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THE Perfect Engine No matter the breed or discipline, good conformation of the hind legs will contribute to more successful performance and a better ride BY LES SELLNOW uch has already been stated in this series about the special concerns involving front limb soundness in the horse since M60-65% 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” var- ies 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. Editor’s Note This is the fourth in a 12-part series of articles on equine anatomy and physiology. Future topics include the hoof, the head and neck, the back, muscles, tendons and liga- ments, the digestive system, the circulatory and respiratory systems, and the reproductive system. DR. ROBIN PETERSON ILLUSTRATIONS 83 as it pertains to feet and legs is the fourth www.TheHorse.com.aspx?id=6623), there edition of Adams’ Lameness in Horses, edit- are several small bones in the lower hind ed by Ted Stashak, DVM, MS, and featuring limb that are important to the function of seven experts in the field as contributors. the leg. Where the cannon bone joins the long pastern bone, two small bones—the Start at the Top proximal sesamoids—lie on the back side We will begin our visual dissection of of the cannon bone and act as pulleys for However, if the discipline happens to be the posterior portion of the horse’s anato- the flexor tendons. And where the short cutting or reining with the Western horse my at the top, or spine, and work our way pastern and coffin bone join, a distal sesa- or dressage or five-gaited action with the downward. Connecting to the spine is the moid bone, or navicular bone, acts in the show horse, it is an entirely different mat- ilium, the largest of three bones in the an- same manner. ter. Although different in nature, these imal’s pelvis. The ilium angles down and four disciplines all put high demands on rearward, and it attaches to the femur or Where Does it Hurt? the horse’s rear end. thigh bone. The angular shape of the pelvis The two key places where rear leg lame- It should also be remembered that, in ad- determines what type of croup the horse ness problems are apt to develop are the dition to being the prime source of propul- has—flat or sloped. stifle and the hock. Both play key roles in sion, the back legs also serve as the horse’s The femur angles slightly forward and the horse’s ability to propel itself forward, brakes. Again, the stress put on those connects with the stifle, forming one of brake to a halt, and move backward. brakes varies with the discipline. It is vast- the more important joints in the rear leg If one were to rate them as to impor- ly different, for example, in a reining horse apparatus. Connecting at the stifle joint, tance in this regard, the hock would per- than it is for one competing in dressage. as we continue our journey downward or haps be placed at the head of the list. The We’ll take a look at how Nature has de- distally, is the tibia, which connects with propelling force is transmitted by tendons signed the rear portion of the horse’s anato- the hock. Emerging from the distal por- passing over the hocks, delivering energy my, especially the leg, in to understand why tion of the hock is the metatarsus or rear from the muscles for forward propulsion. the animal can do what it does. We also will cannon bone. This bone continues down- On the other hand, the stifle serves much take a look at some of the problems that ward until it connects with the long pastern the same function as the human knee. can develop in improperly conformed legs bone, which connects with the short pas- As mentioned earlier, the hock, or tarsal as a result of these stresses. tern bone, which connects with the coffin joint, is where the tibia joins with the meta- An excellent source on equine anatomy bone. As with the foreleg discussion (see tarsal bone. The hock is a like the horse’s The American Association of Equine Veterinary Technicians Seeks Your Support Mission Statement: To promote the health and welfare of the horse through the education and professional enrichment of the equine veterinary technician and assistant. Join today! For more information and to become a member, please visit our web site at www.aaevt.org 84 www.TheHorse.com THE HORSE April 2006 NORMAL AND PROBLEMATIC HIND LIMB CONFORMATION When a horse’s hind limbs NORMAL HIND END STRAIGHT HOCKS SICKLE HOCKS aren’t aligned as they should be, some internal structures are stressed more heavily, resulting in a greater risk of pain and lameness. For ex- ample, the cow-hocked horse will place extra strain in the medial side (inside) of his hocks, resulting in a greater risk of bone spavin, and straight hocks will not be able to absorb as much shock as they should. NORMAL HIND END COW HOCKS BOW HOCKS BASE-WIDE O C D TM Optimal Cartilage Development Are you STILL pretending your horses don’t have bone problems? No surgery. Just OCDTM Pellets and daily turn-out. Backed by a 75-DAY Dramatic Results Guatantee. www.ocdequine.com 800.815.6599 April 2006 THE HORSE www.TheHorse.com 85 HIND LIMB ANATOMY tuber sacrale sacrale enable it to function. The prime function of calcanei the hock is to provide rear leg propulsion. The bones of the hock include: Calca- ilium neus and talus on top of the central tarsal bone, third tarsal bone, first and second tarsal bones (which are fused), and the hip joint fourth tarsal bone. knee on the front leg in that it is comprised The horse’s stifle joint is the largest single of a number of bones—six to be exact. A joint in its body. One of its functions is to ischiatic femur tuberosity difference between hock and knee is the cause the rear limb to become rigid when patella fact that the hock bones lack the degree the foot is on the ground. This is controlled of motion noted in the bones of the knee. via contraction of muscles above the patel- stifle Like the knee joint, the hock joint is held la, which releases the stay apparatus, and joint together by a complex set of ligaments that is the equivalent of a human’s kneecap. tuber calcanei tibia hock tarsal or hock joint bones cannon bone or metatarsus splint fetlock joint bone proximal or first phalanx pastern joint middle or second phalanx coffin joint distal or third phalanx, a.k.a. coffin bone The type of activity with which the horse is involved has a great deal to do with the stresses and pressures that are placed on the hocks. Walking, jogging, or even gallop- ing puts little undue stress on the back legs in general and the hocks in particular. However, when one ups the ante with some activities, it all changes. One type of stress, for example, is placed on the hocks by five-gaited horses, particularly Saddle- breds. When performing at the signature gaits of rack and slow gait, the horse is moving in a four-beat rhythm. This means each leg strikes the ground separately. Put another way, it means that all of the horse’s weight is balanced on one leg during each complete stride. Add to this the fact that five-gaited horses shift weight to the rear and elevate the front end while slow gaiting and racking, and one can quickly conclude that the stresses and strains placed on the rear end multiply. When we consider some of the Western classes, the stress and strain placed on the rear end change in form, but are equally 86 www.TheHorse.com THE HORSE April 2006 severe. The cutting horse, for example, provides propulsion for a soaring jump. distance between its thigh area, as well as works almost completely off the rear end. While we generally don’t think that much between the hocks, should be about the Its back legs serve as both propulsion and stress is placed on the rear end of trail same as the distance between the rear feet brakes as the horse slides to a stop, then horses, being ridden either in English or as they rest on the ground. The back legs shoots forward or sideways as the cow Western tack, there are times when the should be straight from this viewpoint. If a turns and tries to escape. The process of demands are high. An example would be vertical line were drawn on a photo taken turning while weight is suspended over climbing a steep hill or mountain where a from behind, the line would go as straight the rear often places terrific torque on the strong rear end is required for propulsion as a plumb line from the center of the but- hocks and can lead to soundness problems upward as well as forward.

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