Shivers is a poorly understood neuromuscular disorder of the hind limbs. A classic case of Shivers is most obvious when a horse backs up; their hind limbs raise in an exaggerated fashion and the limbs stay raised, hyper-flexed, and quiver and tremble. There are variations on this- standing hyperflexion in which there is exaggerated lifting of the limb if an attempt is made to raise it. There is also a type in which there is hyperextension in which the hind limbs stay positioned to the rear and backing is very difficult. In more advanced cases, Shivers becomes obvious in a horse at rest or in forward motion. In some cases, horses abnormally twitch or move their ears and eyes.

Shivers is most common in draft breeds, and is generally more common in male horses of tall build.

There are various theories about what causes this condition. Historically, it was assumed that the problem was in the hind limb muscle or nerves. Recent research suggests a defect in the Purkinje cells of the cerebellum of the brain. The cerebellum's job is to coordinate more complex movements, including backing.

Diagnosis is by examination, a horse that displays the typical movement abnormalities. It should be differentiated from stringhalt, which typically is also evident at the trot.

There is no treatment for this disorder. Some horses improve on a high fat, low carbohydrate diet.