Which muscle crosses both the hip and knee joints and can flex the hip and extend the knee?

• A. Rectus Femoris
• B. Vastus Lateralis
• C. Sartorius
• D. Biceps Femoris

Answer: (D)

A muscle that spans both the hip and knee joints can influence movements at both joints if its line of pull reaches each joint. The rectus femoris does this: it originates on the pelvis (the anterior inferior iliac spine), crosses the hip joint, and inserts on the tibial tuberosity as part of the quadriceps tendon. When it contracts, it helps flex the hip due to its front-to-pelvis orientation, and it aids knee extension through the knee mechanism.

The other muscles don’t match this specific combination. The vastus group crosses only the knee and extends the knee. The sartorius crosses both joints but primarily functions in hip flexion and knee flexion, not knee extension. The biceps femoris crosses both joints too but extends the hip and flexes the knee, rather than flexing the hip while extending the knee.