The psoas major is the largest muscle in cross section at the lower levels of the lumbar spine.5 It has fibrous attachments to the anterior aspect of all lumbar transverse processes and to the anteromedial aspect of all the lumbar discs and adjoining bodies with the exception of the L5/S1 disc.6 For their relative positions on the spine, the attachments on the transverse processes are named the posterior attachments and those on the disc and bodies are called the anterior attachments. These attachments constitute the individual fascicles.