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 flexion and is the primary connector between the trunk and the lower limbs.
The psoas muscle significantly contributes to stabilizing the spine and maintains posture. It also stabilizes the sacroillac and hip joints contributing to posture.
It's major function is flexion of the thigh at the hip. It can also rotate and abduct the thigh. In action, the psoas muscle allows us to walk, run, sit and jump.
Due to our more sedentary lifestyle (excessive sitting) the psoas muscle becomes short and weak. This causes overuse of other surrounding muscles and often results in low back pain.
Because of it's large size, and shortened length from prolonged sitting, the psoas muscle can override other muscles, especially the abdominal muscles (the abs), when doing exercises like crunches.