multifidus

noun

mul·​tif·​i·​dus ˌməl-ˈtif-ə-dəs 

plural multifidi -ˌdī 

: a muscle of the fifth and deepest layer of the back filling up the groove on each side of the spinous processes of the vertebrae from the sacrum to the skull and consisting of many fasciculi that pass upward and inward to the spinous processes and help to erect and rotate the spine