Myotonia congenita is an inherited muscle disorder affecting dogs. The muscle cells of an affected dog are over-excitable, which causes muscles to remain contracted rather than relaxing after voluntary activity. Signs of the disorder usually present when puppies begin to walk. Their gait may appear stiff and uncoordinated and they may fall frequently. Affected dogs frequently have a “bunny hop” type gait. Episodes do not appear to be painful and the muscle stiffness may improve with increased exercise. Episodes can worsen with hot weather and excitement. Other features include enlargement of the muscles, especially of the neck and limbs, abnormal posture, and an upper jaw that is much longer than the lower jaw. Dogs with this disorder typically have a normal lifespan.