Answer: it would lengthen psoas major and iliacus.
Explanation: The psoas muscle is one of the most important muscles along the vertebral column. It is a long muscle firmly attached on either side of the vertebral column and the edge of the lesser pelvis. At its end, it combines with the iliacus muscle to form the iliopsoas muscle. Both muscles flex and laterally rotate the thigh and anteriorly tilt the pelvis at the hip joint so whenever there's a gentle rotation of the thigh at the hip joint, the two muscles become longer to allow flexibility of the joint.
Basically, the iliopsoas muscle is a large muscle made up of the iliacus and the psoas major which is found in the inner thigh and this is the muscle responsible for flexing the thigh at the hip joint.