Which is the primary hip flexor?

• A. Rectus femoris
• B. Iliopsoas
• C. Gluteus maximus
• D. Sartorius

Answer: (B)

The main idea being tested is which muscle acts as the chief driver of hip flexion. The iliopsoas is the primary hip flexor. It’s formed by the psoas major and iliacus, and it crosses the hip joint from the lower spine and ilium to insert on the femur’s lesser trochanter. Because of its favorable line of pull and large cross-sectional area, it produces the strongest torque to lift the thigh toward the pelvis, especially when the trunk is stabilized or fixed.

Rectus femoris does help flex the hip since it crosses the hip joint, but as part of the quadriceps it’s not the main driver of hip flexion; its primary role includes knee extension and it shares the work with other muscles. Gluteus maximus mainly extends and externally rotates the hip, so it’s on the opposite side of the action. Sartorius can assist in hip flexion and has multiple actions at the hip and knee, but its contribution to pure hip flexion is limited compared to the iliopsoas.