Updated May 01, 2014.
The psoas major is one of the group of muscles called hip flexors, because together they work to bring your leg and torso toward one another into a position of flexion. The psoas inserts in the upper interior of your femur, or thigh bone, at one end, then passes through the pelvis and connects to several of the lower vertebra as it fans out at the other end. Since the psoas is not a surface muscle, it can be difficult to identify and isolate, but its anonymity does not decrease its importance.
Indeed, the psoas is key to the cultivation of core strength and hip flexibility and is particularly important in balancing, forward bending, and back bending.
See recommended poses for the psoas.