Anatomically, it appears to be made up of three distinct sets of fibers though electromyography suggests that it consists of at least seven groups that can be independently coordinated by the nervous system.
Anatomisk ser den ud til at være lavet af tre forskelling sæt af fibre, selvom elektromyografi viser at den består at i hvert fald syv grupper, der kan koordinernes individuelt af nervesystemet.