The Psoas muscle is one of my favourite muscles to talk about and to work with in yoga because it is so, so important to our spine (and lower back health), our hormones, women's health - and lots more...!
The Psoas (sow-as) relates to our posture, our core, our emotions (particularly fear), if female - the menstrual cycle and period pain and the ability for our hormones to flow. As the psoas muscle provides support for the organs in the pelvis bowel, it's length and tone have an effect on the organs and how well they function. If the psoas muscle is tight, the organs don't rest comfortably and therefore won't function at their optimum.
The psoas is very much associated with the 'fear reflex'. The most primal and instinctive reaction to fear is to move into a foetal position, which is our natural position for protection of all the vulnerable parts of the body. The 'flexor' muscles switch on when we experience fear and the psoas is one of the biggest and deepest flexor muscles in the body and plays a huge role in our fear response.
This muscle contracts even if we are not aware of it, when we are confronted with fear.
Fear may not even be an obvious emotion we are aware of, but we can unconsciously experience fear every day, whether it be about paying our rent, dealing with certain people and relationships, having to do particular tasks in our work. I'm sure you might be able to recall yourself or know someone, who gets constipated every time they go away... This is an example of the psoas tightening and the organs not functioning to their full ability. If you experience this, you might think back to the last time this occurred and you may not even recall experiencing fear, however, there is some anxiety about being away from your own surrounds (which is in fact a degree of fear).
Fear lives in our nervous system. We can recall the feeling of fear at any time just by imagining it. When we do, we tighten on a deep level within. Our breathing is also affected and both the diaphram (our main breathing muscle) and the psoas are very strongly connected. Our breath is the first thing to change when we experience any emotion. Working with the breath can often be dangerous because when we try too hard to breathe 'properly' we often make it worse. The best thing with the breath is to just breathe out and let the rest happen. Of course, when the psoas is released, the breath often relaxes and we can breath more naturally.
When the psoas is supple, the skeleton is supported and aligned.