Which muscle causes knee flexion and hip extension with lateral rotation?

• A. Biceps Femoris
• B. Semitendinosus
• C. Semimembranosus
• D. Adductor Magnus

Answer: (A)

When a muscle spans both the hip and the knee and produces knee flexion with hip extension while rotating the leg outward, the long head of the biceps femoris fits best. It originates on the ischial tuberosity, crosses the hip, and inserts on the fibular head, so it can extend the hip and flex the knee. With the knee flexed, it also causes external (lateral) rotation of the tibia, matching the “lateral rotation” part of the action. The other hamstrings (semitendinosus and semimembranosus) also flex the knee and extend the hip but rotate the tibia medially when the knee is flexed, not laterally. The adductor magnus can extend the hip but doesn’t provide knee flexion or lateral rotation in this combined way. So the long head of the biceps femoris is the best match for knee flexion, hip extension, and lateral rotation.