Secondary contracted heels are caused by lameness and disuse of the limb, resulting in hoof atrophy. Crevices and grooves formed by the contracting hoof creates a favorable environment for thrush. Some horses' hooves tend to grow the toe at a faster rate than the heels. Expansion and contraction of the heels during motion increases their wear relative to the toe. The heels are dropped, and the bulb of the heels are prominent.
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.