The iliacus muscle

Iliacus is a triangular muscle of the iliac region which together with the psoas major, comprises the iliopsoas muscle.
The iliacus muscle has a vast origin, the majority of it arising from the superior two-thirds of the iliac fossa. The rest arises from several other origin points, which are the inner lip of the iliac crest, the lateral aspect of the sacrum and anterior sacroiliac and iliolumbar ligaments. The muscle fibers converge distally towards the hip, thus contributing to the triangular shape of this muscle.
At the level of the capsule of the hip joint, the iliacus muscle fibers blend with those of the psoas major muscle, forming a common tendon for the iliopsoas. The tendon of iliopsoas crosses the anterior surface of the hip joint to finally insert to the lesser trochanter of femur. Before insertion, the tendon of iliopsoas receives a small slip called the iliocapsularis muscle, which is a small muscle lying just superficial to the hip joint capsule.
The iliacus muscle is innervated by the femoral nerve (L2-L4).
The iliacus muscle works in synergy with the psoas major muscle to produce movements in the hip joint. When its proximal attachment is fixed, the muscle contributes to the flexion of the thigh. On the other hand, with its distal attachment fixed, the muscle helps to bring the trunk forwards against resistance. These actions are essential for lower limb functions such as walking, running and jumping.