Fill in the blank: When the _____________ is fixed, the Iliacus acts on the pelvis, and when the femur is fixed, the ____________ acts on the lumbar spine.

The correct response highlights the function of the Iliacus muscle in relation to the positioning of the pelvis and the femur. When the pelvis is stabilized or fixed, the Iliacus muscle can effectively engage and move the pelvis. This muscle plays a crucial role in hip flexion and stabilization, which is essential for proper pelvic alignment and posture during various movements.

Additionally, when the femur is fixed, the Psoas muscle—which works in conjunction with the Iliacus as part of the iliopsoas group—actively influences the lumbar spine. This means that the Psoas can facilitate flexion of the lumbar spine, contributing to movements such as bending forward or stabilizing the spine while the legs are in a fixed position.

Therefore, the understanding of how these two muscles interact with different stable points in the body allows for a better grasp of human biomechanics, particularly in activities involving the hips and lower back. This distinction emphasizes the importance of muscle function based on the stability of either the pelvis or the femur, making the choice of "Pelvis; iliacus" the most accurate in describing these actions.