellent source on equine anatomy as it pertains to feet and legs is the fourth edition of Adams' Lameness in Horses, edited by Ted Stashak, DVM, MS, and featuring seven experts in the field as contributors.
Start at the Top
We will begin our visual dissection of the posterior portion of the horse's anatomy at the top, or spine, and work our way downward. Connecting to the spine is the ilium, the largest of three bones in the animal's pelvis. The ilium angles down and rearward, and it attaches to the femur or thigh bone. The angular shape of the pelvis determines what type of croup the horse has--flat or sloped.
The femur angles slightly forward and connects with the stifle, forming one of the more important joints in the rear leg apparatus. Connecting at the stifle joint, as we continue our journey downward or distally, is the tibia, which connects with the hock. Emerging from the distal portion of the hock is the metatarsus or rear cannon bone. This bone continues downward until it connects with the long pastern bone, which connects with the short pastern bone, which connects with the coffin bone. As with the foreleg discussion (see www.TheHorse.com.aspx?id=6623), there are several small bones in the lower hind limb that are important to the function of the leg. Where the cannon bone joins the long pastern bone, two small bones--the proximal sesamoids--lie on the back side of the cannon bone and act as pulleys for the flexor tendons. And where the short pastern and coffin bone join, a distal sesamoid bone, or navicular bone, acts in the same manner.
Where Does it Hurt?