Biceps Femoris action?

• A. Abduction at hip
• B. Flexion at knee; flexion and medial rotation at hip
• C. Flexion at knee; extension and lateral rotation at hip
• D. Extension at knee

Answer: (C)

The biceps femoris is a two-joint muscle that crosses the hip and knee. Its long head crosses the hip, so it extends the thigh, while both heads cross the knee, so they flex the leg at the knee. When the knee is flexed, the muscle can also rotate the leg laterally; and because the long head crosses the hip, it can contribute to lateral rotation of the thigh when the hip is extended. The short head does not cross the hip, so it cannot extend or rotate the thigh. Therefore, the action pattern that fits is knee flexion with hip extension and lateral rotation of the thigh.