The psoas major’s sagittal plane lumbar spine joint action is by far its most controversial function. The term “psoas paradox” describes this controversy.
The frontal plane spinal action of the psoas major is fairly clear; it crosses the spinal joints laterally, so it lateral flexes the spine to that side.
The disc joint is a cartilaginous joint that is composed of outer fibers called the annulus fibrosus that encircle the inner nucleus pulposus.
It is extremely important to be able to locate and palpate the laminar groove of the cervical spine because the thickest musculature is located there.
This 3D Animation demonstrates how the piriformis changes its joint action from being a lateral rotator of hip joint to being a medial rotator.