Grand Prix Guidance

The Equine Hind Limbby Mark R. Baus, DVM

If the front limbs of the horse evolved to support the horse’s mass, the hind limbs developed as the motor for runlimb generates significant amounts of torque on demand. Torque is defined as the force applied to a lever arm to rotate ning, jumping, and everything else we have trained horses to do. This multijointed limb with its impressive muscle mass can propel the horse and rider rapidly over great distances or high fences. It’s easy to imagine that if a special horse can jump a seven-foot wall, every other horse still has remarkable power from their hind limbs.

The real magic of the hind limb starts with the massive muscle mass of the upper limb. In order to control these muscles, the hock and stifle must move in perfect unison. To do this, the horse employs a unique mechanism called the reciprocal apparatus to ensure that the hock and stifle bend at the same angle during motion. The key structure of this apparatus is the peroneus tertius ligament, which connects the femur and the cannon bone. Like the front limbs, the hind limbs also have a group of ligaments, tendons, and muscles called the stay apparatus, which locks major joints in the hind limb. The stay apparatus allows the horse to remain standing with little or no muscle exertion.

Anatomically, the lower limb is similar below the knee of the front limb and the hock of the hind limb. The suspensory apparatus of each limb gradually accepts the body’s weight during motion and like a spring, releases the energy absorbed during the landing phase of the stride by pushing the body up and forward. However, the hind limbs are additionally challenged by the propulsive forces of the powerful hind limb muscles. The suspensory apparatus stores and releases those tremendous forces with each step.

Applying the laws of physics, the hind across a fulcrum or pivot. In the case of

the horse’s hind limb, the forces are exerted from the muscles, the lever arms consist of the bones of the limb and the fulcrums are the joints. Because it is a multi-jointed limb of propulsion, the hind limb generates significant strain forces and compression forces on various parts of the limb.

Katie Hylen

Based on similar laws of physics, the joints under the most strain and compression are the joints between the two longest lever arms: the hock and the stifle. The stifle is a complex joint with the largest amount of synovial lining of any joint. It consists of the femur, patella, tibia, and several interconnecting ligaments and a pair of menisci for cushioning.

The hock is even more complex, consisting of nine bones including the tibia and cannon bone along with five separate joints. Early in the horse’s evolution, the hock landed on the ground during locomotion, like the human ankle. The numerous bones and joints of the hock served the early horse well when it was on the ground, but this complex design posed a challenge as the load bearing shifted to the tip of the phalanx (toes) for greater speed and agility.

The causes of lameness in the hind limb are myriad and often related to the specific athletic pursuits of each horse. Although congenital and developmental orthopedic conditions can occur before they become athletes, horses respond predictably with lameness problems based on their discipline. The lameness conditions associated with high-impact athletes like racehorses, jumpers, and three-day eventers are different from the lameness problems associated with low-impact sports such as dressage, gaited horses, and western performance horses.

Regardless of the nature of their activity, the one predictable source of chronic low-grade pain in the equine athlete is from the lower hock joints. This is expected for several reasons. First of all, the hock is subjected to repetitive forces because it is located between two long bones. Second, the lower joints of the hock are flat-surfaced joints with a very low range of motion. Even though they do not bend much, the fronts of the lower hock joints are subjected to significant compression forces with each stride. This will ultimately cause inflammation and pain from the lower hock joints.

Fortunately, lower-hock joint pain is usually low-grade and highly manageable. Hock pain will diminish during the warmup phase of exercise although it can still affect a horse’s performance after warming up. Due to the nature of joint-related problems, degenerative changes in the lower hock joint are progressive over time and with continued use. It’s common for hock joints to require some form of therapy, usually periodic injections, throughout the horse’s athletic career.

Inflammation in the stifle joint, although not as common as hock-related problems, is not unusual in the equine athlete. It’s generally more common in horses that compete at the higher levels. The sources of inflammation tend to be related to the synovial tissue and therefore more manageable.

Hyperextension injuries of the hind suspensory ligaments (suspensory desmitis) are also common in the equine athlete and are most often chronic injuries rather than acute. This injury is more common in horses with long pasterns and low fetlocks. Horses with this conformation also tend to perform well in dressage and hunter divisions so suspensory desmitis is more common in these disciplines.

The key to preserving soundness is to understand your horse’s limitations based on conformation and preexisting conditions. You can then maintain a workload that’s consistent with those restrictions. Needless to say, your veterinarian’s involvement during the purchasing process and throughout your horse’s athletic career is essential for determining ypeak performance and use levels.

Katie Hylen Mark R. Baus, DVM founded Grand Prix Equine in 2009. He’s provided care to horses in the same region and for many of the same clients for more than 30 years. Mark began the Connecticut Equine Podiatry Group to foster relationships between farriers and veterinarians.