Which muscle is primarily responsible for flexing the hip joint?

The iliacus muscle plays a key role in flexing the hip joint because it originates from the iliac fossa of the pelvis and inserts into the lesser trochanter of the femur. When the iliacus contracts, it pulls the femur toward the pelvis, effectively flexing the hip. This action is essential for movements such as walking, running, and any activity that involves lifting the leg upwards.

While the other muscles listed do have roles related to hip movement, they do not primarily function as hip flexors in the same way the iliacus does. For instance, the gluteus maximus is mainly involved in hip extension and external rotation. The psoas major also contributes to hip flexion; however, the iliacus and psoas major often work together as the iliopsoas muscle group. The rectus femoris, part of the quadriceps group, does assist in hip flexion but primarily functions in extending the knee. Thus, the iliacus is clearly defined as the primary muscle responsible for flexing the hip joint.