There are three distinct varieties of Spasmodic Torticollis: Tonic, in which the head turns to one side, Clonic, which involves the shaking of the head, and Mixed which involves both turning and shaking. 

The turning of the head is generally considered to fall into one of four categories: Rotational, in which the head turns to one side or the other, Laterocollis in which the head is pulled toward the shoulder, Retrocollis in which the head is pulled to the back, or Anterocollis in which the head is pulled forward. In fact, most people's ST is more complex, having a combination of "pulling" from among these four categories.  

Rotational               Retrocollis

Laterocollis             Anterocollis

The muscles