The iliopsoas is actually the blending of two muscles, the large, flat triangular iliacus and the fusiform psoas major. It is considered a single muscle group that begins deep in the back of the abdomen and converges towards its insertion on the back of the femur (thigh bone). It is a powerful flexor of the hip joint. It flexes the thigh when kicking, or the pelvis and trunk when doing sit-ups.
