The Psoas are layerd muscles. The discription Tom Nagel gives in ‘Zen and the horse is:
“Fibres from the higher layers form the outer surface of each muscle, while fibers from the lower layers are buried sequentially deeper within the body of the muscles. As a result of thus layering and the manner in which the psoas attaches to each vertebra, the psoas can operate on the lumbar spine with different opposing functions within it. According to William “Dub” Leigh, founder of Zentherapie, since the psoas attach to both the body and sides of each vertebra, by pulling or relaxing the pull at these attachments, an almost unlimited combination of variable forces is created. The position of the vertebrae can be changed quickly and easily. Therefore, the psoas are able to stabilize the back.”
