The iliacus and the psoas major are located in the pelvis and they merge to form the iliopsoas muscle, which is the main hip flexor. The iliacus begins at the top of the hip bone, called the iliac crest, and at the sacrum, which is the bone at the base of your spine. The psoas major starts along the sides of the lumbar vertebrae. The muscles combine in the iliopsoas and run together to attach on the femur, which is your thigh bone. The iliopsoas muscle is responsible for hip flexion and it helps to stabilizes your posture by helping maintain the normal curve of your lumbar spine.
It’s difficult to try to teach in division when muscles all interact!