Which muscle is the chief flexor at the hip joint?

• A. Sartorius
• B. Rectus femoris
• C. Psoas major
• D. Iliopsoas

Answer: (D)

The hip’s strongest flexor is the iliopsoas, a two-part muscle made up of the psoas major and the iliacus that join to form a single tendon inserting on the lesser trochanter. It crosses the hip joint so it pulls the femur upward and forward with a very favorable line of pull and substantial cross‑sectional area, making it the primary driver of hip flexion. When the thigh is fixed, it also helps flex the trunk, highlighting its dual role.

Other muscles listed can assist hip flexion but are not the main muscle for this action. The sartorius crosses the hip and knee and can contribute to flexion, abduction, and external rotation, but its role as a pure hip flexor is limited. Rectus femoris crosses the hip and knee and does help flex the hip, but it is primarily known as a knee extensor, so it’s not the chief hip flexor. Psoas major alone is a powerful hip flexor, but in most descriptions the combined iliopsoas is regarded as the principal hip flexor due to the contribution of both components.