The psoas muscle doesn't exist independently from the rest of the body. It exists as part of an integrated brain and body whose primary purpose is upright movement and communication (from a neuroscience perspective).

But upright unrestricted movement can be interrupted by sympathetic "fight or flight" activity. This fight or flight activity can arise for many reasons. This video explores the influence of the ventral vagal system (the vagus nerve) on our autonomic nervous system's "fight or flight" branch, and what happens when the ventral vagal system is not functioning smoothly. In particular it explores the connections to the psoas muscles.

The psoas muscles are directly integrated with the diaphragms, they are inseparable. Parasympathetic nervous system activity, "rest and digest" directly relies on diaphragmatic breathing.
So, in order to not be in a sympathetic state of "fight or flight" you must be able to diaphragmatically breathe.

The problem is that when someone IS in fact stuck in a sympathetic state of fight or flight due to neck breathing or cranial nerve dysfunction, hip flexors remain overactive, leading to anterior and lateral pelvic tilts, which are manifestations of the left AIC pattern as described by the Postural Restoration Institute.

Neck breathing, mispositioned jaws, malocclusion, tongue ties, narrow palates, missing teeth as well as visual issues can promote neck breathing due to ventral vagal dysfunction. Neck breathing is sympathetic in nature and will promote fight or flight.

More content about the psoas and how it's related to your jaw, teeth, and vision and movement
Vision, teeth, and feet
   • Releasing the Psoas: The THREE things...  
Braces and posture:
   • The Complete Guide to Postural Restor...