The iliopsoas is composed of two muscles: the iliacus and the psoas.
Some sources will distinguish between the iliacus and the psoas; often, sources are referring to these two muscles together —whether they use the term iliopsoas or psoas.
We follow common usage here, using the word “psoas” to mean iliopsoas.
In addition, there is technically a psoas major and a psoas minor. Typically, the word psoas refers to the psoas major which “may vary, growing out from T12 or out from the lumbar vertebra… There is also a psoas minor which “originates at T12 and grows into a thin tendon at the pelvic rim. A relic of the human organism’s primordial ancestry, it is thought to be a disappearing muscle as the body evolves from a semi-flexed to an upright being. It is possible to have only one psoas minor or none at all.” (Liz Koch)
The psoas muscles are located deep. (Intestines and female reproductive organs are in front of them.)
The psoas is a very large muscle, “almost as big as a wrist.”
The psoas links the spine to the legs. There is one on each side of the spine.
The psoas originates on lumbar vertebrae and attaches at the femur.
From the spine, the psoas moves down and forward across the outer edge of the pubis, then traverses back to the lesser trochanter (on thigh).