The psoas major is the largest muscle in cross section at
the lower levels of the lumbar spine.
5 It has fibrous at-
tachments 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 con-
stitute the individual fascicles. The fascicles of the psoas
major are approximately similar in length throughout the
lumbar spine and have a unipennate fiber orientation.
Muscle fiber length within the anterior fascicles ranges
from 3 to 8 cm and 3 to 5 cm in the posterior fascicles.
7
The fascicles are oriented inferolaterally and come together as a common tendon which descends over the pelvic brim and shares a common insertion with the iliacus
muscle on the lesser trochanter of the femur.