The hock is the large joint in the middle of horses' hind legs. This joint is similar to the ankle in people with the same number and structure of the bones making up the joint. Problems with the hock joint are what cause lameness in a horse. Most problems occur in small hock joints because smaller joints cannot bear the weight or movement of a horse, though horses with large hocks may have problems, too.
Swelling is the first sign of a minor hock problem. When the horse runs or turns, the bones in the joint face more pressure than they can bear. The horse's body responds by padding the joint with fluid. Anytime a horse's hock joint looks larger, pressing on the area to check for possible swelling is necessary.
When a horse is running, it should land on the back, or heel, of the back hoof. The back of the hoof is designed to absorb the shock of the horse's weight. This type of ideal landing also puts pressure on the hock. A horse with a painful hock will begin to land on the toe of the rear hooves. Trainers can check the toe of the hoof for wear or injury to verify that a horse is landing continually on its toe.
Stretching the legs out completely allows a horse to run faster, but the resulting full stride can hurt the hock. Many horses begin to take shorter strides to help alleviate the pain. If a horse's stride seems to be getting shorter, which will be evident in the horse's running time, the best option is to check the hock to see if the horse reacts in pain.