The psoas muscle is a long muscle located on either side of the lumbar vertebra to the lower aspect of the pelvis. It is responsible for hip flection and is the primary connector between the trunk and the lower limbs.
The psoas muscle is a major contributor to stabilizing the spine and for maintaining posture. It also stabilizes the sacroillac and hip joints contributing to postural function.
It's major function is flexion of the thigh at the hip. It can also laterally rotate and abduct the thigh. In action. the psoas muscle allows us to walk, run, sit and jump.
Due to our more sedintary lifestyle (excessive sitting) the psoas muscle becomes shortened and weak. This, in turn, causes overuse of other surrounding muscles resulting in low back pain. Because of it's large size, and shortened length from prolonged sitting, the psoas muscle can override other muscles such as the transverse abdominis (the abs), when trying to tighten theses muscles (crunches) .