Our psoas muscle is a long pelvic muscle. For some mysterious reason, it has now assumed great importance in modern, western yoga. Some are calling it, The Muscle of The Soul, the seat of the soul, and many believe that it is the most important muscle in one's body. And, apparently, it is called the seat of the soul in the Taoist system.
Reptilian part of the brain
I read a weird article "confirming" all this by an author who linked it all back to the part of the brain called the "reptilian" brain. This is the very old part of the brain which controls our vital functions such as breathing, heart rate. So it is very important in terms of us having a lifetime in this physical plane of existence. We need this part of the brain to operate effectively.
These are not our Soul
But, it is important to know that the brain is not your soul. Neither is the physical body. Nor are one's emotions, nor are your thoughts. These are all manifestations of the human experience. Our feelings plus our resulting states of mind and thoughts are important, and, as you know, can change from moment to moment.
But they are not the soul.
The heart and the muscle of the Soul
I would suggest, rather, that the heart is closer to being The Muscle of The Soul, for it is within Anahata (heart) chakra (energy and consciousness spiritual quality), that the symbol of The Soul is revealed.
(the still, golden flame of the Soul)
The Higher Expression of the heart
Furthermore, our physical heart is close to our Inner Essence. When we are experiencing Love, our heart is involved. I have learnt, through experience, that Love is also the great emotion and vibration of expressing via our Higher chakras, which are the heart chakra and the chakras above it.
3rd eye and the heart vibration
Even when we are immersed in Ajna chakra, often referred to as The Third Eye, that Love vibration, we are cloaked in it, and it is in every tiny particle of our being. In these moments, that love vibration, it emmantes from us. I can tell you that if this vibration is not present when you believe that you are seated, in your pysche, in Ajna chakra, then you actually are not there.
Our heart is the bridge between our human experience, our feelings; and our Higher Consciousness. The Higher Consciousness is not a muscle, it is the third eye, Ajna chakra. In Maori lore it is called the Whatumanawa, which means the both the eye of the heart, and the eye of the soul. On the physical level there is a direct relationship with the pineal gland. The Higher Consciousness is indeed very close to one's soul, it is an integral part of it.
The heart is part of our Soul's journey in a lifetime
I find that, for me personally, calling the psoas muscle, The Muscle of The Soul, does not make sense to me. When our heart stops beating, we "die" and the Soul's expression of the current lifetime that we are in, is finished in the manner of expression that it has undertaken. For that life. The heart and the lungs are interconnected, for we also "die" when the breathing is ended, and the heart is still, stopped. We need to have our heart and our lungs working at birth in order to experience a lifetime.
Come close to your soul essence via the heart
For anyone to come close to one's soul, I would say: listen to your heart, for it whispers to you. It calls you back time and time again to your essence.