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.
Psoas Major Function: Hip Joint Actions – Frontal Plane
If the psoas major frontal plane open-chain action is abduction of the thigh at the hip joint, the closed-chain action is depression of the pelvis.
Cervical Spinal Joints
The disc joint is a cartilaginous joint that is composed of outer fibers called the annulus fibrosus that encircle the inner nucleus pulposus.
