<unk>  or<unk>  (from and ) is a state of severe hyperextension and spasticity in which an individual's head, neck and spinal column enter into a complete "bridging" or "arching" position. This extreme arched pose is an extrapyramidal effect and is caused by spasm of the axial muscles along the spinal column. It has been shown to occur naturally in birds, snakes suffering from advanced boid inclusion body disease, and placental mammals, among existing animals; it is observed in some articulated dinosaur fossils.
Opisthotonus
Myoclonus
Clonus
Opsoclonus
Paratonia
Chorea 
Chorea
Dystonia