What is the action of the iliacus?

The iliacus is a large, flat muscle in the hip region. It plays a crucial role in the movement of the hip joint. Here are the steps that describe its action:

1. Origin: The iliacus originates from the upper two-thirds of the iliac fossa, the inner lip of the iliac crest, the ventral sacroiliac and iliolumbar ligaments, and the upper part of the lateral aspect of the sacrum.

2. Insertion: The muscle fibers converge to form a tendon that inserts into the lesser trochanter of the femur, along with the psoas major muscle. This common insertion point is sometimes referred to as the iliopsoas.

3. Contraction: When the iliacus muscle contracts, it flexes the hip joint. This means it pulls the thigh upward, towards the abdomen. This is the movement you make when you lift your knee towards your chest.

4. Other Actions: In addition to hip flexion, the iliacus also contributes to external rotation of the hip (turning the leg outward), and it can help to stabilize the hip joint and pelvis during walking or running.

5. Innervation: The iliacus is innervated by the femoral nerve, which carries signals from the muscle to the brain and vice versa. This allows the brain to control the contraction of the iliacus and coordinate its actions with other muscles.

So, in summary, the primary action of the iliacus is to flex the hip joint, but it also contributes to external rotation and stabilization of the hip and pelvis.