Curb is the inflammation and thickening of the plantar tarsal ligament, a ligament that is located near the hock joint. The plantar ligament runs down the back of the calcaneus and attaches onto the back of the lower tarsal bones and the top of the cannon and splint bones. This resulting in the appearance of a small, firm swelling just below the point of the hock (referred to as the calcaneus), on the back of the leg. It is most apparent from a side view of the leg. In the initial, early stages when the swelling first develops, horses may also show mild lameness.

Curb can occur as a result of trauma such as from getting kicked by another horse or kicking a solid object, sudden overflexion of the hock (often the result of sliding stops), or from chronic strain on the ligament due to faulty conformation (especially horses with sickle hocks).