The psoas major’s sagittal plane lumbar spine joint action is by far its most controversial function. The term “psoas paradox” describes this controversy.
Psoas Major Function: Spinal Joint Actions – Frontal and Transverse Planes
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.
FREE DCOMT video – Piriformis Becomes a Medial Rotator of the Thigh
This 3D Animation demonstrates how the piriformis changes its joint action from being a lateral rotator of hip joint to being a medial rotator.
