Many horses have naturally underrun, weak heels. The heels are lowered more than necessary, creating a broken foot axis. Any foot can become affected, but the condition is more common in the forelegs. The hoof appearance is similar to the shape of a mule's hoof. In some cases the entire hoof is contracted, the sole has more concavity and the frog atrophies.
Some horses develop heel bulb contraction.
Lack of exercise, prolonged stabling in dry stalls, hoof dressings, sand and urine tend to dry the hoof. Lambert 1968 demonstrated rapid loss of moisture and contraction of the heels following regular hoof trimming. The schematic representation is of the area involved in shock absorbtion. The caudal part of the hoof wall functions like a spring, spreading apart as the foot contacts the ground, absorbing the energy of impact. Proper function of this spring depends on resiliency of the keratinized structures of the hoof.