http://www.youtube.com/watch?v=hTskKWeOvtw

From a US health site:

Myotonia congenita is a genetic, neuromuscular ion channelopathy that affects muscles used for movement (skeletal muscles). It is congenital, meaning that it is present from birth. It, amongst other problems, causes delayed relaxation of the muscles (myotonia and rigidity). The disorder is caused by mutations in a gene responsible for shutting off electrical excitation in the muscles, causing muscle fiber membranes to have an unusually exaggerated response to stimulation (hyperexcitability). Symptoms include muscle stiffness (myotonia), hypertrophy (enlargement), pain, and cramping. The disorder is caused by a [genetic mutation] involving the chloride channel of the muscles.

The muscle stiffness, which occurs most commonly in the leg muscles (more often in dominate mutations) as well as the eyelids and hands, is often enhanced by cold and inactivity, and in some forms is relieved by exercise.

The two major types of myotonia congenita are known as Thomsen disease and Becker disease. These diseases are distinguished by the severity of their symptoms and their patterns of inheritance. Becker disease usually appears later in childhood than Thomsen disease and causes more severe muscle stiffness and pain. People with Becker disease often experience temporary attacks of muscle weakness, particularly in the arms and hands, brought on by movement after periods of rest. They may also develop mild, permanent muscle weakness over time. This muscle weakness is not seen in people with Thomsen disease. However, in recent times, as more and more of the individual mutations that cause myotonia congenita are identified, these limited disease classifications are becoming less widely used.

Some form of myotonia congenita is estimated to affect 1 in 100,000 people worldwide.

Early symptoms in a child may include:
Difficulty swallowing
Gagging
Stiff movements that improve when they are repeated

Possible complications may include:
Aspiration pneumonia (caused by swallowing difficulties)
Frequent choking or gagging in infants (also caused by swallowing difficulties)
Abdominal muscle weakness
Chronic joint problems

Fainting goat

A fainting goat is a breed of domestic goat whose muscles freeze for roughly 10 seconds when the goat is startled. Though painless, this generally results in the animal collapsing on its side. The characteristic is caused by a hereditary genetic disorder called myotonia congenita. When startled, younger goats will stiffen and fall over. Older goats learn to spread their legs or lean against something when startled, and often they continue to run about in an awkward, stiff-legged shuffle.

Slightly smaller than standard breeds of goat, fainting goats are generally 43 to 64 centimeters (17 to 25 in) tall and can weigh anywhere from 27 to 79 kilograms (50 to 175 lb). They have large, prominent eyes in high sockets. Their hair can be short or long, with certain individuals producing a great deal of cashmere during colder months. There appears to be no angora strain of the fainting goat. Common coat colors are black and white; however, most possible coat colors are found in this breed. Their life expectancy is 12 to 15 years.[1]

The origin of the fainting goat is peculiar. The goats appear to have arrived in Marshall County, Tennessee, in the early 1800s, courtesy of a reclusive farm worker named Jon Tinsley who was most likely from Nova Scotia. Before he left the area, he sold his goats — three does and a buck — to Dr. H.H. Mayberry, who bred them.

Fainting goats have many other names, including Myotonic Goats, Tennessee (Meat) Goats, Nervous Goats, Stiff-leg Goats, Wooden-leg Goats, and Tennessee Scare Goats. They are smaller and somewhat easier to care for and maintain than larger meat goat breeds, which makes the fainting goat desirable for smaller farms. They are also raised as pet or show animals as they can be friendly, intelligent, easy to keep, and amusing.

Classified as a meat goat as opposed to a dairy goat, it can be raised for chevon (goat meat). This breed is listed as threatened by the American Livestock Breeds Conservancy so the fainting goat is not used as often for chevon as other meat goat breeds; its rarity makes the live goat more valuable.[2] The fainting goat is specifically specialized for smaller production operations as they are unable to challenge fences as vigorously as larger meat goat breeds. This is due in part to their smaller size and also because of the myotonia. Their size makes them easier to care for during chores such as foot trimming and administering medication. Smaller specimens of fainting goats are frequently kept as pets.

Besides the myotonia, another distinguishing feature of the fainting goat is their prominently set eyes. The eyes protrude from the eye sockets, as opposed to recessed eyes seen in other breeds. The profile is straight as opposed to the convex or "roman" profile.

It is theorized that fainting goats were used by their handlers during travel for protecting other livestock (such as sheep) from predators by involuntarily "sacrificing themselves" due to their condition. This allowed the handlers and livestock to escape. However, while there are records of goats being transported with livestock in earlier times, there is no proof that these goats suffered from myotonia nor that they were used for such a purpose.

Even though some people breed these animals for pets, "fainting" is a disorder that most producers try to keep out of their flocks bloodlines, unless they are raising fainting goats.