The Psoas is:
A healthily functioning psoas provides a sensitive suspension bridge between the trunk and the legs. Ideally, the psoas guides the transfer of weight from the trunk into the legs and also acts as a grounding wire guiding the flow of subtle energies. Working properly, the psoas functions like the rigging of a circus tent, stabilizing your spine just as guy wires help stabilize the main pole of the big top.
In addition, the psoas provides a diagonal support through the trunk, forming a shelf for the vital organs of the abdominal core. In walking, a healthy psoas moves freely and joins with a released diaphragm to continuously massage the spine as well as the organs, blood vessels, and nerves of the trunk. Working as a hydraulic pump, a freely moving psoas stimulates the flow of fluids throughout the body. And a released, flowing psoas, combined with a stable, weight-bearing pelvis, contributes to the sensations of feeling grounded and centered.
Think of your pelvis as the foundation of a balanced skeletal structure. For your pelvis to provide this stable base, it must function as part of the trunk rather than as part of the legs. Many people mistakenly think of their legs as starting at the waist, perhaps because so many major leg muscles attach to the pelvis. But skeletally and structurally, your legs start at your hip sockets. If your pelvis tilts forward or back or side to side every time you move your legs, the bones can't bear and transfer weight properly. Your psoas will then be called upon to help protect the spine by stabilizing your skeleton. Since the psoas can contract and release independently at any of its joint attachments, it can compensate for structural imbalances in many ways. But if you constantly contract the psoas to correct for skeletal instability, the muscle eventually begins to shorten and lose flexibility.
Shortening the psoas leads to a host of unfortunate conditions. Inevitably, other muscle groups become involved in compensating for the loss of structural integrity. The pelvic bowl tips forward, shrinking the distance between the pelvic crests and the legs, and the femurs are compressed into the hip sockets. To compensate for this constriction, the thigh muscles become overdeveloped. Since full rotation of the thighbones can no longer occur in the hip joints, much of the rotational torque is transferred to the knees and the lumbar spine—a recipe for knee and lower back injuries. In your yoga practice, if you feel strain in your knees or lower back in seated and standing poses, your body may be telling you that you need to lengthen your psoas.
In addition to structural problems, shortening the psoas limits space in the pelvis and abdomen, constricting the organs, putting pressure on nerves, interfering with the movement of fluids, and impairing diaphragmatic breathing. Finally, by limiting your options for movement and by constricting your center, a shortened psoas decreases both your vitality and your connection to the sensations at your skeleto-muscular and emotional core.