Which of the following best describes the action of the biceps femoris?

• A. Flex the knee, laterally rotate the flexed knee, extend the hip, laterally rotate the hip, tilt pelvis posteriorly
• B. Flex the knee, medially rotate the knee, extend the hip, medially rotate the hip
• C. Extend the knee, flex the hip
• D. Flex the knee, medially rotate the flexed knee, flex the hip

Answer: (C)

Biceps femoris is a two-joint hamstring that crosses both the knee and the hip. Its main actions are knee flexion and hip extension (the long head). When the knee is flexed, the muscle can also cause the lower leg to rotate outward (external rotation) at the knee, and it helps tilt the pelvis posteriorly. Because it pulls across these joints, the description that matches this pattern is knee flexion, external rotation of the flexed knee, hip extension, and posterior pelvic tilt. It does not extend the knee or flex the hip; those would be actions more typical of the quadriceps and hip flexors, respectively.