Consult almost any anatomy book and you will read that the psoas minor, one third of the iliopsoas muscle group, is a devolving muscle absent from fifty percent of the population. After reading that in a number of anatomy books it became my standard line when talking about the psoas minor.
The psoas minor attaches to the sides of T12 and L1, the base of the ribcage and top of the lumbar spine, and the disc in between the two vertebrae. It inserts into the pelvis along the pubic ramus into the pectineal line and the iliopectineal eminence which is where the ilium and pubis of the hip bone meet, as well as into the iliac fascia.
My theory about why it might be a devolving muscle, and this is based purely on conjecture, is that it was replaced by the rectus abdominis in terms of function. In a horse for example the psoas minor functions to flex the spine when still and stabilize the spine in motion. It seems to me that the arch of the lumbar spine created by the psoas major when we came to stand took the psoas minor out of play and the rectus abdominis took over to provide flexion for the spine.
As I mentioned when I first started teaching I would talk about the psoas minor as a devolving muscle. But then a number of years ago I was attending a workshop offered by Bonnie Bainbridge Cohen, the creator of Body Mind Centering. At some point in the workshop she mentioned that she thought everyone had a psoas minor. She had been working with bodies for years and years and she had always felt the presence of the psoas minor.
After that workshop I was happy to start teaching that everyone has a psoas minor. Why not? Her word is pretty much gospel for me. Fast forward a year or two to another workshop where I was chatting with one of the senior BMC teachers, and as it happens at these events, the psoas minor came up.
I shared with her my change in tone after the prior workshop and she related the same thing with a very interesting twist. In between these two workshops this teacher had gone to a cadaver lab where you get to dissect bodies. Before the work began the BMC teacher mentioned Bonnie’s psoas minor thoughts to the leader of the workshop and they went around and checked all the bodies.
Of the thirty-nine bodies in the room only one had a psoas minor. When the BMC teacher related this to Bonnie she was nonplussed and reported, (I paraphrase)” Well, I feel it.”
The iliopsoas is a magical muscle.